The secondary relief angle in end mill geometry is an additional clearance behind the primary relief angle, further reducing contact between the tool and the workpiece. Its purpose is to reinforce the cutting edge while still minimizing friction and heat buildup. This angle is typically shallower than the primary relief angle, providing a transition zone that helps maintain edge strength and tool stability. A properly designed secondary relief angle extends tool life by preventing chipping and excessive wear while still allowing for efficient cutting. The combination of primary and secondary relief angles is carefully balanced based on the material being machined and the desired tool performance.
