Broke a nice 3/16 carbide endmill.

Patrice Lemée

Patrice Lemée

Joined
Aug 13, 2002
Messages
5,703
:(
I have to learn more about using the mill and stop “winging” it. :(

There I was happily slotting along some 304 stainless guards, 3 of them. About ½ inch thick, small slot, maybe 3/4” long. Doing the back relief with said 3/16 endmill, dry (or should I say with minimal WD40 lube) at around 1910 RPM. Plunging most of the time and just cleaning up a little webbing in between. I noticed some movement of the quill, from vibration but did not make much of it since it was going pretty well.
But this is what happened roughly ¾ of the way through the last guard. Looks weird, not a clean break, more like eroding. Now I now that there are just too many variables here that could be the problem (wrong type of ss, speed, lube, quill movement...). ME being the worst of them all. :(

broken_endmill.JPG


Anyone had that kind of break happen before so I could know what to try and fix first?

Thanks

PS: I have a round column import mill
 
if you are plunging and a bit of the carbid snaps off in the gummy "hot SS" then it will just eat up your bit (carbide inbeded in the SS and then jsut grinds down the point)

not sainf that its right but i use HSS to hog and carbide to clean the hole/slot (makes more places for chips to get away and you dont have ans much of them anyhow)
 
With the added sulphur in 303, it is MUCH more free machining than 304.
And when milling guard slots, it's all about small bites. Baby steps.
I never use anything bigger than a 1/8 end mill, and rarely do I break more than 1 or 2 a year.
Whenever I break one - it's always my fault.
 
It sounds like you are doing a full width pass right down the center of the slot.This is tough the cutter try is climb on one side. I find it is better to conventional cut all the way around the slot. Also if your doing a full depth cut break it down to a couple steps.
 
My guess is you're recutting chips. WD40 etc just makes that worse. Try flood coolant or an air blast.

If it sounds like a coffee grinder, that's bad...

This should be done in multiple steeps, probably .080" deep per pass.

Pre drilling holes at the ends of the cut is good, but out in the cut is not helpful.
 
For carbide rigid is good movement is bad. Same rules apply to knifes as they do to tools...Toughness--------------------Hardness are on two ends of the spectrum. Carbide is way off by itself in the Hardness category.

Quick question how were you applying the WD-40?
 
WD 40 vaporizes and leaves a gummy residue that is not good for milling. If you do not have flood coolant or air blast use a cutting fluid like LP #1. However 304 sucks when milling. Like Nathan said, it would be good to do a relief drill at either end of the slot. Plunge at those points. As you found 304 is gummy and tough. Try the 303 or 416 if you can find it. I know McMaster Carr has 303 bar stock.
 
Patrice -- I don't know if you are using a Bridgeport-pattern mill or a mill-drill. IMHO, having owned & operated both, neither mill will do full-depth cuts in 1/2" stainless with a carbide end mill because the end mill isn't stiff enough. I would recommend making light passes (.1" or less) instead of trying to plow through the whole thing all at once.

The way the bit failed looks like you're either re-cutting chip, or you had bad chatter. In stainless, recutting chips is a problem because they work-harden as they are cut, which is brutal on the cutting edges. Chatter is very bad for any end mill, doubly so for carbide because it's so brittle. Some chatter can be cured by using a stiff setup, keeping the quill as far up as possible, gripping the cutter higher up in the collet (no shank showing, but no cutting edges in the collet) using the right speeds and feeds, and changing your depth of cut.

Also, with carbide, you have to be careful about thermal shocking -- an occasional blast of coolant is worse than no coolant at all. As mentioned above, a continuous stream of compressed air will clear the chips and keep the tool cool, but absolutely wear eye & hearing protection when you do this.

Regarding feeds and speeds: it's really worthwhile to get a copy of Machinerys Handbook because they have charts and tables for every possible combination of material and cutting tool metallurgy. They will also have a good discussion of which grade of carbide is appropriate for which material. You might have chosen an inappropriate grade for cutting stainless.

Stainless is difficult to cut because it work hardens if you dally on the cut. It needs low speed, high feed and a constant flow of coolant or cutting oil to keep the cutting edge in normalized material.

WD40 is not a proper cutting oil for anything other than aluminum. I'd suggest visiting your local hardware store and getting a high-sulfur cutting oil from the plumbing department --- it's used for threading black iron pipe. Used liberally on steels, this should help your tool life.
 
Just want to add also, that carbide doesn't like intermittent cooling. Usually either run it dry, or with flood or other full-on coolant options like a vortex tube or mist.

With Cobalt/HSS cutters, you can get some soluble cutting fluid, mix it up in a heavy duty spray bottle and just keep spraying the cut.

Barring that, the best bet is to keep blasting the chips out with compressed air, pretty easy to rig up an air nozzle pointed right at the end mill that you can turn on and off as you cut.


EDIT: Woops, sorry someone covered that already!
 
Yep Nathan, coffee grinder was a good description before it happened.
I will have to give the air blast a try. I already have shop air with a small hand held nozzle right at the mill but I have to find out about a more permanent setup

Carl, I was simply a few squirts of WD40 from the can from time to time. I would love a flood coolant setup like Nathan and AC mentioned but not sure it this would be possible with the small mill I have.

Roadracer, not a Bridgeport, a import round column mill-drill. I was not taking full depth cuts, maybe something like 70-80 thou and that was cleaning webbing between plunges. Guess it was still too much plus faulty coolant/chip clearing setup.:o
I would love to be able to keep the quill in as much as possible but the round column makes it that I don't want to move it up or down so I often have the quill sticking out quite far. I really want a knee mill. But that is not happening anytime soon. :( I do have a copy of the Machinist's Handbook and I only have myself to blame for not reading it and figuring out exactly what I needed as far as feeds and speeds. Well speed anyway cause feed is by hand and that's only a guess.

I will also look into some more forgiving stainless.

I really appreciate all of your help guys. You'll make a machinist of me someday. Well ok maybe not. I'll settle for keeping me out of the emergency room. ;)
 
Pat, stick with it buddy! Milling is always an adventure. I need to spend more time reading the couple books that I have, but trial and error is good too. You learn fast!:D

I'm about a quarter of the way through a big milling project right now.....as I go along I have to keep telling myself to take small cuts, focus, relax, etc. It's not the kind of thing you can rush.

Hopefully all three guards are all good, I assume this is for the little EDC project for you and your brothers?
 
I have similar problems and have decided that the best way for me at least is to use a mist and a couple of the flex nozzles that mount with magnetic bases. A simple jet effect will pick up the coolant to add to the air. My mill bed has drains and will hook up a drain tank.

http://www.grizzly.com/products/Magnetic-Base-w-Two-Coolant-Hoses/G9810

you can actually built a simple jet pick up for the fluid using a small pipe T. The fluid is picked up on the side opening. On the side that the air comes in make an extension on the fitting that goes to the center of the T and smaller in ID to increase the air velocity.
 
Drilling a bunch of hole across the cut just gives the chips a place to get trapped and recut. Plus it makes a load-unload-load cutting condition for the tool. Add some work hardening around the diameter of the holes and you get poor cutter life.

I recommend you just drill holes at the two ends of the cut and leave it solid across.

Feed rate is at least as important as speed in this application. Feed it too fast and you'll break it or damage the cutting edge. Feed it too slow and you'll rub it to death <--- very common problem in stainless. That cutter should probably be fed somewhere between .001 and .0001 per tooth which is going to be around 4IPM at 1910 RPM. You should do the math and figure out how much time it should take to cross that distance at .0005" per tooth and try it out in the air. If you're not in the right ball park you may have found your main problem.
 
Thanks guys.

Jonny, yes this is for the 3 edc and the guards are done.

Jim, I will look into this nozzle thing. I might try air first since coolant is a much more complicated setup but I am looking forward to seeing how you do it cause I eventually want to add coolant.

Daniel, the only problem I have with all these calculation is the feed part. Since I do it manually, how to I factor this in the equations?
 
Patrice Lemée;11908198 said:
Carl, I was simply a few squirts of WD40 from the can from time to time. I would love a flood coolant setup like Nathan and AC mentioned but not sure it this would be possible with the small mill I have.
Click to expand...

That is what I was worried about. Roadracer Al mentioned thermal shock. Carbide is very hard and very brittle. It can withstand the heat buildup that comes with no coolant, of course coolant is better but it has to be consistent.

There is a lot of good info here, read it all and you will be ahead of many.
 
If you don't want to setup for flood coolant, you may want to research different end mill coatings, some coated carbide is designed to be run completely dry. Otherwise, you'll likely be better off with HSS/Co and a spray bottle filled with appropriate coolant. Although inherent chatter or lack of rigidity in your setup could be contributing also. Carbide don't like that.. HSS/Cobalt is much more forgiving in that regard.
 
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