Nobody writing for the WW magazines seems to be in agreement over the when, how and why of the use of the double tenon in joinery(one above the other). In the more extreme examples such as bread board ends on table tops or a two foot wide rail on bed headboard the answers are more clear because of the large amount of expansion and contraction.
Let’s take a typical case of a post and rail end table with a drawer and aprons that are 6″ wide. Why not use a single 5″ wide tenon on the apron? Some say a double tenon is needed because the leg would be weakened too much with such a wide mortise. Others say it’s because the rail would split. Others say, no, the problem is stress failure along the glue line.
On the issue of joint design, some say just use a double tenon, glue both tenons and you’re fine. Others say, no. One must glue only the top tenon and leave the bottom tenon unglued with a tad of breathing room in the bottom mortise. Another view is just cut a 2″ or 3″ tenon at the top and run a long haunch down the rest of the tenon to resist twist. I’ve read, go ahead and make the 5″ wide tenon, but just make a saw kirf through the middle to accommodate movement. And lastly, make the 5″ wide tenon but just keep it very short.
What are your thoughts?
-Rick
Replies
You're describing forked tenons, Hopster. A double tenon is two tenons side by side in a thick rail/member. A forked tenon is two tenons across the width of a wide member, such as the middle rail of an architectural door. Then there are double forked tenons for members which are both wide and thick.
Apart from that, I'll see what other contributors have to say. I already know the answers but don't have the energy right now to go into it. It's almost midnight here, and it's been a very a long day, ha, ha. Slainte.
Sgian,
The terms I've come across are "twin" tenons for two side by side and "double" for the forked style( ex: Joinery by Rogowski, pp.344-345). But, "forked" tenon is a more descriptive term. I'm looking forward to more of your thoughts. In the radio world they call that a "tease".
By the way, I have to confess that during the couple of years I was a lurker here, with a moniker like Sgian Dubh, I thought that you were from India. That is, until I looked up the meaning of sgian dubh!
To your health! -Rick
In the radio world they call that a "tease".
Hopster... that's no tease, it's auld age... with a smidge of cantankerous auld fart t boot.
Forgive him.. he's been in England too long...;)
personally I'd like an answer to this too.... the success / failure of a bed I'm making depends on it...Mike Wallace
Stay safe....Have fun
We'll have to see what the real experts have to say, but as a general rule, I have always thought it best to do more than a single tenon if your stock exceeds 3-4" in width. Gezuar.
Hopster,
I certainly don't know the answer but we might as well give Sagian something to critique when he arrises from his beauty rest...
It would seem to me all of those answers could be correct depending upon the wood and cut your using, the dimensions of the lumber and the intended use of the piece. For instance, thru tennons on A&C is often 4" ...but it is oak often quarter sawn...and therefore less wood movement...so I guess that makes it okay. Recently I made a double drop leaf shaker table with flaired legs....the legs wiere 1 1/2" double tapered....anyhow, I shortened the tennons a bit and pinned them on the inside. I'm still wondering if I should put some support blocks behind the joint. So, my point would be, the joinery depends on several factors....and you listed some of the ways you modify the joinery to avoid complications. Hope I get a passing grade....lol
Hopster,
I think your various explanations for using or not using forked tenons are probably all defensible, in the 6" wide apron you propose. I've seen old work done both ways, that has held up ok. Personally, in a light (weight) table, I'd be inclined to use a single 5"tenon, with a 1"haunch at the top of the apron. In a heavier piece, I feel it would be better to divide (fork) the tenon, with about 1-1 1/2" between the tenons. The added meat will leave the leg stronger, better able to resist the vibrations from dragging the piece across a floor. Cross grain shrinkage doesn't seem to be much of a problem with table aprons, I haven't seen too many showing splits between the forked tenons. Desk lids, or highboy ends, where the width requires using 3 or 4 tenons, need allowance for movement to take place.
The other issue with table legs, is the length of the tenons. I've worked on many old tables with legs broken out around the mortises. The deciding factor seems to me not whether the tenons are forked, but whether the mortises are so deep that they meet inside the leg. I like long tenons, for glue area, but having the top of the leg a shell around the tenons is a weak construction. Better to have shorter tenons (within reason) that are carefully fitted. Let the size of the square at the top of the leg determine the length of the tenons.
Regards,
Ray
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