The primary relief land on an end mill is the narrow, relieved surface directly behind the cutting edge, providing clearance to prevent rubbing and excessive friction between the tool and the workpiece. This relief is essential for effective cutting, as it allows the cutting edge to engage cleanly with the material while minimizing tool wear and heat buildup.
The angle of the primary relief land, known as the primary relief angle, is carefully designed to balance sharpness and strength. A steeper relief angle reduces cutting forces but can make the edge weaker and more prone to chipping, while a shallower angle provides better support at the cost of increased rubbing. The width of the primary relief land is also a factor—too narrow, and it may wear out quickly; too wide, and it may introduce excessive contact and heat.