Bit Trouble

[zorlack]

Hey all,

I had a bit of a bad day today. In the space of an hour I snapped two brand-new 1/4" Onsrud 2-flute upcut spirals.

I'm still a beginner at this, and I don't quite have a handle on the whole chipload math, but I thought I was being quite conservative.


35291.jpg

Here's the stats:
Bit: 1/4" 2-flute upcut spiral. Solid Carbide. (Onsrud 52-285)
Material: 3/4" AC Plywood
Router: PC Router at 21,000 RPM
Cut Depth: 1/3"
Feed Rate: 1.5 Inches Per Second

When the first bit broke I decided that it was user error - maybe it wasn't deep enough in the collet. In both cases the collet was plenty tight.

I also inspected the material pretty well. It's plywood, so if there was metal in it, I expect that I'd see it somewhere near where the bit snapped.

At the moment I'm not sure what to make of this. I'm tempted to say "two bits from the same batch may be a defect". What else might it be?

I have to go in tomorrow and make some more cuts, and I'm pretty nervous about using the other two bits that came in at the same time.

Thanks!

-Dave

[woodworx]

Run those parts tomorrow! You can slow your RPM's down to 10-12K, and set your plunging to no more than a bit's diameter. In this case .25". Feed rate is ok, but it could be sped up. I think you got them too hot by running such high rpm's, and running the bit through the material so slow. Just make sure they are healthy large chips coming out of the cut. Don't be scared of breaking bits yet, at least you were running the bot. You will never forgive yourself if you slide a new piece of material on the bed and realize your bit is too low....SNAP! This is what keeps me up at night! LOL!

[pfulghum]

Dave,
Can you post (or email me) your source file and cut file I can take a look to see if there is any wierdness going on.

What did you use to generate the toolpaths?
-- pat

[zorlack]

Unfortunately I don't have the files with me. The paths were incredibly simple, an 8' long gentle curve. And the blades didn't break during plunge operations, they broke during long stretches of boring cutting. I wrote the cutfiles with Artcam.

Also, I'm using the default ramping values - for whatever that's worth. The two blades broke in different parts of the material as well (about 4 feet away). The blades were hot to the touch, but not so hot that I couldn't hold them lightly.

-D

[beacon14]

If you cranked down on the collet with a bit inserted only partway you may have damaged the collet. How much use has the collet had? It might be worth at least inspecting if not replacing.

[harryball]

Check the collet and probably just replace it, I always replace them when I suspect.

Make sure the bit is properly inserted, not too far or too shallow.

Slow down your RPMS or pick up your cut speed, it sounds like you could easily be toasting the bits. Immediately after cutting the bit should be room temp to slightly warm. Hard to believe but I use an infrared thermometer and on one of my jobs I was toasting a 3/8" roughing bit, reaching 130 degrees as it pulled out at the end of the cut. I changed to 10k RPM and 6 ips and the temp is now 85 degrees when it finishes the same job. BTW, I started researching that job because I was breaking bits. Since making the adjustment, I haven't broken another.

I doubt it's a bit defect, Onsrud is pretty good with QC.

Also, look at your router good. If it's new you should be good but if it is older it may be time for new bearings. Normally you'll hear it or it will be seen as chatter in the cut quality. But I wouldn't rule it out until you rule it out.

Keep us posted.

/RB

[zorlack]

Well,

I took your collective advice. Turned the RPMs way down and picked up the the cut speed. I didn't snap any bits... so that's something.

I'm thinking I'm looking at a couple of problems. First of all, I think there may be something wrong with the collet. Last week a bit got stuck in some wood and got yanked out of the collet. Additionally my router bearings are almost certainly shot. I'm getting a high-pitched squeal whenever the bit touches wood.

Do people generally keep backup collets? I hadn't really even considered it until now.

How involved is the process of replacing the PC router's bearings? I'm thinking about waiting until I finish my current project to do it (for time and budget reasons). Are there different types of bearings?

-D

[zorlack]

Spoke too soon.

Last night my 3rd new bit in a row snapped in half on me. I've now ordered a two new collets and three more 1/4" end mills (from a different mfg).

Last night I was cutting at 12K RPMS, 3IPS and .25" cut depth. Oye.

The bit broke just on the inside of the collet. At the first place that the collet grips. The broken bit was only warm to the touch immediately after the break.

I'm going to replace the collet tomorrow, but in the mean time, I wonder if I'm over-tightening. Is that even possible?

At the moment I'm leaning towards it being a bad collet, but I don't really have any good reason. There's no play at all in the router bearings, so even though they're on the way out, I don't see them causing this.

Urrrg.

-D

[woodworx]

slow your speed to 2ips. Yes collets and bearings do wear out. Replace and try again. You are on the right track, just keep going. JR

[myxpykalix]

Dave,
clearly part of your problem is your bits. You should not have so many bits break regardless of speed (within reason).
I once had a bad toolpath and the bit went down thru 3/4 ply kept spinning and finally the fuse inside the control box blew but the 1/4 endmill never bent, broke, or dulled. I was surprised due the the amount of torque exerted to blow the fuse. I recommend you buy some bits from Fred at
www.centuriontools.com and i promise they won't break.