I've posted the parallel question here:

http://www.mycncuk.com/forums/showth...5781#post15781

Quote Originally Posted by routercnc View Post
Twisting calculations were from Roark's formulas for stress and strain, an old University text book I have about 1" thick full of tables and equations. Excellent book, but heavy going in places. If you search the internet for 'twisting constant K', which is a bit like I values for bending, you might get somewhere.
Thanks, I'll look into it...