Gauges, Threads and Units

Like many in their prime of life (?), I think in feet and work in metres, having lived through the conversion from “British” to S.I. units. For those of you not obliged to convert, a few words of explanation might be a good idea, using round number conversions.

1 kg (mass) = 2.2 lb (mass). Mass is the amount of matter in a thing, whose weight is determined by whatever the acceleration happens to be at the time. Stood still, on Earth, the prevailing acceleration is due to gravity which is about 10 metres per second per second. So the force – or weight – of that 1 kg (mass) is 10 Newtons (N), or 1 kgf. Now kilogram force (kgf) isn’t these days a proper unit to use, but is more convenient to visualise than Newtons. So one refers to 1300kg breaking load, where one should properly say 1300kgf or 13000N. For those who can’t stand kg or N, it so happens that 1 ton – the 2240 lb type – is as near as makes no odds 1 tonne – the 1000 kg type – and the force aspect of things is usually implicit in commonly expressed weights, loads etc. By a neat irony an apple weighs about 1 Newton!

Pressures or stresses are forces applied over an area, such as tonf per square inch, Newtons per square metre (referred to as a Pascal (Pa)), or whatever. Because many of the things we are dealing with are millimetre sized – like wire and bolts and so on – N/mm² is the handiest even though it’s not the pukkah unit. It also happens to be the same numerically as megapascals – MPa – which is what most material properties are specified in. So 20 MPa = 20 N/mm². But use either, and you will need to divide by 10 to get to kgf, and a further 1000 to get to tonnes. The example here should confuse the issue suitably.

There are moments when you wish Napoleon had been a bit luckier, and this is one of them.

Woodscrews are gauged by screw gauge – listed below – where the bigger the number, the bigger the screw