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Generally the number of flutes you use depends upon the chip that is produced by the material being cut, for chip clearance. More teeth the better as far as feedrate for a given RPM. This is the first line of defense in determination of how many flutes to use.

Secondly, it does not matter how many flutes you use to interrupt harmonics that cause chatter, it is the timing of the flutes which will determine it. Although, when you change the number of flutes, you also change the calculation of dynamics, which a formula is given below and this number of flutes could fit into the proper RPM’s of the operation for one to further maximize that operation. But is only by chance that it happens and it certainly does not maximize the operation.

When standard cutters are made they are indexed in a standard set non-variable, such as: A 2 flute cutter will be timed to have 180 degree per flute of index, a 4 flute will be every 90 degree, etc. The trick is to offset these flutes either plus or minus to break up the harmonic effects. One flute + 5 degree, one flute minus 5 degree, example. This will tend to breakup any harmonics of the operation, but it will only maximize the machining operation if it is in a proper RPM, of the natural frequency of the cutter assembly. It might help at your current RPM but will not reach full potential of that cutting operation unless you run the proper RPM which can be calculated by knowing the Natural Frequency of the cutter assembly. Formula: Natural Frequency * 60 / number of teeth.

Three flute cutters work nice because they still give you a deep gullet for chip clearance and are tough on the secondary side (Back side of the cutter) because they have a sufficient amount of material to be strong upon the flute, a 5 flute leaves you with less chip clearance and weakened flutes on the back side, causing breakage.

To learn more about this subject please see MoldMaking Technology Magazine in April – Chatter Myths. Although this article does not refer to number of teeth and timing of it, it will give you the basic outlook of chatter and what it is and how to eliminate it. I will add the Myth of 3 and 5 flute cutters break up harmonics in a later article, seems it is another Myth to be reckoned with.