Grinding Feeds and Speeds

§1Cutting with abrasive

A grinding wheel is a bonded mass of hard abrasive grains, each a minute cutting edge. Spun fast against the work, the thousands of grains each shave an almost invisible chip, together removing metal and leaving a fine finish.

Grinding is therefore machining, but with countless tiny, hard, randomly-shaped edges instead of one or a few defined ones — which is why it can cut materials too hard to machine, such as hardened steel and carbide, and reach finishes and accuracies turning and milling cannot. But those many tiny edges cut best only when moving very fast, so grinding runs at wheel speeds measured in metres per second, far above any conventional cutting speed (§2). The parameters that follow — wheel speed, work speed, the tiny depth of cut, and the wheel’s own wear — all differ so much from ordinary machining that grinding is treated as its own operation, and the wheel speed carries a safety limit no other tool has (§6).

§2Wheel speed

The wheel’s surface speed — how fast its rim passes the work — is quoted in metres per second, and it is the dominant grinding parameter, far higher than any turning or milling speed.

Where a turning cut runs at tens of metres per minute, a grinding wheel runs at tens of metres per second — a typical vitrified wheel around 30 m/s, which is 1800 m/min, orders of magnitude faster. The reason is that each abrasive grain takes so tiny a bite that it needs a very high speed to cut rather than rub, and the finish improves with speed. This high surface speed is why grinding leaves fine finishes and cuts hard materials, and it is set as a surface speed — metres per second — for the same reason cutting speed is quoted for turning: it is the figure that stays constant as wheel diameter changes, from which the wheel’s rev/min follows (§3).

§3Wheel rpm from surface speed

As with any rotating tool, the wheel’s rev/min follows from its surface speed and diameter — the same relationship as turning, adjusted for speed in metres per second.

§4Work speed and depth

Against the fast wheel, the work moves slowly and the wheel bites only microns deep — the two parameters that control removal rate and finish.

The work speed — how fast the workpiece traverses or rotates past the wheel — is slow compared with the wheel, and it trades finish against removal: a slower work speed and lighter cut give a finer finish, a faster one removes more but coarsens the surface. The depth of cut in grinding is tiny, typically a few microns to a few hundredths of a millimetre per pass, because each grain can remove only a sliver — which is why grinding is a finishing process, taking small amounts to close tolerance rather than shifting bulk stock. Together, work speed and depth set the grinding removal rate, much smaller than a turning or milling rate but far more precise. Push either too hard and the wheel heats the work — grinding burn, a bluing and softening of the surface — so grinding is kept light and well cooled.

§5The grinding ratio

Unlike a cutting tool, a grinding wheel wears away appreciably as it works, and the grinding ratio measures how much metal it removes for each unit of itself worn away.

A wheel does not hold a fixed edge like a turning tool; its grains dull, fracture and pull out as it cuts — which is actually desirable, since fresh sharp grains are exposed as old ones go (a “self-sharpening” action). The grinding ratio G captures the trade: a value of, say, 30 means the wheel removes thirty cubic millimetres of work for every cubic millimetre of wheel lost. A high G means an economical, hard-wearing wheel but, if too high, a glazed wheel that rubs and burns; a low G means a free-cutting wheel that wears fast. The wheel grade (its own page) is chosen to keep G in a useful range — hard enough to last, soft enough to keep exposing sharp grains — and the wheel is periodically dressed to restore its face when it dulls or loads.

§6The burst-speed limit

Wheel speed is unique among machining parameters in being a safety limit: spin a wheel beyond its rated speed and centrifugal stress can burst it, throwing fragments with lethal force.

A spinning wheel is under tension from its own centrifugal force, and that stress rises with the square of the surface speed — so exceeding the wheel’s rated maximum, commonly around 33 to 50 m/s for vitrified wheels, can overcome the bond and shatter the wheel explosively. This is why every wheel is marked with a maximum operating speed that must never be exceeded, why wheels are handled carefully and ring-tested for cracks before mounting, why guards enclose the wheel, and why a wheel is run briefly to speed behind the guard before use. It is the same centrifugal law met with chuck jaws on the workholding page — force rising with speed squared — but here the consequence is a bursting wheel, not merely a loosened grip. Wheel speed, alone among the numbers on this page, is chosen with safety first and cutting second.

§7Quick reference

The working core of the page on one card rack.