jackK wrote:
wrote:

This is a dumb question, but what are the relative advantages of
4-Flute vs. 2-Flute end mills, and on what applications should one type
be chosen over the other?

Harry C.

4-flute = steel
2-flute = aluminum

Yeah, it sounds so simple. But, the real problem is the primary and
secondary relief angles cut into the back of the cutting flutes.
You really want more relief for aluminum, but a 2-flute EM does NOT
guarantee that, at all. There are special aluminum end mills that have
less metal backing up the cutting edge, so you can increase the feed
like crazy without the back of the flutes rubbing on the advancing work.
I accidentally made one of these when resharpening some end mills, and
they were awesome! You want just a tiny bit of primary relief behind
the cutting edge, and then a 15 degree secondary relief right behind
the edge. You can tell if the back of your flutes are rubbing if they
have aluminum rubbed onto them after heavy stock removal passes.

Without having to buy $60 Ski-Carb end mills, I generally use small
4-flute end mills in aluminum, as you get twice as many cutting edges
per revolution.

Today, practically all 4-flute cutters are center-cutting, too. Also,
don't forget the 3-flute cutters. They are a nice compromise. You can
even get 1-flute cutters!

Jon