What end mill for brass? Copper?

[mikey]

I believe this is because of the center of the drill, the chisel edge.
The "wings" of the drill will bite into the material but the chisel edge where the cutting lips meet will resist it, due to having to push the material aside to penetrate thus preventing it from grabbing and suddenly trying to take a huge chunk.
So when you use a pilot and remove what the bigger drill would have to 'chisel' out, you're also removing what would've braked it a bit on entry.
At least that's my current theory.

Not sure I agree with this one. You're saying the center has a "braking" effect ...

A non-dubbed drill has variable rake along its cutting edge; more rake at the outer edges and less as you approach center. A dubbed drill has reduced rake that is more uniform along its entire edge. When using a pilot drill, the edges of a non-dubbed drill that contact the edges of a pilot hole begin to cut aggressively and since the rake of the cutting edge increases as you go further out along the lip of the drill, it tends to take a bigger and bigger bite. Hence, the drill drives itself into the hole and eventually stalls. In contrast, a dubbed drill has even rake angles along its entire cutting edge so it tends to cut much more evenly and this tendency to self-feed is greatly reduced.

When drilling with a non-dubbed drill but without a pilot hole, the drill will tend not to self feed because there is no edge for the outer flutes to bite into. Hence, chip formation tends to be much more uniform along the entire flute and the drill not only does not dig; it cannot dig.

The center does not cut at all, nor does it act like a brake in any way that I have heard of. I could be wrong so please feel free to correct me.

 

[mikey]

Yeah the Merchant equation doesn't include nose radius, nor do the follow-ups. This was in a discussion of mthematical models for the cutting tool edge, and immediately was discarded as adding too much complexity to be of much use in calculation. Perhaps I mis-spoke about changing the angle of the shear plane, as it is more an expansion of the shear plane caused by the greater area of contact.

There are a couple of choice illustrations that pertain to this discussion (some of which I haven't reached the text for); if I can get my phone taling to my laptop again, I'll post them.

I wonder if that discussion about radius was pertinent to carbide insert tooling, given that inserts tend to have a radiused edge. Moreover, the fixed lead angle of an inserted tool is more akin to an Orthogonal model so who knows, maybe there is some rationale to it. It makes no sense as applied to oblique cutting tools like our HSS lathe tools or end mills, though.

Do let me know if you sort it out. It would be good to update my knowledge.

 

[gradient]

Yeah the Merchant equation doesn't include nose radius, nor do the follow-ups. This was in a discussion of mthematical models for the cutting tool edge, and immediately was discarded as adding too much complexity to be of much use in calculation. Perhaps I mis-spoke about changing the angle of the shear plane, as it is more an expansion of the shear plane caused by the greater area of contact.

There are a couple of choice illustrations that pertain to this discussion (some of which I haven't reached the text for); if I can get my phone taling to my laptop again, I'll post them.



Back to the OP's main question, I am curious about the experience of other peole with using two-flutes on copper and brass (vs 4-flutes), and whether the coating matters. My thoughts are that an HSS two flute would be the best, but I do most of my work in brass on the lathe, so I can't speak too much from experience here.

Just did quite a lot of milling recently with copper. My experience is that, for milling, two flutes are better than four. Copper is soft and sticky so you need more room for the cut material to clear the cutter especially making slots. The other observation is that copper is much tougher on mill bits than steel because it tends to stick to the mill bits it causing greater local heating of the mill bit cutting edges. the heating dulls them faster than steel cutting. I use a mister to lubricate and keep the bits cool. I also use sharp bits with the misting. Use a good feeds and speeds calculator for your cut width and depth.

 

[Bill Kahn]

OP here...Wow! I am a beginner, was hoping for something like “Click here and buy for slotting brass. And here for copper.” Instead we have a research-level engineering seminar discussion. This list is not to be believed. Wow. I have ordered a 2-flute said to be good for aluminum. A 1 flute said to be good for aluminum. And a chunk of C145–maybe that won’t gum up and stall my mill as readily. Once it is all in, and I have a chance to practice, I’ll post back to tell of my experience. Thank you everyone. -Bill