Technical question on Tightening a Tenon for Bowls-

[Glenn Torbert]

Is there a mechanical advantage for tightening your chuck tenon in vs. out for a bowl?
I have just started turning larger stock and I need maximum holding power. The jaws on most all of my chucks have the ability to expand them out in the tenon cavity or tighten them?

[Scott Hackler]

Glenn, if you need "maximum holding power" you need a faceplate! BUT, if its a choice between a tenon or expansion mode in a dovetail recess... the expansion is going to be stronger. Keep in mind that the chucks true holding power is in a PROPERLY sized tenon or recess. The jaws should be almost completely closed in BOTH methods. If you need a larger tenon or recess....... you will need a larger set of jaws for the chuck.

[John Beaver]

The biggest risk in tightening out is the possibility of splitting the bowl with the outward stress. With a tenon you can usually tighten as much as possible.

[David E Keller]

I prefer a tenon because I can blend it into the finished form if I plan correctly... That gives me the best yield from a piece of wood. I like to turn stuff pretty thin sometimes, and I worry about a recess cracking from excessive expansion. I think jaw size is probably more important than tenon vs. recess.

[Jim Burr]

I to prefer a tenon, it's easier to create a tenon closer to the size of a foot, within limits and you don't run the risk of a crack from the expansion.

[Michael Mills]

I don’t know much about a mechanical advantage but I normally turn a recess and use expansion. That said, I try to have 1” of wood outside of the recess. I do use a tenon for most small items. I think the best approach in either method is a properly sized and formed tenon/recess with a good snugged up fit and not overpower in either direction.
A 2” jaws with tenon will have 3.14” surface area of wood to break.
A 2” jaw in a recess with 1” outside will have about 9.25” surface area to break.
For a 4” jaw – 12.5” on a spigot, 16” using a recess.

[Scott Hackler]

I guess I should have added that I use a tenon 99% of the time and unless I am going over a 12" bowl or such, I only use the 50mm (2") jaws. I like to have either a smooth bottom or concave. With a recess you need either to have planned to remove a BUNCH of material for the smooth bottom OR incorporate the recess into the bottom of the bowl. And while I do believe that ultimately the recess to be a stronger "grip" I prefer the tenon.

[Glenn Torbert]

Thanks for the opinions, I probably should have stated it as a recessed dovetail rather than a tenon, there seems to be alot of opinions on this one.......GT

[Reed Gray]

Personally, I don't think there is any mechanical advantage in either method, tenon/spigot or mortice/recess. The wood is under compression either way, and it doesn't make any difference if the wood is being pushed out from the inside (recess), or in from the outside (tenon). The most important thing is making the attachment properly. This means angles should match as closely as possible, size/diameter should be close to the size of the chuck jaws so when you close, there is little gap between the jaws, and your chuck face should not be in contact with the wood. Dove tailed jaws will give a mechanical advantage compared to straight sides. The dove tail/wedged joint has been used in woodworking since the beginning, for a good reason. I regularly turn and core 16 inch bowls with my big Vicmark chuck, with a recess, and the jaw diameter is 2 5/8 inches. I have been doing that for years.

I don't think one is easier to make than the other, it is more of what you are used to. A specialized dove tail scraper makes easy work out of forming that part of the attachment. Measuring for proper diameter is as simple as a dedicated set of dividers, or marking stick.

I have never split a bowl from using a recess. I have broken off the plastic handle on all of my chuck keys, and replaced them with a more heavy duty key from the local hardware store. When lining up the chuck on the recess/tenon, I think it makes a difference, especially if the wood is green, to line up the chuck jaws and wood grain. You do not want 2 jaws on the end grain, and 2 on the side grain. Main reason, especially on green wood is that the end grain will compress very little, and the side grain will compress more, so the attachment forces are not even. Rotate your jaws about 45 degrees to the end grain.

Having enough shoulder outside of the recess to support the forces is important. 1 inch on a 12 inch bowl is fairly good. You can get away with less, but then have to take care when turning as any catch, or heavy roughing can break the shoulder. This is the same thing that can happen if you use a 2 inch tenon on a 12 inch bowl. Kind of minimal, but it can work.

You can over tighten. The trick is to get it snug, but not 'armstrong' tight. You can blow out a recess if you over tighten, just like you can crush fibers on a tenon. You can also over tighten to the point where the attachment is under a lot of stress, and if you have any catch at all, the joint will fail because that tiny bit of extra force is just too much. You want it snug. I prefer keyed chucks. Snug it up in one key, rotate the bowl, and snug it up in the other key. On bigger pieces, I will rotate and snug it up a couple of times.

I would NEVER use a recess on end grain pieces. That could lead to splitting as it will exert the same type of pressure that an axe would. Tenon only for that.

I have never understood the claim that you save wood depth with a tenon compared to a recess. Most of the time, depth is 1/8 to 1/4 inch max. You do need to trim up a tenon to make the base look good. I just leave the recess in place, sand it out and finish it. If you turn off a 3/16 inch recess, you lose no more wood than you do if you turn off a 3/16 inch tenon. I guess if you are really trying to save wood, you should use a waste block.

robo hippy

[David E Keller]

I have never understood the claim that you save wood depth with a tenon compared to a recess. Most of the time, depth is 1/8 to 1/4 inch max. You do need to trim up a tenon to make the base look good. I just leave the recess in place, sand it out and finish it. If you turn off a 3/16 inch recess, you lose no more wood than you do if you turn off a 3/16 inch tenon. I guess if you are really trying to save wood, you should use a waste block.

robo hippy

I turn quite a few little forms with very small bottoms... I turn away the mark from the tail center, but I keep a portion of the wood from the tenon in the foot. You're right about the glue block though.