# Unique interlocking mortise and tenon joint:Would this work?



## KayB (Jan 12, 2013)

Joined recently to post this question:

I am currently designing a unique piece that is essentially a single, off-centered 4×4 post with platforms. I am designing this piece to also be easily assembled and disassembled (potentially multiple times) with just a screwdriver. Also breaking down a 7ft 4×4 into 20 inch pieces would make it a lot easier to deal with.

Imgur link to joint picture and notes
So I think I designed a joint that will solve this, but unfortunately it is unusual and I can't find something like it. I can't even find any 180 degree end grain type joints to compare it to. So I have a few questions/concerns:

Will it actually work or am I a fool?
What would be the best way (cost/time wise) to make the joint?-Should I just get a hollow chisel mortiser?
Does wood movement make it impossible to ensure year round (dis)assembly ease?
Would there be an easier solution maybe?

Thank you very much for your time and feedback


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## WillAdams (Dec 8, 2012)

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?


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## KayB (Jan 12, 2013)

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?


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## Thalweg (Jan 27, 2009)

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.


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## msmith1199 (Oct 24, 2012)

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.


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## MarkwithaK (Sep 12, 2009)

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.


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## KayB (Jan 12, 2013)

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.


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## Loren (May 30, 2008)

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.


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## KayB (Jan 12, 2013)

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.


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## Thalweg (Jan 27, 2009)

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.


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## KayB (Jan 12, 2013)

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?


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## KayB (Jan 12, 2013)

Sorta like this:


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## JGM0658 (Aug 16, 2011)

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


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## Loren (May 30, 2008)

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.


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## Thalweg (Jan 27, 2009)

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.


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## rexb (Mar 28, 2012)

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.


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