Tech Talk
What are the “Properties of Abrasives”?
For an abrasive to function properly it must be harder than the material being machined. The abrasive particle must be of sufficient hardness and shape to be able to penetrate the surface of the material to be machined and form a chip or particulate. However, the abrasive must be tough enough to withstand both the thermal and mechanical shock of grinding, and at the same time be friable enough to fracture and produce new and sharp cutting edges.
Hardness is a term used in abrasive machining which is often confused. Hardness is usually referred to as a property of the grinding wheel. This will be discussed later. It must therefore be emphasised that hardness is the property of the abrasive grain alone. The relative hardness of some popular abrasives can be shown on the Knoop Scale: Hardened Steel Rc60 – 740; Quartz – 820; Aluminum Oxide – ; Silicon Carbide – 2480; Cubic Boron Nitride – 4700; Diamond – 7200.
Both mechanical and thermal shock resistance are the most important properties of an abrasive. The working abrasive grain endures not only intermittent cutting, but also thermal cycling. The heat of grinding may be detrimental to the abrasive in the presence of certain chemical elements, which could dramatically reduce the sharpness and hardness of the grain by diffusion into the grain matrix at high temperatures. This can be put into perspective when we consider that diamond is the hardest substance that we have discovered, and we assume that it is virtually indestructible. This is not a sound assumption because the hardness was measured by indentation hardness measurement. There was no sliding wear, frictional heat generation, or cyclical mechanical shocks. Hardness does not necessarily mean that the substance has good wear resistance. Diamond conducts heat tremendously well, six times better than copper and with very low thermal expansion. Though the diamond will not expand with increasing temperature, inclusions in the diamond will expand at a rapid rate and destroy the grain. Diamond quality is therefore an important area of concern when specifying natural or synthetic diamonds.
The need for toughness and friability in an abrasive seems to be contradictory. However, an abrasive must possess both qualities to a certain degree. In the extreme, a tough grain with little friability will become dull quickly and the grinding wheel will glaze. Dressing this type of grinding wheel with a single point diamond will tend to pull the abrasive grain from the bond, instead of breaking the grain to leave a sharp edge. Going to the other extreme, a friable grain with little toughness has a very aggressive nature, and will crack and fracture very quickly under load, revealing another layer of sharp-edged grains. This rapid shattering of the friable grain results in a cool cut, but at the expense of high wheel usage.
To reinforce the concept of toughness and friability it is worth considering glass as a material . Glass, which is significantly harder than a piece of mild steel, is a candidate for a tool material to machine steel, but glass has little toughness. If a drill or a milling cutter were able to be made from glass, we would see that the glass would shatter under the machining forces. If the glass were to get hot and then quenched by the cutting fluid, again it would shatter into pieces. Glass is therefore too friable and lacks toughness. Toughness is the ability to hold shape without compromise in hardness.
There are a number of abrasive types as well as forms. Our choice of abrasive will be governed by a number of factors, all relating to the overall economics of the machining process, along with the type of material being machined, the surface finish required, dimensional accuracy, profile detail, and form tolerance. Particularly in the case of precision grinding, the type of machine tool being used and its physical condition also will influence the choice of an abrasive system.
When an abrasive type is first selected, it is graded into grain sizes. The system used to do this is a series of fine mesh wire grids. The mesh size corresponds to the number of openings per linear inch in a wire gauze. The gauze categorisation is carried out for sizes 4 to 240. In many applications where fine finishes are required, the abrasive is categorised into much finer mesh sizes, which are too fine to be segregated by gauzes. The fine grain may be as fine as 4000 on the mesh scale. Wheels of such fine grain are sometimes referred to as “flour” wheel. Grains finer than 240 are separated by a flotation method where the grain is suspended in water. At given time intervals, the grain, which has settled, is extracted and as the settling time progresses, the grain is graded finer and finer. The “flour” grain is so fine that it will float on the surface of the water, held afloat by the surface tension.
Once the grain has been manufactured, processed, and graded into sizes, it is then bonded into a tool. The bonding system is the method by which the grains are held together in the shape of a grinding tool. If the grain is to be bonded into the shape of a precision grinding wheel, then the most common bonding systems are vitrified, resinoid, rubber, metal and plated. Approximately 50 percent of all grinding wheels manufactured are vitrified. If the grain is to be used as a coated abrasive then it will typically be bonded to a paper or a cloth backing.
