The speed of milling is the distance in FPM (Feet per minute) in which the circumference of the cutter passes over the work. The spindle RPM necessary to give a desired peripheral speed depends on the size of the milling cutter. The best speed is being determined by the kind of material being cut and the size and type of the cutter used, width and depth of cut, finished required, type of cutting fluid and method of application, and power and speed available are factors relating to cutter speed.
Factors Governing Speed
There are no hard and fast rules governing the speed of milling cutters; experience has shown that the following factors must be considered in regulating speed:
A metal slitting saw milling cutter can be rotated faster than a plain milling cutter having a broad face
Cutters having undercut teeth (positive rake) cut more freely than those having radial teeth (without rake);
hence they may run at higher speeds.
Angle cutters must be run at slower speeds than the plain or side
Cutters with inserted teeth generally willl stand as much speed as a solid cutter.
A sharp cutter may be operated at greater speeds than a dull one.
A plentyful supply of cutting oil will permit the cutter to run at higher speeds than without cutting oil.
Selecting Proper Cutting Speeds
The approximate values given in table 8-1 in Appendix A may be use as guide for selecting the proper cutting speed. If the operator finds that the machine, the milling cutter or the workpiece cannot be handled suitably at these speeds,immediate readjustments should be made.
Table 8-1 lists speeds for high speed steel milling cutters. If carbon steel cutters are used , the speed should be about one-half the recommended speed in the table. If carbide-tipped cutters are used, the speed can be doubled.
If a plentyful supply of cutting oil is applied to the milling cutter and the workpiece, speeds can be increase 50 to 100 percent. For roughing cuts, the best practice is to reverse these conditions, using a higher speed and lighter feed.
The formula for calculating spindle speed inrevolution per is as follows:
Where RPM=Spindle speed (revolution per minute)
CS = cutting speed of milling cutter (in SFPM)
D = diameter of milling cutter (in inches)
For example, the spindle speed for machining a piece of steel at a speed of 35 SFPM with a cutter 2 inches in diameter is calculated as follows:
RPM= CSx4 = 35x4 = 140
D 2 2 = 70 RPM
Therefore, the milling machine spindle would be set for as near 70 RPM as possible.
Table 8-2 in Appendix A is provided to facilitate spindle speed computations for standard cutting speeds and standard milling cutters.