Roger Wiegand
Beta Tester
Last night I attended another online remote demo, one of many I've been able to do in the last couple of months. The presenter made some fairly dogmatic assertions about tenon size, echoing comments made in other, earlier presentations I've heard. The assertion, in this case presented as an ironbound rule, was that a tenon must be 40% of the diameter of the workpiece. Interestingly, in all three case where I've heard this put forward recently, the justification for the rule was "as Dale Nish told us". With all respect to Mr. Nish, what are the data?
From a practical point of view most folks don't own a chuck large enough to make that happen for bowls bigger than 8-10", and in most cases that gives you a base way too big for the piece, something that you then need to deal with while reverse turning, generally a more fraught situation with regard to holding on to the workpiece.
In practice it seems to be a rule that few seem to actually follow. Since I didn't know the rule I certainly haven't followed it, turning bowls up to 20" on a 3-4" tenon. I've made maybe 100 large bowls in the last two years and I did have a broken tenon on one; that was a flawed piece of wood that I should never have put on the lathe. I also had one spectacular failure with a piece securely screwed to a large faceplate that came apart into three pieces due to a hidden flaw in the wood. Watching many demonstrations recently none of the demonstrators working on larger pieces made tenons that large.
My limited experience suggests that a tenon in the ~20-30% size range seems to work fine. Am I tempting fate in some foolhardy way? What data led to the 40% value?
From a practical point of view most folks don't own a chuck large enough to make that happen for bowls bigger than 8-10", and in most cases that gives you a base way too big for the piece, something that you then need to deal with while reverse turning, generally a more fraught situation with regard to holding on to the workpiece.
In practice it seems to be a rule that few seem to actually follow. Since I didn't know the rule I certainly haven't followed it, turning bowls up to 20" on a 3-4" tenon. I've made maybe 100 large bowls in the last two years and I did have a broken tenon on one; that was a flawed piece of wood that I should never have put on the lathe. I also had one spectacular failure with a piece securely screwed to a large faceplate that came apart into three pieces due to a hidden flaw in the wood. Watching many demonstrations recently none of the demonstrators working on larger pieces made tenons that large.
My limited experience suggests that a tenon in the ~20-30% size range seems to work fine. Am I tempting fate in some foolhardy way? What data led to the 40% value?