13.1.1 Grindability
Workpiece material composition and condition control all aspects of the grinding process. As ever greater demands are placed on productivity and quality, it becomes critical that the engineer understands the metallurgy of the materials being ground and their impact on the grinding process. Grindability is the term used to describe the ease of grinding a given workpiece material, and is akin to the term “machinability” used in milling and turning. Machinability, a more familiar term to engineers, is usually judged by four criteria: tool life, tool forces and power consumption, surface quality (including roughness, integrity, and burrs), and chip form. Comparable criteria exist for grindability; namely, G-ratio, Stock Removal Parameter Л, surface quality (including roughness, surface residual stress, and burrs), and chip form. In broad terms, an easy-to-machine material is usually easy to grind especially when judged in terms of tool life and power.
13.1.2 Effect of Chip Form
However, major deviations can arise when the influence of the chip form is considered. Machining involves using a tool of defined geometry to produce uniform chips of the order of 100 pm to 1,500 pm in thickness. By contrast, grinding is carried out with a random array of cutting-point shapes, generally with a high negative rake angle and random spacings and heights: chip thickness varies from <1 pm to no more than 50 pm even for the most aggressive of rough grinding. An easy-to-machine material would be judged more for the ease of handling and disposal of its chips. A material that produces long stringy chips would be given a poor rating as would one that produced a fine, discontinuous chip. The ideal chip would be a nicely broken chip of a half or full turn of the normal chip helix. In grinding, the greater factor is loading, or the imbedding of metal into the face of the wheel, especially from a long, stringy chip, and a short, discontinuous chip is preferred. Chip breakers can change machining chip form while high-pressure coolant jets reduce built-up edge on tool inserts. Similarly, optimized grind geometry, for example, small de values, combined with high-pressure coolant scrubbers can minimize loading. Under these conditions grindability and machinability based on tool life or cutting energy or the rate of increase in cutting energy become more in step being based on the mechanical properties of the work material.
13.1.3 Chemical Reactivity
Chemical reactivity between a particular workpiece and abrasive material can affect these trends. Diamond is chemically reactive to most transition metals, cubic boron nitride (CBN) is reactive with titanium, while SiC is reactive to titanium, iron, or cobalt. These properties can reduce wheel life by 10 to 104 times from expected values based on their relative mechanical properties. Small amounts of additives such as lead in steel or copper in porous powdered metal components will improve machinability by allowing a more continuous chip formation, but in doing so reduce grindability due to increased wheel loading.