Introduction to Basic Manufacturing Processes and ... - always yours

372 Introduction to Basic Manufacturing Processes and Workshop Technology
19.2.3.1 Micrometers
The micrometers are commonly employed for measuring small dimensions with extreme
accuracy of 0.01 mm. They may be of the three kinds -
(a)
(b)
(c)
External micrometer for measuring external dimensions,
Internal micrometer for measuring internal dimensions, and
Depth micrometer for measuring depths.
For measuring a dimension in external micrometer, the work piece is held between the
fixed anvil face and the spindle face of the micrometer. The spindle of the micrometer is
allowed to move linearly towards the work by rotating thimble. When the spindle will touch
the work piece properly, the ratchet will give its sound. The small locking lever is then
rotated to clamp the spindle so that reading can be taken more accurately. Outside micrometers
are used for measuring the outside dimensions of jobs, such as diameter of a bar, rod and
thickness of plate. Generally, until and unless they are provided with the vernier attachment,
the former can read up to 1/1000 or 0.001 inch and the latter up to 0.01 mm. The former are
known as inches micrometers and the latter metric micrometers, which are gradually replacing
the former due to the introduction or adopting of metric system. Inside micrometers are
commonly used for measuring inside dimensions of the objects, such as inside dia. of a hole,
width of a slot or cavity, etc. The outside micrometers are the most extensively used in
industrial applications. All the micrometers, irrespective of the fact as to whether they carry
graduations in inches or millimeters, are similar in construction. An out side micrometer is
discussed as under.
OUTSIDE MICROMETER
Fig. 19.12 shows an outside micrometer. It consists of the following main parts.
1. Metallic frame
2. Axial graduated sleeve
3. Circumferential screwed spindle
4. Hardened steel anvil
5. Thimble
6. Ratchet stop screw
7. Lock nut
Anvil
Spindle
Lock
nut
Sleeve
Thimble
Ratchet
stop
0 5 10 20
15
Job
Frame
Fig. 19.12 Outside micrometer
Micrometer works commonly on the principle of nut and bolt assembly. The sleeve
carries inside threads at the end, which forms the nut, and the screwed part of the spindle
passes through it. The spindle and the thimble are secured to each other such that by rotating