Chip Load and Cutterhead Characteristics

      Different CNC tool designs call for different chip loadings. May 16, 2005

Question
How do different characteristics of a tool's design affect the chip load? Helix angle? Diameter? Number of edges?

Forum Responses
(CNC Forum)
From contributor M:
The helix angle affects the chip load in two major ways. First, whether the helix is positive or negative (up or downshear) affects chip evacuation. Downshears tend to load the gullets of the cutter more than upshears. Second, the degree of helix has a direct effect on the percentage of the cutting edge that is in the cut for a given depth (a cutter with any helix at all has more cutting edge in contact with the material than a straight fluted cutter).

As for diameters, larger diameters, by nature, have larger gullets and are more rigid, allowing heavier chip loads. The number teeth on any given cutter may require a little less chip load per tooth, but allow you to gain more because of added edges. Example: If you had a 2 flute rougher that you predominantly run at 15,000 rpm and 390 ipm, your chip load is equal to .026 or .013 per tooth. If nothing else changes on the cutter (same grade carbide, same diameter, same rigidity, etc.) except you purchase a three flute version, you may only be able to get .01 per tooth. But now you have 3 teeth. 01 x 3 = .03. At the same given rpm (15,000) you can now feed at 450 ipm. These are just some basic guidelines with no particular cutters or material in mind.



From the original questioner:
Thank you. We have a problem with a finisher edge rounding tool of ours. It produces an even, soft edge when fed at 83 IPMs, but at this slow rate it gets really hot, which affects not only the lifespan of the tool, but also the look of the cut. You should get the even, dark brown colour when cutting compact laminate. The colour of our cut varies from yellow to dark brown. When we feed at 165 IPM, we are close to getting the right colour, but then the surface of the cut becomes unsmooth. We will experiment some more with the current tool, but in order to get a smoother finish without burning, what changes should we test and what changes should we absolutely not test? Today we have a 12 mm diameter tool with 18 mm length of cut, upshear spiral, helix angle 30 degrees, 2 edges.


From contributor G:
Based on what you are telling us, it seems the relief angle is similar to an end mill for steel. We have seen thousands of tools come into Courmatt's service department, only to have a relief angle required for other materials. We have also seen tools designed for wood that have been serviced by companies put the wrong angles on them.


From the original questioner:
Do you think that we should decrease or increase the relief angle when working with compact laminate instead of steel? I suppose that a tool with a straight fluted cutter will work better at a slow feed rate? I'm thinking about trying a 16 mm dia, straight v-flute, 2-edge bit on our 18 mm compact laminate.


From contributor G:
Relief on the primary should be 15 degree. Most metal tools have about 6-7.


From contributor J:
If you cut solid wood without veneer or plam, better to use flat cutter. Try to change teeth to 4 or 5 or tool path to the other direction.

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