Sheet Metal Cutting

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Offset compound snips The angle of the shear tips allows you to keep your hand above the sheet metal as you cut

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SHEET METAL CUTTING (SHEARING)
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Cutting processes are those in which a piece of sheet metal is separated by applying a great enough force to cause the material to fail. The most common cutting processes are performed by applying a shearing force, and are therefore sometimes referred to as shearing processes. When a great enough shearing force is applied, the shear stress in the material will exceed the ultimate shear strength and the material will fail and separate at the cut location. This shearing force is applied by two tools, one above and one below the sheet. Whether Sheared edge these tools are a punch and die or upper and lower blades, the tool above the sheet delivers a quick downward blow to the sheet metal that rests over the lower tool. A small clearance is present between the edges of the upper and lower tools, which facilitates the fracture of the material. The size of this clearance is typically 2-10% of the material thickness and depends upon several factors, such as the specific shearing process, material, and sheet thickness.

Shearing of sheet metal between two cutting edges: 1. Just before the punch contacts work. 2. Punch begins to push into work, causing plastic deformation. 3. Punch penetrates into work causing a smooth cut surface. 4. Fracture is initiated at the opposing cutting edges. The clearance c in a shearing operation is the distance between the punch and die, as shown in Figure 20.1(a). Typical clearances in conventional pressworking range between 4% and 8% of the sheet-metal thickness t. The effect of clearances is illustrated in Figure 20.5. If the clearance is too small, then the fracture lines tend to pass each other, causing a double burnishing and larger 3

cutting forces. If the clearance is too large, the metal becomes pinched between the cutting edges and an excessive burr results. The correct clearance depends on sheet-metal type and thickness.

Effect of clearance: (1) clearance too small causes less-than-optimal fracture and excessive forces; (2) clearance optimal, forces are minimal; and (3) clearance too large causes oversized burr. Ultimate shear strength The amount of shear stress σult a material can sustain, measured in units of force per unit area. Shear strength is commonly expressed as MegaPascals (MPa) of original cross section. The maximum shear stress that a material can withstand before eventually failing is called the ultimate shear strength. So-called Tensile strength is the maximum stress value obtained on a stress-strain curve.

Shearing force The amount of force F required to cut or remove a piece of material through shear, as is done in cutting, blanking or punching operations. The applied force must create enough shear stress in the material to exceed the ultimate shear strength, causing the material to fail and separate. F = τsh·P·δ where τsh – shear strength of the sheet metal (τsh = 0,8·σult), MPa; P – perimeter length of the cut edge, mm, and δ – stock thickness, mm. In blanking, punching, slotting, and similar operations, the minor effect of clearance in determining the value of P can be neglected.

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Stock The piece of material from which the workpieces or blanks are cut. If the workpieces are available in the desired size, the stock dimensions will equal those of the workpiece. However, a larger piece of bar stock or sheet stock is typically purchased and the workpieces are cut from it. The number of workpieces that can be cut from a piece of stock depend on the workpiece size and other spacing parameters (sheet border, web width, bar end, cutoff width, facing stock). The effects of shearing on the material change as the cut progresses and are visible on the edge of the sheared material. When the punch or blade impacts the sheet, the clearance between the tools allows the sheet to plastically deform and "rollover" the edge. As the tool penetrates the sheet further, the shearing results in a vertical burnished zone of material. Finally, the shear stress is too great and the material fractures at an angle with a small burr formed at the edge. The height of each of these portions of the cut depends on several factors, including the sharpness of the tools and the clearance between the tools. A variety of cutting processes that utilize shearing forces exist to separate or remove material from a piece of sheet stock in different ways. Each process is capable of forming a specific type of cut, some with an open path to separate a portion of material and some with a closed path to cutout and remove that material. By using many of these processes together, sheet metal parts can be fabricated with cutouts and profiles of any 2D geometry. Such cutting processes include the following:  Shearing – Separating material into two  Notching parts  Nibbling  Blanking – Removing material to use  Lancing for parts  Slitting  Conventional blanking  Parting  Fine blanking  Cutoff  Punching – Removing material as scrap  Trimming  Piercing  Shaving  Slotting  Dinking  Perforating Shearing Straight-Knife Shearing As mentioned above, several cutting processes exist that utilize shearing force to cut sheet metal. However, the term "shearing" by itself refers to a specific cutting process that produces straight line cuts to separate a piece of sheet metal. Most commonly, shearing is used to cut a sheet parallel to an existing edge which is held square, but angled cuts can be made as well. For this reason, shearing is primarily used to cut sheet stock into smaller sizes in preparation for other processes. Shearing has the following capabilities: 5

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Sheet thickness: 0.1-6 mm Tolerance: ±0.1 mm

The shearing process is performed on a shear machine, often called a squaring shear or power shear, that can be operated manually (by hand or foot) or by hydraulic, pneumatic, or electric power. A typical shear machine includes a table with support arms to hold the sheet, stops or guides to secure the sheet, upper and lower straight-edge blades, and a gauging device to precisely position the sheet. The sheet is placed between the upper and lower blade, which are then forced together against the sheet, cutting the material. In most devices, the lower blade Shearing remains stationary while the upper blade is forced downward. The upper blade is slightly offset from the lower blade, approximately 5-10% of the sheet thickness. Also, the upper blade is usually angled so that the cut progresses from one end to the other, thus reducing the required force. The blades used in these machines typically have a square edge rather than a knife-edge and are available in different materials, such as low alloy steel and high-carbon steel. Inclined-Knife Shearing

Shearing operation: (a) side view of the shearing operation; (b) front view of power shears equipped with inclined upper cutting blade Symbol v indicates motion Rotary Shearing Rotary shearing, or circle shearing (not to be confused with slitting), is a process for cutting sheet and plate in a straight line or in contours by means of two revolving, tapered circular cutters. A rotary shear is a device that cuts with a revolving steel wheel. The wheel is very sharp and produces very clean cuts in a fraction of the time it would take to make the same cut with scissors. A rotary shear can be used on any type of material and even sheet metal and aluminum can be cut with this device. The rotary shear can cut many times faster than a conventional pair of scissors and is able to also cut much more accurately. In a production environment where time is very important, a rotary shear is able to increase productivity by cutting much faster and with less waste. Making more exacting cuts equals less discarded material and thus, more profits for the company. The rotary shear is able to offset any increased initial cost by generating a healthier bottom line in production costs. When cutting sheet metal or aluminum, the rotary shear is able to make much neater cuts without distorting the materials in the process. Many times, cutting sheet metal with tin snips or tin shears results in jagged edges and bent and distorted metal from attempting to make turning cuts. The rotary shear eliminates all of the frustration by making turning cuts with ease. Straight cuts are also much smoother and are completed at a much faster pace.

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Rotary shearing is limited to cutting one workpiece at a time. As in straight-knife shearing, multiple layers cannot be sheared, because each layer prevents the necessary breakthrough of the preceding workpiece.

Rotary shears Blanking Blanking is a cutting process in which a piece of sheet metal is removed from a larger piece of stock by applying a great enough shearing force. In this process, the piece removed, called the blank, is not scrap but rather the desired part. Blanking can be used to cutout parts in almost any 2D shape, but is most commonly used to cut workpieces with simple geometries that will be further shaped in subsequent processes. Often times multiple sheets are blanked in a single operation. Final parts that are produced using blanking include gears, jewelry, and watch or clock components. Blanked parts typically require secondary finishing to smooth out burrs along the bottom edge. The blanking process requires a blanking press, sheet metal stock, blanking punch, and blanking die. The sheet metal stock is placed over the die in the blanking press.

Blanking

The die, instead of having a cavity, has a cutout in the shape of the desired part and must be custom made unless a standard shape is being formed. Above the sheet, resides the blanking punch which is a tool in the shape of the desired part. Both the die and punch are typically made from tool steel or carbide. The hydraulic press drives the punch downward at high speed into the sheet. A small clearance, typically 10-20% of the material thickness, exists between the punch and die. 7

When the punch impacts the sheet, the metal in this clearance quickly bends and then fractures. The blank which has been sheared from the stock now falls freely into the gap in the die. This process is extremely fast, with some blanking presses capable of performing over 1000 strokes per minute. Fine blanking Fine blanking is a specialized type of blanking in which the blank is sheared from the sheet stock by applying 3 separate forces. This technique produces a part with better flatness, a smoother edge with minimal burrs, and tolerances as tight as ±0.01 mm. As a result, high quality parts can be blanked that do not require any secondary operations. However, the additional equipment and tooling does add to the initial cost and makes fine blanking better suited to high volume production. Parts made with fine blanking include automotive parts, electronic components, cutlery, and power tools. Most of the equipment and setup for fine blanking is similar to conventional blanking. The sheet stock is still placed over a blanking die inside a hydraulic press and a blanking punch will impact the sheet to remove the blank. As Fine blanking mentioned above, this is done by the application of 3 forces. The first is a downward holding force applied to the top of the sheet. A clamping system holds a guide plate tightly against the sheet and is held in place with an impingement ring, sometimes called a stinger, that surrounds the perimeter of the blanking location. The second force is applied underneath the sheet, directly opposite the punch, by a "cushion". This cushion provides a counterforce during the blanking process and later ejects the blank. These two forces reduce bending of the sheet and improve the flatness of the blank. The final force is provided by the blanking punch impacting the sheet and shearing the blank into the die opening. In fine blanking, the clearance between the punch and the die is smaller, around 0.03 mm, and the blanking is performed at slower speeds. As a result, instead of the material fracturing to free the blank, the blank flows and is extruded from the sheet, providing a smoother edge. Punching Punching is a cutting process in which material is removed from a piece of sheet metal by applying a great enough shearing force. Punching is very similar to blanking except that the removed material, called the slug, is scrap and leaves behind the desired internal feature in the sheet, such as a hole or slot. Punching can be used to produce holes and cutouts of various shapes and sizes. The most common punched holes are simple geometric shapes (circle, square, rectangle, etc.) or combinations thereof. The edges of these punched features will have some burrs from being sheared but are of fairly good quality. Secondary finishing operations are typically performed to attain smoother edges. The punching process requires a punch press, sheet metal stock, punch, and die. The sheet metal stock is positioned between the punch and die inside the punch press. The die, located underneath the sheet, has a cutout in the shape of the desired feature. Above the sheet, the press holds the punch, which is a tool in the shape of the desired feature. Punches and dies of standard shapes are typically used, but custom tooling can be made for punching complex shapes. This tooling, whether standard or custom, is usually made from tool steel or carbide. The punch press drives the punch downward at high speed through the sheet and into the die below. There is a small clearance between the edge of the punch and the die, causing the material to quickly bend and fracture. The slug that is punched out of the sheet falls freely through the tapered opening in the die. This process can be performed on a manual punch press, but today computer numerical 8

controlled (CNC) punch presses are most common. A CNC punch press can be hydraulically, pneumatically, or electrically powered and deliver around 600 punches per minute. Also, many CNC punch presses utilize a turret that can hold up to 100 different punches which are rotated into position when needed. Punching operation A typical punching operation is one in which a cylindrical punch tool pierces the sheet metal, forming a single hole. However, a variety of operations are possible to form different features. These operations include the following:
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Piercing – The typical punching operation, in which a cylindrical punch pierces a hole into the sheet. Slotting – A punching operation that forms rectangular holes in the sheet. Sometimes described as piercing despite the different shape. Perforating – Punching a close arrangement of a large number of holes in a single operation.

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Notching – Punching the edge of a sheet, forming a notch in the shape of a portion of the punch. Nibbling – Punching a series of small overlapping slits or holes along a path to cutout a larger contoured shape. This eliminates the need for a custom punch and die but will require secondary operations to improve the accuracy and finish of the feature. Lancing – Creating a partial cut in the sheet, so that no material is removed. The material is left attached to be bent and form a shape, such as a tab, vent, or louver. Slitting – Cutting straight lines in the sheet. No scrap material is produced.

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Parting – Separating a part from the remaining sheet, by punching away the material between parts. Cutoff – Separating a part from the remaining sheet, without producing any scrap. The punch will produce a cut line that may be straight, angled, or curved.

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Trimming – Punching away excess material from the perimeter of a part, such as trimming the flange from a drawn cup.

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Shaving – Shearing away minimal material from the edges of a feature or part, using a small die clearance. Used to improve accuracy or finish. Tolerances of ±0.02 mm are possible.

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Dinking – A specialized form of piercing used for punching soft metals. A hollow punch, called a dinking die, with beveled, sharpened edges presses the sheet into a block of wood or soft metal.

Workpiece A piece of material that is secured in a fixture and machined into the final part. The workpiece is often cut from a larger piece of stock material and can be a sheet (blank), a standard extruded shape (solid bar, hollow tube, or shaped beam), a custom extrusion, or any prefabricated part such as a casting or forging. Each workpiece shape has certain dimensions that are used in planning the machining operations. Tolerance Also referred to as dimensional accuracy, tolerance is the amount of deviation in a particular dimension of a part, which results from the manufacturing process. Surface roughness The roughness of a part's surface resulting from a manufacturing process. Surface roughness is typically measured as the arithmetic average (Ra) or root mean square (RMS) of the surface variations, measured in micrometers. A typical primary manufacturing process results in an Ra surface roughness of 800-6500 microinches and finishing operations can lower the roughness to 132 microinches. Punch A punch is a tool that is forced into a piece of sheet metal in order to shear or deform the material. Punches are available in many shapes and sizes and can be used for a variety of processes. Many punches are cylindrical and the punch diameter determines the size of the hole or pocket being formed. In shearing processes (blanking or punching), the punch has a square edge to shear the material. In forming processes (bending or deep drawing), the punch has an edge radius.

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SHEET METAL FORMING Sheet metal forming processes are those in which force is applied to a piece of sheet metal to modify its geometry rather than remove any material. The applied force stresses the metal beyond its yield strength, causing the material to plastically deform, but not to fail. By doing so, the sheet can be bent or stretched into a variety of complex shapes. Sheet metal forming processes include the following:  Bending  Roll forming  Spinning  Deep Drawing  Stretch forming  Stamping  Rubber-pad forming  Superplastic forming  Peen forming  Explosive forming  Magnetic-pulse forming Bending Bending is a metal forming process in which a force is applied to a piece of sheet metal, causing it to bend at an angle and form the desired shape. A bending operation causes deformation along one axis, but a sequence of several different operations can be performed to create a complex part. Bent parts can be quite small, such as a bracket, or up to 20 feet in length, such as a large enclosure or chassis. A bend can be characterized by several different parameters, shown in the image below. Bend line – The straight line on the surface of the sheet, on either side of the bend, that defines the end of the level flange and the start of the bend. Outside mold line – The straight line where the outside surfaces of the two flanges would meet, were they to continue. This line defines the edge of a mold that would bound the bent sheet metal. Flange length – The length of either of the two flanges, extending from the edge of the sheet to the bend line. Mold line distance – The distance from either end of the sheet to the outside mold line. Setback – The distance from either bend line to the outside mold line. Also equal to the difference between the mold line distance and the flange length. Bend axis – The straight line that defines the center around which the sheet metal is bent. Bend length – The length of the bend, measured along the bend axis. Bend radius – The distance from the bend axis to the inside surface of the material, between the bend lines. Sometimes specified as the inside bend radius. The outside bend radius is equal to the inside bend radius plus the sheet thickness. Bend angle – The angle of the bend, measured between the bent flange and its original position, or as the included angle between perpendicular lines drawn from the bend lines. Bevel angle – The complimentary angle to the bend angle. The act of bending results in both tension and compression in the sheet metal. The outside portion of the sheet will undergo tension and stretch to a greater length, while the inside portion experiences compression and shortens. The neutral axis is the boundary line inside the sheet metal, along which no tension or compression forces are present. As a result, the length of this axis remains constant. The changes in length to the outside and inside surfaces can be related to the original flat length by two parameters, the bend allowance and bend deduction, which are defined below. Neutral axis – The location in the sheet that is neither stretched nor compressed, and therefore remains at a constant length. K-factor – The location of the neutral axis in the material, calculated as the ratio of the distance of the neutral axis (measured from the inside bend surface) to the material thickness. The 11

K-factor is dependent upon several factors (material, bending operation, bend angle, etc.) and is typically greater than 0.25, but cannot exceed 0.50. Bend allowance – The length of the neutral axis between the bend lines, or in other words, the arc length of the bend. The bend allowance added to the flange lengths is equal to the total flat length. Bend deduction – Also called the bend compensation, the amount a piece of material has been stretched by bending. The value equals the difference between the mold line lengths and the total flat length.

Bending Diagram

Neutral Axis

When bending a piece of sheet metal, the residual stresses in the material will cause the sheet to springback slightly after the bending operation. Due to this elastic recovery, it is necessary to over-bend the sheet a precise amount to achieve the desired bend radius and bend angle. The final bend radius will be greater than initially formed and the final bend angle will be smaller. The ratio of the final bend angle to the initial bend angle is defined as the springback factor, KS. The amount of springback depends upon several factors, including the Springback material, bending operation, and the initial bend angle and bend radius. Bending is typically performed on a machine called a press brake, which can be manually or automatically operated. For this reason, the bending process is sometimes referred to as press brake forming. Press brakes are available in a range of sizes (commonly 20-200 tons) in order to best suit the given application. A press brake contains an upper tool called the punch and a lower tool called the die, between which the sheet metal is located. The sheet is carefully positioned over the die and held in place by the back gauge while the punch lowers and forces the sheet to bend. In an automatic machine, the punch is forced into the sheet under the power of a hydraulic ram. The bend angle achieved is determined by the depth to which the punch forces the sheet into the die. This depth is precisely controlled to achieve the desired bend. Standard tooling is often used for the punch and die, allowing a low initial cost and suitability for low volume production. Custom tooling can be used for specialized bending operations but will add to the cost. The tooling material is chosen based upon the production quantity, sheet metal material, and degree of bending. Naturally, a stronger tool is required to endure larger quantities, harder sheet metal, and severe bending operations. In order of increasing strength, some common tooling materials include hardwood, low carbon steel, tool steel, and carbide steel. 12

Press Brake (Open)

Press Brake (Closed)

While using a press brake and standard die sets, there are still a variety of techniques that can be used to bend the sheet. The most common method is known as V-bending, in which the punch and die are "V" shaped. The punch pushes the sheet into the "V" shaped groove in the V-die, causing it to bend. If the punch does not force the sheet to the bottom of the die cavity, leaving space or air underneath, it is called "air bending". As a result, the V-groove must have a sharper angle than the angle being formed in the sheet. If the punch forces the sheet to the bottom of the die cavity, it is called "bottoming". This technique allows for more control over the angle because there is less springback. However, a higher tonnage press is required. In both techniques, the width of the "V" shaped groove, or die opening, is typically 6 to 18 times the sheet thickness. This value is referred to as the die ratio and is equal to the die opening divided by the sheet thickness.

V Bending In addition to V-bending, another common bending method is wipe bending, sometimes called edge bending. Wipe bending requires the sheet to be held against the wipe die by a pressure pad. The punch then presses against the edge of the sheet that extends beyond the die and pad. The sheet will bend against the radius of the edge of the wipe die.

Wipe Bending Roll forming machines Three-roll forming machines 13

There are two basic types of three-roll forming machines: the pinch-roll type and the pyramid-roll type. The rolls on most three-roll machines are positioned horizontally; a few vertical machines are used, primarily in shipyards. Vertical machines have one advantage over horizontal machines in forming scaly plate: Loose scale is less likely to become embedded in the work metal. With vertical rolls, however, it is difficult to handle wide sections that require careful support to avoid skewness in rolling. Most vertical machines have short rolls for fast unloading and are used for bending narrow plate, bars, and structural sections.

Pinch roll

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Metal Pyramid Roll Conventional pinch-type machines have the roll arrangement shown in Fig. 2. For rolling flat stock up to about 25 mm thick, each roll is of the same diameter. However, on larger machines, the top rolls are sometimes smaller in diameter to maintain approximately the same surface speed on both the inside and outside surfaces of the plate being formed. These heavier machines are also supplied with a slip-friction drive on the front roll to permit slip, because of the differential in surface speed of the rolls. Therefore, as work metal thickness increases, the diameter of the top roll is decreased in relation to the diameter of the lower rolls. The position of the top roll is fixed, while the lower front roll is adjustable vertically to suit the thickness of the blank. Optimal adjustment of the lower roll is important for gripping the stock and for minimizing the length of the flat areas on the workpiece. The rear, or bending, roll is adjustable angularly (usually 30° off vertical), as shown in Fig. 2. Angular movement of this roll determines the diameter of the cylinder to be formed. All of the rolls are powered in most pinch-type machines. On some machines, however, only the two front rolls are powered and the bending roll is rotated by friction between the roll and the work metal (Fig. 2). This arrangement is usually satisfactory in forming medium-to-heavy stock to large diameters. However, when forming sheet or plate that is thin or soft (or both) or when the diameter is large, the amount of friction is sometimes insufficient to rotate the bending roll. This condition can result in a marred surface if the work metal is soft or has a bright mill finish (aluminum sheet, for example).

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3 roll benders work by pinching the metal between two rolls and curving it as it comes in contact with a back forming roll. This curves the metal workpiece into a cylindrical form, where it is welded together to produce a cylinder. The upper roll is in a fixed position, the lower roll has adjustable movement to perform the gripping function. These are the "pinch" rolls. The third roll (the forming roll) is also adjustable. With a manually opened or hydraulically moved drop hinge, the end of the top shaft is opened to allow removal of the finished work piece, especially a completed tube shape. You may also weld the seam while its still on the machine (ground to part with machine power off). Note: Without a lot of skill, on 3 roll machines, it can be difficult to form metal into tubular shapes smaller than 3 times the upper roll diameter when forming near capacity thickness. 1.5 times the upper roll diameter is often the tightest diameter using thinner and narrower metal. Results vary depending on metal thickness, width, tensile strength and on your level of expertise. Below is a diagram showing how a plate is formed into a tube with a single (initial) pinch plate rolling machine.

Single-pinch (initial-pinch) machines are the most common and may require inserting the workpiece into the machine twice in order to prebend both ends to eliminate flat spots when 16

rolling a full tube shape and to ensure better closure of the seam. To prebend the first end, the operator inserts the plate into the machine, which clamps it and pinches it between the top roll and bottom pinch roll. A rear bending roll, moving diagonally toward the top roll, pushes against the metal to bend the radius. The operator then removes the plate from the bender, rotates the plate 180 degrees to insert the second end into the rolls, then rolls the cylinder to completion. Recommended maximum thickness (or width instead) for prebending is usually 3/4 of the capacity of the machine. Bending thickness can be increased slightly if rolling a narrower width. Consult American Machine Tools Company for advice. Double pinch pyramid machines can prebend both ends of a plate with a single insertion into the bending machine, for reduction in material handling and time. But they cost more and cannot roll as tight a diameter. The wide opening between the rolls allows heavy plate to be rolled. On double pinch machines, the top roll position is fixed and the two lower rolls move in a straight path or an arc toward the top roll. Expensive 4 roll versions can accurately roll metal to tighter diameters and can do it in one pass through because the metal is always held by pinch roll while the 3rd and 4th rolls take turns curving the metal. There also exists 2 roll machines where the top roll is steel and the bottom roll is usually urethane covered. These are custom made for production purposes and have an very expensive price. Forming a full tube shape should have little if any gap in the middle of the seam due to the "crowning" of the rolls. Crowning is where the rolls are slightly larger diameter in the middle which compensates for deflection under load. If the crowning is not enough try wrapping and taping a piece of shim stock around the middle 12" of the bending roll to over-compensate for deflection. Crowning has a side effect of causing very thin metal to have a gap at the ends. If a gap occurs, either in the middle or the ends, it can be pulled together using a ratchet strap, come-along, clamps or the vise grips with chain attachment. If you want to roll thick flat bar, pipe, tubing, angle iron or channel into a curve then go to Pipe and Structural Shape Bending machines. On most of our machines, rolling a sheet metal cone shape can be formed by pre-cutting a flat metal blank with the correct inner and outer radius to form the cone (funnel) shape wanted. Usually the blank is fed on one side so that the inner radius can be held against a cone rolling attachment. The inner radius is supposed to go thru slower than the outer radius. The bending roll position should be independently adjusted lower on on the drive side to match the taper of cone. Cones take a lot of time to make. Contact an American Machine Tools sales engineer for more information.

Figs. 3 and 4 show the detailed operational sequences for thin and thick plate four-roll bendings. They consist of some or all of the following bending modes: 1. Edge setting preparation mode (procedure 1 in Fig. 3 for thin plate bending; procedures 1, 2, 4, 5 and 6 in Fig. 4 for thick plate bending), 2. Edge pre-bending mode (procedure 2, or procedure between 3 and 4, in Fig. 3 for thin plate bending; procedures 3 and 7 in Fig. 4 for thick plate bending), 3. Pre-bending continuous bending mode (procedure 4 in Fig. 3 for thin plate bending; procedures between 3 and 4, and between 7 and 8 in Fig. 4 for thick plate bending) 4. Roller swapping bending mode (procedure between 3 and 4 in Fig. 3 for thin plate bending) 5. Steady continuous bending mode (procedures 5 and 6 in Fig. 3 for thin plate bending; procedure 8 in Fig. 4 for thick plate bending) and 6. Completion rolling bending mode (procedure 6 in Fig. 3 for thin plate bending; procedure 8 in Fig. 4 for thick plate bending). In thin plate bending operation, normally, the procedures in Fig. 3 are followed. The combination of the bending modes depends primarily upon the material flexural rigidity, the bender capacity, and partly upon the size and the required accuracy of a finished tubular section. It is, sometimes, influenced by the personal preference of the operators.

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Operational sequence for thin plate bending process.

Operational sequence for thick plate bending process. From a general comparison between the thin and the thick plate operations (Figs. 3 and 4), it can be seen that the particular differences are: (a) a further pre-bending operation to prepare the finished end is needed for the thick plate bending mode, i.e. procedures 3–7 in Fig. 4; (b) a steady continuous bending mode of thin plate bending can be carried out with either the pre-active side roll operative (i.e. procedure 3 in Fig. 3) or with the pre-inactive side roll in operative (i.e. procedure 5 in Fig. 3)—either operation requires the pre-inactive side roll to complete the bending process; (c) a steady continuous bending for thick plate operation is generally performed with both pre-active and pre-inactive side rolls in operation (i.e. procedure 8 in Fig. 4). For most purposes both side rolls are in operation to optimise the bending effect by altering the top roll contact in the steady-continuous bending mode for thick plate operation. An appropriate positioning combination of the two side rolls optimises the bending mechanics on the rolls.

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Design rules  Bend location – A bend should be located where enough material is present, and preferably with straight edges, for the sheet to be secured without slipping. The width of this flange should be equal to at least 4 times the sheet thickness plus the bend radius.  Bend radius  Use a single bend radius for all bends to eliminate additional tooling or setups  Inside bend radius should equal at least the sheet thickness  Bend direction – Bending hard metals parallel to the rolling direction of the sheet may lead to fracture. Bending perpendicular to the rolling direction is recommended.  Any features, such as holes or slots, located too close to a bend may be distorted. The distance of such features from the bend should be equal to at least 3 times the sheet thickness plus the bending radius.  In the case of manual bending, if the design allows, a slot can be cut along the bend line to reduce the manual force required.

Figure 1 Three-roll bending machines are just one of many plate rolling machine styles available to metal fabricators. The best way to determine the best plate or sheet rolling machine (see Figure 1) for the job is to find out what various machines can do. By obtaining this information, you can properly size and select a machine to fit your particular bending application. Of course, for more in-depth information and application engineering expertise, you can contact plate or sheet metal rolling equipment experts directly. An Overview of Rolling Machines Plate or sheet bending rolls are offered in two distinct categories: single and double-pinch, but they may vary in geometry or style. General machine styles are three-roll initial-pinch, three-roll double-pinch, four-roll double pinch, three-roll variable translating, three-roll pyramid, and tworoll systems. Plate rolls are also built in a vertical format for special applications. Matching the most appropriate machine style to the application is important. Machine capacity is equally if not more important than style. Plate roll manufacturers commonly rate their machines according to baseline material yield strengths of 250 to 270 MPa. However, you need to realize that steel mills are producing materials with ever-increasing yields. When choosing a machine, you must refer to your mill certificates and verify the average yield strength of the plate you are buying. It is not uncommon to find that the “mild“ steel you 19

are rolling will have actual yields in the 330- to 400 MPa range. Remember, machine capacity must match your material, and most plate roll manufacturers can provide detailed capacity-versus-yield tables to assist you. You often will see capacities for both prebending and rolling for any plate rolling machine. Prebending is performed on the plate roll at the leading and trailing edges of the sheet (see Figure 2) and eventually the seam (see Figure 3).

Figure 3 If the prebending is done correctly, the seam should come together nicely. A sheet cannot physically be bent right to the edge, and thus what remains is referred to as the unbent flat (see Figure 4). The minimum flat you can expect is 1.5 times the material thickness and often 2.5 to 3.5 times the material thickness for heavier plate.

Figure 4 Because a sheet cannot be bent right to the edge, an unbent flat results in all roll bending jobs. It is the prebending operation, in attempting to minimize the unbent flat, which takes the most power. That’s why prebending ratings are lower than rolling capacities for any given machine. You must be mindful when reviewing machine capacities that the maximum rolling capacity is usually expressed with the basic requirement of multiple rolling passes and very long unbent flats. You also must note the material thickness and width and equipment characteristics such as cylinder diameter, machine type, yield, and diameter of rolls. Operator proficiency also should be taken into consideration. NCs and CNCs are becoming more common in the workplace. Most NC and CNC machines are four-roll types. Automated controls are recommended for high-volume cylinder or shell production and to roll complex shapes that are not easily reproduced using standard manual controls. Multiple bends, variable-radius bends, and ovals are common examples of these complex shapes. A Closer Look at Machine Styles Three-roll initial-pinch (see Figure 5) or single initial-pinch plate rolls generally are for light-capacity applications and may be electromechanical or hydraulic. They work by pinching the flat sheet between two vertically opposed rolls while the third, offset roll—or bending roll—moves 20

upward to contact and then bend the sheet. When rotation of the rollers is activated, the sheet exits at a given radius. With the sheet cut to the developed length and the bending roll properly positioned, the part is rolled into a cylindrical shape that can then be welded at the seam to produce a closed cylinder.

Figure 5 Three-roll initial-pinch machines typically are used for light-gauge applications. When a cylinder is completely rolled, it is extracted from the top roll. Machines generally are equipped with some type of top roll release mechanism that allows for the cylinder’s extraction. This extraction is accomplished with the help of a forward-tilting or forward-releasing top roll or a removable end yoke. In most applications, these machines require removal and reinsertion of the sheet to prebend both ends. They are cost-effective, but may be more labor-intensive in a production setting than their modern counterparts. Many large, mechanical initial-pinch machines were built during the 1950s and may occasionally be found on the used market. All have cast frames, as modern alloys and welding techniques had yet to be invented. Double-pinch plate rolls are available in light to very heavy capacities and can have three (see Figure 6) or four rolls (see Figure 7). The terminology can be confusing as these units also may be referred to as double-pinch pyramid plate rolls or double-initial-pinch plate rolls. Both three- and four-roll styles have fixed-position top rolls and two offset, or lateral, rolls, one on each side.

Figure 6 Double-pinch rolls have a fixed-position Figure 7 Double-pinch rolls with four rolls top roll and, in this case, two offset rolls. instead of three have an additional bottom roller that constantly pinches the plate during rolling. The four-roll styles have an additional roller underneath the top roll, which constantly pinches the plate during rolling. Double-pinch rolls can prebend both plate ends without removal, as is required with single-pinch rolls. Three-roll machines generally require prebending the leading end, running the sheet through the machine to prebend the trailing end, and then switching roll rotation direction to roll the cylinder body. Four-roll plate rolls have a slight advantage in cycle time because they permit prebending of the leading edge, rolling the cylinder body, and finishing off the trailing prebend, all while rolling in the same direction. Smaller machines can be mechanical, but most are hydraulic and include drop end yokes (see Figure 8) for easy extraction of the workpiece. 21

Figure 8 Easy-to-remove end yokes can make material removal a quick process. Four-roll plate rolls generally are the only equipment with NCs or CNCs because the fourth roll provides constant pinching action, minimizing the chance for slippage. Automatic controls use an encoder to track movement of the plate through the machine. If the plate slips, the bending roll movements will be out of synch with the rolling movement. Variable-geometry three-roll plate rolls are not new, but are gaining in popularity around the world (see Figure 9). They are built to handle medium to extremely heavy plate rolling applications.

Figure 9 Variable-geometry three-roll machines are becoming a more familiar sight in metal fabricating shops around the world. The top roll moves up and down while the lower two rolls each move horizontally. This lower roll movement increases the offset distance from the top roll and in so doing delivers a distinct mechanical advantage in bending. A machine of this type works well over a wide range of material thicknesses. With varying geometry, these rolls can be used like a single-pinch, double-pinch, or pyramidstyle machine that requires minimal sheet movement during prebending. In the past these machines commonly were found in shipyards, but are now being placed in general job shop and manufacturing applications. True pyramid machines are rarely used in cutting-edge facilities. They usually can be found on the used market. They have three rolls (see Figure 10), with both lower rolls fixed in position and the top, or bending, roll moving up and down. In general, they leave a very long unbent flat and are not as user-friendly as other types of rolling machines. Two-roll machines (see Figure 11) are designed for thin-gauge material rolled to reasonably small diameters. They use a large-diameter, urethane-coated pinch roller that moves up with extreme pressure against a small-diameter steel top roll. A mandrel or drum, very close in OD to the desired ID of the finished part, is fitted over the top roll.

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Figure 10 Three-roll pyramid machines cannot deliver the same benefits as more modern equipment.

Figure 11 Two-roll machines are suitable for thin-gauge applications.

Two-roll sheet rolls are extremely fast and will roll round parts even if the blank has cutouts or holes. Because they require a mandrel for each part diameter and material thickness, they are not as versatile as some other machines, but for dedicated high-speed production, they are often the absolute best choice. Comparing four rolls vs three rolls – what are the advantages & disadvantages Four-roll technology has been around almost since the turn of the century: however, it was impractical, as the improved production did not justify the costs. With the advances in Fluid Technology this has changed dramatically. It is now possible to buy a four-roll machine for only about 20% more than a three-roll double pinch machine. Is this an important development to plate roll users? To answer this question, let’s briefly review the functions of the single initial pinch roll, double pinch roll and the basic pyramid roll. Pyramid Roll: Unable to pre-bend, must either live with a large flat area at joining point of metal, or use a press brake to pre-bend prior to rolling. Its primary advantage is that it is inexpensive; however, unless you can live with a large flat area on cylinder, it ends up costing you more in terms of secondary equipment and material handling. It is also very difficult to roll cones. Initial Pinch Roll: Has pre-bend capability and material can be introduced horizontally. The disadvantage is that it is difficult to do cone bending, and plate has to be removed from machine and rotated 180 degrees in order to pre-bend trailing edge. This is a serious disadvantage as it requires that the plate is squared completely once again, and it is during the squaring operation that many of the mistakes are made that lead to bad parts. It is also responsible for many shop accidents when the plate is being rotated 180 degrees. Double Pinch Pyramid: It can pre-bend both ends of a plate without removing plate from machine. The disadvantage is that it takes six different positioning of the rolls to complete a cylinder. Because of the pyramid design, it cannot pre-bend as close to the edge as an initial pinch or a four-roll machine. It is able to roll cones, but with difficulty. For the purpose of comparison, I will be comparing against the initial pinch and the double pinch rolls. The pyramid rolls, while they have a place in the market, are not considered when prebending is a prerequisite. The purpose of this document is to demonstrate that the four-roll has all the desirable features of the other two without the disadvantages – plus, adds additional advantages the other two do not have. Four-Roll Advantages Simplicity: The single biggest advantage that four-roll machines have over the other two machines is simplicity. 23

In order to obtain a perfectly bent pipe with a three-roll double pinch, it is necessary to do three different operations: It is necessary to pre-bend the leading edge of material. This is done by pinching plate between one of the side rolls and top roll. You must also lower the opposing side roll to create the proper geometry for the pre-bend. Because of this it is impossible to load and roll material in the horizontal position. It also requires a much larger area in the shop as the material must pass all the way through the machine in its stretch-out condition for the pre-bending so it requires at least equal distance on both sides of the machine. It is necessary to completely change roll position and move plate back to center of machine and position side roll at correct position to achieve required diameter. This sounds difficult because it is. Remember, every release of rolls is an opportunity for misalignment of plate. To roll a given diameter on a four-roll is extremely simple. You introduce plate into the roll touching it off to the opposing outboard roll for quick and accurate squaring of plate. You then raise lower central pinch roll and the plate is locked into position with no possibility of slipping. After this you back plate up to near tangent point of central rolls, then raise the left or right outboard roll to the correct position to achieve your diameter and begin rolling. When the back edge gets close, simply release the left outboard roll and bring up the right outboard roll until it touches the plate and finish the pipe in one pass. By comparison, a very simple operation. Because the plate is automatically squared and always pinched and not released until the pipe is complete, the net result of this difference is that the four-roll requires 66% less positioning and much less experience on the operator’s part. In most cases, it is difficult to determine the correct position of roll to achieve a given diameter. The operator takes his best conservative guess and moves up from there; however, given the fact that on a three-roll double pinch he must have 6 positioning to achieve a diameter (even a wrong one) it becomes very time consuming with a risk of scraping the material before correct diameter is reached. A four-roll machine, which requires only two positionings, never releases metal and arrives at correct diameter in less than half the time, with much less risk of a scraped piece. Given the above, there is no question the four-roll is the simplest, most productive machine available in rolling technology today. Cone Bending: As cone bending is very difficult on an initial pinch roll it has, up until the last few years, been accepted that the best method to bend cones is with a double pyramid pinch roll. However, it is not an easy process with a double pyramid roll at all. In fact, it is not uncommon for jobbers, as well as manufacturers, to own a double pinch roll and still choose to bump out their cones on a press brake. The only machine capable of bending a cone properly is a four-roll machine. To roll cones on a three-roll machine is very difficult. First you must realize that a cone has to be developed by rolling a plate at two different speeds at the same time. This is a difficult situation to achieve. Both the three-roll and the four-roll machines are capable of inclining the side rolls in a positive attitude, and both have a hardened contrast die to control and slow down the speed of the small diameter. This is an equal comparison as far as it goes; but, by guiding the small diameter and inclining the roll (both of which are necessary to roll cones) you have still created an unnatural situation for rolling cones. Why? Because on three-roll double pinch machine all threerolls are driven, which makes it very difficult for the contrast die to be able to retard the rotation on the small diameter while making the large diameter move faster. This causes lamination and scarring on plate and the roll. So, why can a four-roll, which also has inclinable side rolls and a hardened contrast die do this difficult function better than three-roll? The answer is this: the four-roll (lower central pinch roll) can be inclined in a negative attitude and is also capable of adjusting the force at which it pinches which allows the roll to grip the cone only on the large diameter which needs to turn faster and only with enough force to turn the part. This allows the small diameter to be slowed down more easily. 24

To sum it up, rolling cones properly absolutely requires a lower central pinch roll (fourth roll) capable of a negative inclination and adjustable pinch pressure. Only four-roll machines have this capability but, be careful, not all four-roll machines have it. Be sure to ask the builder about this feature. Also, make sure when inquiring about four-roll machines that the side rolls move independently so that one can be used as a squaring gauge. Handling the Plate: Bending light sheet presents no particular handling problem to either type roll, although the three-roll must be lined up with a groove and then pressure applied to hold this position. On a fourroll, you merely bump the sheet off the back roll which acts as a positive stop and then pinches the plate to insure position. The real problems start with the rolling of long plate. Because the three-roll pyramid has to lower one of the side rolls and pinch and pre-bend with the other, it is really not suited for long plates, as it would drag the ground. This leaves two options; the initial pinch and the four-roll. The initial pinch can require as many as 2 or 3 people to help maintain control of the plate by using cranes, hoists, etc. Also remember, the plate has to be taken out of the machine and turned for the opposite pre-bend operation. Again, this sounds as a tough and time-consuming operation; companies doing this type of work will tell you it is tougher. By contrast, once again, the four-roll is uniquely suited for this type of work. First, like the initial pinch, in a horizontal position allowing for conveyors or support stands, this is the safest, most controllable condition and does not require 2 or 3 men to control plate. Secondly, plate does not have to be turned around. Speed: Because the initial pinch must turn plate for second pre-bend and the three-roll double pyramid must make 6 positioning to roll a pipe, it is conservatively estimated that the floor-to-floor time on making a pipe is 50% faster in production situations on a four-roll with much less operator expertise required. Put simply, if a three-roll can roll a vessel in 20 minutes, a four-roll could do it in 10 minutes, or twice as fast. Even if a company is rolling only a few pipes a day, there is no reason not to do them a rapidly as possible so you can get on with your other work. Automatic Squaring of Material: On a three-roll machine, squaring of plate is a very difficult process and one of the most important. It is extremely difficult to control the squareness of plate over a 6′- 12′ long piece with just one man. Three-roll manufacturers usually put a small groove in the outboard rolls to help line plate up but even with this, it often requires two men to square plate properly. No matter how long it takes, there is no alternative; the plate has to be square or you cannot proceed. This process, on a three-roll, is time-consuming and can be very frustrating. On a four-roll machine, the process is automatic and takes only a few seconds and, equally important, only one operator. This is done by lifting one of the independent outboard rolls and using it as a squaring gauge. Once the material is in contact all the way across, the operator simply drives the lower pinch roll up until it pinches material and, from that point, you can roll complete pipe in one pass. Constantly Pinched Plate: One advantage of maintaining a pinched condition is that the operator has total control of all plate motion. In this condition, it is possible for one operator to roll parabolic curves or boxes without leaving the control and with only one squaring of the plate. This is impossible to do on a three-roll machine. It also isn’t possible to vary the pinch pressure so that you can supply strong force for big plate and less force for thin or soft material and because the plate is driven, it prevents it fro slipping out of position which happens with three-roll machines. Another disadvantage of three-roll double pinch machine is rolling thin sheet (less than 30% capacity) because of the lack of resistance in the material. Again, this is not a problem for a four-roll which is pinching material and creating its own drive force, regardless of resistance in material. Bottom line; a four-roll plate bending machine will improve your production dramatically. Other Considerations for Rolling Machines 25

In terms of optional equipment for rolling machines, the most important items to consider are hardened roll surfaces and cone rolling devices. Today’s harder materials and laser/plasma cutting techniques require hard outer roll surfaces on rolling equipment. Look for a hardness rating from 50 to 55 Rockwell C scale. Hardness in this range will have a reasonable penetration depth and provide long-lasting protection against roll surface wear. Hardness exceeding 60 will have a shallow penetration and result in cracking or crazing of the roll surface. Cone rolling devices, which permit you to roll a conical shape, are standard on some machines. Lateral material supports and overhead supports are also optional, but less frequently requested. Overhead supports prevent light materials from collapsing when rolled to large diameters. A side support can also assist in preventing light materials from recurving toward the floor if the radius is very large. Some machines have extended roll shafts that protrude through the machine frame. Section or pipe dies can be fitted on these stub shafts, but it is not practical to roll angle iron on a plate roll. Angle tends to twist when rolled, and plate rolls do not have outboard, adjustable, lateral material guides to prevent this twist. You should consider using a section rolling machine or angle roll for that type of bending. In general, section dies on plate rolls are good for bending flat bar the hard way, rods, or small pipe. Additionally, most new roll bending machines are equipped with modern safety devices such as emergency-stop buttons; safety trip wires; 24-VAC, low-voltage control circuitry; and detached operator control consoles. However, it remains the responsibility of the owner to ensure the installation and proper use of operation safety guards or devices.

Roll forming Roll forming, sometimes spelled rollforming, is a metal forming process in which sheet metal is progressively shaped through a series of bending operations. The process is performed on a roll forming line in which the sheet metal stock is fed through a series of roll stations. Each station has a roller, referred to as a roller die, positioned on both sides of the sheet. The shape and size of the roller die may be unique to that station, or several identical roller dies may be used in different positions. The roller dies may be above and below the sheet, along the sides, at an angle, etc. As the sheet is forced through the roller dies in each roll station, it plastically deforms and bends. Each roll station performs one stage in the complete bending of the sheet to form the desired part. The roller dies are lubricated to reduce friction between the die and the sheet, thus reducing the tool wear. Also, lubricant can allow for a higher production rate, which will also depend on the material thickness, number of roll stations, and radius of each bend. The roll forming line can also include other sheet metal fabrication operations before or after the roll forming, such as punching or shearing.

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Rolled profiles Roll Forming Line The roll forming process can be used to form a sheet into a wide variety of cross-section profiles. An open profile is most common, but a closed tube-like shape can be created as well. Because the final form is achieved through a series of bends, the part does not require a uniform or symmetric cross-section along its length. Roll forming is used to create very long sheet metal parts with typical widths of 25-500 mm and thicknesses of 0,1-3,5 mm. However wider and thicker sheets can be formed, some up to 1500 mm wide and 6 mm thick. The roll forming process is capable of producing parts with typical cross section tolerances from +/- 0,25 mm to +/- 0,75 mm. Angular tolerances typically +/- 1 degree. Typical roll formed parts include panels, tracks, shelving, etc. These parts are commonly used in industrial and commercial buildings for roofing, lighting, storage units, and HVAC applications. Advantages of roll forming a metal part  the roll forming process allows operations such as punching, notching, and welding to be performed in-line. Labor cost and time for secondary operations are reduced or eliminated, reducing part costs;  roll form tooling allows for a high degree of flexibility. A single set of roll form tools will make almost any length of the same cross section. Multiple sets of tools for varying length parts are not required;  roll forming can provide better dimensional control than other competing metal forming processes;  repeatability is inherent in the process, allowing easier assembly of roll formed parts into your finished product, and minimizing problems due to "standard" tolerance build up;  roll forming is typically a higher speed process;  roll forming offers customers a superior surface finish. This makes roll forming an excellent option for decorative stainless steel parts or for parts requiring a finish such as anodizing or powder coating. Also, a texture or pattern can be rolled into the surface during forming;  roll forming utilizes material more efficiently than other competing processes;  roll formed shapes can be developed with thinner walls than competing processes. Spinning Spinning, sometimes called spin forming, is a metal forming process used to form cylindrical parts by rotating a piece of sheet metal while forces are applied to one side. A sheet metal disc is rotated at high speeds while rollers press the sheet against a tool, called a mandrel, to form the shape of the desired part. Spun metal parts have a rotationally symmetric, hollow shape, such as a cylinder, cone, or hemisphere. Examples include cookware, hubcaps, satellite dishes, rocket nose cones, and musical instruments.

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Spinning Lathe Spinning is typically performed on a manual or CNC lathe and requires a blank, mandrel, and roller tool. The blank is the disc-shaped piece of sheet metal that is pre-cut from sheet stock and will be formed into the part. The mandrel is a solid form of the internal shape of the part, against which the blank will be pressed. For more complex parts, such as those with reentrant surfaces, multi-piece mandrels can be used. Because the mandrel does not experience much wear in this process, it can be made from wood or plastic. However, high volume production typically utilizes a metal mandrel. The mandrel and blank are clamped together and secured between the headstock and tailstock of the lathe to be rotated at high speeds by the spindle. While the blank and mandrel rotate, force is applied to the sheet by a tool, causing the sheet to bend and form around the mandrel. The tool may make several passes to complete the shaping of the sheet. This tool is usually a roller wheel attached to a lever. Rollers are available in different diameters and thicknesses and are usually made from steel or brass. The rollers are inexpensive and experience little wear allowing for low volume production of parts. There are two distinct spinning methods, referred to as conventional spinning and shear spinning. In conventional spinning, the roller tool pushes against the blank until it conforms to the contour of the mandrel. The resulting spun part will have a diameter smaller than the blank, but will maintain a constant thickness. In shear spinning, the roller not only bends the blank against the mandrel, it also applies a downward force while it moves, stretching the material over the mandrel. By doing so, the outer diameter of the spun part will remain equal to the original blank diameter, but the thickness of the part walls will be thinner.

Conventional Spinning vs. Shear Spinning Deep Drawing Deep drawing is a metal forming process in which sheet metal is stretched into the desired part shape. A tool pushes downward on the sheet metal, forcing it into a die cavity in the shape of 28

the desired part. The tensile forces applied to the sheet cause it to plastically deform into a cupshaped part. Deep drawn parts are characterized by a depth equal to more than half of the diameter of the part. These parts can have a variety of cross sections with straight, tapered, or even curved walls, but cylindrical or rectangular parts are most common. Deep drawing is most effective with ductile metals, such as aluminum, brass, copper, and mild steel. Examples of parts formed with deep drawing include automotive bodies and fuel tanks, cans, cups, kitchen sinks, and pots and pans. The deep drawing process requires a blank, blank holder, punch and die. The blank is a piece of sheet metal, typically a disc or rectangle, which is pre-cut from stock material and will be formed into the part. The blank is clamped down by the blank holder over the die, which has a cavity in the external shape of the part. A tool called a punch moves downward into the blank and draws, or stretches, the material into the die cavity. The movement of the punch is usually hydraulically powered to apply enough force to the blank. Both the die and punch experience wear from the forces applied to the sheet metal and are therefore made from tool steel or carbon steel. The process of drawing the part sometimes occurs in a series of operations, called draw reductions. In each step, a punch forces the part into a different die, stretching the part to a greater depth each time. After a part is completely drawn, the punch and blank holder can be raised and the part removed from the die. The portion of the sheet metal that was clamped under the blank holder may form a flange around the part that can be trimmed off.

Deep Drawing

Deep Drawing Sequence Stretch Forming

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Stretch forming is a metal forming process in which a piece of sheet metal is stretched and bent simultaneously over a die in order to form large contoured parts. Stretch forming is performed on a stretch press, in which a piece of sheet metal is securely gripped along its edges by gripping jaws. The gripping jaws are each attached to a carriage that is pulled by pneumatic or hydraulic force to stretch the sheet. The tooling used in this process is a stretch form block, called a form die, which is a solid contoured piece against which Stretch Forming the sheet metal will be pressed. The most common stretch presses are oriented vertically, in which the form die rests on a press table that can be raised into the sheet by a hydraulic ram. As the form die is driven into the sheet, which is gripped tightly at its edges, the tensile forces increase and the sheet plastically deforms into a new shape. Horizontal stretch presses mount the form die sideways on a stationary press table, while the gripping jaws pull the sheet horizontally around the form die.

Stretch forming machine Stretch formed parts are typically large and possess large radius bends. The shapes that can be produced vary from a simple curved surface to complex non-uniform cross sections. Stretch forming is capable of shaping parts with very high accuracy and smooth surfaces. Ductile materials are preferable, the most commonly used being aluminum, steel, and titanium. Typical stretch formed parts are large curved panels such as door panels in cars or wing panels on aircraft. Other stretch formed parts can be found in window frames and enclosures. Rubber-pad forming Also known as flexible-die forming employs a rubber pad or a flexible diaphragm as one tool half, requiring only one solid tool half to form a part to final shape. The solid tool half is usually similar to the punch in a conventional die, but it can be the die cavity. The rubber acts somewhat like hydraulic fluid in exerting nearly equal pressure on all workpiece surfaces as it is pressed around the form block. Rubber-pad forming is designed to be used on moderately shallow, recessed parts having simple flanges and relatively simple configurations. Form block height is usually less than 100 mm. The production rates are relatively high, with cycle times averaging 1 min or less. 30

The advantages of the rubber-pad forming processes compared to conventional forming processes are:  only a single rigid tool half is required to form a part;  one rubber pad or diaphragm takes the place of many different die shapes, returning to its original shape when the pressure is released;  tools can be made of low cost, easy-to-machine materials due to the hydrostatic pressure exerted on the tools;  the forming radius decreases progressively during the forming stroke, unlike the fixed radius on conventional dies;  thinning of the work metal, as occurs in conventional deep drawing, is reduced considerably;  different metals and thicknesses can be formed in the same tool;  parts with excellent surface finish can be formed as no tool marks are created;  set-up time is considerably shorter as no lining-up of tools is necessary. The disadvantages are:  the pad or diaphragm has a limited lifetime that depends on the severity of the forming in combination with the pressure level;  lack of sufficient forming pressure results in parts with less sharpness or with wrinkles, which may require subsequent hand work;  the production rate is relatively slow, making the process suitable primarily for prototype and low volume production work.

Bending process with a rubber pad. (a) Before forming. (b) After forming. While many sheet-forming processes are carried out in a press with male and female dies usually made of metal, there are some processes which utilize rubber to replace one of the dies. The simplest of these processes is shown at the figure below.

Rubber-pad forming. (a) Before forming. (b) After forming.

Rubber-pad formed parts

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Hydro forming Hydroforming, sometimes referred to as fluid forming or rubber diaphragm forming, was developed during the late 1940's and early 1950's in response to a need for a lower cost method of producing relatively small quantities of deep drawn parts. Hydroforming, in simple terms, replaces the punch in traditional stamping with liquid usually water to provide shaping force. Hydroforming refers to the manufacture, via fluid pressure, of hollow parts with complex geometries. Hydroforming can be used to shape tubes or extrusions where it finds its greatest use or to shape sheet blanks. In tube and extrusion hydroforming, the workpiece is inflated by introducing fluid into the cavity while the tube undergoes axial or radial compression. The tube then expands where permitted by the tooling to the die wall. Such hydroforming in many cases is preceded by forming steps such as bending the tube to distribute where it’s needed corner radii, usually for final hydroforming, or bent in order to fit into the die. Hydroforming dies used for tubes or extrusions consist of upper and lower blocks and plates as well as axial units used for sealing and end-feeding of the part. A sheet blank can be formed via fluid applied directly or through a bladder system to force the sheet to assume the shape of the die wall or punch end. Here, the punch may provide additional pressure to assist in the process. The hydroforming process requires specialized presses or specially fitted hydraulic presses and tooling as well as fluid delivery, storage, disposal and reclamation capability. Fluid pressure can range from the about 3,000 to nearly 100,000 psi. Competitive processes Deep-draw stamping, tube bending, fabrication. Applications In automotive, the process delivers hollow parts such as radiator frames, engine cradles, exhaust manifolds, roof and frame rails and instrument-panel supports. Various rails, manifolds and supports find use in aircraft and appliance applications. Parts made through sheet hydroforming, currently a low-volume specialty process, include automotive deep-drawn fuel-tank trays and body panels as well as appliance parts such as panels and sink basins. The process also works well with smaller parts such as fittings and fuel filler necks.

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Benefits Hydroforming results in lighterweight parts in applications where it has replaced traditional stamping, fabrication and assembly methods. In many cases, one-piece hydroformed parts can replace assemblies, thus increasing structural integrity while saving on material costs and reducing scrap. Hydroforming is better suited in producing parts from high-strength steel and aluminum than competing processes. Recently, technology has allowed inclusion of operations such as piercing during hydroforming. Capacities: Part size is dependent on press size. Currently, the largest hydroforming press available can churn out parts to nearly 20 ft. long, but typical parts are less than half that size, and can be produced in sizes down to a few inches. Cycle times are slower than traditional stamping methods. Materials: High-strength steel and aluminum are the materials of choice in hydroforming parts for automotive use. But any sheet material that can be cold formed is a candidate for hydroforming.

Superplastic forming The superplastic forming (SPF) operation is based on the fact that some alloys can be slowly stretched well beyond their normal limitations at elevated temperatures. The higher temperatures mean the flow stress of the sheet material is much lower than at normal temps. This characteristic allows very deep forming methods to be used that would normally rupture parts. Superplastic alloys can be stretched at higher temperatures by several times of their initial length without breaking. Superplastic forming can produce complex shapes with stiffening rims and other structural features as well.

Superplasticity is a term used to indicate the exceptional ductility that certain metals can exhibit when deformed under proper conditions. The term is most often related to the ductile tensile behavior of the material; however, superplastic deformation has the characteristic of easy deformation under low pressures, and compression deformation characteristics are also described as superplastic. The tensile ductility of superplastic metals typically ranges from 200 to 1000% elongation, but ductilities in excess of 5000% have been reported (Ref 1). Elongations of this magnitude are one to two orders greater than those observed for conventional metals and alloys, and they are more characteristic of plastics than metals. Because the capabilities and limitations of sheet metal fabrication are most often determined by the tensile ductility limits, it is clear that there are significant advantages potentially available for forming such materials, provided the high ductility characteristics observed in the tensile test can be used in production forming processes. Complex shapes, fine detail, and close tolerances; forming times are long, and hence production rates are low; parts not suitable for high-temperature use. 33

The process begins by placing the sheet to be formed in an appropriate SPF die, which can have a simple to complex geometry, representative of the final part to be produced. The sheet and tooling are heated and then a gas pressure is applied, which plastically deforms the sheet into the shape of the die cavity. Process Advantages - reduced weight for high fuel efficiency  improved structural performance  increased metal formability and part complexity  near net shape forming of complex shapes reduces part count  cost/weight savings  low-cost tooling  low environmental impacts - non-lead die lubes, low noise Materials used  titanium alloys  aluminum alloys  bismuth-tin alloys  zinc-aluminum alloys  stainless steel  aluminum-lithium alloys Explosive forming Explosive forming has evolved as one of the most dramatic of the new metalworking techniques. Explosive forming is employed in aerospace and aircraft industries and has been successfully employed in the production of automotive-related components. Explosive Forming or HERF (High Energy Rate Forming) can be utilized to form a wide variety of metals, from aluminum to high strength alloys. In this process the punch is replaced by an explosive charge. The process derives its name from the fact that the energy liberated due to the detonation of an explosive is used to form the desired configuration. The charge used is very small, but is capable of exerting tremendous forces on the workpiece. In Explosive Forming chemical energy from the explosives is used to generate shock waves through a medium (mostly water), which are directed to deform the workpiece at very high velocities. Methods of Explosive Forming Explosive Forming Operations can be divided into two groups, depending on the position of the explosive charge relative to the workpiece. Standoff Method In this method, the explosive charge is located at some predetermined distance from the workpiece and the energy is transmitted through an intervening medium like air, oil, or water. Peak pressure at the workpiece may range from a few thousand psi to several hundred thousand psi depending on the parameters of the operation. Contact Method In this method, the explosive charge is held in direct contact with the workpiece while the detonation is initiated. The detonation produces interface pressures on the surface of the metal up to 35000 MPa. The system used for Standoff Method consists of following parts: 1) An explosive charge 2) An energy transmitted medium 3) A die assembly 4) The workpiece. The die assembly is put together on the bottom of a tank. Workpiece is placed on the die and blankholder placed above. A vacuum is then created in the die cavity. The explosive charge is placed 34

in position over the centre of the workpiece. The explosive charge is suspended over the blank at a predetermined distance. The complete assembly is immersed in a tank of water. After the detonation of explosive, a pressure pulse of high intensity is produced. A gas bubble is also produced which expands spherically and then collapses until it vents at the surface of the water. When the pressure pulse impinges against the workpiece, the metal is displaced into the die cavity. Explosives Explosives are substances that undergo rapid chemical reaction during which heat and large quantities of gaseous products are evolved. Explosives can be solid (TNT-trinitro toluene), liquid (Nitroglycerine), or Gaseous (oxygen and acetylene mixtures). Explosives are divide into two classes; Low Explosives in which the ammunition burns rapidly rather than exploding, hence pressure build up is not large, and High Explosive which have a high rate of reaction with a large pressure build up. Low explosives are generally used as propellants in guns and in rockets for the propelling of missiles. Advantages of Explosion Forming  maintains precise tolerances;  eliminates costly welds;  controls smoothness of contours;  reduces tooling costs;  less expensive alternative to super-plastic forming. Die Materials Different materials are used for the manufacture of dies for explosive working, for instance high strength tool steels, plastics, concrete. Relatively low strength dies are used for short run items and for parts where close tolerances are not critical, while for longer runs higher strength die materials are required. Kirksite and plastic faced dies are employed for light forming operations; tool steels, cast steels, and ductile iron for medium requirements. Material of Die Application Area Kirksite Low pressure and few parts Fiberglass and Kirksite Low pressure and few parts Fiberglass and Concrete Low pressure and large parts Epoxy and Concrete Low pressure and large parts Ductile Iron High pressure and many parts Concrete Medium pressure and large parts Characteristics of Explosive Forming Process  very large sheets with relatively complex shapes, although usually axisymmetric;  low tooling costs, but high labor cost;  suitable for low-quantity production;  long cycle times. Transmission Medium Energy released by the explosive is transmitted through medium like air, water, oil, gelatin, liquid salts. Water is one of the best media for explosive forming since it is available readily, inexpensive and produces excellent results. The transmission medium is important regarding pressure magnitude at the workpiece. Water is more desirable medium than air for producing high peak pressures to the workpiece. Very large sheets with relatively complex shapes, although usually axisymmetric; low tooling costs, but high labor costs; suitable for low-quantity production; long cycle times. Explosive forming changes the shape of a metal blank or preform by the instantaneous high pressure that results from the detonation of an explosive. This article is concerned only with the explosives generally termed high explosives, and not with so-called low explosives. Metal tubing up to 1.4 m in diameter in lengths up to 4.6 m have been formed using the explosive forming process. Diameters of 1.4 m or less can have lengths of up to 9.1 m. Typical domes constructed of 6- to 12-piece gore sections fabricated from explosively formed metal can 35

measure up to 6.1 m in diameter. Russian engineers have used the process to fabricate gore sections for a 12 m diam dome. Systems used for explosive-forming operations are generally classified as either confined or unconfined. This article will primarily deal with unconfined systems. Confined systems (Fig. 1) use a die, in two or more pieces, that completely encloses the workpiece. The closed system has distinct advantages for the forming of thin stock to close tolerances, and it has been used for the close-tolerance sizing of thin-wall tubing. However, confined systems are generally used only for the forming of comparatively small workpieces because economic feasibility decreases as the size of the workpiece increases.

Magnetic-pulse forming Electromagnetic forming (EM forming or Magneforming) is a high energy rate metal forming process that uses pulsed power techniques to create ultrastrong pulsed magnetic fields to rapidly reshape metal parts. In practice the metal "work piece" to be fabricated is placed in close proximity to a heavily constructed coil of wire (called the work coil). A huge pulse of current is forced through the work coil by rapidly discharging a high voltage capacitor bank using an ignitron or a spark gap as a switch. This creates a rapidly oscillating, ultrastrong electromagnetic field around the work coil. The rapidly changing magnetic field induces a circulating electrical current within the work piece through electromagnetic induction, and the induced current creates a corresponding magnetic field around the metal work piece. Because of Lenz's Law, the magnetic fields created within the metal work piece and work coil strongly repel each another. The high work coil current (typically tens or hundreds of thousands of amperes) creates ultrastrong magnetic forces that easily overcome the yield strength of the metal work piece, causing permanent deformation. The metal forming process occurs extremely quickly (typically tens of microseconds). The forming process is most often used to shrink or expand cylindrical tubing, but it can also form sheet metal by repelling the work piece onto a shaped die at a high velocity. Since the forming operation involves high acceleration and decelleration, inertia of the work piece plays a critical role during the forming process. The process works best with good electrical conductors such as copper or aluminum, but it can be adapted to work with poorer conductors such as steel. Other high energy rate metal forming techniques include electrohydraulic forming and explosive forming. Instead of using powerful magnetic fields, these forming processes use a powerful shock wave created within a fluid, usually water, to perform the operation. The underwater shock wave is generated by either triggering a powerful spark discharge (using a high voltage capacitor bank) or by detonating a high explosive.

The magnetic pulse forming process which uses opposing magnetic fields to force a sheet of metal onto a mandrel or other form. First, an extremely large current discharge is directed through a coil which creates a magnetic field. Capacitor banks are used to store charge for larger discharges. In the nearby sheet of metal, an opposing magnetic field is induced which causes the metal sheet to be pushed into a form of some shape. The method generates pressures up to 350 MPa creating velocities up to 900 fps. The process production rate can climb to 3 parts a second. Applications 36

 fittings for ends of tubes  embossing  forming Three methods of magnetic pulse forming Swaging - An external coil forces a metal tube down onto a base shape (tubular coil). Expanding - an inner tube is expanded outwards to take the shape of an outer collar (tubular coil). Embossing and Blanking - A part is forced into a mold or over another part (a flat coil) This could be used to apply thin metal sheets to plastic parts.

Electromagnetic forming (EMF) is an assembly technique that is widely used to both join and shape metals and other materials with precision and rapidity, and without the heat effects and tool marks associated with other techniques. Also known as magnetic pulse forming, the EMF process uses the direct application of a pressure created in an intense, transient magnetic field. Without mechanical contact, a metal workpiece is formed by the passage of a pulse of electric current through a forming coil.

The major application of EMF is the single-step assembly of metal parts to each other or to other components, although it is also used to shape metal parts. Within the transportation industry, for example, one automotive producer assembles aluminum driveshafts without welding to save a significant amount of weight in light trucks and vans to meet requirements for reduced energy consumption. Using the EMF process allows the joining of an impact-extruded aluminum yoke to a seamless tube without creating the heat-affected zone associated with welding. Numerous other uses of the EMF process are described in the section "Applications" in this article. In its simplest form, the EMF process uses a capacitor bank, a forming coil, a field shaper, and an electrically conductive workpiece to create intense magnetic fields that are used to do useful work. This very intense magnetic field, produced by the discharge of a bank of capacitors into a forming coil, lasts only a few microseconds. The resulting eddy currents that are induced in a conductive workpiece that is placed close to the coil then interact with the magnetic field to cause mutual repulsion between the workpiece and the forming coil. The force of this repulsion is sufficient to stress the work metal beyond its yield strength, resulting in a permanent deformation. Electrohydraulic forming In electrohydraulic forming, an electric arc discharge is used to convert electrical energy to mechanical energy. A capacitor bank delivers a pulse of high current across two electrodes, which are positioned a short distance apart while submerged in a fluid (water or oil). The electric arc discharge rapidly vaporizes the surrounding fluid creating a shock wave. The workpiece, which is kept in contact with the fluid, is deformed into an evacuated die. The potential forming capabilities of submerged arc discharge processes were recognized as early as the mid 1940s. During the 1950s and early 1960s, the basic process was developed into production systems. This work principally was by and for the aerospace industries. By 1970, forming machines based on submerged arc discharge, were available from machine tool builders. A few of the larger aerospace fabricators built machines of their own design to meet specific part fabrication requirements. 37

Electrohydraulic forming is a variation of the older, more general, explosive forming method. The only fundamental difference between these two techniques is the energy source, and subsequently, the practical size of the forming event. Very large capacitor banks are needed to produce the same amount of energy as a modest mass of high explosives. This makes electrohydraulic forming very capital intensive for large parts. On the other hand, the electrohydraulic method was seen as better suited to automation because of the fine control of multiple, sequential energy discharges and the relative compactness of the electrode-media containment system.

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