What end mill for brass? Copper?

[pontiac428]

Good call out, @mikey . I don't have the books in front of me, but I have the web. With helical cutters, the basic geometries of a lathe bit still apply:

And for brass, we want 0 to -4 degrees rake:

Which is taken from the cutting edge rake angle:

So, by "blunting", or more precisely grinding square and sharp the face of the cutting edge, you are achieving the correction of the rake to suit the material.

 

[mikey]

Thank you, Sir. I sort of understand lathe tool geometry and understand how reducing the cutting edge rake angle would be done on an end mill. It would require that all the flutes be ground back evenly though, I think, so might need a tool grinder.

I find it curious that a drill bit will grab hard in brass if you use a pilot drill before using a bigger one; the bigger one grabs every time because of this. However, if you use that same big drill without a pilot hole, the drill will go in with nary a catch or snag. Done this hundreds of times so I'm pretty sure of the outcome. Similarly, I've plunged holes in brass, slotted, pocketed and profile cut brass with a standard end mill and had no issues so I wondered at this geometry thing.

I am NOT saying you aren't right; just that I wonder if it is really necessary, maybe? Besides, the OP sounds like he needs to slot the material, not use the end so maybe a standard 2 or 3 flute end mill might work.

 

[ThinWoodsman]

I sort of understand lathe tool geometry and understand how reducing the cutting edge rake angle would be done on an end mill. It would require that all the flutes be ground back evenly though

I think it might not be the rake that is being adjusted, but rather the tool tip radius. I've been working my way through Shaw's Metal Cutting Principles (I forget who on the forum mentioned it, or where, but tremendous gratitude to them!), and it seems the tool point radius changes the angle of the shear plane for the cut and reduces the friction at the actual point of the cutting edge.

My metallurgy knowledge is not quite good enough to explain why this is important for brass and copper, though. Still reading

 

[Illinoyance]

You want a SHARP endmill. Use it only on copper or brass. I would use a new or resharpened end mill. I would be inclined to try an end mill designed for aluminum: higher rake and polished flutes.

 

[pontiac428]

I wonder if it is really necessary, maybe?

I would only regrind the mill if there was an issue achieving the finish I was after, or I had to use a very small DOC. I don't need to change the flute helix at all if I want to change the rake angle, just a little off the leading edge by grinding a new facet to make the nominal angle.

it seems the tool point radius changes the angle of the shear plane for the cut and reduces the friction at the actual point of the cutting edge.

I dish my end mills by 1 degree when regrinding to counter this type of effect. The mill and type of gash cut will dictate how it cuts/doesn't cut at the center.

 

[petertha]

My experience with dubbing drills for brass/bronze & similar materials is: apply felt pen to a (cleaned) cutter. Bring your stone to the edge & have a good visual on the relief angle so you can control it & repeat on the other edge(s). As soon as you see a slight new shiny edge come through, that's it. I'm betting less that 0.010" for sure. Its been a while since I saw the Clickspring video, maybe he verbalizes a number or its just apparent by looking at the tool.

Yes, it is sharp. But the critical thing is the edge geometry (the rake angle) is altered. There is nothing wrong with having a sharp tool in these troublemaker alloys, its the 'normal' rake angle that makes them dig in & feed on themselves. I don't think a rounded over edge does much good. It may appear that its not grabbing as much but I doubt its cutting efficiently.

I'm not sure why but on plain lathe turning I have had good luck with sharp un-coated inserts I use for aluminum. Seems to defy the logic.

For whatever reason I haven't had much issue milling brass with regular EM's. Maybe the mill setup is more rigid & the cutter cant influence the machine? In most cases sharp carbide but also the plain vanila HSS. I havent tried bronze either. You never hear about dubbed drills or EM's in production CNC & my only guess is the machines are that much more solid & tight?

 

[mikey]

I think it might not be the rake that is being adjusted, but rather the tool tip radius. I've been working my way through Shaw's Metal Cutting Principles (I forget who on the forum mentioned it, or where, but tremendous gratitude to them!), and it seems the tool point radius changes the angle of the shear plane for the cut and reduces the friction at the actual point of the cutting edge.

My metallurgy knowledge is not quite good enough to explain why this is important for brass and copper, though. Still reading

Interesting. Typically, the Merchant Equation applies to the effect of rake angles on the shear plane. I have not seen the nose radius under discussion with regard to its effect on the shear plane before. I might need to take another look at this as its been awhile. Thanks for the heads up.

 

[mikey]

I would only regrind the mill if there was an issue achieving the finish I was after, or I had to use a very small DOC. I don't need to change the flute helix at all if I want to change the rake angle, just a little off the leading edge by grinding a new facet to make the nominal angle.

Ah, you sneaky guy, I forgot you have one of them there tool and cutter grinder things. It would be interesting to see if a modified rake angle on an end mill really makes a difference. I say that because rake angles on a turning tool used in brass really does have a significant impact on how the tool cuts. This has to do with the shear plane and how that relates to chip formation in this material so yes, I can see how a reduced rake angle could be applicable. I have just never seen it or even heard of it being applied to an end mill, and even if I did it would be for end cutting and not side cutting.

If you ever do modify an end mill like this, please let us know how it works!

We have taken this thread way off topic and I apologize to the OP. The guys were just educating me is all. Now back to our regular programming.

 

[MrCrankyface]

I find it curious that a drill bit will grab hard in brass if you use a pilot drill before using a bigger one; the bigger one grabs every time because of this. However, if you use that same big drill without a pilot hole, the drill will go in with nary a catch or snag. Done this hundreds of times so I'm pretty sure of the outcome.

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.

 

[ThinWoodsman]

Typically, the Merchant Equation applies to the effect of rake angles on the shear plane.

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.

We have taken this thread way off topic and I apologize to the OP.

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.