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.
Chris
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.
Chris