Adequate speed of mould rotation is necessary to fulfil two main functions:
A) to form a perfectly shaped casting and
B) to centrifuge impurities into the bore of the casting leaving it metallurgically sound
For general purpose centrifugal castings a spinning speed for any given diameter should produce approximately 70g. Arriving at this 70g pressure is difficult in view of the varying diameter from inside to outside diameter. The rules for arriving at useful spinning speeds take into account this variation.
Initially, for moderate section castings the spinning speed may be found from a simple formula giving minimum speed where ‘d’ equals the diameter of casting bore (in metres).
Formula Nominal rotation speed N = 300
300 is a factor arrived at by experimentation, enabling a minimum satisfactory spinning speed for most alloys.