Tech Talk
What are Metal Bonded Grinding Wheels?
There are two divisions of metal-bonded wheels: those which have been plated, and those which have been cast. Included in the plating or cast matrix is the grain, usually CBN or diamond.
In the case of a plated grinding wheel, the wheel hub is manufactured very accurately with respect to the form profile and the profile’s concentricity to the bore of the wheel. The profile is machined with an offset in the true shape, which allows for the size of a single layer of grain being plated. The abrasive is then carefully plated onto the form, using a hard, typically nickel or chrome plating to hold the grain in place. The plating does not completely cover the grains, and they are exposed with their sharpest cutting edges outermost. The thickness of the plating, a function of the grit size, corresponds to approximately 1 to 1.5 times the grit thickness. Most metal plated wheels have a periphery of only one or two abrasive grains deep. Such wheels do not require dressing; however, they have to be very carefully assembled onto the machine spindle in order to run true to the spindle rotation. Most wheels of this type have a truing groove to assist in the proper mounting of the wheel. Wheel run out of less than 0.012 mm (0.0005 in) TIR is generally acceptable. Electroplated grinding wheels can run at very high speeds, since the central hub of the grinding wheel can be made from high-strength alloys.
A new technology has been developed called “Diamesh” which allows the superabrasive grain to be deposited and plated in a very tightly controlled pattern. There is no random placement of the grain across the wheel periphery. Diamesh provides a more consistent wheel performance, longer life, and better part surface integrity.
In the case of cast metal bond wheels, they are typically small grinding wheels used for Internal Diameter (ID) grinding and made from a soft bronze or other copper alloy, although there are some made from cast iron. The metal bonding is achieved using a sintering process and provides a very strong, solid bond, which has very good form retention properties. There is little to no porosity in a metal bond, but it can be enhanced somewhat by the addition of graphite fillers. Truing and dressing of these types of wheels is best performed by Electro-discharge Machining (EDM). The EDM process erodes the metal matrix, as well as what is usually a diamond abrasive, and allows relatively complex forms to be dressed into the wheel periphery. The EDM process can be sufficiently controlled to cause a pitting of the surface, which can act as a cavity for chip clearance and cutting fluid flow. Diamond is often used in this bond system, and finds a niche in the machining of hard ceramic materials and glass. Large wheels, above 150 mm (6 in) in diameter, can be manufactured in a sintered metal bond; however, the wheel is generally assembled from a series of separately sintered segments adhered to the wheel periphery. The separately sintered segments suffer from density changes and inconsistencies among the various segments. Such segmentation, and therefore separation, of the wheel periphery can result in high vibration levels and poor surface.
A crush dressable metal bond has been developed, which, by its name, suggests that it can be dressed to a given form profile by a crushing action, but is actually more like a compressing action of a very porous metal matrix.
The bounding and structure of a grinding wheel determine its safe operating speed. The safe speed is printed on every grinding wheel and must never, under any circumstances, be exceeded. Every grinding wheel is tested for safe operation to 1.5 times the Maximum Operating Speed (MOS). Should a grinding wheel burst, there is a high risk of very badly wrecking the machine, the fixture, and the workpiece, and quite possibly injuring the operator. Be very careful, treat the process with respect, and never over-speed a grinding wheel. Maximum speeds are often printed in RPM. However, in new machine tool designs and control systems, wheel speeds are controlled to maintain a constant peripheral speed. This means that as the grinding wheel gets smaller in diameter, the RPM has to increase in order to maintain a constant surface speed. Therefore, the rule is that the maximum operating RPM is with respect to the largest wheel diameter when the wheel is new. It is fast becoming the norm to see both maximum RPM and maximum peripheral speed (MOS in ms1 or sfm) marked on the wheel. Not only should proper care be taken to operate a grinding wheel, but also safe mounting, handling, and storage of grinding wheels should be most important. The American National Standards Institute Safety Code, ANSI B 7.1, outlines the recommended methods for storage and handling.
