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Issue link: https://mms.epubxp.com/i/155490
rapid traverse Machining Technology in Brief thick indicate high cutting forces that could overwhelm and break the insert. Seco lists the proper average chip thickness for a given insert in its standard product catalog. The user's task is to arrive at the feed rate that yields that value. This rate depends on both the radial engagement of the tool (the percentage of the tool's diameter engaged in the cut) and the lead angle of the insert (the angle formed between the insert's outer edge and the center axis of the tool shank). The smaller the radial engagement, the larger the feed rate required to arrive at the proper thickness value. That's because the thickest possible chip for a given tool is generated at the 50-percent-diameter mark, where the insert is buried in the material to the fullest possible extent. So, a cutter with less of its diameter engaged must move into the material at a higher Learn more A video at short.mmsonline.com/avechip depicts a cutter machining the same block of material at various radial engagement levels. Notably, the sound generated by the cut is virtually indistinguishable at most levels. That's because cutting parameters were set with the goal of attaining the same average chip thickness regardless of radial engagement. mmsonline.com 26 MMS September 2013 mmsonline.com As shown here, a lower lead angle spreads the cut over a longer section of the cutting edge. However, feed per tooth must be increased because the chipload is smaller. rate to arrive at the same chip thickness as one that's more fully engaged. For the same reason, calculating average chip thickness is generally not as valuable if radial engagement is greater than 50 percent, Mr. Miller adds. Consider that if the chip is thickest at the 50-percent-diameter mark, it will be thinner at any point beyond that. So, changes to radial engagement at any point above 50 percent make less difference on the average thickness of the chip. It follows that the feed rate adjustment required to compensate is also less significant. The exception to this general principle relates to the other contributing factor: the insert lead angle. Similarly to radial engagement, the lower the lead angle, the higher the feed rate. Consider that at 90 degrees, the ratio of chip thickness to feed per tooth during a full slotting application (100 percent radial engagement) is 1:1—that is, the material removed by each insert on each revolution is equal to the width of the insert's cutting edge. At a 45-degree lead angle, however, the chip thickness is only about 70 percent of the feed per tooth because the edge is slanted relative to the direction of feed. Thus, a user employing a facemill with 45-degree inser ts at 100 percent radial engagement should know that this tool must feed approximately 30 percent faster