Grinding Wheel Selection for Tool Shaping and Sharpening

A properly chosen grinding wheel will rapidly shape and sharpen tools without burning the edge. For the purposes of this article, shaping is establishing the shape of the tool, for example the profile of a gouge or the bevel angle of a plane iron. It is anticipated that a considerable amount of metal will be removed in shaping. Sharpening is defined as the removal of a very small amount of metal, usually up to or near the fine cutting edge, to prepare the tool for honing. This article will provide information for the proper selection of a grinding wheel for shaping and sharpening.

Grinding Theory

When the sharp grits of a grinding wheel engage the tool, slivers of metal should be milled from the surface and ejected. A significant amount of the heat generated is removed in these slivers. One often sees this shower of sparks during grinding. Dull grits plough or skid across the surface without ejecting a sliver. The heat generated is absorbed by the metal part and may cause burning. A properly chosen grinding wheel will maintain sharp cutting grits during operation to minimize heating. Different grit properties are required for heavy and light grinding to achieve the goal of sharp grits and minimal heating.

Grinding Wheel Construction

Typical grinding wheels for bench grinders are made by fusing (vitrifying) abrasive grits into a glass-like matrix called the bond. Both the composition of the grits as well as the properties of the bond affect the ability of the grits to stay sharp during use as well as the ability of the wheel to hold its shape. When choosing a grinding wheel four properties are important to consider

1. grit composition and toughness
2. grit size
3. bond hardness
4. grit density

Grit Composition

For grinding steel, an aluminum oxide grit is normally used. Silicon carbide is use for grinding some metals, but not tool steels. Aluminum oxide can be modified with other elements to control its hardness, resistance to fracture and the way it fractures. Ideally, one wants the surface of the grit to fracture and expose new sharp edges under the grinding conditions used. For coarse grinding, heavy pressure is used to force the tool against the wheel so the grits should be tough and difficult to fracture so that they are not crushed by the heavy grinding pressure. In contrast, for final sharpening, very light passes are made across the wheel. In this case the grits should be friable and fracture under light pressure to expose sharp edges. Each abrasive company formulates about ten aluminum oxide compositions to best address various grinding situations.

A premium ceramic grit composition is available from some sources. It is most suitable for production situations and it probably is not cost effective for typical woodworking applications where high speed and long life are not so important. It is unclear if this material is suitable for sharpening as the literature suggests it is a tough grit.

Grits are colored and the color provides a guide for where they are best used. Dark red (ruby) and blue wheels typically have tough grits that will hold up to heavy grinding pressure for fast material removal. White and pink grits are typically friable and they are best used for light grinding pressure, for example sharpening (one should check the manufacturer's literature to confirm the correspondence of color and application).

Grit Size

The grit grain size is specified by mesh size. Coarser grits, for example 36 or 46 mesh, remove material faster than fine grits such as 60 and 80. The grit size determines the depth of scratches or finish of the ground surface. An 80 grit wheel leaves scratches about 12–18 microns deep, while a 46 mesh wheel leaves scratches over 32 microns deep. Since honing usually follows grinding, ideally the grinding scratches should be easily removed by the following honing step. Coarse grits usually generate less heat than fine grits so grinding should be done with the coarsest grit that will yield an acceptable finish.

Bond Hardness

Grits at the surface of the wheel eventually get dull or crushed. There must be some mechanism for bringing new grits to the surface of the wheel. The bond should fracture and wear away to continuously expose fresh grits under the grinding conditions used. It may seem contrary, but for very hard metals, a soft easily worn bond is required. Hard metals rapidly dull the surface grits. The bond must, therefore, be soft so it wears away to expose new grits. The wear resistance (hardness) of the bond is indicated by letter grade. H is soft and appropriate for hard tool steels. A harder wheel, for example K, would be required in situations where a crisp wheel edge was required, but this would not be a concern for most woodworking tool sharpening. Harder wheels wear more slowly and hold their shape, but they generate more heat because they don't expose sharp new grits as rapidly as softer wheels.

Grit Density

Much like there are open coat and closed coat sand papers, the grit density in the wheel can be varied to facilitate good chip removal and cooler operation.

Decoding the Wheel

Wheels made by reputable manufacturers will have a code on the label that specifies these five wheel properties. For example, the wheel I use for sharpening is made by Radiac Abrasives and it is coded "RAA80-H-VOS". The first letters specify the kind of grit. An "A" in this letter code always means aluminum oxide grit and the other letters specify the toughness of the grit. Each company uses different letters to describe the toughness of their grits. For example, the Norton equivalent to "RAA" is "25A". RAA is a friable pink grit good for cool finish grinding of hard tool steels. The number following the grit type is the mesh size of the grit, in this case 80 grit. The next letter specifies the hardness of the bond, in this case H, a soft wheel for cool grinding of very hard steels. The final letter and number combination specifies the type of bond and the density of the grits in the wheel. This code varies with manufacturer. In this case, VOS specifies a vitrified (melted) bonding material and an open grit structure. V8, common to many wheels, specifies a moderately open vitrified structure .

Recommendations

Many of the factors in wheel selection that would be important in a production situation are of little importance to woodworkers. For us, cool grinding and the resulting surface finish are most important. We are willing to sacrifice the ability of the wheel to hold a sharp edge, the rate at which a wheel wears, or even cost, to get cooler grinding.

Typical wheels for bench grinders are usually made for heavy material removal and long wheel life. The grits are tough, usually ruby or blue, to withstand heavy grinding pressure, and the bond is usually fairly hard , typically J or K, to slow wheel wear and maintain wheel shape. In the coarse grits sizes, for example 46, they are acceptable for shaping tools but a cooler running H hardness wheel would be better. The gray wheels that are often seen on bench grinders are unsuitable for tool grinding. They are usually a cheap, tough grit, and a very hard N bond. They generate a lot of heat in use, but hold their shape and wear slowly under heavy pressure.

For sharpening, a finish with more shallow scratches is desired. Because finer grits are more prone to burning, the best grit size is a compromise between cool grinding and best finish. A 60 or 80 grit wheel is optimum for a 3600 RPM grinder, providing the other wheel properties promote cool grinding. It is difficult to prevent burning with grits finer than 80 on 3600 RPM grinders. A very friable grit is necessary for cool grinding so the wheel should be pink or white and the hardness must be H or softer. An open pore bond also will aid keeping the tool cool.

Many sources sell the typical 1" x 8" blue or ruby bench grinder wheels suitable for tool shaping. Wheels in this size suitable for sharpening are more difficult to find. A much greater selection of wheels is available for surface grinders and these wheels can be used on a bench grinder. These wheels are typically ½" thick and machine shop suppliers sell such wheels, for example Travers, MSC and ENCO. MSC (mscdirect.com) has a wide selection of surface grinding wheels and some suitable wheels are available in 1" widths. An even greater variety of ½" wheels are available. I have found the ½" wheels to be an acceptable alternative to 1" wheels, especially considering their modest cost of about $15.

A good source of ½" wheels is Radiac Abrasives (1-800-851-1095, radiac.com). They are a U.S. manufacturer and they have a wealth of useful information on their web site as well as a helpful tech staff. Radiac will make custom wheels in any size at a modest cost in lots of five or more. For example a custom 8" wheel will be around $30 in lots of ten. Ordering custom wheels might be a good club project.

Summary

Heavy, fast grinding (shaping): Blue or ruby 46 grit, H but up to J or K acceptable, V8 or open pore better. Radiac "RA" abrasive a good choice.

Precise finish grinding prior to honing (sharpening): Pink or white 60 or 80 grit, H, V8 or open pore better. Radiac "RAA" or "9A" a good choice.