Machinery's Handbook, 31st Edition
1226 MACHINING ECONOMETRICS In the 1950’s it was discovered that cutting speed could be raised by a factor of 5 to 10 when hobbing steel with HSS cutters. This is another example of being on the wrong side of the Taylor curve. One of the first reports on high-speed end milling using high-speed steel (HSS) and carbide cutters for milling 6061-T651 and A356-T6 aluminum was reported in a study funded by Defense Advanced Research Project Agency (DARPA). Cutting speeds of up to 4400 m/min (14140 fpm) were used. Maximum tool-lives of 20 through 40 minutes were obtained when the feed/tooth was 0.2 through 0.25 mm (0.008 to 0.01 inch), or measured in terms of ECT around 0.07 to 0.09 mm. Lower or higher feed/tooth resulted in shorter cutter lives. The same types of previously described curves, namely T - ECT curves with maximum tool life along the H -curve, were produced. When examining the influence of ECT , or feed/rev, or feed/tooth, it is found that too small values cause chipping, vibrations, and poor surface finish. This is caused by inade quate (too small) chip thickness, and as a result the material is not cut but plowed away or scratched, due to the fact that operating conditions are on the wrong (left) side of the tool life versus ECT curve ( T - ECT with constant speed plotted). There is a great difference in the thickness of chips produced by a tooth traveling through the cutting arc in the milling process, depending on how the center of the cutter is placed in relation to the workpiece centerline, in the feed direction. Although end and face milling cut in the same way, from a geometry and kinematics standpoint they are in practice distinguished by the cutter center placement away from, or close to, the work centerline, respectively, because of the effect of cutter placement on chip thickness. This is the criterion used to distinguish between the end and face milling processes in the following. Depth of Cut/Cutter Diameter, ar/D is the ratio of the radial depth of cut ar and the cutter diameter D . In face milling when the cutter axis points approximately to the middle of the work piece axis, eccentricity is close to zero, as illustrated in Fig. 3 and Fig. 4, page 1110, and Fig. 5 on page 1111. In end milling, ar / D = 1 for full slot milling. Mean Chip Thickness hm is a key parameter that is used to calculate forces and power requirements in high-speed milling. If the mean chip thickness hm is too small, which may occur when feed/tooth is too small (this holds for all milling operations), or when ar / D decreases (this holds for ball nose as well as for straight end mills), then cutting occurs on the left (wrong side) of the tool life versus ECT curve, as illustrated in Fig. 6b and Fig. 6c. In order to maintain a given chip thickness in end milling, the feed/tooth has to be increased, up to 10 times for very small ar / D values in an extreme case with no run out and otherwise perfect conditions. A 10 times increase in feed/tooth results in 10 times bigger feed rates ( F R ) compared to data for full slot milling (valid for ar / D = 1), yet maintains a given chip thickness. The cutter life at any given cutting speed will not be the same, however. Increasing the number of teeth from say 2 to 6 increases equivalent chip thickness ECT by a factor of 3 while the mean chip thickness hm remains the same, but does not increase the feed rate to 30 (3 3 10) times bigger, because the cutting speed must be reduced. How ever, when the ar / D ratio matches the number of teeth, such that one tooth enters when the second tooth leaves the cutting arc, then ECT = hm . Hence, ECT is proportional to the num ber of teeth. Under ideal conditions, an increase in number of teeth z from 2 to 6 increases the feed rate by, say, 20 times, maintaining tool life at a reduced speed. In practice about 5 times greater feed rates can be expected for small ar / D ratios (0.01 to 0.02), and up to 10 times with 3 times as many teeth. So, high-speed end milling is no mystery. Chip Geometry in End and Face Milling.— Fig. 24 illustrates how the chip forming process develops differently in face and end milling, and how mean chip thickness hm varies with the angle of engagement AE , which depends on the ar / D ratio. The pertinent chip geometry formulas are given in the text that follows.
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online