Basically either :-
machine equal amounts off each side
or
anneal it by heating to red and allowing to cool slowly.
Bright steel bar is finished to size by rolling which locks up stresses in the outer layers of the material.
Machining on only one side takes out the stressed layer on that side leaving the other side still stressed. So the bar bends.Machining both sides equalises any remaining stress so it stays straight. Holding it so you can machine both sides without it moving mid job due to inadequate grip or taking out the bend when remounting to do the second side can require creativity.
Heating to red (all the way through) and cooling slowly lets the stresses work themselves out as the material cools. Rather easier during open fire / soil fuel boiler days as you could dump it in the fire of an evening to get hot and retrieve it from the ashes the next morning when it had cooled down. These days you need to pay for gas to heat it and have something to bury it in for slow cooling.
Clive
PS Jason types faster! That said I'd want it red for longer and very slow cooling. These days some of the bright bar seems to have had a lot of rolling and lots of stress. Several itty bitty cuts are a good way to shorten the life of milling cutter too. So glad I have a shaper! I've been known to weld such pieces to a larger parent bar to keep them straight. Costs you the ends where the weld was but can make life very easy.
Edited By Clive Foster on 17/03/2023 19:03:36