Router bit failure

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So there I was routing a 15" long 5/16 deep slot in some hard maple with a 1/4 inch bit on a 1/2 inch shank and 12 inches into the cut the bit sheared. This bit was one of those green Woodcraft bits that they put on sale from time to time. I was using a pretty slow feed rate, but my guess is that the bit failed due to heat. Any thoughts?
Larry
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wrote:

I go by the rule that the depth of the cut should never be more than the diameter of the cutter.
--
Jack Novak
Buffalo, NY - USA
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I take shallow passes too. But I also have to wonder if the bit was defective. How old was it.
This is one reason I transitioned over to 1/2" shank bits. I had a 1/4" bit fail several years ago and felt a little lucky I didn't get hurt.
RonB
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wrote:

How deep the cut? Any burning on the wood? Did the cutting edges fail or looked burned? Or did the bit shear off below the cutters? Was it sharp?
Inquiring minds want to know.
-Zz
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The cuts was 5/16 deep. No burning on the wood. Cutting edges still look good. Bit sheared off below the cutters. Factory sharp carbide with very little use up to now.
On 10/8/2011 5:32 PM, Zz Yzx wrote:

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On 10/8/11 8:12 PM, Gramp's shop wrote:

I say again, bad bit. Those $5 woodcraft closeout bits are suspect to begin with. Depends on the lot. From what I hear, they were having issues with material later in production and may have switched suppliers when they changed the name of their house brand to WoodRiver.
I still contend that 5/16" on a 1/4" bit is no sweat. With a high quality bit, 5/16" *is* a shallow pass. Only taking 1/2 diameter (1/8") is certainly safe, but also paranoid. Were your sucking out the sawdust?
--

-MIKE-

"Playing is not something I do at night, it's my function in life"
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wrote:

I agree, defective bit.
-Zz
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On 10/8/11 4:05 PM, Gramp's shop wrote:

Bad bit. 5/16" *is* a shallow pass.
--

-MIKE-

"Playing is not something I do at night, it's my function in life"
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On 10/8/2011 4:05 PM, Gramp's shop wrote:

Sounds like you might have been cutting too deep, especially in hard maple. I would have made it in 2 passes.
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I cut everything In two passes, minimum. I rarely hog out the material, _and_ finish the bottom, in the same pass.
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If the finish quality matters at all, always in more than one pass. Single pass in maple guarantees scorching.
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"Gramp's shop" wrote in message
So there I was routing a 15" long 5/16 deep slot in some hard maple with a 1/4 inch bit on a 1/2 inch shank and 12 inches into the cut the bit sheared. This bit was one of those green Woodcraft bits that they put on sale from time to time. I was using a pretty slow feed rate, but my guess is that the bit failed due to heat. Any thoughts? -------------------------------------------------------------------------------------------------
If it was cutting full diameter, that is the hardest cut a router bit can do. Depth should be half the diameter for a cut like that. A 1/4 inch cutter does not benefit by a 1/2 inch shank. The transition in cross section sets up a stress riser that negates the advantage of the larger shank.
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Having worked with a few routers and bits over the last 40 years, a 5/16 deep pass, blind (aka dado) for a 1/4" bit is not very much. BUT... to do such a cut properly, one should use an upcut spiral, because no matter what your're cutting, with whatever kind of tool, it is all about getting rid of the chips. The OP reported no burning, so I can assume that there was a nice balance of cutting speed and feed speed..and keeping in mind that Hard maple can be pretty damned hard...BUT there was no burning and yet the bit broke. The above information tells me the bit was defective/crap... but there is another possibility. Gramp... did the bit scream? ( like a bit can in hard maple)? I mean a high pitched scream? The bit could have been oscillating from side to side, whilst still cutting and not burning.. just whipping. That would be a high frequency sonic/vibration event that broke the bit. Where it broke would support that possibility. Then again, I could be talking shit.
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On 10/8/11 9:21 PM, Robatoy wrote:

You're always talking shit. Even when it's good shit, it's still shit. :-)
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Well I oughtta
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On 10/8/2011 9:21 PM, Robatoy wrote:

Well, I wouldn't call it a scream, but the tonal pitch certainly changed to a higher pitch. There was significant noise coming from me when I discovered the sumbitch sheared and my old replacement bit was pretty much close to dead. I bought a Bosch replacement at the big box, but I do have an upcut bit that I hadn't considered using. I'll try it tomorrow. Thanks!
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Gramp's shop wrote:

Lock it down tight. Those spiral bits can pull themselves out of the collet and all of a sudden you are cutting deeper than you want.
--

dadiOH
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Also, if possible use a featherboard. Usually when I break a bit it's because I didn't have the work's movement solidly constrained.
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wrote:

Also clean the bit shank and the inside of the collet with a little lacquer thinner, this will remove any oil film that can cause a bit to move out of the collet.
I have had a couple of 1/4 inch shank bits fail on me. They were cheap bits bought as a set from Canadian Tire, a source of cheap tools in Canada. I have learned to never push them, even a little.
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Good advice.
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