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Recently in our shop we were testing a new 1/2" Dia. 1/2" shank upspiral cutter in our router table.

We were trying a relatively deep cut in hard Oak - almost 1/2".

The bit came loose and up thru the stock. No injury - no damage - as we have always heard from Bob and Rick that Upspiral bits can go up and to keep fingers and hands far away.

The bit was very securely tightened - as tight as or tighter than our usual HSS shaft bits. We wondered why it came loose..

It seems the coefficient of thermal expansion of silicon carbide is anywhere from 1/4 to 1/3 that of most steels (from 'Materials Selector', Reinhold Pub. Penton/IPC).

So, if you take a deep cut, heating up the bit ((it was way too hot to touch), and also the router shaft, collet and collet nut, the collet and nut will enlarge more than the silicon carbide shaft - making it more likely the bit will come loose.

I've a number of 'router books' and can't remember this being explained anywhere, So I thought it might be worth mentioning.. (But I may be wrong? Comments?)

We learned:

1. REALLY REALLY tighten all solid carbide bits

2. Take shallow cuts and don't heat up the bit

3. Bob and Rick are to be commended for continually mentioning over and over NEVER to get your hands or fingers over a running upspiral bit - regardless of the stock thickness.

Any comments and/or further thoughts appreciated.


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Carbide bits do not require any special tightening. If you follow the 1/4" rule and take off no more than 1/4" in a single pass you should not have any problems. Multiple passes are always recommended; this reduces the load on your router, avoids overheating the bit, eliminates burning the wood and gives you a better finish cut. Perhaps the most common reason for a bit to come loose is it was seated all the way in the bottom of the collet. This does not allow the collet to tighten properly. An easy way to prevent this situation is to insert an 1/8" o-ring with a 1/2" OD into the collet before installing the bit. A thin fawcet washer can also be used. The rubber will compress and allow the collet to tighten properly and it gives you accurate installations of the bit every time.
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Hi Chris

"The bit was very securely tightened" = Can't be

The collet nut will not heat up more than the other steel around it.
That's the only way it would come free, it's like when you use a drill press,the chuck has 3 holes in the chuck to tighten it up on the bit most people think it's made so it makes it handy to lock the bit in place, Not true the chuck is made in 3 parts (jaws) that hold the bit and it should be tighten in all 3 holes, now when it comes to router chucks they have one split in the chuck collet the norm and a morse taper and all it takes is one good turn to lock it down all the way.

Bj :)
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