Machinery's Handbook, 31st Edition
900 Cutter Grinding of grinding marks and improves the quality of the finish produced on the surface being machined. Unless done very carefully, hand stoning may dull the cutting edge. Stoning may be avoided and a sharper cutting edge produced if the wheel rotates toward the cut- ting edge, which requires that the operator maintain contact between the tool and the rest while the wheel rotation is trying to move the tool away from the rest. Though slightly more difficult, this method will eliminate the burr. Specifications of Grinding Wheels for Sharpening Milling Cutters Cutter Material Operation Grinding Wheel Abrasive Material Grain Size Grade Bond Carbon Tool Steel Roughing Finishing Aluminum Oxide 46-60 100 K H Vitrified Vitrified
High-Speed Steel:
Roughing Finishing Roughing Finishing
60 100 80 100
K,H Vitrified H Vitrified F,G,H Vitrified H Vitrified
18-4-1 18-4-2
{ {
Aluminum Oxide
Cast Nonferrous Tool Material
Roughing Finishing Aluminum Oxide
46 100-120
H,K,L,N H
Vitrified Vitrified
Roughing after
Silicon Carbide
60
G Vitrified
Brazing Roughing Diamond Finishing Diamond
Sintered Carbide
Resinoid Resinoid Resinoid Resinoid
100
a a
Up to 500 80-100 100-120
Carbon Tool Steel and High-Speed Steel b
Roughing Finishing Cubic Boron Nitride
R,P S,T
a Not indicated in diamond wheel markings. b For hardnesses above 56 RC on the Rockwell C scale.
Wheel Speeds and Feeds for Sharpening Milling Cutters.— Relatively low cutting speeds should be used when sharpening milling cutters to avoid tempering and heat checking. Dry grinding is recommended in all cases except when diamond wheels are employed. The surface speed of grinding wheels should be in the range of 4500–6500 ft/ min (22.8 to 33 m/s) for grinding milling cutters of high-speed steel or cast nonferrous tool material. For sintered carbide cutters, 5000–5500 ft/min (25.4 to 27.9 m/s) should be used. The maximum stock removed per pass of the grinding wheel should not exceed about 0.0004 inch (0.010 mm) for sintered carbide cutters; 0.003 inch (0.076 mm) for large high- speed steel and cast nonferrous tool material cutters; and 0.0015 inch (0.038 mm) for narrow saws and slotting cutters of high-speed steel or cast nonferrous tool material. The stock removed per pass of the wheel may be increased for backing-off operations such as the grinding of secondary clearance behind the teeth since there is usually a sufficient body of metal to carry off the heat. Clearance Angles for Milling Cutter Teeth.— The clearance angle provided on the cut ting edges of milling cutters has an important bearing on cutter performance, cutting effi ciency, and cutter life between sharpenings. It is desirable in all cases to use a clearance angle as small as possible so as to leave more metal back of the cutting edges for better heat dissipation and to provide maximum support. Excessive clearance angles not only weaken the cutting edges, but also increase the likelihood of “chatter,” which will result in poor finish on the machined surface and reduce the life of the cutter. According to The Cincinnati Milling Machine Co., milling cutters used for general purpose work and hav- ing diameters from 1 ⁄ 8 to 3 inches (3.18–76.2 mm) should have clearance angles from 13 to 5 degrees, respectively, decreasing proportionately as the diameter increases. General
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