Upcut vs Downcut on a ball-nose end mill

[beatle567]

I can't seem to find much info on the differences between upcut and downcut on ball nose end mills.  I have a few upcut ball nose bits and wonder if it would be worth investing into some downcut ball nose bits.  When cutting with downcut end mills, the top surface is left with a nicer finish, so if you are using these mostly on detailed surface finishing passes, shouldn't it follow that a downcut ball nose bit is superior in 99% of applications?  Or am I missing something here?

[oldtech]

Downcut bits are useful for some material such as plywood, but would be a bad choice for cutting aluminum or harder materials because chip removal helps to control heat buildup. And heat buildup can even happen in wood so I would reserve compression and downcut bits to plywood and thinner materials where chipping is a problem.

BTW: Ball nose is superior to flat end mills because it's engagement with the material is more uniform. Just consider a plunge cut. A flat end mill goes from cutting nothing to full engagement in an instance while the ball mill goes from no engagement to full engagement in a ramp. This really comes into play if you need to work with small bits (< 3mm) to keep from breaking them.

Of course, if you need a flat surface, a flat end mill will give better results. One strategy is to rough cut with ball nose and finish with flat.

I consider the up cut ball nose, and it's cousin the bull nose, the first choice for removing material.

[beatle567]

How about ball nose upcut vs ball nose downcut for finishing passes rather than rough cuts? Thanks for the insight on the engagement, I haven't thought about it that way. Most of my work for now has been in plywood, just bought some maple and apple blocks to try out though!