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I am practicing dovetails and so I have made various stools of differing sizes consisting of three pieces of rectangular wood: one for the seat and two for the legs, being joined with dovetails.

I want to put a joining piece between the two legs to prevent splaying under heavy load.

I can't find much information on what the proper joint for this would be. I would rather not go all the way through the legs for something like a pull-through dovetail just because I haven't done it before and don't want to ruin the stool (which is for my dad).

I was thinking of just a simple T-butt joint but I have been reading that it is very poor for a joint which needs to take any tension. Does adding some small dado to the accepting end of the butt-joint help increase the tensile strength of the joint in any substantial way?

In terms of my tools... pretty sparse. I have chisels, hammer, and a dovetail saw; along with copious amounts of sand paper :P

Since I have gotten this far without any metal I am also going to avoid any mechanical fasteners.

Edit: I added a rough picture showing the current three parts and where I want to put the additional piece (green). Thanks.simplestool

jbord39
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    I'm having trouble visualising this from your description, it's late here so it might be me but a quick sketch would be helpful I'm sure. One thing you can be sure of, a butt joint without reinforcement isn't strong enough. Some joint with a mechanical advantage would be best, but the strength of the joint at the top of the two legs is possibly more important. – Graphus Feb 02 '17 at 01:43
  • I'm not 100% certain, but I think jbord39 is simply referring to stretchers between the legs. – rob Feb 02 '17 at 07:21
  • @rob Yes I think so too, but wanted to confirm. Helps the Question be clearer for future searchers apart from anything. – Graphus Feb 02 '17 at 10:44
  • @Graphus: I added a picture. After googling stretchers: yes that is what I was thinking of. I didn't know the name for it, thanks. The top two joints are pretty sturdy; I haven't glued them yet but can sit or jump on it and the joints hold strong (I will glue them once I get this other piece ready). – jbord39 Feb 02 '17 at 17:11
  • How thick are the legs? If the dovetail joints are snug and the legs are thick enough (say 1 1/2"), it may be possible that a stretcher is not necessary. Also, how high is the stool? – Ashlar Feb 02 '17 at 21:32
  • @Ashlar: Thanks. The wood on top is claro walnut and the legs are paduak, both 7/8 of an inch thick (not quite as thick as you recommended). Due to limited resources I ended up just putting a 1/8 of an inch dado slot on the inside of both legs with a chisel as a small recess for the stretcher to fit into. It fits pretty snug and even without glue holds itself. I am going to shape it and glue it later. – jbord39 Feb 02 '17 at 21:36
  • Aha! I'm glad I asked as that wasn't quite what I was imagining. – Graphus Feb 02 '17 at 21:40

1 Answers1

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Since your legs are less than an inch thick I am not confident that the dovetail joints will handle the lateral loads that the stool will experience, so the stretcher is a good idea. To understand what type of joint is needed, consider the way the loads will be applied. Vertical loads from the person sitting on it will be easily carried by the legs and the leg/seat joints are fine. As weight is added, the legs will want to splay in or out to the outside spread eagle or lean in parallel to one side or the other. The shallow dado connection you have proposed will certainly prevent the legs splaying inward where the stretcher is compressed, however the joint is little more than a butt joint and will not resist being pulled apart if the legs splay out or lean. One or both ends of the stretcher will be pulled out of the leg. This pulling force is what must be resisted.

You can do so using a mortise/tenon joint which extends deeper into the leg thickness (say 5/8") or by a through tenon with a pin on the exterior. Through Tenon

Another alternative is to screw the stretcher to the leg from the outside using a countersunk screw. You can plug the screw hole with a wood plug. A cutter bit for your drill is inexpensive and allows you to match your project wood. When you align the grain, the plug will all but disappear.

Wood plug cutter

Ashlar
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  • I didn't want to suggest a M&T because of the extra work involved, but ideally the stretcher does need to resist tension better than a shallow glued housing joint will. I was thinking of proposing dowelling in through the sides instead of screws because the screws will enter end grain on the stretcher. But given the structure here I'm not sure that would be a big deal since I don't imagine the 'legs' will want to splay that much. – Graphus Feb 02 '17 at 23:04
  • @graphus Thanks, I just finished a project with a lot of plugs so the dowels didn't even occur to me :) – Ashlar Feb 03 '17 at 00:30
  • Another alternative would be to use a 3-4" high stretcher glued into a dado about 5/8: thick. It should have enough strtength to handle the stresses. – Ashlar Feb 03 '17 at 00:32