Paul Lazenby Design and Development

Reasons for using the S.I. units August 31, 2006

It may seem a bold step to promote Model Engineering from a mostly metric view-point but I firmly believe that we must make a definitive effort to move with the times. After 34 years working within the engineering industry I cannot think in, relate to, or even remember Imperial Units readily.

I am a Time-served Fitter/Machinist and a qualified Draughtsman (C & G) 1973-78. From my first day-release at Technical College (Kitson, Leeds) I worked in Metric units. After a few lessons, the obvious supremacy of the SI Units made me resent all those years at school, wasted learning in Imperial units. I wish that I had been taught in Metric all along.

The firm that I served my apprenticeship with officially switched to SI units in 1976. That was 30 years ago.

I often think that one of the reasons why model engineering does not attract so many young people to it’s ranks as it should is because they perceive it as being cobwebbed by old codgers, so set in their ways that the inability to move with the times is a science. That is of course until it comes to using digital calculators in favour of Logarithms & Slide-rules. The resistance to the SI units (used virtually throughout iIndustry and schools) only compounds this impression.

Metric machines are usually graduated 0.02mm thus they are more accurate than Imperial ones. This is easy to relate to, and is slightly smaller than 0.001″. Metric Machines, Tools, Cutting Tools (especially HSS) Stock, Fasteners, Tubes and Fittings are all readily available from industrial suppliers, often local and we should take advantage by incorporating these into our designs. How much would a box of 500 M3 Nuts cost you compared to 6BA?

Whilst it may seem hypocritical, I will maintain ME 26, 32 & 40 Threads mostly for Pipe and Tube Fittings, at least for the present time.

This brings me to the final point – that of drawings for ME’s. A good number of these leave a great deal to be desired since they have not been drawn by professional draughtsmen. It is little wonder that it often takes ME’s years to build a model because the drawings and/or designs are so chronic. Why do ME designers dimension in fractions of an inch, if the part to be produced is to be machined, usually to 0.001″? I have to add up a string of fractional dimensions, find the lowest common denominator, then decimalise it all. CAD has sorted some of this, but has brought about, even in iIndustry, the ‘CAD Jockey’ – someone who has no engineering and/or drawing training but learns how to use the software then jumps on board for the ride. 3D CAD has exacerbated this de-skilling of the prawing profession. For example, a trained draughtsman does not need 3D to produce a design for say – Laser-cut Stretchers. 2D CAD and the skill of the draughtsman to project elevations suffice.

Whilst designing the 76XXX I am working nominally to a minimum of 1/64″. This means that in order to maintain accuracy I must type in units to 6 decimal places! This is so as to prevent progressive/accumulative error since when dimensioning to suit Imperial machines the dimension style is set to round up to 3 decimal places.

When working in metric, nominally 0.5mm minimum, I usually have only to type 1 decimal place with the dimension style set to round up to 2 decimal places and a suitable tolerance as required. This is far easier.

One thing I will guarantee. If I produce Metric & Imperial versions of a design – they will be separate designs within their own right – not one converted from the other.

Paul Lazenby