Tolerances

Before 1815, every gun issued to British troops was craftsman made. There was no other way of making it. Each metal piece was cast or hammered and then carefully filed to fit perfectly.

Both Thomas Newcomen and James Watt continually complained about the bad workmanship of the people who made parts for them. One 900mm (3 feet) long cylinder was delivered to Watt with a bore 25mm (1inch) wider at one end than the other. All the parts were made by hand and the workmanship varied. What was needed was for parts to be made by machine, so that they were turned out to a constant specification.

There are four processes for which machine tools were first invented; metal planning, milling, lathe work, and screw cutting, which is a specialised form of lathe work. The first machine tool was a metal planer, invented in 1751 in France by Nicolas Focq.

In 1774 John Wilkinson patented "A New Method of Casting and Boring Iron Guns or Cannon." Besides weapons, Wilkinson's machine could bore cast iron cylinders such as those used in James Watt's steam engines. The simple idea that made for accurate boring was to hold the rough cast cylinder at both ends. Wilkinson had invented the modern lathe.

The first improvement to the metal lathe was the addition of a tool holding piece to create a turret lathe. A turret lathe is a normal lathe where the tool holder is in the form of a turret, with different tools attached to it. Rather than replacing a tool when a change in operation was needed, the turret was turned until the correct tool was facing the workpiece. This allowed complex pieces to be produced quickly and to exact specification. But this still left a problem. Each company could make the screws and bolts which held machines together, any size they fancied. Industrialisation required standard gauges for fasteners, standard gauges for metal thickness and standard sizes for timber.

The original British gauge for bolts and screws was the Whitworth gauge, first used in 1841. This was measured in 1/32s of an inch, from 1/16 to 4 inches. It was quickly adopted by the railway industry, which used huge numbers of bolts and screws and, as a result, was soon adopted across most industries. It was still being used for some aluminium fasteners in the 1970s, despite the adoption of metric measurements in the late 50s and early 60s.