Use of Ball Radius end milles

At a guess, it's because conventional milling requires cutting into stock from the thin side of the removed material, making it more likely that the cutter will deflect, smear, or skip before it has enough tooth load to dig in and cut, but a round endmill exagerates this even further because you're cutting an even thinner chip down on the radius, and that switching to climb milling loads the teeth enough to put the cutter back into a reasonable cutting depth.

--Glenn Lyford

Good guess. Also implied in what you're saying is that the cutting speed near the centerline of the cutter is 'way too low, which makes the problem worse.

With climb milling you'll get a more adequate chip load near the center, because all of the deflection is working in your favor.

-- Ed Huntress

Glen & Ed, I have thought about that as well, but if cutting speed were the key, I would have noticed a finish difference across the width of the cut, but I do not. In aluminum, it actually galls. I think it has something to do with chip clearance, but I don't understand it. Steve

What's special about ball and radius endmills? The same (to some extent) applies to all endmills doesn't it?

Chip geometry and clearance are not easy to imagine in a case like this, but it's true that the periphery of the cutter sweeps across the area cut by the center of a radiused endmill, wiping out some surface roughness that may appear at the centerline of the cut. However, that's not the case with a ball-nose endmill. Surface finish issues are common with ball-nose cutters but the problem usually disappears in practice because most applications involve multiple passes to flatten the cusps left after rough milling.

In any case, a ball-nose cutter cutting at feeds and speeds that are appropriate for the outer diameter of the cutter is turning way too slow for cutting at the center. And conventional milling makes it worse. If a machine has the ridigity for climb milling, that will solve some of the problem.

-- Ed Huntress

Only when plunge cutting. With side milling (climb or conventional) the center of flat bottom endmill does virtually nothing. With a ball end the center is still cutting *some* material.

I assume there's some good reason a ball end mill doesn't just have a single flute at the very bottom? Take a regular ball end mill, grind back all but one flute close to the center/bottom.

Seems like that would let the slower (surface feed rate) part take a bigger bite, but the rest of the mill would have appropriate rates.

I Imagine one drawback would be you'd have to go slower on the plunge parts.

Dave

Greetings Dave, Some router cutters are made this way except they remove completely one flute. I have used these on aluminum. But grinding away one flute does make the cutter less rigid. So use on steel might be precluded due to excessive deflection. ERS

Exactly right, thin to thick and thick to thin, milling cutters (all) like thick to thin but the lash in the machine after a certain point does not. Cutters (all) last longer if you climb cut.

Matt