Re: [CAD_CAM_EDM_DRO] Re: real granite machines!

----- Original Message -----
From: dave engvall <dengvall@...>

> Granite has compressive strengths of about 19 Ksi whereas it is easy to
cast the desired shapes in concrete you will >loose some strength. Using
some of the newer techniques it is possible to get concrete with compressive
strengths in the >12 Ksi range. Embedding steel in this and providing
decent anchors should not be too difficult.
>
The problem with concrete is that it shrinks as it cures, maiking it hard to
fabricate a piece to finished size.

>Likewise one could drill holes in granite and mount components using a
filled epoxy to set anchors and maybe come out >OK.
>
This is how some custom machines are built - works great.

> The local tombstone company would seem to be a better source than the lab
grade stuff. I have seen 4' x 4' by 1' pieces for >a couple thou. (Boeing).
>
Enco sells a 24"X36" surface plate for $160. 36"X48" is less than $300. We
were going to use one of these for our gantry router but decided we needed a
much bigger table.

> My thinking goes more along the line of an L shape with cast iron ways,
etc anchored in masses of concrete. Has anyone >tried this?

Some of the early attempts at cheap CNC machines used concrete filled
weldments. The concrete provided excellent vibration damping but has a
tendency to separate from the steel housing, again due to that shrinkage
problem. Using the concrete for mass is a good idea if you can get around
this.

Doug