How to choose which grinding wheel to use when grinding different metals.
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.
Published in: Do-It-Yourself