I'm generally following Raymond Benwood's method of broaching and that requires regrinding several store-bought broaches. The broaches are HSS steel, which is a pain to grind, and there's a lot of material to remove, so this is a time consuming and generally not fun part of the project.
I tried several different ways of grinding the broaches. Raymond suggests grinding them in a mill using a white grinding wheel. Well, I will disagree with Raymond on his choice of wheel -- white grinding wheels are not the best for HSS. Wheels come in different colors, grits, hardness, etc. for different applications. For grinding hard materials, I've had better luck with pink or blue/gray. After briefly trying a white wheel on the mill, I switched to pink and it worked much better, cutting faster and cooler. But even with the pink wheel, the mill was still painfully slow. It took me all day to grind down the height of one broach using the mill. Ugh.
Raymond also suggets using a sander with zirconia belts. Well I tried that and it was too slow to be practical, though I did end up using a belt sander to fine tune the final dimensions on the broach height, and to chamfer the ground edges.
Raymond also suggests using an angle grinder. I tried it using aluminum oxide disks and they barely made a dent in the HSS. So, I ordered some zirconium disks from LehighValleyAbrasives.com, Metabo #616785000. The zirconium disks were an improvement though still slow -- it's much slower than grinding mild steel. Still, the zirconium disks removed material about 5 times as fast as using a mill and pink disk.
The method I settled on for grinding the broaches is as follows:
1) scribe a line in layout fluid or marking pen to use as a guide while grinding with the angle grinder.
2) with the broach clamped in a vise, use an angle grinder with the previously mentioned zirconium disk remove the bulk of the material from the backside of the broach. Stop about 0.020" shy of the scribe line.
3) remove the remaining 0.020" of material in the mill using a coarse pink disk.
4) then use a surface grinder to thin the sides of the broaches. My surface grinder happened to be set up with a pink/grey #60 wheel and that worked well enough. While still slow, a surface grinder is nonetheless faster than the mill and does a better job (flatter, more predictable cuts) than grinding on a mill.
Why not use the surface grinder for step #3? Because I did not have a way to hold the broaches in the right position for that operation. Knowing what I know now, if I had to do it over again I would take the time to make some sort of fixture to hold the broaches in the surface grinder for step #3, because a surface grinder is faster and does a better job than the mill.
I took lots of pictures while grinding the broaches, then unintentionally deleted them.
Oh well, here are the finished 7/16" broaches. The reason one broach is shorter is simply because it is a different brand -- the part that cuts is still the same length. Both the finished taper and the height are critical, and I'll discuss those dimensions in more detail when I actually start broaching. The short version is that, because the action is much longer than the broaches were intended to cut, we have to compensate by taking smaller bites.
Showing the cross-sections after grinding.
The next step will be to make shims for the broaches. The standard shims sold for keyway broaches are too short and too thick for our application. A standard shim is 0.063" thick which means that every subsequent pass will remove 0.063" -- if you try removing that much in a rifle action the broach will get stuck trying! Raymond used 0.020" shims. Looking through my stash of scrap metal, it looks like I have some 0.012" and some 0.019", so I'll make some of each.