Machinery's Handbook, 31st Edition
Milling Cutters
871
MILLING CUTTERS Selection of Milling Cutters
The most suitable type of milling cutter for a particular milling operation depends on such factors as the kind of cut to be made, the material to be cut, the number of parts to be machined, and the type of milling machine available. Solid cutters of small size will usu ally cost less, initially, than inserted blade types; for long-run production, inserted-blade cutters will probably have a lower overall cost. Depending on either the material to be cut or the amount of production involved, the use of carbide-tipped cutters in preference to high-speed steel or other cutting tool materials may be justified. Rake angles depend on both the cutter material and the work material. Carbide and cast alloy cutting tool materials generally have smaller rake angles than high-speed steel tool materials because of their lower edge strength and greater abrasion resistance. Soft work materials permit higher radial rake angles than hard materials; thin cutters permit zero or practically zero axial rake angles; and wide cutters operate more smoothly with high axial rake angles. See Rake Angles for Milling Cutters on page 901 . Cutting edge relief or clearance angles are usually from 3 to 6 degrees for hard or tough materials, 4 to 7 degrees for average materials, and 6 to 12 degrees for easily machined materials. See Clearance Angles for Milling Cutter Teeth on page 900 . The number of teeth in the milling cutter is also a factor that should be given consider ation, as explained in the next paragraph. Number of Teeth in Milling Cutters.— In determining the number of teeth a milling cut ter should have for optimum performance, there is no universal rule. There are, however, two factors that should be considered in making a choice: 1) The number of teeth should never be so great as to reduce the chip space between the teeth to a point where a free flow of chips is prevented; and 2) The chip space should be smooth and without sharp corners that would cause clogging of the chips in the space. For milling ductile materials that produce a continuous and curled chip, a cutter with large chip spaces is preferable. Such coarse tooth cutters permit an easier flow of the chips through the chip space than would be obtained with fine tooth cutters and help to eliminate cutter “chatter.” For cutting operations in thin materials, fine tooth cutters reduce cutter and workpiece vibration and the tendency for the cutter teeth to “straddle” the workpiece and dig in. For slitting copper and other soft nonferrous materials, teeth that are either chamfered or alternately flat and V-shaped are best. As a general rule, to give satisfactory performance the number of teeth in milling cutters should be such that no more than two teeth at a time are engaged in the cut. Based on this rule, the following formulas (valid in both SI and English system of units) are recommended: For face milling cutters, (1) For peripheral milling cutters, (2) where T = number of teeth in cutter; D = cutter diameter in inches (mm); W = width of cut in inches (mm); d = depth of cut in inches (mm); and A = helix angle of cutter. To find the number of teeth that a cutter should have when other than two teeth are in the cut at the same time, Formulas (1) and (2) should be divided by 2 and the result multiplied by the number of teeth desired in the cut. T W 6.3 D = T D 4 d 12.6 D cos A = +
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