Length measurement:

Engineer’s rule

These are made from hardened and tempered steel marked off with high accuracy in lengths from about
10-3Ocm with folding rules up to 60cm. They are used for marking off, setting callipers and dividers, etc.
When used directly, the accuracy is &0.25mm, and when used to set a scribing block the accuracy is
f 0.125 mm.

Feeler gauge (thickness gauge)

These consist of a number of thin blades of spring steel of exact, various thicknesses. They are used for
measuring small gaps between parts.

Micrometers

Micrometers are used for the measurement of internal and external dimensions, particularly of cylindrical
shape. Measurement is based on the advance of a precision screw. The ‘outside micrometer’ is made in a
variety of sizes, the most popular being 25mm in 0.01-mm steps. It has a fixed ‘barrel’ graduated in

1-mm and 0.5-mm divisions screwed with a 0.5mm pitch thread and a ‘thimble’ graduated around its
circumference with 5W.01 mm divisions.An ‘inside micrometer’ has the fixed anvil projecting
from the thimble; extensions may be attached. A ‘micrometer head’ is available consisting of the barrel
and thimble assembly for use in any precision measuring device.

Reading a micrometer
Reading shown:
Reading on barrel = 5.5 mm
Reading on thimble = 0.28 mm
Total reading = 5.78 mm
Thimble

Vernier calliper gauge

This is used for internal and external measurement. It has a long flat scale with a fixed jaw and a sliding jaw,
with a scale, or cursor, sliding along the fixed scale and read in conjunction with it. Two scales are provided to
allow measurement inside or outside of the jaws.

Reading a vernier calliper gauge
Reading shown:
Reading on main scale=43.5 mm
Reading on cursor = 0.18 mm
Total reading = 43.68 mm
Main scale

Dial test indicator (dial gauge)

The linear movement of a spring-loaded plunger is magnified by gears and displayed on a dial. Various
sensitivities are available and a smaller scale shows complete revolutions of the main pointer. A typical
indicator has a scale with 1OO-O.01 mm divisions and a small dial reading up to 25 revolutions of the pointer,
Le. a total range of 25 mm.

Gauge blocks (slip gauges)

These are hardened, ground and lapped rectangular blocks of steel made in various thicknesses of extreme
accuracy and with a high degree of surface finish so that they will ‘wring’ together with a slight twist and
pressure and remain firmly attached to one another.They are made in a number of sets; BS 888 recommends
metric sets, two of which are given in the table below.

Protective slips are provided for use at the ends of the
combinations.

Measurement of large bores

The size of very large bores may be measured by means of a gauge rod of known length slightly less than the
bore. The rod is placed in the bore and the ‘rock’ noted. The bore can be determined from the amount of
rock and the rod length.

Accuracy of linear measurement

The following table gives the accuracy of different methods of linear measurement.