The Whitworth Screw Thread
The first successful industrial standard anywhere in the world.
Origin
Proposed by Joseph Whitworth to the Institution of Civil Engineers in 1841, and adopted across British industry within two decades.
The catch
Before Whitworth, every workshop cut its own threads. A bolt made in Manchester would not fit a nut made in Leeds, and a machine could only be repaired by the firm that built it. Standardising the thread did not merely improve manufacturing; it made interchangeable parts possible at all.
| Size | Threads per inch | Core diameter (in) | Typical use |
|---|---|---|---|
| 1/8 in | 40 | 0.093 | Instrument and light fittings |
| 1/4 in | 20 | 0.186 | General light engineering |
| 3/8 in | 16 | 0.294 | Machine assembly |
| 1/2 in | 12 | 0.393 | Structural bolting |
| 5/8 in | 11 | 0.508 | Heavy machinery |
| 3/4 in | 10 | 0.622 | Locomotive and marine work |
| 1 in | 8 | 0.840 | Heavy structural |
| 1 1/2 in | 6 | 1.286 | Bridge and girder work |
Whitworth fixed the thread angle at 55 degrees with rounded crests and roots, a form chosen because it was strong and, crucially, because it could be reproduced by ordinary workshops. He arrived at it by collecting bolts from manufacturers across the country and averaging what he found, which is a rather democratic way to write a standard. The British Standard Whitworth form governed British engineering for over a century, and was only displaced by unified and metric threads after the Second World War.