Whenever you want!
Typically a 2 flute is ground with end clearance and a center cutting gash is made so it will drill a hole if required. Drill, then mill.
Some 3 and 4 flute cutters are also ground as "center cutting" but if not, then you cannot drill into the work with them.
Any cutter that is not ground for "center cutting", you can ramp into the cut at a shallow rate of Z descent, but that may require simultaneous 3d interpolation that your mill might not have available. But, you can start milling from an existing opening with these.
So it boils down to the surface finish that you want. The more flutes you have, the finer the effective feed per tooth. The 2 flute cutter has typically about the same feed rate capacity as a three or 4 flute, but has larger flutes, and can handle a larger chip, perhaps with less tendency to plug up in soft materials.
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(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management) |