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Proportional vs. Non-Proportional Tooth Spacing

Last Updated: Jun 05, 2015 04:04PM EDT
Non-proportional pitch endmills do not eliminate chatter (they do not suddenly make an endmill assembly 100% stiff). They do, however, shift the location of the best spindle speeds. If you do not know where the stable lobes are on a proportionally spaced tool and you have chatter, changing to a non-proportional pitch tool of the same size, number of teeth and projection may shift the lobes enough to stabilize the tool. This can be useful for transfer lines, for example, where no change in operating parameters is allowed. Interestingly, you also get the same spindle speed shift by changing the stickout length of the tool in or out of the toolholder. However, if you measure the frequency response of the proportionally spaced endmill and locate the most robust stable speed and depth of cut, you should be able to replicate or exceed the performance of the non-proportional pitch endmill.

The feed for a non-proportional (or variable) pitch tool has to be reduced from the feed for a proportional pitch tool. That is because the chip load is limited by the strength of the edge and one edge is taking more load than the others. One edge is fully used and the remaining edges are underused. The amount of the feed reduction depends on the severity of the non-proportionality. The feed must be reduced by a factor of the ratio of the spacing between teeth on the proportional tool over the largest spacing between teeth on the non-proportional tool.For example, on a proportional four flute tool the spacing is 90 degrees and the permitted feed is (10,000 rpm)*(4 teeth per rotation)*(0.15 mm per tooth) = 6000 mm/min. Let's say that the biggest spacing on a non-proportional four flute tool is 110 degrees. The feed for the non-proportional tool can't be more than (6000 mm/min)*(90/110)= 4909 mm/min. This is more than an 18% reduction. You have to be able to increase the depth of cut by at least that factor to be even in terms of metal removal rate.

Tool life is also compromised with the teeth staggered at the same amount as the above scenario (two at 110 degrees and two at 70 degrees). Two of the teeth are in the cut 18% longer than those of a proportionally spaced tool and will wear out sooner by a similar amount. One tooth (or two) fails and the entire tool needs to be replaced. 

On top of that, a non-proportional spacing that works well for one machine may be worse for another. It depends on the frequency response of the machine tool and the tool assembly.

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