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Comment by shawnz | Hacker News Reader

But the holes are spherical, not cylindrical. How could you do that with an end mill the size of the hole?

With a ball end mill, theoretically.

Not that it's a good idea - you need to be able to actually evacuate material from the surface you're cutting, and milling is different than drilling in this respect. But I imagine you could come up with a very fast two step program using a cylindrical end mill for plunging/roughing and a relatively large ball end mill for finishing.

"milling is different than drilling in this respect"

Is it?

With drilling you need to evacuate material also.

Or am I misunderstanding something, I am far from an expert in such things.

With drilling, the fluted drill bit evacuates material for you. When milling, you don't have the same luxury.

Right ok, thanks :)

Also not an expert here, but doesn't an end mill have flutes too?

It does, but the ability to evacuate material straight out of a hole is very limited. Some are designed for side milling only, others can do plunge cuts but it's mainly done to expose a new "shelf" so that the bulk of the material removal can be done using the side of the tool.

Could you perhaps have an end mill with catch-holes on the insides between the teeth(?), leading to a central hollow in the end mill, on which a vacuum is pulled to pull the material through and out, allowing the end mill to just bore continuously? (Sort of a very tiny solid-state version of a tunnel-boring machine, if you can picture that.)

That’s not feasible for a few reasons. The size difference between a cutting tool and the chip it produces is much smaller than the difference between a digging machine and rocks/gravel. The relative strength is also an issue, strings of steel or aluminum aren’t going to be sucked into a hole as readily as loose material. Lastly the geometry of a cutting head would be tricky if you need the chip to go towards the center instead of being thrown out. I guess it could work if you were milling something that turns into a fine material, like graphite. (Now THAT was a messy day in the shop)

I can see the problem with wide chip, yeah.

Specifically re: strings, though, I’ve always wondered whether—as a separate problem—it’d be possible to have an end mill or drill that dices long stringy chip into shards. Seems like it’d make operation/maintenance easier in some respects. Maybe a bit with two parts, where the tip (either end mill or drill) has counter rotation against teeth running down the rest of the bit, such that the chip gets sheared as it encounters the interface between the two parts?

My understanding is that using a ball end mill to "drill out" a hemisphere while perpendicular to your surface will create very high forces that are bad for the tool. I'd imagine you could offset the tool axis and then achieve the same effect, but using a smaller tool might end up being easier.

Please note that I am far from even a beginner machinist.

Right, that's the idea I was trying to express. Don't expect anything good to happen if you try to plunge a ball end mill straight down into the material.

The thought was only that a large-radius ball would be able to approximate the sphere to within tolerance with larger stepover than a smaller tool. After bulk material removal with a more suitable cylindrical tool of course.

With a little bit of googling I discovered a different class of end mill called a circle segment tool, which could potentially make short work of the problem in a 5-axis machine:

https://www.moldmakingtechnology.com/videos/circle-segment-t...

Would love to see how Apple does it, but it's never happening.

I'm not really a machinist either, but let's not allow that to stop us from incorrecting each other on HN!

The stock material is aluminium alloy, it’s not that hard on tooling.

Ball end mill.

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