A micrometer converts the turning of a screw into a measured gap: with a 0.5 mm pitch screw and a thimble divided into 50 parts, each division is 0.01 mm of spindle travel. Its readings can be no better than the screw’s pitch and the flatness of its two faces, so it must be checked against gauge blocks at several points along its range rather than trusted because its scale looks fine.
Part of Precision Tool Making
Building a screw-based measuring instrument that reads to 0.01 mm, and verifying what it actually delivers.
The Screw Is The Scale
On a vernier, the scale is engraved on a bar. On a micrometer, the scale is the thread. One revolution advances the spindle by one pitch, so the graduated thimble divides that pitch into readable parts. With a 0.5 mm pitch, a thimble of 50 divisions reads 0.5 / 50 = 0.01 mm per division (Precision Tool Making).
The consequence, which Precision Measurement states bluntly, is that a micrometer with a varying pitch, rough thread or wobble is useless; your first one will be crude, and you use it to make better screws, which make a better micrometer. Pitch error is the micrometer’s error.
What You Need
- A C-shaped rigid frame that does not flex under measuring pressure
- A hardened, lapped, flat anvil, and a hardened spindle with a lapped flat face
- A 0.5 mm pitch screw cut on the best lathe, with its flanks lapped (Master Lead Screw)
- A barrel graduated in 0.5 mm steps and a thimble divided into 50 parts
- Gauge blocks to calibrate against (Gauge Blocks)
Method: Build And Verify
Step 1: Forge and machine the frame. The opening sets the range.
Step 2: Cut the screw and the nut as accurately as you can. Micrometers describes a simpler build around an M10 x 1.0 tap and die (Micrometers); with a 1.0 mm pitch you need 100 thimble divisions to read 0.01 mm, since 1.0 / 100 = 0.01, and 50 divisions would read 0.02 mm. Choose pitch and divisions together.
Step 3: Harden and lap the faces flat. Without hardening, faces wear in hours of use; case-hardened to 60 HRC minimum, through-hardened tool steel preferred.
Step 4: Divide the thimble into 50 equal parts (Dividing and Indexing).
Step 5: Zero check. With the faces closed, the reading must be zero.
Step 6: Check at five points. Build gauge-block stacks of 5, 10, 15, 20 and 25 mm, wring and clean them, measure each, and write down the deviation at each point.
Worked Example
Reading: the barrel shows 12.5 mm and the thimble line 37 is at the index. Reading = 12.5 + 37 x 0.01 = 12.87 mm, the example given in Micrometers.
Error budget: if the screw’s pitch error stays within +-0.01 mm per 100 mm of travel (the lead-screw target in the carrier), then over a 25 mm range it contributes up to +-0.01 x 25 / 100 = +-0.0025 mm, if it accumulates in proportion to length. Micrometers says a good one should read within +-0.003 mm of the gauge-block value over its range and that errors above 0.005 mm indicate wear or damage; that article’s figures, not verified here. So the screw alone could use most of the allowance, and flatness and parallelism of the faces have to fit inside what is left.
The five-point check reveals pitch error that grows with travel (different deviation at each size) separately from a fixed offset (the same deviation at every size). A fixed offset is a zero error you can adjust; a trend is the screw.
Common Mistakes
- Assuming the thimble division gives the accuracy. It only gives the step.
- Measuring with excessive force. The frame flexes and the screw springs; use light, consistent pressure.
- Mixing pitch and thimble divisions. Work out which combination reads 0.01 mm.
- Never checking after wear. Soft faces degrade in hours; re-check against blocks.
Safety
Lathe work and lapping dust
The screw is cut on a lathe and its flanks lapped with abrasive: keep sleeves clear of the chuck, know the lathe’s stop, and treat lapping compound as a respiratory hazard (see Lapping for Ultra-Precision). Case-hardening and quenching involve hot steel and oil.
What’s Next
- Learn next: Gauge Blocks - the reference that calibrates it
- Combine with: Master Lead Screw - where the screw’s accuracy comes from