RE: [CAD_CAM_EDM_DRO] Re: Flat as a (very precise) pancake...

>-----Original Message-----
>From: Tom Murray [mailto:thomasm923@...]
>Is there a way to calculate the proper length to thickness ratio to
>give me my desired "flatness"?

Well, you can use the beam deflection formulae in Machinery's Handbook to see
how much the thing will sag under its own weight. It would be almost impossible
to finish a surface to a finer precision than the amount of self-induced sag.

On the other hand distortion caused by further aging or by irregular thermal
expansion is probably impossible to predict.

This all leads me to the next question, though:
You want these "really flat" pieces for mounting a relatively floppy linear
guide rail. The overall assembly will, of course, be about as stiff as the flat
piece itself.
But what holds up the flat piece? I assume it will be attached to a frame, or
directly to legs, etc. How can you do this attachment without distorting the
flat piece?
Also, how much flexing will occur under the forces imparted by the operation of
the finished unit (acceleration plus cutting forces, if any)? As long as these
forces are small compared to the weight of the flat piece, the deflections they
impart will be small compared to the sag that limits the flatness of the flat
piece. OTOH if these forces are much greater than the weight of the flat piece,
its no-load flatness will be largely irrelevant.
-Kevin Martin