Breaking endmills

I’ve had my PRO4896 for several years now, and up until recently, I’ve never broken a 1/4" endmill. Recently, the Porter Cable 7518 that I had on the machine died, so I replaced it with a Triton 3.25HP router. Same horsepower, same dimensions as far as I can tell. Since then, I’ve been snapping endmills left and right! What’s going on? I snapped four 1/4" endmills today cutting plywood at 40ipm, 0.166" DOC! Those are tame numbers! The only real difference I can see is that the Porter Cable router used is own proprietary collet, vs the new Triton uses an ER20, but I can’t imagine why that would make a difference. Anyone have troubleshooting tips?

Those are light cuts, I would think it is a tool holding issue. Can you tell if the tool is slipping before it breaks? Is it breaking at the collet?

It’s reliably breaking right at the collet. No slipping as far as I can tell.

can you give us more details? Does it happen immediately? In a corner? Etc. did you re-tram your spindle when you made the change?

No, it doesn’t happen immediately, it’s not super predictable. Could break in 30 seconds, could break in 15min. I haven’t re-trammed, but I didn’t touch the router mount since swapping out the router, but before that it was within a thousandth. I guess I’ll check that again tomorrow but I have a hard time imaginging how it could’ve changed.

I would suspect a bad collet, maybe some burs on the inside, right at the edge.

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I suspect this is the problem. This is a very slow move speed. I bet you’re cooking the bits and snapping them that way. I’d increase move speed to at a MINIMUM of 150 IPM at about 12-14k RPM with a minimum DOC of .25.

You have to make big enough chips to evacuate the heat and at those slow speeds you’re likely making dust and recutting chips causing a lot of heat.

You can check the runout on the new router with a dial indicator to see if it’s got bad bearings, but I suspect that’s not the case given you feedrates.

I think Eric is exactly right. I had this happen to me a few years ago before I fully committed to learning about feeds and speeds. It’s counterintuitive and can be scary, but faster is sometimes better.

The other thing to consider is the router’s torque curve. When you reduce the speed on a router with dc motor, the torque drops off pretty fast and the resistance from what you’re cutting can quickly slow the motor down to the point that the bit is unable to cut fast enough to handle even a slow feedrate. So, unless you’re running the router at a fairly high RPM, this could also be a culprit. A spindle with VFD handles lower speeds much much better in this regard

Usual preface: I’m with PreciseBits, so while I try to only post general information take everything I say with the understanding that I have a bias.

It’s almost certainly a bad or wrong collet. Barring edge cases like a large amount of runout on a very long tool breaking at the collet face is in our experience either a damaged collet or the wrong size. In both cases it makes the collet grip the shank of the tool at a single point and “wiggle” at the face. Once there’s enough force it snaps. Not a 100% accurate test but take the collet out of the nut and try to put a tool in it. If it doesn’t slip in with almost no force then it’s either damaged, very poorly made, or the wrong size.

It’s a guess at best for how slow you’re running as feeds and speeds are just trying to get to chipload and surface speed. So at a minimum need to know the RPM and flute count. As an example if you are cutting at 10KRPM at 40IPM with a single flute cutter that works out to a 0.004" chipload. Very conservative but not to the point of burning up a tool in most materials. On the other hand if we are running at 21KRPM at 40IPM with a 3 flute cutter you are only running a chipload of 0.0006" and almost certainly rubbing.

The above being said, if you had a LOT of runout on a collet or router you could break the bit as runout adds to the chipload in plunge and in multi-flute tool in feed. e.g. if running at 20KRPM with 0.002" runout on a 2 flute cutter you would be adding ~80IPM of feed in the worst case. If there’s a LOT of runout in your collet a simple test is to load a tool into the spindle and turn the router on and off. Watch the tool spinning and if you can see it wobble you have probably at least a few thou of runout. If it’s really obviously wobbling then a lot more.

One other thing to check for. If these are ER20 collets (I’ve never been able to confirm this for the Tritons) the collet needs to be snapped into the nut before mounting in the router. Failure to do this will keep the collet from seating in the angle ground in the nut and prevent proper seating of the collet.

Hope that’s useful. Let me know if there’s something I can help with.


I tip my hat to you sir, once again an excellent and thorough response!

As usual, I think you’re right. I polished/burnished the inner lip of the collet, and haven’t had issues since. Thanks everyone for the advice!

I had one a couple weeks ago that couldn’t hang on to a half inch end mill. I looked inside and it had a couple of burs. Not sure how they got there, but I tossed it and got a new one. Problem solved.

I just lost a 0.25" bit (broke at the collet). It had worked its way out a little before it broke. The collet had some odd wear spots and so did the little flanges on the inside of the nut (ER20 collet and nut). I replaced both of them and suddenly the machine was running a lot quieter. Apparently it had been slowly getting worse and I hadn’t noticed.