Cutting a blind keyway

[TomG]

Pete,

Actually, four flute end mills are more rigid than two flutes. As an end mill feeds, the side force created by the teeth is perpendicular to them and right through the weakest section of the end mill, the web. With a four flute, that weak section is braced by the other web. If I'm milling a slot with a width close to the diameter of the end mill, I'll always opt for a four flute over a two flute.

I have access to a force gauge at work. It might be fun to see how much these end mills do deflect relative to each other.

Tom