Sawdust

Chris Clark From the editor's desk,
Issue no. 65

Cutting the thread on the centre post of the piano stool got me thinking about the mixture of measurement systems we use in woodwork. The threadboxes for screw-cutting in wood are still manufactured to Whitworth’s pattern in inch fractions. In 1820 Whitworth was apprenticed to the greatest English machinist of his time, Henry Maudslay. Maudslay produced metal lathes with lead screws so accurate that they could cut threads for the finest astronomical observation equipment. Whitworth noticed that each machine was assembled out of a variety of handmade nuts and bolts. By standardising the pitch of the thread to 55º and introducing increments in diameter, Whitworth revolutionised the machine making industry. For the first time machines could be designed, built and maintained using mass-produced nuts and bolts.

As an apprentice, Whitworth would have walked to the gates of Greenwich Observatory in London and copied the imperial foot and inch divisions from the bronze plaque embedded in the gateposts.

French apprentices made a very different pilgrimage. The French revolution replaced their ancient system of measurement with an enlightened system based on natural phenomenon. A line was drawn from the North Pole, through Paris and onto the equator. This meridian was then divided by 10,000,000 to produce the metre unit. A platinum bar was then engraved with this standard metre length. A volume 100 x 100 x 100mm of water at 4ºC was then defined as both a litre and a kilogram.

Timber is still cut in imperial sizes and then machined down to a metric dimension. Timber lengths of 0.9, 1.2 and 1.5m are equal to the 3’, 4’ and 5’ lengths of old. A 6” x 1” board will be machined to 140 x 19mm. Plywood made for the US market will be 4’ x 8’ (1220 x 2440), however MDF and plywood made for the Australian market will be 2400 x 1200.

The reason we still use Whitworth threads is that the pitch is just right for cast iron and other brittle materials. Dowel is universally machined to Joseph Whitworth’s standard imperial diameters from 1830.

Good woodworking!

Chris Clark
Editor