The choice of diameter of milling tools is decided by milling machine itself and milling dimension of workpiece.
1) Face milling tools
The most important factor when choose face milling tools is the power should be within the power of milling machine, and it also can be determined by the diameter of spindle. Diameter of milling tools D=1.5d(d is diameter of spindle). For large quantity producing, the diameter of milling tools could be 1.6d.
2) End milling tools
The milling dimension of workpiece is very important for choosing end milling tools, and make sure the power consumption is within the power of machine. Take small diameter end milling tools for instance, the Max. RPM of machine should match Min. cutting speed(60m/min).
3) slot milling tools
diameter of slot milling tools and width should be chosen according to dimension of machined workpiece,. And make sure power consumption of cutting is within power of machine.