grinder wheel evaluation for sharpening
bill tindall
>This discussion will focus on using the bench grinder for sharpening, where a small amount of metal must be removed to establish a wire edge prior to edge refinement by honing. An in depth discussion of grinder wheels has recently been posted in the Articles section and only the highlights will be repeated in this study of three wheel types.
Background When sharpening a dull edge enough metal must be removed to establish a wire edge. The appearance of the wire edge ensures that all of the worn edge has been removed. This step can involve removing a significant amount of metal, a slow process on a bench stone. To speed this process woodworkers have considered the advantages of a "power assist" in the form of some sort of sharpening machine. The versatile, and inexpensive, bench grinder may not be considered for this task for fear that it will heat the edge and destroy its hardness. I will show that with proper wheel selection, the bench grinder is both a quick and inexpensive way to use power assisted sharpening without fear of overheating.
The act of grinding generates heat which, if not removed from the tool, will heat the edge and soften it. Softening can occur at temperatures as low as 400 F depending on the steel, so heating during grinding is a serious concern. If the abrasive particle contacting the steel is sharp, a metal shaving is milled from the steel and ejected. A significant amount of the generated heat will be removed in this shower of red sparks. A dull grit ploughs across the steel and transfers much of the heat to the steel. So the "secret" to cool grinding is to renew sharp grit particles at the grinding wheel surface.
Two factors determine the sharpness of the particle during grinding. "Particle friability" is the particle's tendency to fracture under grinding pressure. The abrasive particle can be designed to fracture under the particular pressure used for grinding, heavy pressure for heavy material removal and light pressure for sharpening. As the particle edges break off, new sharp edges are exposed. Obviously, this process can not go on forever because the particle grows smaller as its edges break off. The bond holding the particles together can be designed to slowly fail under the pressure used in grinding and thereby slowly release the dull particles from the wheel surface. As these dull particles erode away, fresh sharp particles are exposed. "Bond hardness" is used to describe this wheel property. It is designated by a letter grade, G or H for a very soft bond, to N for a very hard bond typical of the gray wheels that come with bench grinders. It follows that particle friability and bond hardness BOTH must be tailored to the grinding pressure conditions expected as well as the hardness of the metal being ground. Heavy material removal, such as tool shaping, require a less friable particle than light pressure grinding such as would be used for sharpening and hard metals require a softer bond.
The typical wheel sold for bench grinders is optimized for heavy pressure grinding and soft steel. When such a hard, less friable wheel is used for sharpening at light pressure the surface particles dull, the wheel loads up with metal dust and vast amounts of heat are soon generated. It is little wonder that many people have concluded that bench grinders are inappropriate for sharpening.
The appropriate abrasive particle as well as the optimum bond- both must be considered for sharpening- are available in wheels used in surface grinder machines. Unfortunately, none of the woodworking sources have stocked such wheels sized for bench grinders. Many places stock "white wheels" which have abrasive of the proper friability, but the bond in these wheels is too hard for sharpening tool steels(see test results below). Perhaps as customers become knowledgeable about what is best, woodworking sources will begin to stock appropriate wheels. Indeed one source has already pursued this opportunity.
I recently posted some favorable results for grinder-sharpening using a surface grinding wheel from Radiac. Joel, Tools for Working Wood, suggested an alternative wheel that he and the folks at Norton were developing. Joel arranged for me to try this Norton 3X wheel.
The test Three wheels were used in this test, all 7", 80 grit, on a 3600 rpm grinder.
1. Norton 3X , bond hardness I. This wheel is 30% Norton SG aluminium oxide particles and the remainder white aluminium oxide, probably 9A. The combination is friable. The SG component is expensive to make, and long lived while maintaining sharp edges, and the bond is somewhat soft.
2. Norton White 9A, bond hardness J. This abrasive is very friable but the bond is fairly hard.
3. Radiac RAA abrasive, bond hardness H. This abrasive is very friable and the bond is quite soft. Radiac claims it is nearly equivalent to Norton 25A abrasive. It is pink from chromium and pink is a different abrasive than red or ruby which is not very friable. Chromium is supposed to make this abrasive better for abrasion resistant steels, eg A2 and CPM 3V.
A thin A2, a thick O1 and a very thick CPM 3V plane iron and a 3/8" CPM 3V bench chisel were sharpened with these wheels. The bevel angle was 28 degrees and grinding extended to the cutting edge of the tool. It could be said that this was a "cutting edge" grinding evaluation. (Groan!) Grinding technique was "touch and go", that is while moving the tool from left to right and right to left it was contacted with the wheel as it moved past the wheel surface. On purpose considerably more metal was removed than would have been the case with normal sharpening. This grinding provided a severe test of edge heating, especially with the thin plane iron and small chisel, which provided little mass for heat removal.
Results The 9A wheel with its hard bond quickly loaded up with metal, the surface dulled and I could not control burning. This test illustrates that a friable grit is insufficient. It must be combined with a soft bond to be cool running.
No evidence of burning was encountered with the other wheels. Only the wire edge was discolored, which is unimportant. The Radiac wheel cut a bit faster which may not be an advantage if fine control of grinding is important. The softer bond of the Radiac wheel shed particles in use. In my experience the Radiac wheel sheds particles at such a rate that wheel dressing is never necessary, but shed particles can be a housekeeping issue, especially if they land on a board to be planed. The Norton 3X wheel slightly loaded with metal after some use. It might eventually be necessary to dress it, but maybe not. It shed particles much less than the Radiac wheel (because of harder bond). The finish from each wheel was the same as best I could tell. A few honing strokes on 15 micron diamond removed the grinder scratches in either case .
Conclusions Either the Norton 3X or the Radiac RAA wheel can be used to sharpen edges without burning, at least under the conditions tested. It is expected that a Norton 25A in a soft bond would be equivalent in cool grinding. A 9A or equivalent might also be just fine in a soft bond, say G or H. It is my impression that the Norton 3X wheel is in beta testing and will soon be available in a variety of sizes and grits at places where Norton wheels are sold. The Norton 25A and 9A may be had at tool places that sell to metal shops, eg MSC, but it can be problematic getting the wheel in sizes best for bench grinders. The Radiac wheel must be custom ordered from Radiac. Our club recently ordered a batch of custom sized Radiac wheels, which solves the minimum order size problem for custom wheels.
Because these wheels provide a cheap alternative to sharpening machines, some costing hundreds of dollars, there is a fine marketing opportunity on the table for someone to stock wheels of optimum grit composition and wheel bond hardness in bench grinder sizes.
