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2 flutes for aluminum and non-ferrous material , 4 for ferrous. You can get by with a 3 flute, but in general they are more expensive. Make sure the bit is capable of center cutting! I can't emphasize that enough. My chassis team leader last year took a non-center cutting bit and tried to route out the pocket required for butt joints in 80/20 - and ruined EVERY piece of our chassis.
If you're doing multiple passes, that end mill geometry selection is okay. cutting all the way thru, 1/4" x 1/4", is a bit aggressive.
3/8" shank bits, between 1/8" to 3/8" in my experience tends to be more economical. My team mostly uses valuebrand HSS bits, with solid carbide bits reserved for special applications. The bits are uncoated. This is due to budget restrictions. TiN is a good choice for most tooling application because of it's lubricating properties on the cut. TiAlN and TiCN are better for high speed commercial operations (10k RPM or above on machining centers) but most people are not equipped to run at these speeds (flood coolant is mandatory).
For coolant we use NewLube, which is a vegetable oil based lubricant. I don't want my kids to have to deal with petroleum distillates.
Make sure that you climb cut your alumninum for a better finish. Off the top of my head, aluminum should be cut at 600 to 800 S.F.P.M. Do the calculation for RPM translation for a 1/4" cutter. (Roughly 3000 RPM is my off-top-off-my-head number).
Lemme know if this helps
-=- Terence
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