Yup. Stresses released during machining.

By the very nature of things all matrials have surface stresses induced during the production process. Solid equivalent of surface tension in a liquid. So any asymmetrical machining, such as on one side but not the other will leave stresses trying to bend the material. If the residual stresses from production are small they probably won't have any effect but if large things can bend in a pretty spectacular manner.

Hot processed material is bought to size and left too cool down so pretty much all the surface stress sorts itself out as it cools. Hot rolled steel in particular has a rather poor finish which is usually machined off to leave something nicer so the (lightly) stressed part is taken off anyway further reducing the small effects so bending is extremely unlikely.

Cold processed materials, like bright steels, are effectively squashed to shape cold under high pressures. The closer to the edge the more the squash. So there is a lot of stress locked up close to the surface of the material. As made it usually stays straight as stresses are pretty much the same all round. Taking a decent cut off one side removes the stressed layer that side so the other one pulls, or pushes, it bent.

The exact depth of the stressed area is material and process dependant in monumentally complicated ways. In our sort of sizes you probably need to take off something over 0.04" / 1 mm or so to reduce any residual stresses in bright materials to safe, non distorting levels. Which may need some workholding creativity to stop it bowing half way through.

Clive

Edited By Clive Foster on 12/11/2019 20:28:10

You can also heat up the bar to a nice red heat, hold it there for a couple of minutes and allow to cool slowly. This redistributes all the locked-in stresses created during the drawing process and distortion should be reduced.

I've milled a lot of hot rolled steel and never had a problem with distortion. Although there was a thread recently where someone had experienced distortion after milling hot rolled steel. If I recall it was quite a thin section. I'll see what happens when I mill the small hot rolled section to make the beading for my traction engine tender.

Cold drawn is a different animal, and not just steel. Cold drawn brass will distort as well. If I need to mill cold drawn steel I normally heat it to 800°C and let it soak for 30 minutes or so per inch of thickness. I then let it cool slowly. Of course I have the luxury of an electric furnace so soaking and slow cooling are fairly simple. I've never had a distortion problem after heat treatment.

Andrew

Unless it was bowed to start with.

Martin