[jim@edmik]

I am new to the cnc zone. I have to mill a 12" x 12" by 1 thick down to .750" +-.005. Past experience says 304 warps, I have tried insert mills, carbide end mills, fliping part over mill equal amounts off each side, it still seems to warp. Any sugestions would be greatly appreciated. Sugestion for speeds and feeds and cutters. I also have to mill some large slots and boss .625 deep. I know you have to keep the stress out of material when milling to avoid warping. thanks jim@edmik

[Geof]

It is not a case of keeping stress out; the warping is caused because the milling unevenly relieves stresses that are in the material from the manufacturing process.

The best solution to prevent warping is to have the material normalized to remove all pre-existing stresses. However, this removes the work hardening and lowers the yield strength of the material.

If it is necessary to leave the material in the worked hardened condition removing equal amounts of material from both sides in a series of small cuts might minimize the warping.

[tminnig]

Something that has worked well for me in the past is a 3-flip method.

First - Face the first side deep enough to get past the material skin, as this seems to relieve a lot of the stress right off the bat.

Second - Then flip it and face it again, leaving .050-.100" on the overall thickness. If you have some open tolerances, you could probably get away with finishing the outer contour and some of the features. I tend to like side milling at least two sides of the part at this stage. Use some machinable vise jaws and you can mill some reliefs to allow for the cutter path for the side milling and still allow you a deeper step for optimal clamping surface.

Third - Flip it again and finish the overall thickness and mill any remaining features. I normally will do a 90 deg rotation here so I can finish the remaining two sides of the contour.

In the first and second holds, I normally do them in a single vise with a M0 to flip the part the first time. The reliefs can be used as a visual guide to locate the part in the X direction.

For the third hold, I like a second vise so I can add any additional locators and vary my workholding as necessary for finishing the part. Depending on the part configuration, you could potentially use reliefs for the contouring cutting (along the remaining sides of this op) as a visual guide for an X locator, same as the second flip. However, if you finished any other features on the previous op you will need to use them to provide accurate locating, of course.

The key is not to hold down on the part. If you use downward pressures, you will simply mill the face flat relative to the table, then release the pressures of the clamping and the material will spring back to its previous state. You need to use vises or some other side clamping device, and may even need to do a re-clamp or two to release the material to its 'desired warp' before finishing the faces.

Good luck,
Travis