Re: [CAD_CAM_EDM_DRO] Re: measuring scales and thermal compensation

----- Original Message -----
From: Jon Elson <jmelson@...>
> The glass (or quartz) scale idea is that the scale is fixed at a
> reference point at
> one end, and allowed to sit without thermally induced stress being
> applied to
> it. (Only the better scales actually implement this, but many use
> resilient mounting
> techniques.) Since the thermal expansion of glass (and especially
> quartz) is
> much less than steel, the measurements of machine position will be more
> accurate as temperature varies. I think, overall, that a glass scale is
> more
> accurate,

But this won't be the case. The problem here is that if the workroom isn't
at 20C - and mine, for instance, can often be 10 or less when I start
working in it - then the machine itself will have contracted more than the
glass scale. So, if you work to the scale, the part will come out big.
Conversely, of course, if the shop is above 20C or the machine has warmed up
through use, the machine will be bigger than the scale and the parts will
come out small. The workpiece may be the same temperature as the machine or
it may be different due to the coolant temperature which, in a cold shop,
may increase the problem. So, make the scale out of the same stuff as the
bulk of the machine, and they both change length by the same amount.

Ian

--
Ian W. Wright
Sheffield UK