This document discusses various linear measurement instruments used in engineering. It begins by classifying linear measurement devices into low, medium, and high resolution. Low resolution devices include graduated rules and calipers. Medium resolution devices discussed are vernier calipers, micrometers, and dial gauges. High resolution devices able to measure to within microns include slip gauges and comparators. The document provides detailed descriptions and examples of how each of these tools is used to perform precise linear measurements.

1. Engineering Measurements
Linear Measurements

2. Introduction
• Widely used measurements are
– Linear Measurement
– Angular measurement
• The Linear Measurement includes measurements of length,
diameters, heights and thickness
• The Angular measurement includes the measurement of angles or
tapers
• Dimensions of the measuring instruments are classified as follows
– Low resolution devices (up to 0.25mm)
– Medium resolution devices (up to 0.0025mm)
– High resolution devices (less than microns)

3. Linear Measuring Instruments
• Low resolution devices
– Graduated devices (Steel rule)
– Non graduated devices -Calipers
• Medium resolution devices
– Micrometer, Vernier caliper, Dial gauge
• High resolution devices
– Gauges such as Slip gauges, Comparators, Optical flats

4. Low resolution devices

5. Graduated Rules and Scales
We often use the terms “ Rule” or “Scale” to mean the simple device we use
for length measurements
However, there is a clear difference in the actual meaning of these two familiar
words. Scale is a graduated in proportion to a unit of length. For example, the
scale used by architects gives inches and feet as shown below. The plumber’s
scale would have divisions in terms of 1/32 or 1/64 the of an inch.
The divisions of a rule, on the other hand, are the unit of length, its divisions and
multiples. Typically, the rules with which we are familiar have graduations in term
s of cm, mm or inches, and their decimal divisions throughout the length.

6. Graduated rule and scales

7. Calipers

8. Calipers
To provide a reading, the calipers must be checked against a
measuring device such as a ruler or micrometer.

9. Medium resolution devices

10. Vernier caliper
Vernier Caliper - Parts

11. Vernier caliper
• Widely used in engineering workshops
• The line of measurement does not coincide with the line of
scale. Therefore the accuracy depends upon the straightness
of the beam and the squareness of the sliding jaw with
respect to the beam.
• To ensure the squareness, the sliding jaw must be clamped
before taking the reading. The zero error must also be taken
into consideration.
• Instruments are available with a measuring range up to one
meter with a scale value of 0.1 or 0.2 mm.
Vernier Caliper - Use

12. VERNIER CALIPER
Suppose 1 division of main scale is equal to 1 mm
Also suppose 9 divisions of the main scale is equally divided into 10
divisions in the vernire scale.
Thus, 1 division of vernier scale is equal to 9 mm/10, that is 0.9 mm.
Now, the smallest distance that you can differentiate using the
combination of this two scales is due to the coinciding of 9 main scale
divisions with 10 vernier scale divisions is 1 – 0.9 (= 0.1mm). This is the
least count of the caliper
Vernier Caliper - Resolution

13. Vernier Caliper
Main scale of a Vernier caliper is graduated in 0.5 mm. Nine divisions of the
main scale is divided into 10 divisions in the Vernier Scale.
Find out the least count of the caliper
Answer
Nine divisions of the main scale = 0.5mm x 9 = 4.5mm
Measurement of 1 division in Vernier Scale = 4.5mm/10 = 0.45
Smallest difference between main scale division and Vernier scale division =
0.5 – 0.45 = 0.05mm
Therefore, Least Count is 0.05mm
Vernier Caliper – Example

14. Vernier Caliper
Following diagram shows measurement of an object with the Vernier caliper
mentioned above. What is the value indicated in the caliper?
Reading of the instrument = MS div + (coinciding VS div x LC)
= 2.5 + 3 x 0.05 = 2.65mm
Vernier Caliper - Example

15. Vernier Depth Gauge
A vernier depth is very
commonly used precision
instrument for measuring
depth of holes recesses,
slot and step.
Its construction and
method of reading are
similar to those of a
vernier caliper.

16. Vernier height gauge
1. Base
2. Beam
3. Vernier slide
4. Fine setting device
5. locking screws
6. scriber
1
2
3
4
5
6
Vernier height gauge is used to
measure height of an object

17. Outside Micrometer
• A micro meter is a
precision instrument
used to measure an
object, generally within
an accuracy of 0.01mm.
• Micrometer used to take
the outside
measurements are know
as outside micrometer.

18. Outside Micrometer
• Spindle: The cylindrical component which moves when you rotate the
thimble. This touches the object which is to be measured and holds it
tight.
• Anvil: Spindle moves towards the anvil when in order to hold the object
and take the measurement.
• Lock ring: This is used to tighten the movement of the spindle so that it
doesn’t move while taking the measurement.
• Thimble: It is the component which is turned by thumbs and which makes
the spindle movement.
• Ratchet knob: The device which is at the end of the instrument and is
responsible for exerting only allowable pressure to the object and not
more than that.
• Screw: It is the main component on which the micrometer works and is
hidden from the sight.

19. Outside Micrometer
The basic principle of operation of a micrometer
• The amount of axial movement of a screw is measured by the amount of
rotational movement of it. This is because the pitch of the screw is
constant.
• Part rotation is taken by the vernier scale on the thimble
Suppose 1 rev of screw gives
1mm movement and Vernier
(circumference of thimble) is
divided into 50 divisions, then
smallest measurement is
1/50 (0.02mm)
Reading of the above micrometer is 16mm + 0.02 x 16 = 16 + 0.32 = 16.32mm

20. Types of micrometers
Inside Micrometer
Depth Micrometer
Outside Micrometer
Thread Micrometer

21. Dial Gauge

22. Dial gauge
• Linear displacement is converted into rotary movement of the
two hands by rack and pinion mechanism
• Suppose 1 mm of spindle movement causes one full rotation
of the long hand (arm)
• If the dial is divided into 100 equal divisions and then each
division represents a spindle movement of (1/100) 0.01 mm.
• The short hand registers the number of full turns made by
long hand
• In reading measurement, first read the reading in the short
hand in mm, then the divisions in the dial using the long hand

23. Dial gauge
Dial gauges are also used for checking flatness of
surfaces and parallelism of bars and rods.
Dial gauges are used for measuring linear displacements of
objects (not like static length as a vernier caliper or a
micrometer)

24. High resolution devices

25. Slip gauges
• Slip gauges are rectangular blocks of steel having a cross-
section of about 30 by 10 mm manufactured with very high
accuracy
• They can be used to measure tolerances in the range
of 0.001 to 0.0005 mm very accurately.
• Slip gauges are the universally accepted ‘standard of length’
in industries. These are the simplest possible means of
measuring linear dimensions very accurately.

26. Slip gauges
• Slip gauges are used for
– checking the accuracy of vernier calipers, micrometers
and other measuring devices
– setting the comparators to a specific dimensions.
– direct precise measurement where the accuracy of work
piece is important
– measuring angle of work pieces with sine bar
– checking the gap between parallel locations

27. Slip gauges
Grades of Accuracy of Slip Gauges:
• Slip gauges are made in five grades of accuracy.
– Calibration grade, grade 00, grade 0, grade I, and grade II,
in the decreasing order of accuracy.
• Grade 0, grade I, grade II are used for general
workshop purpose and are known as working gauge
blocks
• Calibration grade (master gauge blocks) and grade
00 (Inspection gauge blocks) are used only for
checking other types of blocks

28. Slip gauges
Slip Gauges Sets:
Gauge blocks are available in sets with steps with steps of 10, 1,
0.1, 0.01 and 0.001 mm.
Some available Metric (mm) sets are:

29. Comparators
• A comparator is a precision instrument used to
compare the dimension of a given component with a
given standard, rather than obtaining an absolute
value of measurement
Introduction

30. Comparators
Basic principle
• Initially, the comparator is adjusted to zero on its dial with a
standard job in position.
• Then the standard job is replaced by the work-piece to be
checked
• If there is a change in the dimension the comparator
deviates form the zero position.

31. Comparators
Uses
• In mass production, where components are to be
checked at a very fast rate.
• For inspection of working gauges and standards
• Attached with some machines, comparators are used as
working gauges to maintain required tolerances at all
stages of manufacturing.
• For grading components depending upon their
tolerances

32. Comparators
1. Mechanical comparators
2. Optical comparators
3. Electrical and Electronic comparators
4. Pneumatic comparators
5. Fluid displacement comparators
Types of Comparators

33. Mechanical Comparators
• A mechanical comparator
employs mechanical means
to get the magnification for
example, lever, gear system
etc.
• Its manufacturing requires
high degree of accuracy. The
magnification of mechanical
comparator ranges from 250
to 1000.
Rack and Pinion
Cam and Gear train
Lever with toothed gear
Compound Levers
Mechanisms
Twisted taut strip

34. Mechanical Comparators
Reed Comparator
Reed type comparator uses
the frictionless reed
mechanism for magnifying
small motions of the spindle
In the following figure:
1. A - Fixed Block
2. B- Floating Block
3. C- Reeds

35. Mechanical Comparators
Reed Mechanism

36. Mechanical Comparators
Sigma Comparator
Works on leverage principle in which high magnification is
obtained

37. Optical Comparators
• In optical comparators
small displacements of
the measuring plunger
are amplified first by a
mechanical system
consisting of pivoted
levers
• The amplified
mechanical movement
is further amplified by a
simple optical system
involving the projection
of an image

38. Optical Comparators
Mechanical-Optical Type Comparator

39. Optical Comparators
Optical flat
Optical flats can be used to
measure the flatness of objects

41. Electrical Comparators
• In electrical comparators, the movement of a contact
is changed into an electrical signal and then this
signal is recorded by a device.
• For converting movements into a electrical signal,
several electrical principles or devices are used
– Wheatstone bridge circuit
– Linear Variable Differential Transformer
– Slide Potentiometer

42. Electrical Comparator
Electrical comparator based on Wheatstone Bridge

43. Electrical Comparators
Linear Variable Differential Transformer for Electrical
Comparators

44. Electrical Comparators
Linear Potentiometer

46. Pneumatic comparators

47. Gauges
• Gauges are precise measuring devices that provide a
description of qualities such as thickness, length, diameter
and pitch of a screw thread.
• Each gauge is purpose built for a specific application.
• The value of the quantity to be measured is determined by
the observation of the coincidence of known values in gauges
• Gauges are often used with trial and error to estimate or
determine various aspects of an object.
Examples
Limit gauges
Screw Thread gauges
Radius gauges
Feeler gauges

48. Gauges
• In component manufacture we use tolerances in
dimensions. For example the diameter of a hole, the
minimum and maximum values acceptable are
20mm – 0.02mm ( 19.98mm) and 20mm + 0.02mm
(20.02mm)
• Limit gauges are used for checking limits of the
dimension of the component.
• The “Go gauge” should pass through the hole
while “Not Go gauge” should not pass through the
whole.
Limit Gauges

49. Gauges
Radius gauge
A radius gauge is a hand-held tool which has many precision rounded
curves (radii) machined into its edges. This is also referred to as a fillet
gauge.
Common metric gauges provide measuring ranges from: (0.75 - 5 mm) 18
blades with 0.25 mm increments. Or (5.5 - 13 mm) 16 blades with 0.5
mm increments

50. Gauges
Thread gauge ( Screw thread gauge)
Thread gauges are used to check the pitch of screw threads
Screw pitch gauge has several leaves .The leaves are clearly imprinted
with the size and are used to determine the pitch of various threads.

51. Gauges
Wire gauge
Wire gauges are reference tools used for selecting the appropriate diameter
wire for a specific purpose.
Wire diameter size is given by
British standard Wire Gauge
(SWG) No.
SWG #
Diameter
(mm)
Area
(mm2)
7/0 12.700 126.6769
6/0 11.786 109.0921
5/0 10.973 94.5638
4/0 10.160 81.0732
3/0 9.449 70.1202
2/0 8.839 61.3643
0 8.230 53.1921
1 7.620 45.6037
2 7.010 38.5989
3 6.401 32.1780

52. Gauges
A feeler has a number of folding metal strips (also known as blades, or
leaves) which are machined to specified thickness levels.
Feeler gauges can take measurements as small as 0.02mm (0.0008")
and as large as 5.08mm (0.200").
The gauge gives you an idea of how wide a gap is by physically filling
the space with metal strips which have a known thickness value.
Feeler Gauge

53. NEXT Angular Measurements