Material Removal Rate — Turning

Speed times feed times depth. Three numbers multiplied together, and the product is the volume of metal leaving the part per minute — the only honest measure of how hard a roughing pass is actually working. It is the number to compare two setups with, because feeds and speeds quoted separately can flatter almost anything.

The three terms are mathematically equal and economically nothing like equal, and that asymmetry is the whole strategy of roughing. Depth of cut is nearly free. Doubling it doubles the metal removed and barely touches tool life, because the extra load spreads along more of the cutting edge rather than concentrating anywhere. Feed is close behind — it costs some tool life and it costs finish, since theoretical roughness rises with the square of the feed. Cutting speed is the expensive one: Taylor's exponent means a 20% increase can halve the tool life, for a 20% gain in rate.

So the order of attack is settled: take the deepest cut the machine, the fixture and the insert's usable cutting edge will stand; then the heaviest feed the chipbreaker and the finish allow; and only then raise the speed. Beginners do it in exactly the reverse order, because speed is the knob that feels like it makes the machine work harder.

What actually limits the depth is rarely arithmetic. Spindle power is the obvious ceiling, and the specific-cutting-energy page turns a removal rate into the kilowatts it demands. But before power, most cuts run into rigidity — the deflection of a slender workpiece, the overhang of a boring bar, the grip of the workholding — and into chatter, which is a stability problem rather than a strength one and can appear at a depth well below anything the power calculation objects to. And a depth of cut deeper than the insert's usable edge length is not a heavy cut; it is a broken insert with the corner buried in the part.

One quiet arithmetic point. On a lathe, the depth of cut is taken off the radius, so it reduces the diameter by twice as much. A 2 mm depth of cut takes a 50 mm bar to 46 mm. Half the diameter mistakes in a first-year shop come from this and the other half come from forgetting it in the other direction.