Measuring Instruments and Bench Work
This domain tests least count, correct reading of vernier and micrometer, zero-error correction, and the sequence of bench operations.
Least count
- Vernier caliper: least count = value of one main scale division divided by the number of vernier divisions. With 1 mm main divisions and 50 vernier divisions, LC = 0.02 mm. Calipers of 0.05 mm and 0.1 mm also exist.
- Micrometer: pitch 0.5 mm, thimble with 50 divisions, so LC = 0.5/50 = 0.01 mm.
- Vernier bevel protractor: 1 degree main scale and 12 vernier divisions give 5 minutes.
- Dial indicator LC is usually 0.01 mm; also know height gauge, depth gauge, feeler gauge, screw pitch gauge and radius gauge.
Reading procedure
Clean the measuring faces, close the jaws or anvils and check zero error, then record it. Note the main scale value just before the vernier zero, find the coinciding vernier division, multiply by the least count and add. For a micrometer, read the barrel first, then the thimble.
Bench work
Marking off from a datum, centre punching, filing flat, square and parallel checked with a try square and surface plate, sawing, chipping, scraping and lapping. File on the forward stroke with the file flat, and check squareness at intervals rather than after the final cut.
Worked example
A 0.02 mm vernier caliper reads 12 mm on the main scale with the 34th vernier division coinciding: 12 + 34 x 0.02 = 12.68 mm. If the closed jaws read 0.04 mm, the true size is 12.68 - 0.04 = 12.64 mm, because a positive zero error is subtracted. For a micrometer showing 7.5 mm on the barrel and 22 on the thimble: 7.5 + 0.22 = 7.72 mm.
Traps that cost marks
- Skipping zero-error correction.
- Reading the vernier from the wrong division.
- Parallax from viewing at an angle.
- Measuring a hot job, or forcing the micrometer instead of using the ratchet.
- Mixing up 0.02 mm and 0.05 mm calipers.