How Machines Learned Precision

Accuracy from Iron in the Industrial Revolution By Gary Linscott · Discuss: This is a screw-cutting lathe of the kind Henry Maudslay built in London around 1800. A labourer turns the large wheel to spin the bar in the middle, and a cutting tool moves along beside it on a carriage that slides on straight iron rails.

By the 1850s, Joseph Whitworth had built a machine that could detect a difference of a millionth of an inch. In this article we'll follow how workshops learned to make flat surfaces, cut accurate screws and measure their parts, using interactive figures to explore each step (try rotating the lathe above with two fingers, or pinching to zoom indragging the lathe above, or zooming with ⌘/Ctrl + scroll). Let's start with the cylinder of Watt's steam engine, which we built in the previous article. The cylinder Newcomen's engine filled a cylinder with steam, then condensed it with a spray of cold water. The atmosphere pushed the piston down into the partial vacuum, but the spray also chilled the cylinder. Watt moved the condensation into a separate cold vessel so that the cylinder could stay hot, saving about two thirds of the coal. This also meant he had to change the way he sealed the piston. Below, we can compare the two engines. Newcomen kept a pool of water on his piston to fill the gaps against the uneven cylinder wall. Water would have cooled Watt's cylinder, so he used dry packing instead. The tightly rammed packing couldn't give way enough to fill the gaps, so the hole through the cylinder, called the bore, had to be round and straight along the whole stroke. The cylinders of the 1760s were far from that. The eighteen-inch cylinder of Watt's test engine at Kinneil, in 1769, was about ⅜ of an inch wider one way than the other at its worst point. That is nearly a centimetre of error in a cylinder less than half a metre wide. The traditional way to make two parts fit was to press them together and file away the bright spots where they rubbed. The workman doing this was called a fitter. He could make the small surfaces of a valve or a gun lock fit closely, but a piston had to fit at every point along its stroke. Filing one part of the bore wouldn't make the rest match, and most of it was deep inside the cylinder, where a file couldn't reach. To see why this was such a challenge, let's go through how the cylinders were built. The thick iron walls were made in one piece by pouring molten iron into a mould, a process called casting. The mould formed the outside, while a core in the middle left a hole through the iron.1 Below, we can watch a cylinder being cast in a sand mould. Molten iron is far denser than sand, so it pushes the core upward against whatever holds it down. If it lifts even slightly, the casting is ruined. Even a core that stays in place leaves a rough hole that isn't straight or round. A boring mill cuts away the inside of the casting to bring it to the right shape and size. The boring mills of the time held the cutter on the end of a long bar supported from one side, like a broomstick held at arm's length. The bar sagged under its own weight,2 and the uneven wall pushed it from side to side as it cut. Instead of making a straight hole, the cutter followed much of the crooked one. Large cylinders were bored four times, turned a quarter turn between passes, and still did not always come out round.3 Around 1775, the ironmaster John Wilkinson found an ingenious solution. He built a boring mill with a much heavier bar that ran right through the cylinder and turned in bearings at both ends.4 The casting was clamped in place, and the cutter head slid along the bar as it turned. The cutter now followed the bar the way a pencil follows a ruler. Supporting both ends greatly reduced the bending, so the cutter could make a straight bore through a crooked casting. In 1776, Watt's partner Matthew Boulton wrote that a fifty-inch cylinder Wilkinson had bored for them “does not err the thickness of an old shilling in any part.” A worn shilling was a little under a millimetre thick, which puts the error at less than one part in a thousand across a cylinder wider than a metre. Relative to its size, that was about thirty times more accurate than Watt's test cylinder from seven years earlier! The coin was thinner than the smallest mark on a workshop rule. Even a common modern tolerance for a precision bore that large is only about seven times tighter.5 Wilkinson had made a cylinder accurate enough for Watt's dry packing to seal. For the next twenty years, nearly every engine Boulton & Watt built had one of his cylinders.6 The slide rest Most other round parts, like shafts and piston rods, were made on a lathe. The metal spinning in a lathe is called the work, and the person running the lathe is the turner. In the 1770s, the turner rested the cutting tool on a T-shaped support and guided it by hand, much as a woodturner steadies a chisel. But cutting iron takes a lot of force, and a turner's arms couldn't always hold the tool steady.

Below, we can watch the same cut in slow motion, first with the tool braced by hand and then clamped to cast iron. A slide rest clamps the tool in a carriage of cast iron and advances it with screws. The carriage slides along two rails called the ways, which run the length of the lathe's bed. The work is held between the headstock and tailstock, which sit on the same ways. The headstock carries the spindle, the shaft that turns the work. If the ways are straight and the two stocks line up, the tool travels parallel to the work's axis. With a slide rest, the forces of the cut go into iron instead of the turner's arms. The spinning surface drags down across the cutting edge, pressing the tool and carriage onto the bed, while the ways resist the push away from the work. In our model, the iron support bends about a hundred times less than the turner's braced arms,7 so a cut that set the hand-held tool chattering runs smoothly. Slide rests weren't new. Instrument makers and French mechanics had used them for decades.8 Beginning in the 1790s, Henry Maudslay made them rigid enough for heavy cuts in iron and combined them with the screws and gears of a practical screw-cutting lathe. The tool now followed the ways, so any bend in them would be copied into the work. A workshop could check the ways against a flat reference surface, but first it had to make one. Flat The usual way to make a surface flat was to file it by hand.