A note on 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.

 

S.I. “handy” “British”
Max allowable stress 500 MPa 500 N/mm2 32 tsi
Size of plate .025m x .008m 25mm x 8mm 1” x 5/16”
Area of plate 2 x 10-4 m2 200 mm2 5/16 in2
Max load 1 x 105 N 100,000 N or 10,000 kgf or 10 tonnes 10 tonf

A note on gauges etc.

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

 

Wood Screw Data

Nominal Diameter Clearance Pilot Hole
Guage Decimal inch mm Hole Hardwood Softwood Metal
2 .082 2.08 3/32” 1/16” – –
3 .094 2.39 3/32” 1/16” – –
4 .108 2.74 7/64” 1/16” – 5/64”
5 .122 3.10 1/8” 5/64” – –
6 .136 3.45 9/64” 3/32” 1/16” 3/32”
7 .150 3.81 5/32” 3/32” 1/16” –
8 .164 4.17 11/64” 3/32” 5/64” 7/64”or 1/8”
9 .178 4.52 3/16” 7/64” 5/64” –
10 .192 4.88 13/64” 7/64” 5/64” 9/64”or5/32”
12 .220 5.59 15/64” 1/8” 3/32” 11/64”or3/16”
14 .248 6.30 1/4” 9/64” 7/64” 13/64”or7/32”
16 .276 7.01 9/32” 5/32” 1/8” –
18 .304 7.72 5/16” 11/64” 1/8” –
20 .332 8.43 11/32” 3/16” 9/64” –

Copper nails, Gripfast nails and metal sheet are dimensioned by wire gauge, where the bigger the number the thinner the nail or sheet.

 

Wire Gauges

16g 1.626mm 10g 3.251mm
14g 2.032mm 9g 3.658mm
13g 2.337mm  8g 4.064mm
12g 2.642mm 6g 4.877mm
11g 2.946mm 4g  5.893mm

Pilot Holes for nails

Gripfast Boat nails
Gauge Hardwood Softwood
14 1/16” 1/16” 3/64”
13 – 5/64” 1/16”
12 1/16” 5/64” 1/16”
11 – 3/32” 5/64”
10 5/64” 3/32” 5/64”
9 – 3/32” 5/64”
8 3/32” 7/64” 3/32”
6 – 1/8” 7/64”

 

Thread Comparisons – Bolts and machine screws

Metric Imperial
ISO metric TPI Pitch (mm) UNC Whit. UNF Inch dia
M3 (0.118”) 51 0.5 – – – –
M4 (0.158”) 36.3 0.7 – – – –
M5 (0.197”) 31.8 0.8 24 24 – 3/16”(0.187)
M6 (0.236”) 25.4 1.0 20 20 28 1/4”(0.250)
M8 (0.315”) 20.3 1.25 18 18 24 5/16”(0.312)
M10 (0.394”) 16.9 1.5 16 16 24 3/8”(0.375)
M12 (0.472”) 14.5 1.75 13 12 20 1/2”(0.500)
M16 (0.630”) 12.7 2.0 11 11 18 5/8”(0.625)