Instruments for 0.001 mm readings, measuring thread pitch diameter and checking with a DTI.
Vernier micrometer
An outside micrometer with an additional vernier scale on the sleeve. It gives readings to 0.001 mm.
10 vernier divisions = 9 thimble divisions
LC = 0.01 ÷ 10 = 0.001 mm
Reading = Sleeve + Thimble (× 0.01) + Vernier line coinciding (× 0.001)
LC = 0.01 ÷ 10 = 0.001 mm
Reading = Sleeve + Thimble (× 0.01) + Vernier line coinciding (× 0.001)
Screw thread micrometer
- Measures the effective (pitch) diameter of external threads.
- Spindle has a conical point and the anvil has a V-groove; they fit in the thread form.
- Anvils are changed according to the pitch range of the thread. Least count 0.01 mm.
Dial Test Indicator (DTI)
A comparator – it shows the difference in size or position, not the actual size. Small movements of the contact point are magnified and shown by a pointer on a dial.
- Plunger type dial indicator – plunger moves in a straight line; for height, run-out, end play.
- Lever type DTI – small swinging stylus; for checking in bores and difficult places.
- Common least count 0.01 mm (also 0.001 mm types).
- Mounted on a magnetic stand or surface gauge.
Uses of DTI
- Checking flatness and parallelism of surfaces.
- Checking run-out / eccentricity of shafts.
- Setting jobs true in a 4-jaw chuck or on a machine.
- Comparing heights with slip gauges.
Quick Revision
- Vernier micrometer LC 0.001 mm.
- Screw thread micrometer → effective diameter (cone and V anvil).
- DTI is a comparator; LC 0.01 mm; used for run-out, parallelism, setting.
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