Tech Talk
What is "Eccentric Relief on an endmill"?
There are three basic methods of resharpening end mills and milling cutters, depending on the type of clearance desired behind the side cutting edges: 1.flat grinding, 2.concave grinding and 3.eccentric grinding. Flat grinding is performed with a cup wheel whose axis is perpendicular to the axis of the end mill. Hollow grinding, also called concave grinding, is performed with a pain grinding wheel whose axis is parallel to the cutter axis. Hollow grinding results in a nearly flat but slightly concave surface. Eccentric grinding produces a slightly convex land behind the end mill cutting edge, and provides a little more support behind the cutting edge since slightly less metal is removed. Sometimes called radial relief, eccentric grinding can be performed in resharpening operations or, if the operator prefers, flat or hollow grinding can be used to resharpen a tool originally ground with an eccentric relief. Eccentric relief leaves slightly more metal behind the cutting edge (shaded area) than flat relief, making for greater edge support.
Regardless of the grinding method used, the clearance angle is established by properly locating the wheel, the setup must be adjusted to the diameter of the grinding wheel. (If the end mill teeth face downward, the cutter axis is below the grinding wheel axis; if the end mill teeth face upward, the cutter axis is positioned above the grinding wheel axis.) When grinding with a cup wheel, the cutter and grinding wheel axis are in the same plane and the clearance angle is determined by the height setting of the toothrest.
Where secondary clearance angles must also be ground, the flat grinding method is preferred since the secondary angle can be ground using the same setup. For this operation, the wheelhead is tilted slightly to produce the required angle, or the toothrest can be offset to accomplish the same effect.
Chamfers, tapers, and angles can be ground using the hollow ground method. End cutting teeth are typically flat ground.
