Which joint is stronger?

[Packard]

Assuming the stock is identical for both joints, say 3/4” x 3-1/2” poplar, which of these joints would be stronger?  Stronger in which direction of strain?

Biscuits?

Or

Dominoes?

My guess is that the dominoes would provide greater strength against the changing of the 45 degree angles to make the 90.

And the biscuits would provide greater strength maintaining the flatness of the two pieces of stock.

Thoughts?

I make this joint frequently and use two 3/8” x 1” dowels to add strength and to maintain alignment.  Dowels are my fall back joinery method.  I often don’t even ask myself if it is the best method for the project.

Do you ever find that you are using your default joinery method without considering your other options?

I mentioned in an earlier post that I was going to try out biscuits to join my face frames to my cabinet box. 

A notable exception is that I always use pocket hole construction to assemble face frames.  Whereas years ago I made cabinets entirely with pocket hole construction, and the results were perfectly adequate.  Though I did not like to have the pocket holes visible in the final product.

 

[Sparktrician]

Have you considered using lock dominoes?  In the case of screen doors, for example, the lock dominoes should be at the top and bottom of the doors to keep them out of sight. 

 

[Michael Kellough]

Have you considered using lock dominoes?  In the case of screen doors, for example, the lock dominoes should be at the top and bottom of the doors to keep them out of sight.

I forgot about your lock domino idea, thanks for posting the link.

Packard, the same thing can easily be done with a dowel. Why are you considering not using dowels for the joint?

 

[woodferret]

I actually like the biscuit better because of twist resistance across almost the whole mitre.  The domino joint can have issues at the peaks because they are very inward.  PVA creeps.

edit: Assuming this is a picture frame, not a playground set for a gorilla.

 

[smorgasbord]

I would think you'd have problems double-stacking two biscuits in stock that's only 3/4" thick.

That's 19mm. A biscuit slot is 4mm. So, that's 11mm split 3-ways, which means the biscuits are thicker than the space between them.

Som maybe you only put 2mm between the two biscuits. That leaves you 4.5mm on either face to the biscuit. Depending on the wood, you might get "puckering" from the biscuit and glue swelling/drying.

If you really want strength, then make the inside domino as long as possible, and make the outside one as close to the edge as you can without it drilling through.

Then again, if you want strenth, a bridle joint, which can be done mitered, is probably the strongest, along maybe not the edge treatment you want to see.

 

[Duckler]

I am pretty sure that some youtube testers have shown biscuits are not very strong. Biscuits have some benefits, but from a strength standpoint mortise and floating tenon seems superior.

 

[simonh]

I'd guess from another perspective it would be which joint is strong enough for the use it will be put to?

Most structural engineers are going to work from that basis when designing a house. I know myself, not been a structural engineer, we tend to over-engineer many things and that just ends up costing more, or been complicated, or taking more time. I recently got some structural engineering software for a garden summer house I'm building and was amazed at some of the time/cost savings where I would have been way over doing things.

 

[Michael Kellough]

I'd guess from another perspective it would be which joint is strong enough for the use it will be put to?

Most structural engineers are going to work from that basis when designing a house. I know myself, not been a structural engineer, we tend to over-engineer many things and that just ends up costing more, or been complicated, or taking more time. I recently got some structural engineering software for a garden summer house I'm building and was amazed at some of the time/cost savings where I would have been way over doing things.

[member=66875]simonh[/member] I’d like to know more about this subject. Do you have a link?

 

[Packard]

Have you considered using lock dominoes?  In the case of screen doors, for example, the lock dominoes should be at the top and bottom of the doors to keep them out of sight.

I forgot about your lock domino idea, thanks for posting the link.

Packard, the same thing can easily be done with a dowel. Why are you considering not using dowels for the joint?

I am happy with the dowels.

What I am not happy with is that I don’t consider all my joinery options for all my joinery needs.  I always fall back on dowels except when it is particularly difficult to execute.

I think domino users probably default to dominoes like I default to dowels.

As mentioned the only place I use pocket holes is on face frames.

I do use Confirmats, especially where clamping is difficult or where I would run out of bar clamps because I was assembling too many boxes at a time. 

I can’t think of the last time I even considered biscuits.  I’d have to do some math to see if biscuits have more glue area than dowels.  My guess is that glue area will determine the strength.

 

[Packard]

I actually like the biscuit better because of twist resistance across almost the whole mitre.  The domino joint can have issues at the peaks because they are very inward.  PVA creeps.

edit: Assuming this is a picture frame, not a playground set for a gorilla.

No gorilla. [big grin]

I use miters at the corners of Shaker doors.  It is faster and easier for me, but mostly I don’t like the way paint looks on end grain.  With the miters all the perimeter is face grain.

I use two dowels to prevent twist.  I only used these photos because they were the only stacked biscuit photos I could find. 

But basically, what I am asking is:  Will stacked biscuits be as strong as a single domino?  Obviously, a single domino will be faster to cut than two biscuits.  But biscuits are a lot cheaper than dominoes. 

 

[Packard]

Have you considered using lock dominoes?  In the case of screen doors, for example, the lock dominoes should be at the top and bottom of the doors to keep them out of sight.

I use Hafele’s lock dowels.  But the Festool version looks substantially more robust.  I don’t use the lock dowels as structure, but rather as a replacement for clamps.  It allows me to produce a cabinet box and glue and assemble on site.  I worked out a way that I only need a 3/16” hole exposed for clamping.

 

[smorgasbord]

I'd guess from another perspective it would be which joint is strong enough for the use it will be put to?

This, of course, is the real question. What's strong enough given the complete situation?

YouTube tests of isolated individual joints miss that within a structure, the stresses on the joints may be quite different and/or provide additional strength (like having a back on a frame or carcass). They also miss that the geometry of the specific joint members can restrict being able to structure joints certain ways. Like being able to fit two dowels but stock too narrow for two dominos.

And then there's the environment. Outdoor versus indoor, but for a garden shed, being in a climate where it snows means roof needs to support additional static weight. Being in a windy area means more shear stresses on the entire structure. Temperature swings affect glue as well as wood stability.

 

[Svar]

Just a note, not all biscuits are created equal.
People look down on them, because the stuff they buy in a hardware store has poor fit and looks like it's made of compressed toilet paper. If you make biscuits from solid hardwood with correct grain orientation they make an excellent joint. Their glue and cross section area is not much different from any other floating tenon or spline.

 

[Crazyraceguy]

Just a note, not all biscuits are created equal.
People look down on them, because the stuff they buy in a hardware store has poor fit and looks like it's made of compressed toilet paper. If you make biscuits from solid hardwood with correct grain orientation they make an excellent joint. Their glue and cross section area is not much different from any other floating tenon or spline.

The issue I have with biscuits is the width-depth ratio. They do ok for aligning panels, hardwood edges, even face frames to each other, but not so good for smaller parts. Even though they do make those tiny FF biscuits, they are essentially worthless. I would build a face frame with pocket screws before even trying it with a biscuit jointer, with the DF500 being king there.

There is also thickness. Biscuits are only 5/32" (.156) (4mm) so, there's only so much they can do. Their use is limited to a small window. I had a DeWalt many years ago (and was glad to have it) but when it was destroyed in a fire, I never replaced it. The DF500 that I bought a couple of years before eclipsed it.

 

[simonh]

I'd guess from another perspective it would be which joint is strong enough for the use it will be put to?

Most structural engineers are going to work from that basis when designing a house. I know myself, not been a structural engineer, we tend to over-engineer many things and that just ends up costing more, or been complicated, or taking more time. I recently got some structural engineering software for a garden summer house I'm building and was amazed at some of the time/cost savings where I would have been way over doing things.

[member=66875]simonh[/member] I’d like to know more about this subject. Do you have a link?

It's UK based so might not been be applicable as the grading systems are probably different. However:https://www.timberbeamcalculator.co.uk/

 

[Mike Goetzke]

I've used small Dominos for frames and don't know how but couple have hit the floor and the corner joints failed. I purchased a Woodpecker's spline jig. This may be your best joint.