Re: Flat as a (very precise) pancake...

---hi,
there is a product out there that has been used successfully for
years
, it is called 'shim'. Perhaps a little old fashioned and low tech
for
the group but it does have its merits.
T.

In CAD_CAM_EDM_DRO@y..., "Kevin P. Martin" <kpmartin@t...> wrote:

> >-----Original Message-----
> >From: Tom Murray [mailto:thomasm923@a...]
> >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