All Replies on Unique interlocking mortise and tenon joint:Would this work?

The top of the middle section is going to be really fragile, as will the bottom of the vertical tenon—- which direction(s) will it be stressed in?

How much weight should the whole thing support?

This is for a cat tree, the biggest platform would be an almost 2’ by 4’ piece of half inch osb and the biggest possible cat on it would be 20 pounds at the most.

I have a feeling that separate, the post pieces would have very weak points, but once assembled it’ll be solid wood, basically.

I have also learned about “drawboring” since I have been on this site, and I think maybe my “screw through the whole thing” design might have some of the benefits of a real drawbored joint. Am I just being silly?

Neat idea, but very challenging joinery. With that many joints, and no glue, any “play” would be magnified significantly. The top of your post would probably move a lot. Cutting the mortice in the end-grain of the vertical post would be quite difficult, and probably weak, so I think you’d want to build your vertical post as a box instead of a 4×4 post and then use loose tenons in it as opposed to cutting normal mortices and tenons. Post pictures if you do this. It’ll be interesting.

My question would be why make it so complex for a cat tree? You could easily screw brackets to a 4×4 to hold up a shelf and then unscrew those brackets and take things apart when you need to move it.

-- Mark Smith, Tracy, CA., http://www.markscustomwoodcrafts.com

I agree. Seems a bit elaborate for what it’s intended use will be but it is your project and tackling a seemingly difficult task may end up a learning experience/skill builder. Just because there are easier ways of doing something doesn’t mean you shouldn’t try the more difficult method. That being said I also think that, as stated above, the mortise is going to be on the fragile side.

-- If at first you don't succeed then maybe skydiving isn't for you.

Thalweg-thanks for the loose tenon idea, I was also thinking about that sort of set up but thought it would maybe, almost take as much time (as making 4 inch endgrain mortise/tenons?!?) and also not be as natural or “tree-like” I guess, as a solid piece of wood. If that makes sense.

And good question Mark. This is actually the second iteration of this design geared towards lower cost and simplified assembly for a customer. My first prototype I made had hardware like shelf brackets and right angle tie downs but this hardware added up when there are five platforms. Also shelf brackets limit the movement of the cat around the post, which is necessary for the cat to get up n down. Also joinery is just so much cooler.

I think I am going to redesign this to be more of a lap joint with interlocking mortise/tenons for the horizontal supports. I will post the sketchup pictures soon.

Making unusual mortise and tenon joints is
not usually a good way to reduce costs.

You can do it like you’ve drawn (I’m confused
by what you’ve got in mind with the 1×3s
but that’s a different issue), but you’ll need
a way to pull and fasten the joint snug… a
peg, wedge or perhaps a metal doohicky.

Excavating that mortise into end grain is
going to be a pain.

Here’s a new idea, same 4×4 1×3 combo but no more mortise:

Would this work? Should I switch the orientation of the 1×3s 90 degrees?

Loren-”Making unusual mortise and tenon joints is not usually a good way to reduce costs.”
Hah, good call.

Jonathan-Interesting joint, very interesting.

Cool, another creative joint. I like it. However I think there is a weak point. I don’t think screws will be strong enough to hold the two big half-laps together without glue. I think you’d have to use bolts running all the way through the 4×4.

Thanks for the input Thalweg, much appreciated.

I am afraid of that weak point too. Shooting from the hip, I feel like if there were two honkin #12 screws that went through even the two 1×3s, it might be enough to tighten everything together.

I was also thinking if I rotated the 1×3s in relation to the 4×4, so that the through 1×3 would go through both halves of that lap joint? Would that make it stronger?

Sorta like this:

Your second idea would work just fine, there are similar joints in Japanese joinery and construction.

-- Just because you’ve always done it that way doesn’t mean it’s not incredibly stupid.

You can put threaded inserts on the inside of the joint
and drill holes through which the assembler inserts
machine screws to engage the inserts.

I think this would be kind of fun to try. My wife has been bugging me to make cat furniture for years. You’ve given me some ideas.

I like where you’re going with your second and third iterations, but in my opinion (FWIW) Thalweg and Loren are right. Thru bolts or at least machine screws into threaded inserts would be much better than wood screws in this case, especially after it has been taken apart and reassembled a few times. Think about how much leverage the top platform (around 7’) would have on the bottom joint (20” off the floor).

The best would probably be thru bolts with spring washers (cone or split) to take up any slack as the wood moves with the weather and keep the joints tight.

-- Rex