I have never file fit a shoulder for a guard, do have respect for people that do it. Have always done it on a mill, this is the best way I have found, I made the fixture that goes in my 6" mill vice, it will drop in with in a .001 or you can use a dial indicator on it, if you have a knife with a straight back it will give you a 90 % I first put the knife flat in the 6" vice and take off .020 or .030 depending on how thick the blade is, take equal amounts off each side make them match best you can, then put them in the fixture vice, now you can take a mill bit and go all the way around the tang of the knife, this will give you a good shoulder, don't try to cut more than .010 at a pass two on the last pass. If I have a long tang have a long bit, some times after etching I will take a diamond cup and touch it up. Before I heat treat take a paragraver and relieve the centers so my guard is only sitting on the outer edges, hope this will make life easier for some people, a friend made one fore his mill drill and it wants to chatter, think the reason is that you need to raise the head so high that you lose your ridity, I tried it in my mill and it worked fine.
