Somewhere in my learning journey I recall someone mentioning the size of a tenon in relation to bowl size when turning green wood. I have always gone with the bigger the better especially when coring. Is there a rule of thumb?
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Tenon Size
- Thread starter Don Penny
- Start date
It varies a lot but I’ve heard a tenon about 1/3 the diameter is a good rule of thumb. Definitely bigger is better when coring but you don’t want a tenon so big it limits the shape of the outer bowl either.
It can vary a lot, but I would consider 1/4 to be a bit minimal, 1/3 to be pretty good, and 1/2 to be overkill. Depends on the wood some what, and on your turning skills as well. One big catch can peel a 1/4 diameter one right off the chuck. Don't think that is possible with the 1/2 diameter tenon.
robo hippy
robo hippy
Using a ratio is an oversimplification. 3” diameter bowl with a 1:2 ratio for a tenon: I have seen many a catches take a piece airborne with a 1 1/2” tenon. Even with a 3” tenon.
Take a deep 12” bowl of 6” deep vs 12” that is 3” deep. Density/weight of wood also may impact a safe tenon size selection. All in all, I think “bigger is better” when your inexperienced or unsure of the physics.
Take a deep 12” bowl of 6” deep vs 12” that is 3” deep. Density/weight of wood also may impact a safe tenon size selection. All in all, I think “bigger is better” when your inexperienced or unsure of the physics.
I agree bigger is safer. Just keep in mind you don’t have to treat the tenon as waste. It is possible to have your outside curve extend into the tenon. Good way, with a little planning, to save a half inch or more of height/depth.
Tenon size I use is 40% of diameter. Very important is that the face of the chuck jaws have total contact with the bottom of the bowl.
There are 2 different aspects to tenons, just as there are 2 different aspects to turnings - diameter and depth.
When work starts extending further from the jaws, a larger tenon can help, but a longer tenon can help also. HF’s such as vases with L/D > ~1 or so I use tower type jaws with ~ 1” tenon length. These are typically endgrain and are not likely to break across the grain like face grain, but are more apt to vibrate or pull out, which the longer tenon resists.
As to large tenon dia limiting the piece design, it can can be accommodated. There are different approaches. Many times I start with a large tenon and end with a much smaller one, cutting the piece down in stages.
When work starts extending further from the jaws, a larger tenon can help, but a longer tenon can help also. HF’s such as vases with L/D > ~1 or so I use tower type jaws with ~ 1” tenon length. These are typically endgrain and are not likely to break across the grain like face grain, but are more apt to vibrate or pull out, which the longer tenon resists.
As to large tenon dia limiting the piece design, it can can be accommodated. There are different approaches. Many times I start with a large tenon and end with a much smaller one, cutting the piece down in stages.
I think it's important to match the tenon size to your chuck jaws. If you open smaller jaws to grab a large tenon, you are only gripping by the corners of the jaws. The points dig in and like wedge can weaken your tenon. If the jaws are nearly closed, you have full contact all the way around. Same with a mortise, size it so the fully closed jaws just barely fit inside.
I like the idea of a simple ratio, but it seems pretty meaningless. It feels like it all goes out the window when you start thinking about hollow forms.
I like the idea of a simple ratio, but it seems pretty meaningless. It feels like it all goes out the window when you start thinking about hollow forms.
I use a 50 mm on smaller bowls and 100 mm on larger bowls. We had a master turner friend at our club that recently passed away that turned everything with a 50 mm tenon and never had a problem.
I was working on a 6”x3” cherry bowl blank today. Outside all done. Reversed it onto a tenon for 50mm jaws. Tenon broke off cleanly when I started first cut on interior at center of bowl. Almost looked like I had parted it off. Not sure why. I have cut much bigger bowls on this size tenon. RPM was low, so it fell on floor. Cut a new tenon for the 100mm jaws and finished it.
My guess is that was an internal flaw in the wood just coincidentally at the base of your tenon.
You can overtighten. Same with a recess. What that does is put a lot of stress on the tenon/recess. If there is the slightest catch, the extra stress is relieved by cracking, same as with drying the wood. With a 6 inch bowl, I would use a 2, probably 2 1/2 inch diameter tenon. On a bowl that size, when using a recess, my recess would be 1 5/8 inch diameter, and a shoulder of about 1/2 inch.
robo hippy
robo hippy
Glenn Lucas recommends 1/3 the diameter. That is why Vicmarc chucks are great. They have jaws up to 9-inches in diameter.
Rough turned this Box Elder piece yesterday. The shallow bowl is about 17 inches across, and the shallow tenon is 7 inches.
I can continue my curve (sight) right through the corners of the tenon, and just about touch the bottom. My intent is to either make it a round bottom, or pull the curve in just a bit to make a footed bowl. In either instance the eventual bottom will be within about 1/8 to 1/4 from the chainsaw cut.
Why give up another 1/4 or 1/2 inch to overhead, strategizing how small of a knob will keep the piece on the lathe?
I can continue my curve (sight) right through the corners of the tenon, and just about touch the bottom. My intent is to either make it a round bottom, or pull the curve in just a bit to make a footed bowl. In either instance the eventual bottom will be within about 1/8 to 1/4 from the chainsaw cut.
Why give up another 1/4 or 1/2 inch to overhead, strategizing how small of a knob will keep the piece on the lathe?