Greenfield along with Cutting Tools Chicago aka General Cutting Tools defines controlled feed rates when thread milling. For thread milling applications, we recommend setting controlled feed rates within the program for the centerline of the spindle or tool. This method ensures absolute feed control of the tool through the entire thread milling operation.
As the tool moves around the centerline of the bore, the tool centerline must traverse at a slower IPM rate than the periphery of the tool at the cutting edge or major thread diameter. If the standard feed rate of RPM x feed per flute x number of flutes was applied to the spindle centerline, the tool would have an effective feed rate at the cutting edge that may exceed the capability of the thread mill, resulting in poor tool life and possible breakage. The programmed feed rate for the centerline is based upon the thread mill diameter selected. The larger the tool diameter, the greater the reduction required to ensure the proper operating parameters on the periphery.
Feed Compensation Formula for Internal Threading:
Centerline ‘PM2 = ‘PM1 x (Major Thread Diameter – Tool Cutting Diameter) Major Thread Diameter
Feed Compensation Formula for External Threading:
Centerline ‘PM2 = IPM1 x (Major Thread Diameter + Tool Cutting Diameter)
Major Thread Diameter