Measurements and Measuring
Tools
Measuring can be considered to be the most important process in
engineering. Without the ability to measure accurately, we
cannot:
• Mark out components (Marking out is, essentially, drawing
on metal so as to provide guide lines for a fitter or a
machinist to work to).
• Set up machines correctly to produce components to the
required size and shape.
• Check components whilst we are making them to ensure that
they finally end up the correct size and shape.
• Inspect finished components to make sure that they have
been correctly manufactured.
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Linear measurement
• When you measure length, you measure the shortest
distance in a straight line between two points, lines or
faces.
• It doesn’t matter what you call this distance (width,
thickness, height, breadth, depth or diameter) it is still a
measurement of length.
• Measurement of length is the comparison of the size of a
component or a feature of a component and a known
standard of length.
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Steel rules
The steel rule is frequently used in workshops for measuring components of limited
accuracy quickly. The quickness and ease with which it can be used, coupled with
its low cost, makes it a popular and widely used measuring device.
Metric rules may be obtained in various lengths from 150 mm to 1000 mm (1 metre).
Steel rules may be ‘rigid’ or ‘flexible’ depending upon their thickness and the ‘temper’
of the steel used in their manufacture.
When choosing a steel rule the following points should be looked for. It should be:
• Made from hardened and tempered, corrosion resistant spring steel.
• Engine divided. That is, the graduations should be precision engraved into the surface
of the metal.
• Ground on the edges so that it can be used as a straight edge when scribing lines or
testing a surface for flatness.
• Ground on one end so that this end can be used as the zero datum when taking
measurements from a shoulder.
• Satin chrome finished so as to reduce glare and make the rule easier to read, also to
prevent corrosion.
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two scribed lines
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two faces using a hook rule
two faces using a steel rule and
a tool bit as an abutment
Calipers and their use
Calipers are used in conjunction with a rule so as to transfer the distance across
or between the faces of a component in such a way as to reduce sighting errors.
Firm-joint calipers are usually used in the larger sizes and spring-joint calipers
are used for fine work.
The accurate use of calipers depends upon practice, experience, and a highly
developed sense of feel. When using calipers, the following rules should be
observed:
• Hold the caliper gently and near the joint.
• Hold the caliper square (at right angles) to the work.
• No force should be used to ‘spring’ the caliper over the work. Contact should
only just be felt.
• The caliper should be handled and laid down gently to avoid disturbing the
setting.
• Work should be stationary when taking measurements. This is essential for
safety and accuracy.
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Vernier calipers
• Calipers are tools used in home, small shop, and industrial settings.
• They are used to make precise length measurements.
• There are three types of calipers.
1. The vernier caliper.
2. The dial caliper.
3. The digital electronic caliper.
The vernier caliper.
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The dial caliper
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The digital electronic caliper.
Construction
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Use
The VERNIER Scale (0.1mm)
• Notice that 10 divisions of the Vernier scale have the same
length as 9 divisions of the main scale. The smallest division
on the main scale is 1mm so the divisions on the Vernier
scale are 0·9mm each.
• Precision = 1mm - 0.9mm = 0.1mm
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The VERNIER Scale (0.02mm)
• Precision = 0.5mm - 0.048mm = 0.02mm
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Examples
Ans. = 169 + 0.1 * 4 = 169.4 mm
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Although more slow to use and rather more difficult to read, the vernier
caliper has three main advantages over the micrometer caliper.
• One instrument can be used for measurements ranging over the full
length of its main (beam) scale.
• It can be used for both internal and external measurements.. Remember
that for internal measurements you have to add the combined thickness
of the jaws to the scale readings.
• One instrument can be used for taking measurements in both inch units
and in metric dimensional systems.
The measuring accuracy of a vernier caliper tends to be of a lower
order than that obtainable with a micrometer caliper because:
• It is difficult to obtain a correct ‘feel’ with this instrument due to its
size and weight.
• The scales can be difficult to read accurately even with a magnifying
glass.
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Micrometer
• Most engineering work has to be measured to much greater accuracy
than it is possible to achieve with a rule, even when aided by the use
of calipers. To achieve this greater precision, measuring equipment of
greater accuracy and sensitivity has to be used. One of the most
familiar measuring instruments used in engineering workshops is the
micrometer. The constructional details of a typical micrometer are
shown in Fig..
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Inside Micrometer Caliper
Depth Gage Micrometer
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METRIC MICROMETER
• The figure below shows a 0.01-millimeter outside metric
micrometer
Note: The thimble has a scale that is divided into 50 parts. One
revolution of the thimble moves 0.5 millimeter on the barrel
side. A movement of one graduation on the thimble equals
1/50 of 0.5 millimeter along the barrel
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METRIC VERNIER MICROMETER
• The figure below shows a flattened view of a metric vernier
micrometer
Reading a metric vernier micrometer is the same as reading a 0.01mm
micrometer except for the addition of reading the vernier scale.
The vernier scale consists of five divisions. Each division equals
one-fifth of a thimble division, or 0.002 millimeter
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Examples
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Gauge blocks
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Slip gauges are blocks of steel that have been hardened and stabilized by
heat treatment. They are ground and lapped to size to very high standards of
accuracy and surface finish. They are the most accurate standards of length
available for use in workshops.
Measurement Ranges (mm) Jump (mm) No of gauge
blocks
1.0005
1
From 2.005 to 2.009
0.001
9
From 2.01 to 2.49
0.01
49
From 0.5 to 9.5
0.5
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From 10 to 100
10
10
Sum =
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88 block
Example (1)
29.874
- 2.004
Example (2)
16.9955
- 1.0005
27.87
15.995
- 2.37
- 2.005
25.5
13.99
- 5.5
- 2.49
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11.5
- 20
- 1.5
00
10
- 10
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00
Measuring angles
• Angles are measured in degrees and fractions of a degree. One degree of
arc is 1/360 of a complete circle. One degree of arc can be subdivided into
minutes and seconds (not to be confused with minutes and seconds of
time)
Right angles
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The try-square (a), its use (b) and (c)
Angles other than right angles (plain bevel
protractor)
The plain bevel protractor (a), and its use in checking angles (b)
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Angles other than right angles (vernier
protractor)
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