Modern Machine Shop

SEP 2013

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rapid traverse Machining Technology in Brief Average Chip Thickness Dictates Milling Performance by Mat t DanforD I n any milling application, setting a feed rate that yields the correct average chip thickness for a particular insert is critical. Yet many manufacturers overlook this basic principle, says Todd Miller, manager of rotating products for Seco Tools. The typical result is unexplained problems with premature insert failure, poor surface finish, work hardening, vibration or deflection. The key word here is "average," Mr. Miller explains. Unlike turning applications, inserts on a rotating milling tool encounter different forces at different portions of the cut. (For example, the insert cuts progressively shallower and encounters progressively less resistance as it rotates out of the cut from full depth during climb milling.) The result is chips that are thicker on one end than on the other. Given that the very nature of the milling process makes it impossible to generate chips with a consistent thickness, the only option is to aim for an average. That average can be calculated from the size of the undeformed (uncut) chip at all points where the insert is oriented at a right angle to the workpiece. "If you could take a chip and lay it out flat, the average thickness would be right in the middle," he says. The ideal average chip thickness for a particular insert depends on the insert's edge preparation. Compared to the sharp angle formed by the rake and flank faces of a typical turning insert, Unlike turning, milling generates chips that vary in thickness according feed rate, the percentage of the tool diameter engaged in the cut, and the inserts' edge preparation and lead angle. As shown here, average chip thickness—calculated from the size of the undeformed chip at right angles to the cutting edge—decreases with the cutter's radial engagement. 24 MMS September 2013 mmsonline.com a milling insert's cutting edge generally has a small chamfer to protect against the shock of repeated material entry. Chips should be at least as thick as this edge protection (typically in the form of a T-land or hone) to properly dissipate heat. Chips that are too thin indicate that the cutting action—and the heat it generates—is constrained to a relatively small portion of the insert edge. This can lead to premature cratering, thermal cracking or flank wear. Chips that are too

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