Twin tenons

[James Pallas]

There was a recent thread about twin tenons that started me thinking. I usually start thinking about twin tenons when the rail is wider than than it is tall or when the mortised piece is thin leaving not much length for the tenons. I don't remember any rules of thumb about it. How do you decide when to do twin tenons? I would gues that this would also apply to dovetails at the top of post connections also.
Jim

[Brian Holcombe]

Jim,

I use them for shoji, but also use them fairly often outside of shoji. I basically prefer them in most cases where I can fit them, very often especially in 8/4 stock where the layout lends itself to a layout of twin 3/8" tenons. Same in shoji where the layout lends itself to a 1/4" layout since the stiles are 30.5mm. Sometimes I shrink the shoulder slightly to make the layout, but I try not to do so.

I don't do it in situations where I can't fit it easily into the layout, like 7/8" thickness doors.

Personally, I think twin tenons are considerably better for the material than one large tenon and as Stan pointed out in the other thread they seat square even with minor inconsistencies to them. When making by hand this can be a considerable advantage.

[James Pallas]

Thanks Brian
i don't often work with 8/4 material. If I were doing so I think that I would use twin tenons until the depth exceeded the width or possibly a little more and then would go to a single tenon as it becomes more difficult to cut longer tenons. I have successfully used twin tenons of 1/8 when joining 3/4 stock to 3/4 square stock. I do that to get more glue surface because of the short tenons. I don't have any real reason just because it feels right. I don't know of any rules on it or have heard many speak about any guidelines.
Jim

[Stanley Covington]

Somewhere, somebody did the calcs and the destructive testing necessary to determine when twin tenons are justified. I don't have that information, even as rule of thumb.

Twin tenons, and their matching mortises, are more than twice as difficult and time consuming to make as two single tenons/mortises. They are not decorative, but serve practical engineering and durability purposes only. They have always been the choice of craftsmen throughout the world when making doors and windows and other highly-stressed joints that needed to last a long time. So when deciding whether or not to use them, you need to consider the pros and cons.

Twin tenons lock in very tightly even without glue. Lots of friction. As an added plus, they have twice the glue area of a single tenon in the most effective direction, in typical designs. When glue was less than reliable, this was a big advantage. Not so much nowadays.

As an aside, there are some folks that eschew glue strength wanting the option to easily disassemble mortise/tenon joints on shoji, fusuma, itado, doors and windows to make it easier to repair damaged kumiko or mullions. I suspect such people have never tried to disassemble/reassemble a properly-cut twin tenon joint. IMO, this philosophy is akin to using plastic bolts to manufacture a truck so dinged body panels will be easier to replace someday. The repairman might appreciate it, but it is not a responsible strategy for the original manufacturer, and the original purchaser will not thank him. There can be no acceptable excuse for a joint coming lose and a door rattling apart just to save a repairman a few extra minutes.

The biggest advantage of twin tenons IMO is the resistance to torsion (twisting) they provide. This is true in any configuration. This is a big deal for shoji, fusuma, doors/windows, table legs, etc where the rails and/or stiles are subject to eccentric loading. Doors, table legs, and chairs especially are subjected to a lot of torsion, much more than timber frames. If the tenon does not resist twisting adequately, the joint will eventually fail, and the assembly will fall apart. You need to think about this loading situation when designing. Most people don't, with predictable results.

I encourage you to use twin tenons where they are justified, especially if you want your work to last more than just a few decades. You will find that they are more difficult to cut by hand than you might imagine, but one cannot honestly claim to be a first-class joiner until they can cut this joint with some confidence.

For big twin tenons, such as 36" doors for instance, a router and tablesaw make the job easier, and provide the opportunity for dramatic screwups.

[James Pallas]

Thank you Stanley. I'm thinking that there is no hard and fast rules. I'm going to continue as I have been. If the tenon seems not long enough or the piece is wider then tall and there is room a twin tenon it will be. Of course things can dictate otherwise.
Jim

[Phil Mueller]

James, here’s an article where Chris attempts to give some rule of thumb to M&T joints:

https://www.popularwoodworking.com/techniques/joinery/tenons-rule-so-here-are-the-rules-on-tenons

[Brian Holcombe]

Chris Hall has excellent material on this subject.

[James Pallas]

Chris Hall has excellent material on this subject.
Thanks Brian. I have looked at Chris Hall's site many times. His work is very impressive. I have learned a lot there. I guess I was just wondering if there was anything like "1/3 the width" or "4 or 5 times for the length". I am comfortable with what I do such as "if I cannot get twice the length" or the tenoned part is wide and the mortise side is too narrow for a sliding dovetail.
Jim

[James Pallas]

James, here’s an article where Chris attempts to give some rule of thumb to M&T joints:

https://www.popularwoodworking.com/techniques/joinery/tenons-rule-so-here-are-the-rules-on-tenons

Thanks Phil. I've seen that before. I think sometimes he writes like he is thinking out loud and can't bring it in for a landing. Maybe he will settle in after he builds another dozen or so workbenches.
Jim

[Phil Mueller]

James, lol. I hear you. Wasn’t sure how much looking around you had done...

[Prashun Patel]

Stan,

"Twin tenons, and their matching mortises, are more than twice as difficult and time consuming to make as two single tenons/mortises."

Can you elaborate? I don't understand how two single tenons are different from twin tenons?

[Brian Holcombe]

Thanks Brian. I have looked at Chris Hall's site many times. His work is very impressive. I have learned a lot there. I guess I was just wondering if there was anything like "1/3 the width" or "4 or 5 times for the length". I am comfortable with what I do such as "if I cannot get twice the length" or the tenoned part is wide and the mortise side is too narrow for a sliding dovetail.
Jim

Chris’. Carpentry drawing series gets into the details like that, I have found it incredibly helpful.

[Stanley Covington]

The practical tolerances for twin tenons are much more severe than a single tenon. If a single tenon is a bit slanted, or a mortise a bit fat on one side, it can still be made to work. In the case of twin tenons, the 2 tenons and 2 mortises must be cut quite precise or they will fight against each other.

There is less room for error, compared to a single tenon, if a good fit is necessary.

With a single tenon, you only need to fit one tenon to one mortise, not two tenons and two mortises at the same time. Ergo, more than twice as difficult and time consuming.

I have a double- bladed mortise chisel that cuts twin mortises at the same time and very precisely. It speeds things up a lot. Withou it, you need to be pretty good with saw and chisel if working by hand.

[James Pallas]

The practical tolerances for twin tenons are much more severe than a single tenon. If a single tenon is a bit slanted, or a mortise a bit fat on one side, it can still be made to work. In the case of twin tenons, the 2 tenons and 2 mortises must be cut quite precise or they will fight against each other.

There is less room for error, compared to a single tenon, if a good fit is necessary.

With a single tenon, you only need to fit one tenon to one mortise, not two tenons and two mortises at the same time. Ergo, more than twice as difficult and time consuming.

I have a double- bladed mortise chisel that cuts twin mortises at the same time and very precisely. It speeds things up a lot. Withou it, you need to be pretty good with saw and chisel if working by hand.
Twin tenons are indeed harder to fit. That chisel you speak of must be Japanese. Somewhere in my cloude memory, from the 1980s, was someone selling them. I think it may have been Garrett Wade. Never bought one. I can do them. Not sure they are great but seem to work.
Jim

[Stanley Covington]

Twin tenons are indeed harder to fit. That chisel you speak of must be Japanese. Somewhere in my cloude memory, from the 1980s, was someone selling them. I think it may have been Garrett Wade. Never bought one. I can do them. Not sure they are great but seem to work.
Jim

Here are some pics. Not my chisel, but very similar. Difficult to tune to perform well.

https://auctions.c.yimg.jp/images.auctions.yahoo.co.jp/image/dr000/auc0311/users/4/7/1/7/heavysrus-img1024x768-1509673894i2oyba17274.jpg
https://auctions.c.yimg.jp/images.auctions.yahoo.co.jp/image/dr000/auc0311/users/4/7/1/7/heavysrus-img1024x768-1509673894is72g717274.jpg

[sean contenti]

That chiselis awesome, and also crazy.

Build the piano, then tune it, all with one implement.

[Stanley Covington]

That chiselis awesome, and also crazy.

Build the piano, then tune it, all with one implement.

Ha Ha :p It doesn't make a tuning fork sound, but it does make nice mortises once you get the dimensions adjusted, and get accustomed to using it. A very specialized joiners (tategushi) tool.

[Pat Barry]

Here are some pics. Not my chisel, but very similar. Difficult to tune to perform well.

https://auctions.c.yimg.jp/images.auctions.yahoo.co.jp/image/dr000/auc0311/users/4/7/1/7/heavysrus-img1024x768-1509673894i2oyba17274.jpg
https://auctions.c.yimg.jp/images.auctions.yahoo.co.jp/image/dr000/auc0311/users/4/7/1/7/heavysrus-img1024x768-1509673894is72g717274.jpg
If a guy is going to have a tool like that then he MUST HAVE a 4 blade tenon saw to go with it or he just isn't being productive.

[Stanley Covington]

If a guy is going to have a tool like that then he MUST HAVE a 4 blade tenon saw to go with it or he just isn't being productive.

Now that’s a great idea!