Fine Woodworking published the results of joint strength tests in their January 2009 issue (#203). They tested eighteen joints: half laps, bridle, mortise and tenon, floating tenon, miter, splined miters, dowel max, bead lock, domino, biscuits etc. They attached a rail to a stile with various joints and tested them in a lab under controlled conditions for lever strength. The range of the results ran from 1660 pounds of breaking strength for the half lap joints to a meager 200 pounds for the stub tenon. Traditional mortise and tenon rated near the top with a breaking strength of 1444 pounds followed very closely by floating tenon at 1396 pounds The bad news is that the domino placed #14 out of 18 at a lowly 597 pounds, just above biscuits at 545 pounds.
I replicated these tests for half lap, mortise and tenon, floating tenon, biscuit, and of course domino. I used used 1x2 1/2" stock and a simple vise and my arm. I performed the tests twice for each joint, once in red oak and once in white pine. I used Tightbond's original glue and allowed each joint to dry for three days. Obviously I can't give a psi specification for breaking strengths, but I can tell you that my results paralleled those published. The half lap was incredibly strong , The mortise and tenon and floating tenon were also very very strong. The biscuit sheared off easily, but this was no surprise. Sadly the domino (#8 50 floating) joint broke with sickening ease. The domino (#8 50 tight) joint was a bit tougher, but was not very impressive. Using two dominos if you can work them in substantially improved the joint, but fell well short of a mortise and tenon or floating tenon. The shortcomings of the domino surprised and saddened me. The questions for Festool Domino owners is why so weak and what do?
In my opinion, the weakness results from the relatively slim glue surface of the domino. This shortcoming has in my opinion been exacerbated by
the impressed pattern on the Festool dominos. Initially, I assumed that the beech domino was meant to swell like a biscuit and the impressed pattern on the domino would swell up and disappear. But this was not the case. On the joints I tested these impressed patterns did not swell out from immersion in water based glue, but remained impressed. I do not know what Festool hoped to achieve by adding these impressions, But they result in a further reduction of glue surface. They reduce the wood to wood contact by creating a series of gaps along the whole glue surface thus weakening the joint.
So what to do? The answer is simple; increase the glue surface by widening the glue surface and getting rid of the impressions. You do this by using the domino joiner to make traditional floating tenon joints and making up wide smooth floating tenons. I recently made up red oak floating tenons 1 3/4" wide x 2" long for the #6 domino bit. Making up yards and yards of tenon stock is simplicity itself with a band saw and a planer. I cut matching mortises in rails and stiles by lining up 2 Domino machine cuts to make a 2" mortise including the corners which provides a 1 3/4" flat surface. I then glued up the joints using the smooth, wide, oak tenons. The resulting joints are enormously strong: far, far stronger than using the dominos. This method was quick and easy and produced true chair-strength joints. So the Domino machine is a great addition to my shop, but without the dominos!
I replicated these tests for half lap, mortise and tenon, floating tenon, biscuit, and of course domino. I used used 1x2 1/2" stock and a simple vise and my arm. I performed the tests twice for each joint, once in red oak and once in white pine. I used Tightbond's original glue and allowed each joint to dry for three days. Obviously I can't give a psi specification for breaking strengths, but I can tell you that my results paralleled those published. The half lap was incredibly strong , The mortise and tenon and floating tenon were also very very strong. The biscuit sheared off easily, but this was no surprise. Sadly the domino (#8 50 floating) joint broke with sickening ease. The domino (#8 50 tight) joint was a bit tougher, but was not very impressive. Using two dominos if you can work them in substantially improved the joint, but fell well short of a mortise and tenon or floating tenon. The shortcomings of the domino surprised and saddened me. The questions for Festool Domino owners is why so weak and what do?
In my opinion, the weakness results from the relatively slim glue surface of the domino. This shortcoming has in my opinion been exacerbated by
the impressed pattern on the Festool dominos. Initially, I assumed that the beech domino was meant to swell like a biscuit and the impressed pattern on the domino would swell up and disappear. But this was not the case. On the joints I tested these impressed patterns did not swell out from immersion in water based glue, but remained impressed. I do not know what Festool hoped to achieve by adding these impressions, But they result in a further reduction of glue surface. They reduce the wood to wood contact by creating a series of gaps along the whole glue surface thus weakening the joint.
So what to do? The answer is simple; increase the glue surface by widening the glue surface and getting rid of the impressions. You do this by using the domino joiner to make traditional floating tenon joints and making up wide smooth floating tenons. I recently made up red oak floating tenons 1 3/4" wide x 2" long for the #6 domino bit. Making up yards and yards of tenon stock is simplicity itself with a band saw and a planer. I cut matching mortises in rails and stiles by lining up 2 Domino machine cuts to make a 2" mortise including the corners which provides a 1 3/4" flat surface. I then glued up the joints using the smooth, wide, oak tenons. The resulting joints are enormously strong: far, far stronger than using the dominos. This method was quick and easy and produced true chair-strength joints. So the Domino machine is a great addition to my shop, but without the dominos!
