Checking a caliper and a micrometer with gauge blocks

Caliper: outside jaws

1. Let the caliper and the blocks reach the same temperature on the bench; handle the blocks with gloves or tongs, not bare hands.
2. Close the jaws and check zero (daily care and the zero check are in caliper care and calibration).
3. Put a block between the outside measuring faces and read. Error = reading − calibrated size of the block.
4. Repeat at five or more roughly equally spaced points covering the range of the main scale and the vernier (NPL GPG 40).

For a 0–150 mm digital caliper, 30, 60, 90, 120 and 150 mm would do; this is the example caliper used in how to read a calibration certificate. On a vernier caliper, also choose sizes whose decimal part falls on different vernier lines, so the vernier scale is checked as well as the main scale.

Parallelism and the inside jaws

Put a block at different points along the jaw, near the root and near the tip. Use two blocks of different length so the check is made at two places in the range, one close to zero and one close to full range, and check that clamping the slider does not change the result.

For the inside jaws, build a known internal size from a gauge block with a holder and accessory jaws, measure it with the inside faces and subtract the block size in the same way.

NPL also checks the squareness of the fixed face to the guiding edge with a knife-edge square, the flatness of the faces with an optical flat and their straightness with a lever-type dial indicator.

Micrometer

A micrometer is checked with a special set of gauge blocks whose sizes are chosen to show both the periodic error of the screw (within a turn) and its progressive error (along the travel), and with optical parallels for the flatness and parallelism of the faces. Put each block in turn between the measuring faces (anvil and spindle), using the ratchet as when measuring, and note the reading. A micrometer stand keeps your hand off the frame.

NPL’s good practice guide gives this example of a micrometer calibration (positions from 2.5 to 25 mm):

Its expanded uncertainty is ±0.002 mm (k = 2, about 95 %). The same guide notes that most organisations send their micrometers to an accredited laboratory rather than calibrating them in house.

Recording and deciding

Record the block set and its certificate number, the temperature, each reading and error, the maximum permissible error used, the decision, who checked and the date (see gauge management). Comparing each error with the MPE and writing pass or fail is verification (in China, calipers are verified to JJG 30 and micrometers to JJG 21). A calibration certificate from a laboratory reports the errors and their uncertainty instead.

Planned for the lab

Gauge blocks of 10, 20, 25, 30 and 40 mm are in KELIBRON as setting standards for the dial indicator. A micrometer can be closed on them, but the lab’s calipers and micrometers have no error of indication that varies over the range yet (only the zero-error demonstration, a constant offset), so a block check would find nothing to record. Until that is added, practise the habits on the scenes below: check zero first and read to the instrument’s resolution.