Page:Practical Treatise on Milling and Milling Machines.djvu/112

106 Clearance on Cutters. The clearance or relief of milling cutters is the amount of material removed from the top of the teeth back of the cutting edge to permit the teeth to clear the stock and not scrape over it after the cutting edge has done its work. On formed cutters, the clearance does not have to be considered in resharpening. This is because the teeth are so formed that when ground on the faces, the clearance remains the same.

The angle of clearance depends upon the diameter of the cutters, and must be greater for small cutters than for larger ones. The clearance on the teeth of plain milling cutters should be 4° for cutters over 3 inches in diameter, and 6° for those under 3 inches diameter. The clearance of the end teeth of end mills should be about 2°, and it is well to have the teeth a little hollowing, making them .001 or .002 inch lower near the centre than at the outside, so that the inner ends of the teeth will not drag on the work. This can be done by setting the swivel on the cutter grinder slightly away from 90°.

Vibration of Cutters. If the clearance of a cutter is too great, vibrations are likely to occur in operation, and this should be corrected by regrinding the teeth. "Chattering" is a serious drawback to successful milling, as it impairs the quality of the work, limits the capacity and injures a machine, and reduces the life and efficiency of a cutter. While it is impossible in many cases to eliminate it, every precaution should be taken to reduce it to a minimum.