University of North Carolina Manual

Object : - Study of various measuring tools like dial gauge, micrometer, vernier caliper and
telescopic gauges.

Apparatus Required : - Vernier Callipers, Dial Gauge and Micrometer.

Theory : - Measurement is an essential part of development of technology and as technology
becomes more complex the techniques of measurement become more sophisticated.
For every kind of quantity measured there must be unit to measure it. This will enable the
quantity to be measured in number of that unit. If we say that “A” is longer than “B” until and
unless we answer how much, our statement is incomplete. We must be able to express this
difference in quantitative terms i.e. in terms of numbers.
Linear measurement applies to the measurement of lengths , diameters, height nd thickness,
covering both external and internal measurements.
The instruments used for linear measurements can be classified 2 groups : -
1. Non precision instruments such as steel rule, caliper etc.
2. Precision instruments such as Vernier calipers, micrometers , slip gauges etc.

Vernier Calipers
Vernier Calipers is a valuable measuring tool that enable us to increase our degree of precision
when measuring the size of objects. However, learning to read callipers can be somewhat
confusing, thus requiring a certain amount of practice.
Vernier caliper is a measuring device used to measure precise increments between two points.
Vernier calipers can measure internal dimensions (using the uppermost jaws), external
dimensions using the lower jaws, and depending on the manufacturer, depth measurements by
the use of a probe that is attached to the movable head and slides along the centre of the body.
This probe is slender and can get into deep grooves that may prove difficult for other measuring
tools.
The vernier scales may include both metric and inch measurements on the upper and lower part
of the scale. Vernier calipers commonly used in industry provide a precision to a hundredth of a
millimetre (10 micrometres), or one thousandth of an inch. A rod extends from the rear of the
caliper and can be used to measure the depth.






Least count (L.C) is the smallest reading we can measure with the instrument. It is the difference
between the values of main scale division and Vernier scale division. It can also be calculated by
ratio of value of minimum division on main scale to the number of divisions on Vernier scale.



L.C = one main scale division – one vernier scale division
L.C = 1mm – 0.09mm
L.C = 0.1mm = 0.01cm
Least Count = Value of the smallest division on Main Scale
Total number of division on Vernier Scale
L.C = 1mm / 10 = 0.1 cm / 10 = 0.01cm




Sourcer of error : -
Parallax Errors - can be improved by looking directly over the scale; not at an angle.

Zero error = can be improved by ensuring the same locking force is used when taking a reading.
Moving Jaw Tilt Error : -
If the moving jaw becomes tilted out of parallel with the fixed jaw, either through excessive
force being used on the slider or lack of straightness in the reference edge of beam, a
measurement error will occur as shown in the figure.



Dial Gauge/ Dial Indicator
An indicator is any of various instruments used to accurately measure small distances
and angles, and amplify them to make them more obvious. The name comes from the concept
of indicating to the user that which their naked eye cannot discern; such as the presence, or exact
quantity, of some small distance (for example, a small height difference between two flat
surfaces, a slight lack of concentricity between two cylinders, or other small physical deviations).
Many indicators have a dial display, in which a needle points to graduations in a circular array
around the dial. Such indicators, of which there are several types, therefore are often called dial
indicators.
Non-dial types of indicators include mechanical devices with cantilevered pointers and electronic
devices with digital displays.
Indicators may be used to check the variation in tolerance during the inspection process of a
machined part, measure the deflection of a beam or ring under laboratory conditions, as well as
many other situations where a small measurement needs to be registered or indicated. Dial
indicators typically measure ranges from 0.25mm to 300mm (0.015in to 12.0in), with
graduations of 0.001mm to 0.01mm (metric) or 0.00005in to 0.001in (imperial/customary).

Dial Gauges are one of the most commonly used instruments in all types of metal working
industry. Various names are used for indicators of different types and purposes, including dial
gauge, clock, probe indicator, pointer, test indicator, dial test indicator,drop indicator, plunger
indicator, and others.
There are several variables in dial indicator like Analog versus digital/electronic readout
(most are analog).
Nomenclature : -




The dial gauge has long been an standard with engineers, artisans, and do-it-yourself enthusiasts
for taking very fine measurements on precision parts. High levels of accuracy are possible in
extremely small increments with typical measurement ranges running from 0.015 inches to 12
inches (0.25 – 300 mm) in increments as small as 500 thousands of an inch (0.001 mm). There
are two basic dial gauge formats; the first is the plunger or lever type gauge. In this case, a spring
loaded plunger or lever at the bottom of the gauge transfers workpiece surface height deviations
to the gauge. The second type is the vernier dial gauge which receives its measurement input
from the movement of the jaws of a conventional vernier.
Plunger type dial gauges are typically held in a fixed position while taking measurements.
Specially designed weighted or magnetic clamps or stands are used to support the instrument
while the workpiece is rotated or moved. This process generally involves zeroing the gauge,
adjusting its position until the plunger or lever rests on the workpiece, and then rotating or
moving the work piece to check for anomalies. The contact tip of a lever type gauge is generally
relatively small and measures narrow grooves where the plunger would not fit.

MICROMETER
A micrometer sometimes known as a micrometer screw gauge, is a device incorporating a
calibrated screw widely used for precise measurement of components in mechanical
engineering and machining as well as most mechanical trades, along with
other metrological instruments such as dial, Vernier, and digital calipers. The spindle is a very
accurately machined screw and the object to be measured is placed between the spindle and the
anvil. The spindle is moved by turning the ratchet knob or thimble until the object to be
measured is lightly touched by both the spindle and the anvil.
Micrometers are also used in telescopes or microscopes to measure the apparent diameter of
celestial bodies or microscopic objects. It is a precision measuring instrument used to measure
distances between surfaces in thousandths of an inch.
Most micrometers have a frame, anvil, spindle, sleeve, thimble, and ratchet stop.
Micrometers are used to measure the outside diameters; inside diameters; the distance between
parallel surfaces; the depth of holes, slots, counter bores, and recesses; and the distance from a
surface to some recessed part.
Basic Types : -
Outside micrometer typically used to measure wires, spheres, shafts and blocks.
Inside micrometer, used to measure the diameter of holes.
Depth micrometer, measures depths of slots and steps
















Operating Principle : -
Micrometers use the principle of a screw to amplify small distances (that are too small to
measure directly) into large rotations of the screw that are big enough to read from a scale. The
accuracy of a micrometer derives from the accuracy of the thread-forms that are at its heart. In
some cases it is a differential screw. The basic operating principles of a micrometer are as
follows:
1. The amount of rotation of an accurately made screw can be directly and precisely
correlated to a certain amount of axial movement (and vice versa), through the constant
known as the screw's lead. A screw's lead is the distance it moves forward axially with
one complete turn (360°). (In most threads [that is, in all single-start
threads],lead and pitch refer to essentially the same concept.)
2. With an appropriate lead and major diameter of the screw, a given amount of axial
movement will be amplified in the resulting circumferential movement.
For example, if the lead of a screw is 1 mm, but the major diameter (here, outer diameter) is
10 mm, then the circumference of the screw is 10π, or about 31.4 mm. Therefore, an axial
movement of 1 mm is amplified (magnified) to a circumferential movement of 31.4 mm. This
amplification allows a small difference in the sizes of two similar measured objects to correlate
to a larger difference in the position of a micrometer's thimble. In some micrometers, even
greater accuracy is obtained by using a differential screw adjuster to move the thimble in much
smaller increments than a single thread would allow.

EXAMPLE MEASURE READINGS

Using the first example seen below:
1. Read the scale on the sleeve. The example clearly shows12 mm divisions.
2. Still reading the scale on the sleeve, a further ½ mm (0.5) measurement
can be seen on the bottom half of the scale. The measurement now reads 12.5mm.
3. Finally, the thimble scale shows 16 full divisions (these are hundredths of a mm).
The final measurement is 12.5mm + 0.16mm = 12.66