How to choose the wheel and process — and avoid thermal burn
How to choose a grinding wheel and process, with the reasoning, the risks, and a confidence level. Pair with the Grinding Parameters calculator to set wheel speed and specific removal rate, then source from the machine tool directory.
Abrasive: aluminum oxide for steels and ferrous; silicon carbide for cast iron, non-ferrous, and carbide; CBN for hardened steels and superalloys (cool, very long life); diamond for tungsten carbide, ceramics, and glass. Grit: coarse (24–46) for fast stock removal and soft/ductile materials; fine (80–220+) for finish and hard materials. Grade (bond hardness): a soft grade for hard materials, a hard grade for soft materials. Use an open structure on soft, gummy metals to avoid loading.
The abrasive must be harder than and chemically compatible with the work — which is why diamond is never used on steel (carbon diffuses into iron at grinding heat, so CBN is the hard-steel choice). Wheel grade is the counter-intuitive part: hard materials call for a soft-bonded wheel so dull grains break away and expose fresh cutting edges, while soft materials call for a hard wheel that holds each grain longer.
Too hard a wheel on hardened steel glazes over and burns the part. Too soft a wheel on a soft material wears and loses form fast. An aluminum-oxide wheel on carbide barely cuts; diamond on steel destroys the wheel.
Use creep-feed grinding (very deep cut, very slow table feed, one or a few passes, heavy coolant) for form grinding, deep slots, and heat-sensitive superalloys such as turbine components. Use conventional reciprocating grinding (shallow cut, fast traverse, many passes) for general flat work and simpler setups.
Creep-feed's long, gentle contact arc means each grain removes very little per unit of travel despite the deep cut, and continuous high-volume coolant carries heat out of the zone — ideal for materials that burn easily and for finishing a form in one pass. Conventional grinding is more flexible and simpler to set up but slower for deep forms and more prone to leaving the part with residual heat if pushed.
Creep-feed without properly aimed, high-pressure coolant burns the part almost instantly. Trying to hog a deep form with conventional reciprocating passes invites chatter and thermal damage.
Surface discoloration or bluing, fine surface cracks, and a metallurgically altered layer — a hard, brittle re-hardened "white layer" or an over-tempered soft zone — that cuts fatigue life. The metallurgical damage is often invisible until a nital etch reveals it.
Specific material-removal rate (Q'w) too high, a glazed or dull wheel, a wheel that is too hard for the material, and insufficient or mis-aimed coolant.
1) Dress the wheel — a sharp wheel cuts cool. 2) Move to a softer or more open-structured wheel. 3) Reduce depth of cut and feed to bring Q'w down. 4) Flood coolant directly into the contact arc (through-wheel or high-pressure), not just near it. 5) For heat-sensitive materials, switch to CBN, which runs markedly cooler.