In general, as cutting speed is increased, cutting temperatures rise and tool wear develops and shortens tool life. Conversely, when the cutting speed is reduced, cutting temperatures also reduce and tool wear development is delayed, thereby lengthening tool life.
This also applies to end mills so cutting speed should be set at a point where tool life and machining efficiency are well balanced.
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Cutting speed of ball nose end mills
The cutting speed of solid ball nose end mills is set according to the depth of cut, workpiece material, tool material and the diameter. Usually, the depth of cut is less than the ball nose radius.
As seen in the image below, when machining on an inclination the maximum cutting speed changes according to the depth of cut.
The maximum cutting speed is found by formula (1) for level faces and formula (6) for inclinations.
When machining steep inclination
As shown in figure below, when machining a steep inclination, the position where the cutting speed becomes maximum is near the periphery, thus dictating that the cutting speed and feed rate can not be increased by much. However, with a shallow inclination, the cutting position is near the centre, therefore the cutting speed has to be increased and the feed rate can be increased when considering machining efficiency.
The cutting speed of indexable insert ball nose end mills is set according to the insert grade and workpiece material in the same way as square indexable end mills. Cutting speed of some indexable insert ball nose end mills is set also by additionally considering the machining mode and diameter.