Machine tools and precision

Two plates lapped together can both be curved — one convex, one concave — and still fit. Three plates cannot all fit each other in every pairing unless all three are flat.

Take three roughly flat cast-iron plates, labelled A, B and C. Coat one with marking blue — lampblack or red oxide mixed with oil. Rub A on B, and scrape the high spots the blue shows on both. Then A on C, then B on C, rotating the pairings every time. Keep going. This produces a true plane from no prior reference at all — the single most important trick in manufacturing, and it is free.

Scraping is taking off only the high spots the marking shows, with a hardened flat scraper, a few microns at a time, in short strokes in a crosshatch pattern that changes direction each pass.

Measure progress by counting contact spots in a 25 × 25 mm square: a rough surface shows a few; a good machine way shows around 10–20; a reference surface plate more. The spots must be evenly distributed, not concentrated in one area. Keep one plate as the settlement's master and use the others for work.

From a flat, derive everything else. Flat, straight ways let a slide move in a true line; a straight slide past a rotating spindle cuts a true cylinder.

The lathe is the only machine tool that can largely reproduce itself: it turns its own spindle, bores its own headstock, and cuts its own leadscrew. Scrape the bed ways against the master flat, align the headstock so a turned test bar is the same diameter at both ends, and build the rest of the shop with it. See The Lathe and Its Uses.

A screw cut by a screw inherits its errors. The escape: cut the best rough screw you can, then lap it against a long split nut — a nut many threads long, clamped lightly, charged with fine abrasive and oil — running it back and forth along the whole length.

The long nut touches many threads at once and averages their errors away, wearing down the high parts. Use the lapped screw to cut the next screw, lap that, and repeat. Each generation is more accurate than the last. Measure the pitch error over a long length against the master length standard.

Establish one physical standard length for the settlement — see instruments — and derive every gauge from it.

Then build the ladder of measurement: calipers → a micrometer → gauge blocks. A micrometer is a precise screw: a pitch of 0.5 mm and a thimble divided into 50 parts reads 0.01 mm. Steel grows about 11–12 thousandths of a millimetre per metre per °C, so a 1 m part measured 10 °C warm is over a tenth of a millimetre long — measure at a stated reference temperature, conventionally 20 °C. See Measuring Tools.

Boring a true, parallel cylinder is what makes a steam engine an engine rather than a bomb. A boring bar supported at both ends, passing through the cylinder, with a cutter that feeds along it, cuts a bore that follows the bar rather than the casting's distortions. A bar supported at one end sags and cuts a taper.

Historically, cannon-boring machinery with a rigid bar was what made Watt's engine possible. Check the bore at several depths and angles.

Accurate spacing of marks around a circle gives gears, screw pitches and every instrument scale. Start from the largest, most accurate dividing plate you can make — see woodworking — and move to a worm-and-wheel dividing head: a worm turned by whole revolutions advances the wheel by exactly one tooth, so errors in individual teeth average out.

Check a division by stepping round the full circle — the error shows up as a gap or overlap at closing.

Pick one thread series — angle, pitch per diameter, form — and never deviate. Make master taps and dies and keep them as standards, copying working sets from them.

Make go/no-go gauges for every standard dimension: the part passes if the go end fits and the no-go end does not. Then files, taps, dies and reamers turn we made one into we can make many that interchange. Interchangeable parts are the difference between repairable machinery and disposable machinery.