Aerospace Grinding Wheels

Abrasive grinding wheels consist essentially of abrasive grains held as a mass by a binder. Classified by bond, the principal types are vitrified, silicate, rubber, shellac, and resinoid or synthetic resin wheels: Phenol-formaldehyde resins are the most important type of resinoid grinding-wheel binders.
They operate at high speed, up to 16,000 surface feet per minute.
The principal requirements for phenolic grinding-wheel resins are high tensile and impact strength in the finished product, resistance to generated heats without breakdown, and adaptability to standard wheelmaking practice.

Phenol-formaldehyde resins developed as grinding-wheel bonds during the twenties to meet certain shortcomings in materials then used and to permit the development of new classes of wheels. Resinoid wheels are used today in a great variety of shapes, sizes, and grain types, but in general their use falls into two classes-in cutoff wheels and similar designs where a certain amount of resilience is required, and in grinding wheels intended for high-speed operation with heavy work removal. Such wheels often operate as high as 9,500 surface feet per minute compared with an average of 5,000 and a maximum of 6,500 surface feet per minute for vitrified wheels. 

Aerospace cutting off wheels

The strength of the wheel depends entirely on the bonding agent. Since grinding efficiency depends to a considerable extent on the speed of the wheel and the strength of the resin determines the maximum operating speed. resin characteristics are closely related to grinding efficiency. A wheel in use is subjected to great centrifugal force, tending to tear the mass apart. The ability of a resin bond to withstand these stresses is its most important property.
Impact strength is likewise important, since in service the wheels are in intermittent contact with the work being ground and are subjected to repeated shock. Another important characteristic of a bonding resin is the nature of the adhesion between it and the individual grains. As grinding surfaces wear, it is desirable that new cutting points and edges be bared. In most types of wheels, the resin-grain adhesion is such that a worn grain is released by the resin under the greater tearing force set up by a dull grain in contact with the work. Recently, however, so-called " friable " grains have been used, which break when worn and so bare new cutting points. In such grinding wheels a higher than normal adhesion is desirable. Actual tests on finished wheels are used to determine these characteristics, the results being expressed as the ratio of weight of the metal removed to the weight of the wheel lost in removing it. This value is particularly important in resins for snagging wheels, widely used for rough finishing and scale removal on steel billets and castings.
Heat resistance of the resin bond is a critical value since very high heats are developed at the cutting surface. Too soft a resin tends to flow and "glaze" the wheel, shielding the abrasive grains and necessitating frequent redressing.
The strength advantages of resinoid wheels were not realized until grinding machines were developed that utilized higher wheel speeds. The fundamental advantages of such wheels lie in the greater efficiency this makes available through high-speed operation. Only in recent years have machine tools been available that can provide such high speeds.
The fabricating requirements arise from the shop practices followed in the manufacture of resinoid grinding wheels. Phenol-formaldehyde resin is supplied in two forms, a dry powder and a liquid. The powdered resin is the actual bonding medium, while the liquid acts as a wetting agent providing adhesion between grain, resin, filler, and other ingredients during the fabricating cycle. Grain, dry resin, filler, and desiccants are mixed with the liquid resin, the amount of liquid resin and the time of mixing depending principally on the size of the grain and the viscosity of the liquid component. Some procedures omit the liquid phenolic resin and use an alternative wetting agent.
The mixed charge is then molded to the required shape and size in a hydraulic press, and strengthening disks and bushings are inserted. After pressing, the molded forms are put through an ovenbaking cycle which first produces the sufficient flow in the resin to make immediate contact with all the grains and then cures the resin to a thermoset material.
Resinoid wheels are somewhat more costly than other types. Despite this factor, the advantages of resinoid wheels have gained for them approximately 35 per cent of the grinding wheels market.