Milling out a bend in GM casting

I have milled both sides of a gunmetal casting (loco frame stretcher) and it is now very slightly curved (max deflection approx 1.5mm in middle of 100mm of total length). As the stretcher will carry valve guides I would like it as straight as possible.

Am I correct in thinking that if I return it to the mill vise, hammer it flat on parallels and then take another 'skim' off the convex side, this will relieve the internal stress and remove the bend?

Also, when milling off material for the above situation how much is enough? Obviously I don't want to take too much off end end up with a bend in the opposite direction. Or is it a case of take off 0.1mm remove check/repeat?

Any advice appreciated, Thanks.

Very strange for a gumetal casting to distort when machined. Is your milling cutter sharp and not rubbing on the metal? That might cause surface stresses over a fairly thin, flat component. Otherwise, I think you'll have to "suck it and see" by taking off a light cut on the side you need to be flat and see what happens. Don't distort the casting by over-tightening the milling vice.

Edited By Clive Brown 1 on 07/07/2022 15:39:39

Did you distort the casting by clamping an uneven surface to something flat. Always best to shim under any casting so it sits solidly to the mill table or vice jaw. If you don't it will spring back when clamping pressure removed.

You will probably have to bend beyond flat to allow for springback.

Edited By JasonB on 07/07/2022 15:44:27

I think my main question here is when a casting has been machined and a bend becomes evident does extra material need to come off the Convex or Concave side to equalise the internal stresses and remove the bend as this is the part I'm unsure about? I would have thought the convex side?

In answer to the above; No, the casting was not subjected to any uneven clamping that could have resulted in a spring-back after clamping removed.

Also a new end mill was used so no issue with rub.

I think that the probable answer is both sides. Internal stresses are balanced affairs, so if strained material is removed from one side, the remainder will adjust to a new state. That's why I suggested that "suck it and see" in small stages might be the best way forward.

Or else try heat the casting up to 3-400 deg. C for a short while before re-machining flat.