Festool Domino pull-apart strength in face to end grain butt joint

[Bennett Ostroff]

When using the domino joiner to join a 90 degree butt joint, I understand the domino that goes into the end grain is very strong because the majority of glue surface is long grain to long grain. But the other end of the domino that goes into the face is long grain (domino portion) to end grain, with the wood exception of the bottom and the sides of the mortise which is a small portion of it.

The sheer strength would seem to be very strong, but what about the pull-out strength? I’ve done a lot searching but can’t find this specific question addressed. I’m inclined to use a coupe of screws to increase the pull-out strength but I’m not sure if that’s necessary. This is a bookcase which won’t be seeing a lot of abuse, but it’s around 75 lbs and moving it would likely be done by applying that pull-out force lifting the majority of the weight.

The wood is 3/4” thick and 11” wide and I’ll be using four 5x30 dominos in each butt joint with Titebond II or Quick n Thick.
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[Simon MacGowen]

The only concern I would have of the bookcase shown in the images is racking if you move it with heavy content in it. None of the domino butt joints (cabinets, bookcases, drawers, wall shelves, chests/boxes, tables, and what have you) has ever failed.

I wouldn't want to spoil that piece with screws.

Simon

[Wade Lippman]

Everything looks like compression; I don't see why you should be worried about joint strength.

Racking might be an issue, but I've done worse.

[Art Mann]

I fail to see what forces might be pulling the joints apart in the photo you posted unless you try to carry the thing with heavy weights resting on certain parts. Furthermore, I am not convinced that screws would be stronger than the Dominos if racking forces are applied. I think it is always worthwhile for a woodworker without a whole lot of experience to construct some sample joints and then proceed to destroy them in order to convince himself of the integrity of his work.

My appraisal of that design is that it is not particularly robust and is at risk of racking forces. The strength of the joints used will only help so much. I would be tempted to install steel "L" straps on the back where they are not visible.

[Dan Friedrichs]

Beautiful piece! I agree that I wouldn't worry about it, but am curious how well it resists racking. Is it fairly sturdy?

[Bennett Ostroff]

Thanks all. I'm glad to hear you're confident about the domino strength. Art, I am far from a "woodworker without a whole lot of experience." I'm not sure what gave you that impression. Even experienced woodworkers have questions.

The piece pictured in my original post was a previous version I made using only screws (at a slight angle as to not go straight into end grain - and then plugged with dowels for aesthetics) and it is *extremely* rigid. The vertical supports in the middle help a lot. I'm not the least bit worried about racking forces. The pull-out force that had me concerned is if someone were to lift the piece by one or both of the shorter horizontal pieces, the horizontal piece would release from the dominos because those domino surfaces are glued up against end grain.

[Jim Becker]

I do not believe there is any difference here using the Domino vs using traditional mortise and tenon. I agree with the poster that the biggest risk is likely racking and that's a design thing rather than a joinery issue.

[Prashun Patel]

I like that design. In fact, I am building something similar for my son's wall.

I have not calculated this, but it is my experience that racking is significantly increased by the depth of the tenon (loose or fixed). I made a dining table that is done completely counter to my intuition: a 2" slab top upon two metal legs connected by a metal plate, lag screwed to the top. The table is 108" long and there is no stretcher. There is almost no racking after about 4 years. I suspect the key is using enough screws (or dominos) and making them as deep as possible.

[Art Mann]

Pardon me if I offended you but the very nature of your question offered ample evidence of your woodworking experience. I began serious woodworking in 1977 and I have had many years to observe my own design failures. I was just trying to spare you a little of that pain. You have now gotten four replies from experienced woodworkers trying to tell you that your real problem in the long run is going to be racking. You can choose to learn from that or you can learn like I did..


Thanks all. I'm glad to hear you're confident about the domino strength. Art, I am far from a "woodworker without a whole lot of experience." I'm not sure what gave you that impression. Even experienced woodworkers have questions.

[Bennett Ostroff]

I do not believe there is any difference here using the Domino vs using traditional mortise and tenon. I agree with the poster that the biggest risk is likely racking and that's a design thing rather than a joinery issue.

With almost all mortise and tenons, the tenon goes into the face grain oriented so that the tenon is glued against almost all long grain (pic 1). In this domino situation, since the wood is only 3/4" thick, they must be oriented 90 deg resulting in the mortise's glue surface being almost all end grain (pic 2).

It sounds like despite this, they're incredibly strong. I glued some test dominos last night and tried to pull them out with a pliers with all I've got...nada. I'm content.

Thanks Prashun - it makes sense that the depth of the domino would be positively correlated to the racking resistance.

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[Carl Beckett]

I guess if the thicknesses allowed it, you could put the smallest dominos in sideways and then have face-face glue surfaces...??

[Dan Friedrichs]

Looking at this further, I just want to reiterate how cool I think the design is. Apparently a lot of people think it "shouldn't be possible" due to racking, but it sounds like that's not an issue, which is I guess why the design is so neat :) Very nice work...

[Robert Engel]

We always need to be open to suggestions no matter how much experience we have. From my limited amount (25yrs) of experience I am confident of one thing: Wood does what it wants to do & we need to design things with this in mind.

With this in mind, my first thought wasn't racking, but the amount of unsupported wood you have there.

But regardless, if the wood is throughly dry, acclimated and stress free, you'll be OK.

That said, it is a nice looking piece.

[Frank Martin]

How about miter joinery with dominoes? That may be stronger.

[Tom Bender]

Hi Bennett
Nice design, I like the look of the metal legs, but I cringe a little for your floor.

Even experienced woodworkers can benefit from a test to destruction, and it can be fun and use up some of that heap of offcuts I know you have.

Tom

[Brian Holcombe]

Add a board turned 90 degrees at the back of the sides and make one single center upright of 8/4 material with the tenons aligned along with the grain.

Even experienced people run into these kind of questions, in fact I think it makes a great discussion.

[Randy Heinemann]

The Domino makes designs like yours easy to make with sound, strong joints. Isn't that the genius if the Domino? It just makes making unique designs easy.

[Brian Holcombe]

It's a tenon, it should be planned like any other tenon in that you want face/long grain contact to be the major feature.

[John TenEyck]

Add a board turned 90 degrees at the back of the sides and make one single center upright of 8/4 material with the tenons aligned along with the grain.

Even experienced people run into these kind of questions, in fact I think it makes a great discussion.


^ is the right idea if greater rigidity is needed; add one or more boards at 90. But I've seen a lot of MCM designs similar to the OP's. Most (all?) of those were constructed with doweled joints, often in 3/4" veneered plywood designs. A lot of those pieces are still around. I suspect the OP's design will be just fine.

About racking resistance of the joint. The depth of the joinery is only important if the connector fails in tension. In that case, a longer tenon, dowel, Domino, screw, etc. would have offered higher strength. The rigidity of the joint against racking, before failure, is more related to the surface area of the joint where the two members meet. Joints made with thicker, wider stock will have more racking resistance than those with thinner, narrower stock. Orientation of the joint members is key, too, which relates to Brian's recommendation. Orienting the members perpendicular to the racking forces reduces the stress the joint is subjected to. Sorry, I don't mean for this to sound preachy, just how I see it.

John