How about an off the wall solution? Is your vise keyed to the table? I had the same problems last year and it was because I was tightening the vise on the table too tight in order for it to stay in tram. The table on my PM mill is not flat. It varies about .002 up and down in both Y and X. It could take up to an hour to replace the vise and tram the jaw face and base into a good couple of tenths reading using shims. I finally made vise keys to speed up the project. Solved almost all of my problems. I was over tightening the vise to keep it in place and it was torquing the body which caused the jaws to act funny when tightened. The vise is a Kurt D40.
With the keys. the vise spans any imperfections in the table. It is fastened around snug. With a 3/4" x 1" x 9" parallel clamped in the vise, I get .0002 in X (face) and Z (top). and a 1" x 2" x 3" clamped in the Y gives a .0001 across the top. Takes around 1 minute to reinstall the vise. Pretty much everything now mills to .001 or less (if I'm lucky) across 4-6" which is the majority of my work.
The post above on metal stress is spot on. I brought the same problem up a couple of months ago about 1018 getting a belly in it when machined. The standard posts are to buy better metal that is stress free. You can do your stress relief if you have an kiln big enough. One guy posted an old time trick which I tried and like. He said that they used to mill .1 oversize and then let it sit there for a week or two. I mill .1 oversize and then put it outside in the sun for a few days. I leave it out overnight if it is not going to rain. The surface rust is easily sanded off and you are going to mill through it anyway. Sure has improved the dimensions on most pieces of steel. Some pieces still warp and I remake them, but very few.
Don't know if this helps, but it sure solved my problem.
With the keys. the vise spans any imperfections in the table. It is fastened around snug. With a 3/4" x 1" x 9" parallel clamped in the vise, I get .0002 in X (face) and Z (top). and a 1" x 2" x 3" clamped in the Y gives a .0001 across the top. Takes around 1 minute to reinstall the vise. Pretty much everything now mills to .001 or less (if I'm lucky) across 4-6" which is the majority of my work.
The post above on metal stress is spot on. I brought the same problem up a couple of months ago about 1018 getting a belly in it when machined. The standard posts are to buy better metal that is stress free. You can do your stress relief if you have an kiln big enough. One guy posted an old time trick which I tried and like. He said that they used to mill .1 oversize and then let it sit there for a week or two. I mill .1 oversize and then put it outside in the sun for a few days. I leave it out overnight if it is not going to rain. The surface rust is easily sanded off and you are going to mill through it anyway. Sure has improved the dimensions on most pieces of steel. Some pieces still warp and I remake them, but very few.
Don't know if this helps, but it sure solved my problem.
