Pushing a router bit to the breaking point

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If your bit was solid carbide you were using the wrong bit, I have found that solid carbide bits tend to break/shatter when you get vibration, over torque on bit or heat build up. The bits are too brittle to handle much stress. Craftsman sometimes seems to have bits with case hardened shafts which act like solid carbide bits and break easily, any stress on the bit and you have a two piece bit. To do what you were doing I would use a HSS bit which seems to take stress better then carbide bits or bits with carbide cutters. You will change bits often though. Your best bet is as stated in other post is to use multiple cuts to reduce stress on the bit. If you wish to make only one router cut you could hog out the slot with a drill bit and use the router for clean up but I still believe that a decent quality bit with carbide cutters and multiple cuts would be the best way to go.
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On 5/11/2009 6:49 AM sweet sawdust spake thus:

>

Not a solid carbide bit, but a seemingly well-made standard (brazed?) bit with steel shank.
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Solid Carbide bits are wonderful when they work and well worth the money, but when they get stressed they aren't worth a dime. I have never dulled one but have broken several and the ones I have left just set in their boxes. I know that craftsman does not always get the best in a lot of items and their router bits seem to be/have been one of the areas of concern. Of course like Harbor Freight its hard to say what they will have in quality from day to day. With a good bit properly made you should be able to make the cut you were describing with out problems. However if anything goes wrong your are in trouble, and it sounded like something went "wrong" and will probably never be able to figure out what it was nor will anyone else and it will probably never occur again, the same way anyway. That is why the multiple cuts for each slot, if something goes wrong it is usually so minor that you never notice it, where as in a deep cut the same problem will be devestating, as you noticed.
It sounds like you are cutting free hand with the router rather then using a table, this can make slot cutting tricky, You might try making a slot cutting jig for the job. it will give you much better repeatability and accuracy especially when making multiple cuts needed to do a deep slot. Good Luck and have fun
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On 5/11/2009 12:11 PM sweet sawdust spake thus:

Nope, using my trusty old homemade router table. Makes great cuts with reliable repeatability. (I'd never attempt anything like this "freehand".)
I was just pushing the bit beyond its limits was the thing.
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I have found that end mill bits work well, last much longer, and are less expensive.
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I was using a 1/2" diameter carbide bit to cut some 7/16" OSB. While it did not break off, there was quite a bit of smokin going on.
I went to Lowes and found a similar bit with three flutes and thought that might have helped.
I needed the bearing on the bottom of the bit so a spiral up/down bit would not have helped.
But a 1/2" diameter shank bit would likely do better than a 1/4"
I was cutting out the waste siding for a window opening. For the second window, I used a jig saw to rough cut then trimmed it up with the router bit.
Like the man said, it for trimming, not hogging out!
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