Marking the center of the piece of steel helps get the bevels equal. You can get a scribe or a height indicator for the best tool, but a small drill bit held on it side on a good flat surface will work if you drag the blanks edge along it, turn the blank over and do the other side. With the correct size drill you will get a line scratched doen the center, if thee drill size is a little off you get 2 lines close to the center with the exact center in the small gap between them. I actually scribe 2 lines on purpose on do my rough grind to each sides line. If I do a good job on the coarse grits the fine grits will stay equal as I bring it to an edge.. You can make a cheap file guide by taking 2 matching pieces of good tool steel about 1/4 to 3/4 thick and at least 3/4 wide and 3 inches or so long. clamp them together and drill through both ends with a drill bit that will work with a 1/4 20 tap. Tap one bar and drill the other out with a 1/4 drill.. Now they should bolt together. Using a real flat surface with sand paper on it sand the bolted pieces to a perfect match. Use a punch or something to mark one end of the set so you always reassemble them the same. Now take the bolts out and harden the 2 pieces and do a 350f temper. If the hole shrink a bit polish the bolts a bit with the bolt in a drill and strips of sand paper. till you have a tight fit. I haven't had a set of threads not work yet. Maybe with tight thread engagement and a real hard bolt it might be a problem the first couple times you work it. Once done a few times thew will work out. I made my first set just like that and they worked OK, just not on a grinder. They can be improved by making them longer and adding a "dowel" on each end just inside the bolts. Just drill a second set of holes just like the first set before hardening and tap the threads into the same bar as before and 1/4" drill the other. Then run a bolt up to the unthreaded shank hard into the inside tapped holes. Cut off the bolt heads and bevel the ends a bit. Now those will slide into the matching 1/4" holes and when you add the bolts they will stay in very tight alignment. Now harden as before.
This will work for a file guide. If you file in the initial plunges and work the coarse grits carefully once you get to the real fine grits that have a flexible backing you can run the belt over the edge of the platen and grinding edge up You can look down on the spot where the plunges meet and fine tune. The fine grits remove metal very slowly, your eye is a precision tool. Keep the angle of the length of the knife the same as you do both sides each on their side of the platen. Don't just run the plunge into the belt and hold it or you will end up with a grind line 2" from it. Keep moving back and forth into it pushing against the edge of the belt covered platen edge with the plunge. It does take practice not to mess up the rest of the grind when you try this. Being able to slow the belt way down is a big help.
