Bit Trouble

[zorlack]

Hey all,

I had a bit of a bad day today. In the space of an hour I snapped two (http://www.flickr.com/photos/zorlack/2971025464/) 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.


6807

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 (http://www.flickr.com/photos/zorlack/2971024652/) 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 (http://www.centuriontools.com) and i promise they won't break.

[gerryv]

On my metal lathe and mill I can check runout to see if it's the cause but I'm too new to wood routing to know if runout can be checked on a router or spindle. Maybe someone else knows?

[chodges]

Cutout a small (~3" diameter) circle, then look at the edges of the material. If you have any runout, the edges will look "choppy" instead of a smooth arc.

Also, with your control box ON, grab each end of your gantry and see if you can move it at all. If you can, check the set screws on the gears the fit on the motor shafts (ours has 2 screws per gear). Our set screws were loose recently, and we had a lot of the same symptoms.

[scottcox]

You haven't mentioned your hold-down method. It could be that your material is flexing too much and vibrating itself back into the bit. This happened to me with 1" PVC. Something I didn't expect.

I added some screws to hold the material better near the area where the bit broke and finished the cut with an identical bit, same speed and feed.

Worth looking at. Good luck.

[zorlack]

Thanks for the feedback everyone!

I didn't do the circular cut, but I did try to "wiggle" the x-car with the controller on. The x-car wouldn't budge. So that's a good thing. I guess that means the problem is somewhere closer to the bit. IE: bearings, collet, bit or holddown.

Scott, you may be on to something. Both times I was cutting 3/4 AC ply I think I was holding down the whole sheet with two or three screws, so its certainly possible that the material was bowing as it cut through...

Is there an shop manual for replacing the router bearings? Are there different types of bearings?

Thanks everyone!

-D

[blackhawk]

Dave - Here is a link to a discussion on router bearings. You can buy them from MSC.

http://www.talkshopbot.com/forum/messages/26/34075.html

[mitch_prest]

is it possible that there are some bad bits from onsrud.. i had a couple of new bits.. right from the original set from shopbot snap on me. One of them was fairly long and still straight, so i chucked it up to try it one more time and it snapped again. both times clean breaks ...

mitch

[david_white]

Is the collet "snapped"into the nut? If not you will have the problem you are describing.

Dave

[mitch_prest]

collet is snapped in.. and I had been using dozens of other bits with no trouble till i tried one of the bits that came with the machine..

mitch

[sneakers]

I have had the same problem. 2 onsrud bits that came in the starter kit snapped the first time I used them. I have cut many other things including cabinets, signs and carvings with bits from centuriontools.com with no breakage and the same settings in the tool paths.
Gary

[nschlee]

Carbide at times, has a tendency to break easily for seamingly no reason. This is why most bit mfgrs. DO NOT guaranty them. Just collet up another and hope it's a good one! :-) If you break another then I'd start looking at collets, excessive run-out or other reasons.

[gene]

I dito Gary. I too had some of the starter bits to do the same thing . I have yet to break one of Fred's bits . The Centurion bits seem to hold their edge a long time also. Try one and you will buy more.

[woodworx]

I cut some of this Exterior sheeting plywood today. I can tell already the reason for your bits breaking is because of your hold down techniques. These sheets move too much when you are cutting them. The sheets bow, which make it a bad choice for routing. The sheets I nested today had a lot of cuts close to the end of the sheet and left small strips of material that the vacuum wouldn't hold. When the bit was slapping those strips, I did think the bit would break. You are using an upspiral bit so you probably don't have a lot of chips between your cuts, which will leave your parts with a lot of room to move. I would try to find a more suitable material to cut from. You can get sheets of Oak at Home Depot for $40. Exterior sheets goods are like $25. Spend the extra $15 and get a good substrate. It's not good to have your small/thin parts slap your bit while cutting, especially if the machine is older and doesn't cut as smooth as it once did.

[centurion]

There are a couple of things to look for when breaking bits. First thing is the collet, to check TIR place a solid blank in the collet and use an indicator to check TIR near the collet, and at the end of the blank. Second, if your part moves while cutting, this will generally cause the bit to snap, next thing is the length of the cutter, if your are cutting at the very end of the cutter, this can cause a lot of side pressure, and if collet and bearings are bad snap goes the cutter. Chip load is also important, as stated by someone earlier, you need chips coming out, not dust, if you see dust, you are creating a lot of heat, this can cause the cutter to break, and fires can also be created! don't know if this helps, just wanted to put in my 2 cents

Fred

[pfulghum]

Just went through this thread...
One thing not mentioned is how much of the bit is exposed?

Cutting 3/4" ply use a 2 flute spiral with 1" cutlength and only expose that 1".

(I always use downcut).

-- Pat

[zorlack]

Thanks to everyone who gave feedback in this thread. After breaking three Onsrud bits in a row I couldn't take it anymore. I bough a new collet and a couple of Bosch bits and haven't had a problem since.

I haven't really changed my machining strategy (in terms of speed or hold-down), so I'm guessing it was either bad bits or a _very_ bad collet. I'm guessing that a weak collet combined with sloppy hold-down was what was causing my problems.

-D

[beacon14]

To be fair you might try another Onsrud bit in the new collet, my guess is it will perform quite well. That way you can verify that it was the collet causing the problem.