Cubic inch per minute (in³/min)

The cubic inch per minute is a unit of volumetric flow rate equal to one cubic inch passing a point every minute. Its symbol is in³/min. In the machine shop it is better known by its initials: MRR, the material removal rate, the single figure that says how fast a cutting process is turning solid metal into chips.

The calculation behind it is simple multiplication. Depth of cut times width of cut times feed rate gives the volume removed per minute, so a 0.2-inch depth at 1 inch wide with a feed of 30 inches per minute removes 6 cubic inches per minute. Every variable a machinist can change appears in that product, which is why it is the number used to compare one strategy against another.

Material decides what is achievable. Roughing aluminium on a rigid machine can exceed 100 cubic inches per minute, mild steel manages perhaps 10 to 20, stainless steel less, and nickel alloys used in turbine parts only a few. The limit is set by spindle power, by the rigidity of the setup and by the heat the tool can survive, in whichever order runs out first.

Spindle power connects directly to it. Each material has a specific cutting energy, roughly 1 horsepower per cubic inch per minute for steel and a third of that for aluminium, so a 20-horsepower spindle can sustain about 20 cubic inches per minute in steel. A quoted removal rate that exceeds the machine's power is arithmetic that the machine will settle in its own way.

Beyond cutting, the unit describes small imperial flows generally. Hydraulic pilot lines, lubrication systems, fuel injection test benches and adhesive dispensers on American equipment are all rated in cubic inches per minute, where the metric world would say millilitres per minute.

For scale, 61 cubic inches per minute is about a litre per minute, and 231 cubic inches per minute is exactly one US gallon per minute — the gallon being defined as 231 cubic inches. That exact relation is why hydraulic engineers move between the two units without a conversion table.

One cubic inch per minute equals about 16.39 cubic centimetres per minute, about 0.01667 cubic inches per second, or about 0.004329 US gallons per minute.

Cubic inch per minute conversion table