Re: [CAD_CAM_EDM_DRO] Achieving flatness on a large gantry table

kdoney_63021 wrote:

>I just checked my table for flatness and was appalled. Max .025"
>error over a foot at certain points. The gantry itself is within
>reasonable tolerance. I have THK slides, machined extrusions and ball
>screws. I checked it every step of the way and it looked great; until
>mounting the table. This is a moving table design from Bob Campbell's
>plans (a nice system). I first used standard extrusions clamped
>together, I then went to slat wall, then tried to wedge together
>machined t-slotted 6061 (not mic6).
>

If you want it flat, you have to MAKE it flat. Unless you start with a
surface plate, every bolt you put in distorts the structure to some degree.
So, even if it started out with a flat plate, when you start assembling the
components, it starts to distort, and just keeps getting worse. Apparently
you have some method to check flatness, so you then need to use that
information
to correct the errors. If different materials are combined (like
aluminum plate
with steel rails and supports) then temperature changes can make the
distortion
change. Release of internal stresses can also make it drift over time,
either
getting worse, or if you are really lucky, better.

Jon