Broken bit woes

[sawkerf]

Today was a bad day to be a compression bit at my shop. Broke three brand new ones and two slightly used ones. I haven't changed a thing in my cutting strategy. I do wonder if you guys using the MK router have had this happen. Just after I got my MK I had problems with bits slipping out and gouging my plenum. Now the problem with breaking new bits is gonna break me. These things are $20 bucks a piece. Anyway, I noticed the bottom of the collet was a little rough around the opening so I switched to a new collet, bad thing is, I don't have another bit to test my theory of the bad collet. Any of you guys had this happen?

[patricktoomey]

Kip, I use a spindle but the only similar thing that I've had happen did end up being a bad collet. I lost a few 1/16" bits way earlier than usual, changing the collet got it back to normal.

[paco]

I have the MK and never have this happen to me.

My first check would be with a new collet and nut assembly. If it's still slip and if you are sure you are tightening the bit properly, get in touch with MK support to see if they would investigated the chuck (which they call the collet shank). This part was very affordable when I looked for spare on the MK Web site ( LINK (http://www.milwaukeeconnect.com/webapp/wcs/stores/servlet/PartsPricingView?storeId=27&catalogId=40027&langId =-1&p1=02-04-0852&p2=02-04-2006&p3=22-18-0075&p4=45-10-0081&p5=&p6=&p7=&p8=&p9=&p10=)).

[jeffreymcgrew]

Sadly, this same problem has been hitting us badly. Went through 2 of them, all breaking in the same spot and getting hot. So we got a new collet, and it still broke a bit just two sheets of plywood later.

Are we driving these bits too hard? We're cutting 3/4" 13-ply Birch, with a 1/4" compression bit, using two rough passes that leave about an eighth inch thick 'skin' with a third last cleanup pass takes off. PC Router at 21,000 rpm. We're moving at 1.70-1.75 inches per second. Are we cutting too deep with each pass, and should only hold to 1/4" max depth? Are we moving too fast? Is the new collet we got not working, for the chuck itself in the PC router is 'bad'? How can you tell?

Thanks, we need to figure this out soon, in the middle of a big job,,,

[richards]

You're probably heating up the bit by spinning it way too fast (or by feeding it way too slow).

Are you familiar with the Chip Load Calculator in the tools menu of the Shopbot program? With a chip load of 0.015 to 0.020, you should be moving a 2-flute cutter at aboout 8-ips with a spindle speed in the 12,000 to 16,000 range. A 1-flute cutter could be moved at 4-ips at those same feed speeds.

[jeffreymcgrew]

ok, I was wondering about the rpm's on the router. With other bits, I was using the chip calculator, but the Amana compression bits say in the bit catalog "21,000 rpm" so I (probably stupidly) thought that was the required speed. Now that I think about it, that's probably the maximum speed.

I'll try running them slower RPM's and see how they do...

Thanks!

[paco]

Jeffrey,

there's "nothing" wrong at spinning the bit at 21,000 but you need to feed it some material to compensate this setting. Most operator prefer to spin it less with more conservative feed.

About the 1/4" CED compression, if that's a single flute, that might be your problem. Such small CED single flute compression does vibrate a lot under high load or into hard material. I would suggest to either try a 2 flutes or a larger CED if that your case.

Next keep the CEL as short as possible if you happen to have some choice. Too long CEL break more easily and cause more vibration.

[jeffreymcgrew]

These are two-flute bits we're breaking.

So it sounds like I need to:
A. slow down the RPM on the router
B. make the shopbot go slightly faster
C. not cut much more in depth than the CED.

We've got some bits on the way that are a 1/2" CED instead of the 1/4", however for this job there is a fair amount of detail work that will require the 1/4" bit.

Thanks for all the help, I should know by this afternoon or tomorrow if we solved the problem...

[patricktoomey]

I use 1/4" compression spirals for all my plywood cutting. I cut 3/4 prefinished Chinese birch in 3 passes (2 at .340 and the last pass takes off the last bit of skin) I generally run at 10,000 RPM at 3 inches per second, 12,000 RPM at 4 inches per second or 15,000 RPM at 6 inches per second. At those settings I get 40-50 sheets of cabinet parts out of one bit and it generally runs warm to the touch but not hot.

[jeffreymcgrew]

Patrick, thanks a ton! That's exactly the kind of info I'm looking for. We'll give it a try as soon as UPS delivers the bits (should be this afternoon) and I'll report back.

What a community! Shopbot rocks!

Thanks again

[jeffreymcgrew]

Hey just a follow-up.

So the Chip Load calculator is important. Heh. But anyways, by using it we got a better RPM to cutting speed and haven't had any more problems.

Now with the Gecko upgrades running the compression bits is fast and easy, and we're getting great results.

Thanks for the help everyone.