# What end mill for brass? Copper?



## Bill Kahn (Dec 3, 2019)

I am going to mill about 100 notches into brass.  1/4” end mill.  3/8” full depth (cannot step down bit by bit to the 3/8”).  Have to go through a 3/8” piece.  (Will be notching out a 3/8 by 3/8 square by 1/4 wide). What end mill would be best?  I am guessing a two-flute, like for aluminum but is there something better? Speed isn’t really important—I have a PM25 which does 2500 rpm and am a hobbyist, so I have time. Or, is brass so easy to machine anything will do fine? What might I gain from something like  http://www.2linc.com/endmills_aluminum_1fl_std.htm.   

I hate working in copper, though the end result can be beautiful. Is there an end mill for the above that will work well with copper without grabbing/chattering/gumming?

 Thanks.  -Bill


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## Bob Korves (Dec 3, 2019)

I probably do not know your answer, but it will help all of us to know what alloy of brass you are using, if you know what it is.


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## pontiac428 (Dec 3, 2019)

The wisdom of the reference books says to blunt the edges of a standard end mill with a stone, similar to how you would modify a drill bit for brass or plastic.  I believe Karl Moltrecht and Harold Hall cover this in the cutter sections of their books.  It may even be in the Army technical manual.  Anyway, the idea is that you present a square or negative raked cutting edge to soft metals.


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## chips&more (Dec 3, 2019)

I would use a new and sharp (not blunted) carbide end mill. Carbide because it’s more rigid and will not flex as much when you make that cut all at once. And choose your lubricant wisely, especially when cutting copper.


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## Karl_T (Dec 3, 2019)

I agree, do the above (stone edge of HSS EM). I also find liberal amounts of WD40 for coolant work well on copper. Its gummy stuff.

I find brass just plain easy to machine, no special requirements. I do use HSS and hi feeds.


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## Mitch Alsup (Dec 3, 2019)

I believe the word you are looking for is "sharp".

Clickspring mentions (in one of his videos) that filing of copper and brass require files that are sharper than would be necessary for aluminum and steel.


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## Bill Kahn (Dec 3, 2019)

Bob Korves said:


> I probably do not know your answer, but it will help all of us to know what alloy of brass you are using, if you know what it is.


Planning to buy 360, but will be happy to buy something else if it would make a difference.

Would like to use C101, maybe C110 copper, but if like C111 or C182 machines much better I'll look for that.


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## KMoffett (Dec 3, 2019)

C145 is the best to machine. Mills like brass. Not cheap though.


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## Janderso (Dec 3, 2019)

Wow, I was interested in the answers to your question.
Please let us know what you choose and how it works out.


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## mikey (Dec 3, 2019)

pontiac428 said:


> The wisdom of the reference books says to blunt the edges of a standard end mill with a stone, similar to how you would modify a drill bit for brass or plastic.  I believe Karl Moltrecht and Harold Hall cover this in the cutter sections of their books.  It may even be in the Army technical manual.  Anyway, the idea is that you present a square or negative raked cutting edge to soft metals.



I'm not disagreeing with you here but typically, cutting edges on tools used with brass are blunted to prevent the tool from catching or grabbing and pulling the work into the cutter. We have all had this happen with drills. However, I've never seen it happen with an end mill and wonder if you know how Mr. Moltrecht and Hall justify their recommendation. I've milled a fair amount of brass over the years and always used a sharp end mill with no problems but maybe there is something I should know here?


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## pontiac428 (Dec 3, 2019)

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.


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## mikey (Dec 3, 2019)

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.


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## ThinWoodsman (Dec 3, 2019)

mikey said:


> 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


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## Illinoyance (Dec 3, 2019)

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.


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## pontiac428 (Dec 3, 2019)

mikey said:


> 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.



ThinWoodsman said:


> 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.


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## petertha (Dec 3, 2019)

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?


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## mikey (Dec 3, 2019)

ThinWoodsman said:


> 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.


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## mikey (Dec 3, 2019)

pontiac428 said:


> 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.


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## MrCrankyface (Dec 4, 2019)

mikey said:


> 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.


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## ThinWoodsman (Dec 4, 2019)

mikey said:


> 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.



mikey said:


> 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.


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## mikey (Dec 4, 2019)

MrCrankyface said:


> 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.


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## mikey (Dec 4, 2019)

ThinWoodsman said:


> 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.


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## gradient (Dec 4, 2019)

ThinWoodsman said:


> 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.
> 
> ...


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.


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## Bill Kahn (Dec 6, 2019)

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


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