Joints... Nope not the kind you smoke, maaaaaan! (1 Viewer)

[Brett DiMichele]

I was just curious is there any documentation that says
what type of joint really is the strongest for wood?

I never see anyone here build enclosures with Mortise And
Tennon Is that just due to the time it takes or the fact
that a butt joint is just as strong and much easier to do?

What about Dovetails and Rabbets?

I never see much discussion on varying types of joints so
please fill me in!

 

[JesseSilver]

I've definitely seen rabbets, and I've definitely seen dados, particularly used with bracing, and I think both are good ideas. Mortise and tenon would be a bad choice, in my opinion, since the tenon is meant to be load bearing, like the bottom rail of a park bench, and wouldn't be of particular use in holding one board at an angle to another. As for dovetail, I actually saw this once, but I consider it to be a waste of time. It looks cool, but for all the time and effort spent, it's even more likely than a simple butt joint to have air leaks in it, and the simple butt joint, glued with regular wood glue, or especially something like Gorilla glue, is more than capable of holding together for longer than you'd want to keep the speaker anyway!!

It's a good question, but in my opinion, dados for bracing, small rabbets for sides and tops, and that's plenty strong.

 

[Brian Bunge]

I like dados for bracing in some cases and I like rabbets for enclosure alignment. I think it helps square up the joints better. I know some people like lock-miter joints, but as fragile as MDF is you have to be careful with them. But 99% of the time we just use butt joints with wood glue and a nail gun. Butt joints will be more than strong enough for MDF enclosures. And as Hank always likes to point out, the glue joint will be stronger than the MDF itself.

 

[Joe Ku]

Biscuits also work really well for MDF, very strong joints.

 

[Hank Frankenberg]

I've done rebates (rabbits) and I've done some dovetail drawers on my Jointech router table system, and I've done lock mitre joints and glue joint router bit joints. BUT, in many years of building boxes, I've definitely settled on butt joints. Butt joints only. No screws. No biscuits - they don't add much strength. The right amount of PVA (carpenters) glue applied to accurately cut MDF boards will give you a joint that's at least as strong as the wood itself. Polyurethane glue is very expensive and doesn't give you a better joint than carpenters glue on surfaces that mate well. Note that I'm talking about MDF here, not plywood. Plywood joints need help, as in rebates.
My 2 Hz.
Damn, between the time I started typing my answer and posting it after an interrupting phone call, three people posted answers. Advice is always waiting in the wings.

 

[Chris Tsutsui]

Hey Brett, you planning your Line Array assembly out?

I like Lock Miter joints with laminated MDF, but the edges are always very fragile and can splinter. After the panels are cut, the edges also become sharp. Another lesson I learned is you must be very precise running every piece perfect or else things get messy.

That's why I like butt joints. The easiest method for me is to use a nail gun and yellow titebond glue. I apply the glue with a rubber roller on the edges of the panel to be glued. The brads are then applied to hold the box together while the glue dries, and even if a panel wasn't nailed right, a simple hit with a mallet will fix it before the glue sets. (Glue sets in like 30 min)

If the box requires to be strong, I use drywall screws that are countersunk added to the box once it is completed with brads and glue. I predrill right next to a brad so the screw goes in so the head hides the brad indentation. It is faster than clamps because you don't have to wait 30min for the glue to dry although one setback could be having to fill in the countsunk screw holes with body filler at the end.

Biscuits add strength to a butt joint but then you'd have to spend more time adding those. Dovetails might be one of the stronger joints to use but could take a long time setting it up.

 

[PaulT]

Here are a few descriptions/details. I'm sure you are familiar with them. In most cases the strongest joints would maximise any gluing surface as well as provide mechanical holding ability with or without glues. The 'dovetail' and/or finger joints are generally considered the strongest wood joints for those reasons.

http://www4.district125.k12.il.us/Fa...oodJoints.html
http://www.am-wood.com/joints/joints.html

 

[Brett DiMichele]

Chris,

Nope not planning anything. I am doing the LA's with Butt
Joints Glued and Pocket Hole Drilled simply because I would
need 60 clamps per cabinet to hold em together

I just wanted to start a threat a little different than the
norm because I never see people discussing joints here.

There is no doubt about it that a square and flush butt
joint glued with Carpenters glue *IS* stronger than the
actual MDF and unless you drop the box on the floor from
say 5' in the air chances are it's never going anywhere.

 

[Pete Mazz]

Yep, anything other than a properly cut and glued butt joint is overkill for speaker enclosures. Doing some extra work to help alignment is more a matter of what tools/clamps you have to work with. Some mechanical fasteners, if not done right, will actually weaken an MDF joint. Non-predrilled screws, swelling bisquits, over torqued screws, large guage nails, for instance, can all weaken a joint.

Pete

 

[Brett DiMichele]

Pete,

Yeah you find out the hard way with MDF that if not careful
with Screws and Nails you can easily ruin your work.

I know MDF certainly hates screws penetrating into the end
grain (layers).

Pocket Holes drilled with a Kreig Pocket Hole Drill Jig
seem to be the way to do screws in MDF if you lack the
clamps required (and I do..)

It's going to be cheaper for me to buy the pocket hole
drill than the required clamps would cost.. Sure I'd like
to have 60 clamps though