Horsepower required can be calculated using the following formula which is based on a metal removal rate of one cubic inch per minute per horsepower.
The machinability constant 'C' allows 25% for cutter dulling and 60% power efficiency at the cutter spindle.
Depth of cut x width of cut x T x FT x rpm
Depth of cut x width of cut x F
Kilowatts = HP x 1.34
Machinability Constant 'C' = Aluminium and soft alloys = 4.0, Grey cast iron = 1.5, Steel up to 300 HB = 0.6, Steel up to 400HB = 0.5, Brass (duplex) = 2.5, Low Carbon steel = 1.0, Bronze = 2.0