What the heck is the problem with 300 series Stainless Steel? It doesn't seem to matter how many times I take a mill cut on it, it never stays flat.

Two things you can do:

1) If you have larger, intricate parts that have some difficult flatness callouts, the material must be solution annealed. I don't want to go into a big long story of how I came across this, but trust me it works. The process involves bringing the raw material up to a temperature of 1900° farenheit and after it has reached that temperature it must be held there for one hour, then quick cooled in a nitrogen atmosphere. If the parts are substantially large, material should be cut to size before annealing. It is well worth the extra price per pound to have this done. Check with your local heat treater.

2) If you have a smaller 300 series SS part that requires flatness on one side, rough the part in, leaving .010" - .015" of material (you be the judge). Release the part in the vise and then snug up the vise just enough to hold the part for the finish cut. The last cut must be made with sharp tooling. A beneficial side effect of this process (using a sharp tool on your finish pass) is that the burrs almost fall off the part. By the way, dull tools used in the roughing process will put more stress in the part which will affect final flatness.

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