A comb joint combined with an appropriate adhesive will afford the strongest, the joint provides the greatest contact area for the adhesive..

I've used Gorilla glue with good success…..

if you want an adhesive that soaks into the joint, then Superglue will wick its way into the timber fibres very well….

*my advice comes from experience rather than hearsay….I use superglue in my model aircraft pursuits as well as Gorilla glue….*

Edited By John Rudd on 26/01/2017 11:07:39

Neil,

'Box joints' / 'comb joints' … i.e. the square equivalent of through dovetails … are probably best.

Waterproof white PVA glue.

MichaelG.

.

… Two others got there before me, but  we're not far apart in our thinking.

Edited By Michael Gilligan on 26/01/2017 11:10:49

While dovetails are likely the strongest, what is wrong with box joints? The more the merrier in this respect as glue area is increased with number of segments. As a box, it presumably has one end closed (like a lid?) Which will assist with rigidity?

I have, more recently, used 'gorilla glue' as it expands to fill the joint. Cascamite is not really waterproof, but is good for filling joints. ( Water resistant) PVA is good as a thin film timber adhesive and should work well, as long as the joints are fairly well made (no sloppiness or gaps at dry fit stage).

If it is really 'big' why not make it in thicker ply? None of my beehives have fallen apart, but they are made in 18mm timber with one pair of sides fitted in a rebates in the other two. These 'boxes' have neither top nor bottom as they are simply stacked one on top of the other.

Another alternative is to machine let-ins on the inside for, say 1-2mm corner brackets, if none can be fitted outside the box.

Since no-one else has mentioned it.. aerolite 306 is good stuff or get traditional with hide glue granules (if you can stand the stink of boiling them), though I'd use brown glue (of a cheaper brand than gorilla).

If it's cheapo far eastern ply then the glue holding the laminations is the weak point so you need joints. If it's a really good marine ply then it'd hold with a simple butt joint.

you need box joints or similar as otherwise you are joining to end grain which is generally very weak

why use ply – 12mm MDF cut very square can be drilled to take screws (remember to drill to core dia of screw (prob 3.5mm screw with approx 2.5mm core as MDF will split otherwise) then put any decent wood glue on edges before rescrewing box

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There is a cabinet maker in the membership. Perhaps he would know the best solution.?

Although he's very quiet and seldom posts.

Failing that you could get Ajohnw to TIG weld the wood for you.!

Nick

There ya go… no room for internal bracing so weld steel angle all the way round outside and lace steel wire through the sides

Can't help with the joint form but for glue, I have been impressed by the D4 structural PU adhesives (I think Gorilla glue is one of these). I have used it for model making and structural work in my house and it is the same stuff they use in glu-lam products these days. It is water proof and fast setting – oh, and it is strong! Just don't get it on anything you don't want sticking….

Mark

PS. In my house I needed to stiffen a large span floor and had little scope to do anything with steel (always my preferred option). I increased the floor stiffness significantly by adding a 6 x 1 timber to the bottom of the joists (a significant increase in the area moment) and almost completely removed the "bounce" from the floor. I have also used it to add "plants" to doors when I was unable to get a suitable size in standard oak veneer doors etc.

Edited By Mark C on 26/01/2017 12:31:18

A lot of the strength will depend on the quality of the ply wood you are using, cheap DIY softwood cored ply will not make up into as strong a box as one made from decent quility birch throughout ply.

Finger/box/comb joint call it what you will is liekly to be the strongest, dovetails would have more strength in one direction but equal in teh other and as you say hardet to cut. Titebond II alphatic resin glue or what the yanks call yellow glue would be my choice unless its going outside.

What length are the sides of the box? if you can stand them end on under your mill that makes a very effecting and easy to use comb joint cutter, this little one was done that way.

If you were able to cut decent mitre joints then a splined mitre joint would be another option but would want quite a few splines to give the same strength as the comb joint

J

PS if you are worried that your efforts at cutting dovetail joints may not look good to the eye you could always do a mitred dovetail which will hide them

Ah yes, glue sniffing for the health freak! Seriously though, it loses a lot of strength if you do actually boil it. Kempe's year-book for 1929 recommends a maximum temperature of 140 degrees Fahrenheit. Subtract 32, divide by 8, multiply by 5…

George

Neil, you just squeeze Titebond out of the bottle same sort of consistancy as PVA, small brush to apply to all surfaces. Out of interest Titebond also do a liquid hide glue that can be used straight from the bottle but teh Titebond II will be better.

Birch ply is generally WBP bonded and being a denser timber that the far eastern hardwoods gives a better board. Also (depending on grade) has less voids and surface blemishes so will also be stronger and better looking.

MR (Moisture Resistant) MDF is available and is what I use for most things inside as is the even more durable External MDF.