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I'm designing a loft bed (here's a rough picture without the cross supports and other details). Right now, most joints will be mortise and tenon.

I'd like to be able to take the loft down to parts, and the simplest thing to do would be to disassemble it at the M&T joints. But that would mean they couldn't be glued, and my current understanding of M&T joints is that their strength comes in large part from the glue.

Am I missing something? Is there a way to build M&T joints that aren't glued but are still full strength?

Edit 2013-11-16: I'd like to make these blind tenons. That makes @Jack's sliding dovetails really interesting. Since the joist structure will be visible from the underside, the aprons and hangar bolts for the corner are also interesting -- I wonder if there could be something like that to secure the non-corner joists...

Edit 2013-11-16: After watching a video about making dovetail joints, I'm going to go with @Jack's answer because they depict the answer to the problem. @HerrBag's comments (specifically stopping the dovetails) are, however, what make it a usable answer.

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The loft bed in that other question looks dodgy, even if glued/screwed together, with no cross bracing on the legs. –  whatsisname Nov 15 '13 at 17:39
    
@whatsisname The OP apparently knows this. In related questions he acknowledges the need for bracing. –  bib Nov 15 '13 at 23:07
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4 Answers 4

up vote 2 down vote accepted

It might be good to consider a sliding dovetail.

enter image description here

Edit 11/16/13 To assemble your bed frame, the M&T joints with captured bolts with nuts to hold the corners together with the legs.Captured nut Just as a mention, a good hardwood needs to be used for the assembly. regular 2X4, 2X6s and other similar materials used in the construction of homes will be too soft, and when the joints are put under stress SPF, Hem fir, etc is going to compress and loosen the joints. To use this material you need glue and screws to hold all this together. That is not an option for you I think?

To assemble this, white oak, ash, Black walnut and a dense variety of mahogany would be a good choice, if it is not cost prohibitive. There are other hardwoods too, just give a look. Red oak and Douglas Fir are hardwoods, but very prone to readily split under the stresses you are going to expose the joints to.

Another good thing about some hardwoods, is that you will not need the same dimension of material to have the same strength as 2X framing lumber.

The sliding dovetails are ideal for the cross members into the side rails. To use the same joint for the attachment of the legs would produce a weak joint in my opinion, I have seen it fail in pedestal table legs.dovetail at leg

To dissemble this in time, and to ease the assembly, which sliding dovetails can be a bear to slide together, I suggest tapering the dovetail so as it goes in, the joint gets tighter, it doesn't need much, a 1/16th of an inch over 4 inches on each side would dramatically increase the ease of assembly/disassembly. A router jig with a bushing guide on the router would make the female cuts in the rails, and a router table with a tall fence and a shim on the appropriate corner of the support for the slats would cut the male dovetail. A push bock would be handy to control the top heavy stock for this cut.

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I'm new to all of this, so I've never seen a sliding dovetail. It seems ideal -- what's the downside? Why don't people use it more often? –  LoftyGoals Nov 16 '13 at 17:09
    
In your case, it would need to be stopped (the slot stops 1/3 to 1/4 above the bottom. A router table would make fast work of the slots and pins (2 different bits). You should seal the surfaces to minimize moisture differences through the season. You should still adjust the fit for the moisture content of the wood and the season you make them. Spring and Fall are the best, as MC is in the middle. Kiln dried wood is a requirement for this joint. –  HerrBag Nov 16 '13 at 17:29
    
I revised the answer a bit, yes as Herrbag mentioned rightfully so the dovetail notches should not go all the way through. I tried my first sketch with sketchup as I have seen used here but still need to learn how to convert it. That drawing had the stopped cut, just couldn't get it out there –  Jack Nov 17 '13 at 1:58
    
This is a fantastic answer, particularly the information about the wood that I didn't ask but really needed to know. I'm a little afraid I won't be able to pull off the wedged dovetails, but maybe I can use a combination of captured nuts and blind tenons. I'm also wondering if I can use a turnbuckle and cables to keep everything in tension, but I posted that as a different question. –  LoftyGoals Nov 17 '13 at 6:00
    
There are 2 jigs that would be made to create these joints. that would eliminate the human error part, after a few test pieces are made. –  Jack Nov 17 '13 at 14:25
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You could use one of the many variations of the mortise and tenon joint, depending on the look you're going for. Though I agree with HerrBag, that the strength of a mortise and tenon joint should not rely on glue.

Tusk Tenon

This is a through tenon variation, where a wedge is used to lock the joint together.

Assembled Tusk Tenon
Exploded Tusk Tenon

Fox tail (Wedged) Tenon

This is another through variation, though the tenon ends up flush on the outside of the joint. In this joint slots are cut into the tenon. and wedges are driven into the slots after assembly. This causes the tenon to expand, and tighten in the mortise. Usually the mortise will be slightly tapered, to allow additional expansion and to create a stronger joint.

Assembled Fox Tail Tenon
Exploded Fox Tail Tenon

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The tusk tenons would be good for the slats. –  HerrBag Nov 15 '13 at 18:43
    
+1 for tusk. Fox tail would be much more of a challenge to disassemble. –  bib Nov 15 '13 at 23:05
    
Can any of these be done blind? That was my original aesthetic, but the tusk tenons might make for a more rustic look... –  LoftyGoals Nov 16 '13 at 3:04
    
Foxtail joints were meant to be permanent. Blind foxtail joints were used on chair legs in one of their uses. A skilled craftsman could drill the seat, work up the fit and tailor the wedge to completely tighten and permanently fasten the front legs to the seat as the top of the pocket drove the wedge into the slot. –  Fiasco Labs Jan 11 at 4:39
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Mortise and tenon joints do not need glue to be strong. If they fit tightly and are oriented properly to carry load through the post (like a stud) and the tenon stabilizes lateral loads, it only need to be secured with a peg or screw.

Tester101's tusks would be good for the slats. I was envisioning table type aprons for the corners.. This photo shows mortises, corner block (using a hanger bolt) and pocket screws for a solid yet disassemblable joint: enter image description here

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I really like this! Is there some equivalent of aprons for the non-corner joists? –  LoftyGoals Nov 16 '13 at 17:09
    
You could make the blind mortise (like these), but make it with over and under pocket screws to hold it fast without glue –  HerrBag Nov 16 '13 at 17:18
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One really strong option to consider is drawbore pegs.

cross-section of a drawbore peg

These use a slightly offset hole to pull the joint tight. They can be done fairly easily with home-center oak dowels. I found them to be reasonably easy as a beginner, and they even compensate for a loose joint if you don't quite get the fit right.

Google "drawbore joint" for all the details you could ever want.

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And it's not a peg, but a trunnel or trenail. This is one of the oldest post and beam or ship-building joints once a way of "easily" boring holes in timbers was perfected. –  Fiasco Labs Jan 11 at 4:37
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