I made some flat (no teeth) jaws for my 3" Wilton Machinist vise out of O1 Oil Hardening Steel, and it worked just fine. Ordinary saws, drills, and mills work just fine, but don't stint on the black sulfur cutting oil.
I plan to harden the jaws, but have not yet gotten around to it, and they don't seem any the worse for it.
The key is to make sure that the new jaws rest firmly in their pockets in the vise, so the clamping force is transferred from jaws to frame without undue concentration. In the Wilton, this required that one long edge be beveled (so it wouldn't bottom on the inside radius of the pocket) and the elimination of all burrs and irregularities.
All this can be done with hand tools, but is a lot easier with a mill. The hardest part is making the counterbores to accept the 1/4-20 Filister head screws that hold the jaws to the frame.
I replaced the original screws with hex socket head machine screws so I could change the jaws without taking the vise apart using a standard hex key.
I have a bunch of spare parallels about the right size, figured I'd try using two of those if that plate stuff I have is too narrow. I'm not sure what those plates are actually. They are about 3"X12", in thicknesses of approx 1/16 - 3/8". They were individually wrapped in oilpaper. They are *very* hard steel. I'm not confident I can cut them.
I might have missed that, thanks
This one had SHCS already, so I'll go back with those. the fixed jaw had been milled back over time until the heads of those two screws were all but gone. Took a bit to coax them out.
Stop! You will *not* be able to cut them with ordinary drills, saws, and end mills. You will instead destroy your cutting tools.
For the record, the parallels are probably something like W1, but hardened. W1 is dirt cheap.
Life will be far easier if you buy some precision ground O1 flat stock from MSC or the like, already in the correct size. Then all you need to do is cut to length and drill and counterbore some holes.
O1 is a far stronger steel than W1, so even unhardened will be stronger than W1. For small quantities, the cost differential isn't great. Buy enough O1 so you can throw the first jaw away. It happens.
Hardening O1 is easy. Heat to orange-red with a torch, dunk in corn oil (cheapest suitable oil, get from grocery store) in a metal container (so no disaster if you drop the red hot workpiece). I hold the workpiece by threading some heavy iron wire through the screw holes. Polish scale off with wet wet-dry sandpaper, temper with torch by heating slowly until it turns pale blue. After torch is removed, color will deepen to dark or darker blue, which is the traditional spring temper.
On machining counterbores, I found that it was best to do this in two steps. First, rough the counterbore out. Then blow all the chips away, liberally daub with black sulfur oil, and then make the final cut. This procedure prevents the chips from scouring and thus mangling the surfaces upon which the heads of the cap screws will rest.
I forgot to mention that it's a good idea to smear the threads liberally with anti-seize compound.
It's also necessary to ensure that the screws do not control where the jaws end upshould in the sockets in the frame - the jaws should be loose. All the screws do is clamp the jaws in place, but not attempt to locate the jaws.
I would just have at it. The worst that will happen is that you will throw your first attempt or two away. I have one of those too. Nice paperweight.