Hot rolled mild steel (A36) is too gummy and is difficult to get a good finish, even when grinding it. Cold rolled mild steel like 1018 works good, but can bend like a banana if you cut only one face, due to internal stresses. 6061-T6 or 7075-T6 (or T651 for either) will work well. 7075 is harder and stronger aluminum than 6061. I used 1/2 x 2 x 6+" 1018 for my 6" Bridgeport vise, and milled the working faces true after installation with the side of a sharp 1" carbide end mill. They still look and work fine after 5+ Years of use. No complaints, they grip well and do the job they need to do. Should I ever crash a cutter into them, it will be less dramatic than if they were hardened.
