When are double tenons needed? (Old Dog trying to learn a lot of new tricks)

[David Castor]

For my workbench build, my legs and stretchers will both be about 5" x 2.75" (Douglas Fir). So I'm wondering if a 5" long (by maybe 1") tenon will cause issues due to expansion and contraction in the mortise. Is there some rule of thumb for when to switch to a double tenon? I've read several references on M&T joints and why double tenons are used, but not finding many specifics on when exactly they are needed.

Thanks,

Dave
Corvallis, Ore

[Cliff Polubinsky]

You know, I've tried to understand this and I can't seem to get my head around any of the explanations I've seen on how splitting a large tenon into 2 smaller ones helps with wood expansion. Whether you have one 5" wide tenon or two 1 1/2" tenons with a 2" space between them, the outside edges of the tenon(s) are going to move exactly the same. After all, you haven't split the rest of the board so that 5" of wood the tenon is attached to will still be expanding and contracting moving the edges of the tenons along with it. And with a split tenon you now have to worry about the movement of the inside edges as well. Can someone please explain how this is supposed to help?

Cliff

[David Castor]

My limited understanding is that each of two smaller tenons will have less movement than one longer tenon -each less than half, so the impact on each mortise is reduced. Although I suspect with the fit of my joints there should be plenty of room. 8-)

[george wilson]

I used double tenons side by side on both of my wooden work benches on the leg joints. On a drop leaf 18th. C. style table,I have used a "haunched" double tenon. It is a single ,tall,vertical tenon which has an upper and lower tenon,which is,say,2" deep into the legs,with a web of wood about 1" deep in the middle . The tenons of the apron go in deep enough to have 45º angles on their ends,which come fully together inside the leg,for maximum tenon depth. This shallower step in the tenon is for the strength of the leg,so it isn't so prone to splitting from such a tall(about 5" tall) tenon. That is why the tenon is less deep in its central part.

This explanation is probably not clear: A picture would be needed. Maybe Derek has an illustration. I don't make furniture on a regular basis.

Most likely the "haunched" term is English,as I learned it from the old English furniture restorer,Mr. Sims, in the early 70's. He was the sole member of the furniture restoration department of the museum in the 70's. Roy Underhill many years later,had mr. Sims's tool box on his show.

[David Castor]

Thanks, George - I must be thinking about this wrong. I was considering making two 2" tall tenons, one above the other instead of one 5" tall tenon in the stretcher. But I was only worried about the tenon expanding and contracting, not the mortise. I hadn't thought about a haunched tenons for this. I've seen them described, but never quite understood the logic for them. Your explanation has helped me understand why they might be used.

[Cliff Polubinsky]

My limited understanding is that each of two smaller tenons will have less movement than one longer tenon -each less than half, so the impact on each mortise is reduced. Although I suspect with the fit of my joints there should be plenty of room. 8-)

That's the explanation I've always heard.
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In the example above there is only 1 1/2" between A and B and between C and D. But there is still 5" of wood between A and D and that in my humble opinion will be the controlling dimension for the expansion. Taking some of the wood out between B and C will not make the rest of the material expand less. So it seems that one wide tenon or two narrow tenons makes no difference.

What am I missing?

Cliff

[Chris Vandiver]

Thanks, George - I must be thinking about this wrong. I was considering making two 2" tall tenons, one above the other instead of one 5" tall tenon in the stretcher. But I was only worried about the tenon expanding and contracting, not the mortise. I hadn't thought about a haunched tenons for this. I've seen them described, but never quite understood the logic for them. Your explanation has helped me understand why they might be used.

It is very common to use this method when your tenon piece is wide. The member that is mortised is stronger this way and of course, there is slightly less to mortise. The bottom rails of a mandoor, being wide, are traditionally done this way.

[Salem Ganzhorn]

I am not sure what the canonical answer is but I can tell you why I have used them :).

When I built the mobile base for my table saw (oh heck this is a neander forum!!!) I broke a tenon up into two pieces to increase the long-grain surface area of the joint:
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This doubles the long grain surface area of the joint.

And just so I don't get kicked out of this forum: although the base was for a power tool all the joints were cut with hand tools :)... arg except some of the tails I cut on the bandsaw but don't tell!!
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Salem

[george wilson]

Cliff,the only thing you might be missing is that the haunch leaves the wood in the leg thicker,so it it stronger than with just 1 wide tenon. The haunch still adds glue area to the tenon,more than 2 separate tenons would do. And,making them so long that you have the tenons meeting inside the leg adds even more glue surface. Then,you pin them,so even if a crack develops,the tenon is still in there pretty strong.

[Wilbur Pan]

My Douglas fir workbench has 3-1/2" x 1" tenons on the stretchers connecting them to the legs, and 5" x 1" tenons on the top of the legs connecting them to the underside of the bench top. I haven't had any issues with expansion/contraction.

http://lh5.ggpht.com/_a3R4RhWn1Is/S1OWz1WeVSI/AAAAAAAAA6E/jv2cFZZ791s/s800/IMG_3445.JPG

http://lh5.ggpht.com/_a3R4RhWn1Is/S1OW03NCLrI/AAAAAAAAA6I/uko8Q9K_01g/s800/IMG_3446.JPG

If you're worried about the joint working itself loose over time, you can always drawbore the joint, which should make it pretty much bulletproof. I drawbored the stretcher M/T joints, as you can see, but I didn't do it for the leg/benchtop connection.

More details on my workbench and the tenons here (http://www.forums.woodnet.net/ubbthreads/showflat.php?Cat=&Number=4088180&page=&view=&sb=5&o=&fpart=all&vc=1).

[David Castor]

Thanks for the responses. That's a nice looking mobile base, Salem - but tells me I have a lot to learn. Wilbur, thanks for the info and the link. Based on your experience, I think I'll go with a full length tenon. I was thinking about drawboring the joints - another new trick to learn - but that was one of my goals in building the bench.

Dave

[Chris Friesen]

Here's my take on it. For a really wide (or really long, but this occurs less often) single tenon you can get problems due to wood expansion/contraction on the glue line. There are two main strategies:
1) Use a narrower tenon but with unglued haunches to resist twisting.
2) Split the tenon into two, glue only one of the two and give the other one a bit of extra room to move. The second one can be pegged/drawbored with an elongated hole in the tenon.

Chris Schwarz has a blog post on classical M&T sizing rules of thumb at http://www.popularwoodworking.com/techniques/joinery/tenons-rule-so-here-are-the-rules-on-tenons

[Jim Neeley]

David,

Looking at Wilbur's beautiful work brings back one more consideration, if you are just getting started with M+T's. Wilbur's full-width tenons provide the maximum strength however this also places the end of the mortise right at the edge of the stretcher board. This meaning, any error in cutting the end of the mortise will be exposed. Taking even a small amount of the width off of the tenon hides the intersection behind the stretcher so if the end of the mortise (inboard) is less than perfect it will not show.

Either way, if it is a through tenon, the outside (outboard) end will be exposed.

Just my $0.02.

Jim

[Jim Stewart]

The advantage of double tenons is simply more surface area that is glued. That is for the side grain..not end grain applications where that would not add glue strength.

[Garrett Ellis]

When people say that multiple tenons increase glue area, doesn't this only apply to double tenons as shown in wilbur's picture? Where the tenons are one behind the other?

I don't see how 2 tenons in line increase glue area... seems to me it would actually reduce glue area.

[george wilson]

I wouldn't bank on soft wood tenons not getting broken ahead of draw bored holes. If they are too offset,fir can just split on each edge of the hole,and move the slug of wood sideways.

I don't understand "2 tenons in line". I make my double tenons side by side. Maybe you refer to the tall,haunched tenon described above. In that case,yes,2 tenons would decrease the glue surface,but you also must consider the strength of the leg will be badly affected with just 1 tall tenon. At least haunching it provides an extra region of wood in the leg to help it not split.

[David Castor]

For what it is worth, this is what I was trying to describe originally (if this image comes through OK). I thought this might have some advantages over one longer tenon in terms of the expansion of the tenon.


Dave

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[george wilson]

Since any form of tenon that tall is still coming from the parent plank,I don't see that it would be less susceptible to expansion,possible glue line cracking,etc. Just pin it too. I had no problem with the tables I've made,but I only ever use pretty old wood that I've accumulated. My walnut was cut in 1941,and is my age! My mahogany is twice as old.

[Wilbur Pan]

I wouldn't bank on soft wood tenons not getting broken ahead of draw bored holes. If they are too offset,fir can just split on each edge of the hole,and move the slug of wood sideways.

When I did a dry run before making these joints, I found that if the holes were offset too much, the main issue was that the peg (I used 3/8" white oak) simply wouldn't go through and stop short. For these particular joints, the size and location of the hole compared to the thickness of the tenon really made splitting a non-issue. And when I did the drawboring, I could see the tenon piece move in towards the mortise, so I'm pretty confident that everything did its job.

I could see splitting the tenon being a problem if the holes were too big or too close to the end of the tenon. Luckily, it's easy to avoid that situation.