To test my theory I took a big piece of 1/8" steel and put the mill on top of it, helping to decouple the mill from the bench. That fixed the problem quite nicely.
To make the fix more permanent I took a 1/2" steel plate and, with the help of another machinist with a bigger mill, milled both sides of the plate flat.
|1/2" steel plate milled flat, with locations for posts marked.|
I then drilled four countersunk holes in the plate for the mounting holes in the mill's base and JB Welded M6 studs into the holes. The mill was then placed on the plate and nylock nuts were loosely tightened down. They're left a little loose because I want the mill to be able to move slightly on the plate in order to keep it decoupled from the bench.
|The new base in place and bolted down.|