Re: Re: Scratch-built CNC mill

Ian,

the pages with this link give an unbiased look at all aspects of this
question as the company involved makes scales with many different
coefficients of thermal expansion
http://www.heidenhain.com/phaise2/lindt.html
for scales with a glass base: (the LF, LS, LB, and LIP, LIF, LIDA
codes are product family idents)
LF: 10 ppm/K
LS: 8 ppm/K
LB: same as scale mounting surface or 10 ppm/K

And for scales with a Steel tape base
LIP: 8 ppm/K or 0 ppm/K (Zerodur�)
LIF: 8 ppm/K
LIDA: same as scale mounting surface or 10 ppm/K

the whole Tech Data section of their site is a gold mine of basic
(and advanced) info on the principals of how DROs work as well as
general metrology:
http://www.heidenhain.com/techdata.html
the link from that page to "Principles of operation" is the place
with most of the meat... really a great source no matter who's DRO
gear you use...

>From: "Ian W. Wright" <ian@...>
>
>CG wrote:
> > Why don't we just use glass scales??
>
>The question really should be - why *do* people use glass scales. Close
>contacts with a company which made precision measuring products taught
>me that the material which machine scales are made from should match
>either that of the machine itself or the material being processed. In
>this way, its not necessary to take any account of changes in
>temperature. That is - a steel scale on a steel machine will remain
>proportional at all temperatures. If you are working hard on a large
>metal part it gets hot and will consequently expand, so, logically, if
>you are relying on a scale which does not expand as much or is in a
>position where it is not affected by the heat of the work, the work will
>finish up undersize when it cools down. opinions anyone??

Randolph Lee boss@...
---------------------------------------------------------
Windshadow Engineering Nantucket Island, MA USA
---------------------------------------------------------