Re: Steel plate for hexapod bed

The thinner the plate, the more difficult it becomes to get it flat in
its free state via grinding. Milling a large piece of thin plate flat
is virtually impossible, as the stresses induced into the surface by
the cutter generally make a bad situation worse with each successive pass.

Grinding a 2x2 part and ending up with it flat at 1/4 would require at
least 5/16 plate as a starting point. You have to initially put it on
the chuck with the convex side down and shim between the chuck and the
part prior to powering up the chuck. The object is to get one side
flat with the chuck turned off, and because thin plate is so flexible,
this is an extremely time consuming process. Once one side is flat,
the remainder of the job is a piece of cake.

For a point of reference of just how flexible 1/4" plate really is,
you can take a 2x2 square with .015" or so of crown and put it on a
surface plate, and push it flat with thumb pressure.

Anyone looking for a good starting point for a machine base in this
size range should keep an eye out for a pallet from an HMC that's been
scrapped. Quite a few of them around and they bring only scrap metal
prices. They're flat within tenths, very stiff due to the fact that
they're usually 3 or more inches thick, and they're already t-slotted.
Production HMC's often use multiple pallets arranged in a circle with
a robotic changer sitting in the middle of the circle. When the
machine is scrapped, the entire pallet changing system goes out the
door too. A pallet on its original stand is still a junk item, and
you then have the precision pallet mounted on a very rigid fabricated
stand, complete with leveling screws in the base.