Festool Domino - oversized mortise width settings?

[Michael Heffernan]

I just picked up a used Festool Domino DF500. Read through the manual and did a few test mortises. Can't wait to use it for my next project.
However, I have a question about the oversize mortise width settings on the Domino. One is 6 mm (1/4") and the other is 10mm (over 3/8") wider than the tenon. Other than using it for floating tenons in a breadboard end, what would you use these settings for? I see in the manual, that they recommend it for butt joints and panel glue ups, mortising the outside mortises with the normal (tight fitting) setting and the inner mortises wider. But, 6mm or 10mm? Those are huge 'fudge factors' in the event of misalignment of your mating pieces. It would be nice if you could incrementally increase the width of the mortise, by say 1mm or 1/32".
Any experts out there might have some insight into this function on the Domino.
Thanks

[Wade Lippman]

Your strength is on the faces; it doesn't make much difference if the sides are tight or not. The loose fit is to ensure that the pieces go together if you don't get the registration perfect.
In general you use one tight fit to make sure the pieces go together as you want them, and the rest loose. I normally do one side tight and glue them in that first, and the second side loose.

[Mike Henderson]

As far as I can tell, the extra width is for slight mismatches in the location of the mortises on the two faces. So let's say you were using dominoes to glue two pieces of plywood together. You might make the first mortise on each side with the small setting, then make the others a bit larger to allow for slight misalignments. If you want to, you can do one side all narrow and the other side wider (except for the first one). The first one (one each side) keeps the two pieces from sliding, while the wider ones make sure all the dominoes fit into the mortises.

As far as strength, the strength is in the glue surface on the faces of the dominoes, not on the glue surfaces on the sides.

Mike

[Sam Murdoch]

I have complained to Festool that this margin is tooooo large. I wish they offered one in between the 1st and 2nd setting and call it good. In the case of gluing up boards for a table top or attaching a solid wood edge to a ply shelf I typically make all the slots on one edge the good fit size and then the centerline mortise on the opposing side tight while leaving all the other slots on the big size. This certainly helps the ease of the glue up. The dominos do fit very tight otherwise and a little bit of play left to right does not compromise the joint. The face to face alignment is excellent. Often I don't change the settings but rather do two plunge cuts on one of the faces, each slightly off my mark. This seems like more work but I am never in that much of a hurry and I prefer the slots not be so oversized.

[Steve Baumgartner]

I have complained to Festool that this margin is tooooo large. I wish they offered one in between the 1st and 2nd setting and call it good. In the case of gluing up boards for a table top or attaching a solid wood edge to a ply shelf I typically make all the slots on one edge the good fit size and then the centerline mortise on the opposing side tight while leaving all the other slots on the big size. This certainly helps the ease of the glue up. The dominos do fit very tight otherwise and a little bit of play left to right does not compromise the joint. The face to face alignment is excellent. Often I don't change the settings but rather do two plunge cuts on one of the faces, each slightly off my mark. This seems like more work but I am never in that much of a hurry and I prefer the slots not be so oversized.

Agree 100% with all these suggestions. I do the same (and have the same complaint).

Also, the dominos are notorious for being a bit over thick - perhaps due to swelling a bit - especially the 6mm ones. Many folks dry them in an oven or microwave to shrink them, or else sand or plane the surfaces. Without this a joint with a lot of dominos can be pretty hard to push together.

[Brian Kincaid]

...a joint with a lot of dominos can be pretty hard to push together.

I use a large rubber mallet for all my domino assemblies. I knock the domino's into the holes and knock the mating piece down tight before clamping. The wider setting is nice if you use them to attach table tops. The idea is you glue one side normally, and you overcut the opposite side to allow wood movement (end-grain). I agree that it is probably way too wide though. The 2nd setting is fine even for the table top attach.

-Brian

[Prashun Patel]

What's the downside of the mortise being too long? The huge upside I like is that you hardly have to mark the two sides. You can practically eyeball both sides of the joint (at least you don't have mark that carefully) and still get a perfect glue up. It just makes the tool incredibly forgiving with no apparent (to me) downside.

[Wade Lippman]

Assuming your glue holds, there is no downside. And if your glue doesn't hold; well, the joint is broken anyhow.

[Larry Fox]

If you want the slot to be wider by a very small amount (like 1mm) you could also stroke the domino itself over your block plane a few times on each side (not each face) and it will do pretty much the same thing. Kinda a pain if you have a lot to do but it will get you there. For me, I just cut the dominos tight on one side and in the middle setting on the other side and rely on the strength of the face glue to hold. I have gone through boxes and boxes of dominos and never had even one fail.