Advice needed - snapping cutting bits

I’ve managed to snap two bits in my Lowrider and I’m not quite sure what the issue is.

FWIW, it’s actually the one endmill bit I’ve broken twice, the first time I assumed it had just come loose and fallen out, there was enough shank remaining to reinsert it into the collet. Only when I went to remove it the second time did I find the piece of shank in the collet from the previous break. As I didn’t realise I’d broken it the first time I didn’t bother to play with any settings, just tightened the collet more.

Both times it appears to have broken flush with the collet, about 20-30mins into a job.

I’m cutting plywood, running at the minimum speeds recommended on the milling basics (about 8mm/s feed) with a depth of cut around 1.4mm. Not sure what RPM my router is running at (currently sourcing an optical tachometer), but on the box it says it can do 11,500-32,000 RPM and I’ve probably got it set to about 80% of maximum.

Not sure if I need to adjust the feeds or something else. Reading about cutting plywood it sounds like I can get away with a much faster feed rate, however in my mind this puts more lateral force on the cutting bit. Is it possible that I’ve over-tightened the collet?

Just wanted to get some advice with I wait for my replacement bits to turn up. Apologies if this isn’t the right place to post this, I appreciate that this issue probably isn’t strictly related to the Lowrider hardware.

I think you might be running the router too fast. I’d actually try to run the router close to the minimum speed. There’s a whole science on feedrate, router speed, and chip size… if you don’t move fast and turn the router bit fast, then you make saw dust rather than ‘chips’ and that results in a the router bit getting hot. This could lead to it breaking. The lowrider (and Maslow cnc with which I have experience) can’t run a fast enough feedrate to produce ‘optimum’ chip load at the router’s minimun bit speed. So I just run the router close to the low-end and go with it.

Plywood doesn’t work harden though. I’d be surprised if that is the cause.

The second time doesn’t count. It sounds like maybe the bit wasn’t completely in the first time. It should be almost touching the back of the collet.

FWIW, I’ve done a lot of stupid things with my CAM and I haven’t broken a bit during cutting. I’ve even cut through hold down screws…

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What bit are you using? How large, how many flutes and where did you get it?

Not sure how I should be positioning the bit in my setup. The router I’m using has a 1/4" collet which is almost 40mm deep. I have a 1/4" tp 1/8" step down collet, however it’s only about 23mm deep. Finally, the shank on my bit is about 21mm long. Do I just push everything as far back into the router as it will go, or do I need better matched gear?

I’m using a 1/8" 2 flute upcut endmill sourced from a local seller here.

The only broken bits I’ve had were the ones that came with the DW660, which I later realized were evidently not endmills but drill bits. But the bits you have look to be legit endmills. Did it break in exactly the same spot the second time? That might be additional evidence like Jeff said that there is something funny going on with CAM, maybe a sudden lurch. Did it make an unhappy sound before breaking? Did the workpiece show signs of abuse? Plunging for example can be awful and shake the whole thing violently and put huge loads on a bit. Not sure what else to suggest – that depth and speed should cut like a breeze with low loads on the tool.

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You want everything as far into the collets as possible. You also don’t want it to bottom out. I’m not sure how the adaptor works, but the adaptor being further out supporting the bit is more important than the collet adaptor being all the way in.

The load is about the same when you’re going 2x faster and 1/2 as deep. But the load is focused on the end of the bit if you’re doing shallower and faster cutting. Thus wears down that part of the bit a lot faster and then when it has dulled, it will put more torque on the bit. So maybe you’re going too fast and not deep enough.

Is there a lot of burning? I’ve had bits get hot enough to burn, and I’ve had toolpaths with enough load to skip steps, and still, not broken a bit. That’s anecdotal, but I suspect it is really something mechanical with the but holding, and not the CAM.

Edit: Except, that if you’re going very fast at the very tip, that is something that is CAM, I guess. I haven’t done that.

No, different spots both times. The first attempt got a little further into the job (maybe 35mins), second broke quicker, around 20mins. Best I can tell the machine was running fine up to that point, no lurching and motors sounding normal. I’ll admit I don’t think I’d be able to tell if the cutter itself sounds out of the ordinary, just sounds like machine tool cutting to me.

No burning at all, the cuts otherwise look ok.

Thanks for the suggestions. I’ll report back once I get some new cutting bits and have had a chance to give them a go.

New cutting bits arrived in the mail this week and I had a chance to test them out today.

I had everything inserted as far into the collet as possible and turned the router right down to it’s minimum setting, which turns out to be around 14,000RPM. Running a few test cuts at this setting was so much quieter than my previous set up (both from the router motor and the cutting bit itself), I’m sure my neighbours will be grateful.

Anyway, I settled on a feed rate of 15mm/s and cut depth of 1.5mm and ran the long job that was causing trouble before. The job completed successfully without issue!

I think my problems may have been related to heat buildup at the router when run at the high RPM setting. The whole thing (motor body and collet) would get quite warm even after a couple of minutes. Now running it on low for this job (1.5hrs) the router was barely warm at the end.

As usual, thanks for everyone’s assistance.

Interesting. I didn’t catch what router this is. DW611?

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No, I’m using an Ozito 850W router (see here) in a custom 3D printed mount.

Definitely not a “pro” tool, Ozito is a store brand for the main hardware store chain here in Australia. However the price was right, we’ll see how long it lasts.

I have found Adam’s Bits to provide both excellent bits and outstanding service. Good choice.

I was using the Ozito on speed 2 and the only time I’ve snapped a bit is when it came loose then caught. I found I had to jam a piece of plastic in to lock the speed control as after a while the vibration caused it to move.

I’ve always been told to not “bottom out” the bit in the collet, leave a little space. When things get hot they’ll expand and this can push the bit outwards, in effect making the tool longer part way through the cut.

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