Re: Re: Scratch-built CNC mill

Hi,

Thanks for all your comments. I should perhaps have clarified that the
situation I was referring to in which I had seen scales of the same
material as the machine was on equipment for the production of rules and
scales. As you know, most rules are calibrated to be correct at 20 deg.
C and are guaranteed accurate to very small tolerances, particularly
such things as vernier height gauges but the factory in which these
products were made (Chesterman's) dated back over 100 years when close
environmental controls were unheard of. For making precision steel
rules, for example, the old equipment was still used until the factory
closed a couple of years ago and the process was to coat the rule blanks
with a film of resist and then to scratch through this the various
divisions prior to acid etching. The marks were made by hand using a
long 'bench' which had ways like a lathe and a carriage carrying the
scriber head. This worked against a steel scale which had been
accurately cut at 20 deg. C and which was intimately fastened to the
machine bed. Consequently it didn't matter what the ambient temperature
of the room was as the master scale and the rules being produced would
both have expanded or contracted to the same degree and so both would
still be accurate at 20. As most of us do not have access to close
environmental controls, I had wondered how appropriate to our situation
this 'old' technology may be.

Ian

Ted Robbins wrote:

> Years ago I worked at a place that machined to 50 millionths, long before
> it was done any way except by grinding.

--

Ian W. Wright LBHI
Sheffield Branch Chairman of the British Horological Institute.
Bandmaster and Euphonium player of the Hathersage Brass Band. UK.
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