Sharpening Blades:

Coulter's Smithing Home.

Some pictures of a knife grinding jig for the big lathe, and results. Simple to make and to use, this is nice to have around. We also made a jig for the milling machine and got some interesting results from that. Blades come off the lathe very very sharp already, then we put a final sharpening on them to slightly alter the profile right at the edge. The net result of the new profile is that for awhile, the edge wears to a sharper state yet. Doug just loves things like this, when nature works in the direction you want with only a little hint required.

Lathe jig The jig is two parts. One is a normal grinding wheel supported on a shaft between center and the chuck, the other is just a piece of angle iron with an added flat piece with tapped holes in it, used to clamp the blade to the angle piece. This whole thing mounts on the lathe instead of the tool post. Once it is set up, it makes very nice straight edges. Setup is time–consuming, however, so there is some room for improvement here. For one thing, each blade has to be set at the same height at both ends, or the grind won't be the same all over. The jig itself of course needs to be perfectly parallel to the ways. For the former, I think I will add some setscrews rather than use the tap with hammer measure, and repeat method any more. The lathe is set to fairly low rpm for this, and the traverse is set very slow, so it doesn't matter that the grinding wheel is actually coarse. Only that one highest grit does any work, and the traverse per turn is less than the width of that grit, so the result is shiny. We don't have to worry about drawing the temper at this stage, as we are going to heat treat anyway, but it just does a better job on cool metal that is stiff.

Lathe jig in lathe Here it is mounted and ready to rock. Not shown for this picture are the rags that normally cover the lathe ways when doing this. Grit and lathe ways should never mix. Once setting the jig straight, the main work is getting the blade in there perfectly level and at the correct height to get the edge angle desired. This jig does not make that easy yet. If it is assumed that only 1/6" by 1/2" stock will be used, one could mill out a slot in one of the jig faces to help set this. For more versitility I'll probably just add a way of supporting the blade with some adjustable set–screws at the ends.

A2 steel blade sharpened on lathe Here is a partially finished knife with a hollow ground blade made on the lathe fixture. It was then nickel plated up to near the edge for corrosion resistance and slipperyness. We use a neat trick here for mounting blades into wooden stocks. The blades are 1/2" by 1/16" stock, in general. We use a forstner bit in a lathe to drill a half inch hole in the center of the stock. We then pick a piece of grain–matching wood and turn a half inch dowel out of that. We split the dowel, drill a couple of 3/16" holes in it to match ones in the blade, and put a couple of same sized brass pins through the holes. That assembly is then epoxied together, put back on the lathe and turned back down to a half inch. We then glue the assembly into the stock using regular titebond wood glue, and it is plenty strong. It looks like the blade "just grew there" when we finish the shaping on the end. The advantage of this is that we don't have to split the main stock and try to make a longitudinal glue line disappear. The scratches on the blade shown are on the A2 stock when I get it, due to its being "precision ground". Next run I will try buffing them out before hardening perhaps. This knife is awaiting a visit by the customer so I can fit the stock to his grip.
ripple edged knife from milling machine Here is what happened when I tried to use a 3/8" mill bit and a jig to hold a piece of 1075 steel in the mill at an 8° angle. First side cut went smootly, second side small cut the same, then when I took a fairly meaty cut of about 30 mils, this happened. The edge is still about 10 mils thick -- it just bent out of the way of the mill cutter in this wavy pattern. I think I will try holding it up to the belt sander next and see if I get sort of an odd steak knife out of it. This was attempted because milling metal off (cutting it) is a lot faster and more efficient than grinding (rubbing it off) is. Well, maybe you just have to do it the slow way to get the result you want, or maybe use a grinder for that last pass on the mill? This has piqued my interest, expect some new results of tests up here pretty soon. I did try sharpening this on the belt sander after pounding out some of the ripple, but it was still too much and left more of a saw blade than a knife blade. Too hard to clean even if it cuts good, as the edges grab and hold bits of whatever is being cut.
cheap but good tool for sharpening Money can't always buy you love, or something like that. After trying most of what's on the market, this is what we found that works the best, no kidding. We've used waterstones with angle jigs, and all the other stuff I'll probably get email about as well. We don't usually use metal that develops a wire edge (E.G. crummy stainless steel), and so rarely need a buffer. Actually, the best edge I've ever seen was put onto a piece of aluminum by hydrochloric acid. It seems that suspending aluminum in foaming acid (it makes its own due to the violence of the reaction) does this just so. Of course, that edge didn't last long.

When our blades come off the lathe jig, they are very acute at the edge. This tool puts a somewhat more obtuse angle on. The resulting profile has shoulders where the angles meet. This is where most of the wear takes place according to what the microscope shows me. The result is that for awhile, the blade actually gets sharper with use! A lot of times, especially in certain materials, once a cut is begun, it continues mostly by wedging action; there is a little crack formed in the material ahead of the actual blade edge due to this. In fact, this is how the basic carbide lathe tool removes material. Once a cut is begun, most of the wear and heat happen just behind the edge, not right at the edge. This is good for the tool, as the edge is by definition thin and not able to conduct heat away very well.

One of the things they don't tell you with this tool is a couple of neat tricks in its use. First, mount the darn thing in a bench vise. That's much safer! Now, if the knife is drawn through the coarse side with the point down (eg not horizontal) and the right force is used, the carbide cutter will "chatter", giving you kind of a micro steak knife edge. If you do this, skip the fine side.

Coulter's Smithing Home

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