Domino tenon strength.

[rjwz28]

How much strength of the Domino tenon joint is from the rounded over sides? The reason I ask is that I want to make custom wider tenons and want to know if I need to make the rounded sides exactly the same radius as the stock ones or it doesn't make difference as the holding power is from the glued opposite wide surface areas of the tenon?

thanks,
Rob

 

[jacko9]

Rob,

I square up the Domino Mortises and leave the tenon with cut shoulders.  I find it easier to do a little chisel work than router table or sanding.

Jack

 

[PaulMarcel]

The Domino radiused edges seem grooved by compression (instead of by cutting); dunk one in water and watch it and the grooves flatten a bit.  But they don't press hard enough on the sides to give you the good glue joint (unless you use a gap filling glue).  I think the biggest reason to either radius the sides to match the mortise or square the  mortise to match the tenon like Jack does is so the tenon itself doesn't have to rely on glue to not move.  If you put a square-sided tenon in a radiused Domino mortise, you could wiggle it side to side.  Doing that relies on the glue to do all the work.

 

[barnowl]

How much strength of the Domino tenon joint is from the rounded over sides? The reason I ask is that I want to make custom wider tenons and want to know if I need to make the rounded sides exactly the same radius as the stock ones or it doesn't make difference as the holding power is from the glued opposite wide surface areas of the tenon?

thanks,
Rob

IMHO, I don't think that the sides add much strength. The lion's share is from the top, and bottom.

I base that on the fact that the Domino has wider settings (position 2 and 3) for the cutter.

And it is recommended to cut only the first Domino at the regular width, for alignment, and all the others at a wider width, for ease of assembly.

I don't think Festool would have designed it that way if the integrity of the joint would be compromised by very much.

 

[Jesse Cloud]

Another thing to consider is that once you get past a couple of inches in width, the wood movement will be significant enough that "exactly matching the tenon to the mortice" becomes meaningless.  I round over shop made tenons, but I'm not trying for a perfect fit.

 

[fritter63]

In a traditional M&T joint, all of the strength comes from the glue surfaces on the cheeks and the mechanical joint of the shoulders ,and none is expected from the short (don't have a name for it) part of the tenon. And if you think about this makes sense, as that would be an end grain joint anyway which has ZERO strength in a glue joint.

When I used to do M&T the hard way, I'd rout my mortises with rounded ends, then chamfer the tenons so they'd go in and miss the curves, but never glued them.

However, given the tight fit of the domino's on the narrow setting, I'm sure there is some *mechanical* strength added.

 

[jacko9]

All good answers however, you might want to consider what loads the joint will encounter.  If your making a apron to table leg joint for a large table you want to make sure that you design for racking stresses that the shoulders of the tenon help support the glue joint over many cycles of loading.  The best m&t joint has shoulders and is wedged top and bottom to offer full mechanical strength.  There are many studies to support joint strength both in new joints with an applied load and long term cyclic loading.  But, if your only making a light load joint don't worry about it the glue face joint will suffice.