are round tenons bad

[Dave Macy]

Making a stool for the lady down the street. Are round tenons bad? I imagine that over time the pieces could rotate since there's no edge like in a mortise and tenon. But this is what they use for chairs right? If I have a 1"x1" square, would I use a 3/4" tenon to keep as much strength as I can? Thanks

[Doug Garson]

By round tenon do you mean dowel?

[David Eisenhauer]

3/4" round tenons should be strong enough to resist breaking, but, yes, can rotate at some point down the road if the glue joint breaks down. Rotating will eventually result in falling out. There are tons of round legged stools out there that last a long time without failure, though, and I guess it depends on the amount/severity of use. But now I am rethinking the whole thing and wonder if a heavy person were to use the stool (with 3/4" tenons/legs) on a frequent basis if the legs would in fact hold up. Don't most round-legged stools seem to have bigger diameter legs than 3/4? Aren't lots of stools made with the full diameter of the leg (larger than 1" diameter?) attached to the seat via through mortise?

[Prashun Patel]

On stools with round tenons, the rotation is stopped by the stretchers. Windsor chair construction uses round tenons. You can wedge the tenon or (more difficult) taper the mortise and tenon to produce a very strong, durable joint.

Windsor chair style seat construction is known to be extremely durable.

[Andy Giddings]

I would have thought that round tenons from an engineering perspective are stronger than tenons with sharp corners. Fewer stress points that may result in failure. Of course the rotation issue that Prashun mentions is there but easy to stop

[Prashun Patel]

The reason (at least in theory; I have not tested, and have heard some published tests were flawed) that round tenons are inferior to square-sided ones is glue area. More face grain is available on square faces than round tenons which have only two points of face-grain contact, the rest varying degrees of mitered connection.

[glenn bradley]

Engineering discussions of round and squared material and their appropriateness for certain purposes can go on longer than a Saw Stop thread ;-) Prashun has you answer; round tenons are fine but, use stretchers. You could also pin them from the side but, this weakens rather than strengthens a well made joint.

[Robert Engel]

No expert, but I have repaired many chairs literally coming apart due to failed tenons, and most do it even with stretchers. So I don't think the problem is rotation forces, I think its simply glue failure which IMO is due to the fact most the joint is endgrain glueing.

With someone sitting in the chair, the forces on the tenon are tension forces due to the legs wanting to splay which essentially pulls the tenon out of joint.

I have also found paint or finish on the tenon from the factory which needs to be scraped off.

I epoxy (never wood glue) and pin the tenon from the bottom. I think this is essential, especially in a stool which has no stretchers.

Pin I use either a screw or 1/4" dowel and be sure you go all the way through the tenon into the wood above. If you do this I guarantee it be permanent. Be sure you have everything clamped before you pin.

[Prashun Patel]

Robert, I'm talking out of school here because I've only read a book on Windsor chair construction, and have not built one myself. However, I believe this is precisely why those chairs taper the tenons and mortises. A tapered tenon - not a wedged tenon - actually seats more securely into it's socket every time the user sits on it. The splay angle may cause slight torque on the leg, but it's being driven deeper into the tapered socket. While counter-intuitive to me, I see that this technique works very well on machined parts like the lathe tail stocks.

In a windsor construction, the stretcher is not holiding the legs together, but rather driving them apart by virtue of being a little over-long. Every time you sit in the chair, the tenons are reinforced back into the seat, which keeps the assembly from failing. If the stretcher were shorter and the seat tenons normally round, then once the seat tenons loosened, they would release the stretcher. Conversely if you used an oversized stretcher without tapered seat mortises, the stretcher force would act to weaken the seat joint. So the taper, and compression stretcher seem to need to be used together to make a solid joint in the absence of strong glues or epoxies or pins.

[pat warner]

Typically round tenons are tapered, acting as wedges.
Morse tapers are wedges and compression only adds to their security.
Try to pull the chuck of your drill press x hand.
On compression it wedges x 360°, a formidable connection.

[Brian Holcombe]

They can also be made to go through a joint and be wedged.

I just replaced a set of 'round floating tenons' otherwise known as dowels on a restoration with more appropriate square non-floating tenons. The square tenons offer long grain glue surface on two sides, much more than that offered by dowels.

Square is strong for wood because you have grain, and can take advantage of characteristics of flat grain or edge grain to improve resistance against certain forces. For instance, a rectangular axe handle provides face grain on it's sides to resist shear forces.

[Mike Henderson]

For a round tenon, effectively half of the surface area is long-grain-to-long-grain surface area. So, taking a 3/4" round tenon, 1 inch long (to make the computations easy), the total surface area is 2.36 square inches. Half of that is long grain or 1.18 square inches.

[The formula for area of a cylinder (excluding the top and bottom) is 2pi*r*h or pi*d*h. I made h=1 to make the computations easy.]

For a square tenon with each side 3/4", we have a total surface area of 3 square inches, but only 1.5 square inches are long-grain-to-long-grain surface area.

So the 3/4" square tenon provides more long-grain-to-long-grain glue surface area than a 3/4"round tenon. And strength of a M&T joint is determined by the long-grain-to-long-grain surface area.

The difference is usually more striking when comparing a rectangular tenon to multiple dowels of the same width as the rectangular tenon.

Mike

[In this case, the round tenon is tapered, and gains from pressure into the joint, but the square tenon can be tapered, also. It's usually easier to make a tapered round mortise and a tapered round tenon than the equivalent square M&T.]

[Joe Bradshaw]

I have not done any chairs, but, am working on a straight chair now. I have made a lot of stools and I use a tapered tenon on my stools. I use a bridge reamer to shape the holes in the seat and turn the taper on the legs. I use a jig to get my taper right. On stools 20" and lower, you don't need stretchers.
Joe

[lowell holmes]

Joe,

Did you get an answer to your question? :)

[Ben Rivel]

Joe,

Did you get an answer to your question? :)Ha ha! Yea really!
I love threads like this though as this kind of discussion really cuts down to the foundation of a topic and helps me get the best understanding of why something is the way it is and how it works.