Successful grinding of HSS toolbits is best achieved by the use of a wheel that is readily friable and open in structure to insure free cutting and proper removal of swarf. Grain selection can be a matter of choice, but is often dictated by the availability of wheel selections. If more than one wheel is at one's disposal, a 46 grit wheel is a good choice for roughing a tool, but the finish is not as desirable as one produced by an 80 grit wheel. 80 grit wheels tend to be a poor choice for roughing, cutting slower and hotter than a 46 grit wheel, so it's not a good choice if only one wheel is available. It has long been my custom to use a 60 grit wheel, which serves both purposes adequately, and is an excellent compromise.
Hardness rating of grinding wheels is based on surface speed. The typical vitrified wheel is generally rated @ 6,000 sfpm, and behaves softer as the diameter decreases. Armed with this idea, it's wise to select wheels with the largest possible diameter, without exceeding their recommended maximum speed rating. As wheels get smaller, useable life diminishes rapidly, but it can be recovered to satisfaction if wheel speed is increased. If you're fortunate to have control of the speed of your grinder, insure that you don't exceed the 6,000 sfpm threshold, to insure that the wheel doesn't come apart. Most bench and pedestal grinders don't have the ability to change speed, but many cutter grinders do.
The typical bench or pedestal grinder is normally equipped with wheels that are too hard for such an application, and wheels are not readily available in the more desirable grades for such use. To resolve this problem, a 3600 rpm grinder can be adapted to run wheels that have a 1¼" diameter hub. Such wheels are commonly used on cutter and small surface grinders and are readily available in a broad variety of styles and selection of grits, hardness's and structure.
Type of grinder is beyond the scope of this post, but it's important that the reader understand that the vast majority of problems one encounters when grinding HSS are related to wheel choice, and while success of sorts can be achieved with almost any wheel, there are solutions to the problems that can turn a rather unpleasant task into one that is quite tolerable.
Rule of thumb for grinding is to use a hard wheel for soft materials, and a soft wheel for hard materials, and further dictated by silicon carbide's ability to dissolve in steels at high temperatures such as found when grinding. While silicon carbide far exceeds the hardness of aluminum oxide, it is not suited to grinding HSS for that reason. In keeping with these guidelines, aluminum oxide wheels become the acceptable choice for grinding HSS---with, perhaps, the odd exception, which is unlikely to be an issue. Silicon carbide wheels are rarely used in the shop unless non-ferrous materials are to be ground. They are not intended to be applied to steels or steel alloys.
Wheel hardness is not related to the hardness of the grinding media. Hard or soft wheels share the same material, but are assembled with more or less bonding material to dictates the hardness of the wheel. A wheel that requires greater effort to dislodge dull bits would be considered a hard wheel, while one that permits the bits to be dislodged easily would be considered a soft wheel. That is the friability index of a grinding wheel, and is noted as a letter of the alphabet, A being soft, Z being hard.
Another feature to be considered when making a choice is the structure index. Abrasive grains are spaced close together in a tight wheel, and spaced at greater distance in an open wheel. This index is referenced by numbers, with 0 being a closed wheel, and 12 being an open wheel. Open wheels provide a space for swarf and help in keeping the temperature of the grind at a lower level. A wheel that is not open enough will "load" and not unload, yielding a wheel that displays all the characteristics of a wheel in need of dressing because it's dull. It's clear, a balance of hardness and structure are important to successful grinding, so they should be considered when selecting a wheel.
Wheels are bonded by various methods, but the one that is of most concern to the home shop type would be a vitrified bonded wheel. Vitrified wheels are bonded with selected clays, which are high fired, and are identified by a V in the wheel designation, following the hardness and structure designation. The process builds minute lengths of glass that bond the bits to one another upon vitrification. Hard wheels have a greater amount of bonding agent than soft wheels.
