Wringing Gauge Blocks: Building Any Length from a Box of Them

Wipe each block with a lint-free cloth and a little isopropyl alcohol, and handle them as briefly as you can. Do not touch the lapped faces with bare fingers if you can avoid it. A fingerprint is a few micrometres of oil and salt. On a block whose length is certified to a fraction of a micrometre, that is a large error sitting directly in the measuring path — and the salt will etch the surface within days, which turns a temporary error into a permanent one. A trace of oil is wanted for wringing; a fingerprint is not the same thing. The difference is a uniform film you put there on purpose versus a lumpy one you left by accident.

Lay one block across the other at right angles so only a small area touches, press lightly, and rotate one onto the other while keeping the pressure up. You will feel it take hold. Done properly the pair will not fall apart under its own weight, and you can pick the box up by the top block. If it slides without gripping, or grips in one spot and rocks, take it apart and clean both faces again — a bad wring is dirt nine times out of ten. Never leave blocks wrung together for long. The same forces that hold them will, given days, make them very difficult to separate, and forcing them apart damages the faces. Slide them apart the way you slid them together, never prise.

Build stacks at four or five sizes spread across the micrometer's range — near zero, a quarter, a half, three quarters and full — and measure each on the ratchet exactly as you would measure work. Record the error at each size, with its sign. What you now have is a CALIBRATION, not a pass or fail. A micrometer reading consistently 0.004 mm large across the range is a perfectly useful instrument with a known offset. One that reads correct at zero, small in the middle and large at the top has a worn thread and is useful only for comparisons. Write the errors on a card and keep it with the micrometer. An uncalibrated instrument is not wrong — it is unknown, which is worse, because unknown reads exactly like right.

Separate every stack, wipe each block, apply a thin film of the oil supplied with the set, and return each to its own place in the case. Steel blocks rust, and a rusted lapped face cannot be recovered by cleaning. Ceramic blocks do not rust and are far more forgiving of a wet workshop, which is most of the reason to own them. Returning each to its own numbered recess is not tidiness. It is how you notice one is missing before it turns up under a machine, and how the next person finds the block the algorithm asked for without reading every label in the box.