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Tusk tenon for trestle table

Greg_Millert's picture

I'm designing a trestle table with tusk tenons on the stretchers and need help with proportions. The leg assemblies for the table will have two uprights, each 2 1/8 inch square. Stretchers 4 inches by 5/4 thick will run between the legs and will be about 13 inches off the floor. (there are another pair of stretchers at the top of the leg assembly, directly under the table top.) I've currently designed the tusk tenons on the stretchers to fit into a through mortise that will be 7/8 inch thick by about 3 1/2 inch high. I used 7/8 inch because its slightly more than 1/3 the total width of the leg and is thick enough to allow me to cut the mortise for the wedge. The mortise for the wedge will be 3/8 inch wide, leaving 1/4 inch of wood on each side of the mortise.

Questions: 1.) Is the mortise for the tusk so wide that it will structurally weaken the legs? If so, what would be the right width? 2.) Is 3/8 inches to wide for the mortise for the wedge? If so, what would be a good width. 3.) Authors seem to be all over the lot as to the right angle for the slope on the wedge. Ian Kirby says 5 degrees, others say 6 or 7 degrees, and at least 2 authors say 10 degrees. I'm inclined to go with the 5 degrees because Kirby has a lot of experience.

Note: The leg assemblies are already dimensioned and mortise and tenons cut to mount the legs into the top and bottom of the leg assemblies so I don't want to change the 2 1/8 inch square legs. Wider might have been better but this width looks good in the design.

Sgian_Dubh's picture

(post #106258, reply #1 of 3)

It sounds to me like you've got the proportions about right. You are leaving plenty of meat in the leg, i.e., 5/8" either side of the mortise.

You might consider reducing the width of the mortice for the key or wedge to 1/4" to retain a little more strength in the stretcher's through tenon. But whether you do, or keep it at 3/8", it will be rather tricky to cut the angled through mortice through 3-1/2". Not impossible by any means, but tricky all the same, and only you know your own capabilities.

One other possibility worth considering might be to drive your wedge- or opposing wedges, perpendicular to the wide face of the stretcher tenon. This is easier to cut, but may not be aesthetically pleasing for you. That's down to your judgement.

As to the angle of the wedge, the steeper angles tend to work loose sooner, and once a little looseness develops, the quicker the wedge will work its way out of the slot. Conversely, a lower angle requires more accurate workmanship, but is more effective once assembled. The difference between the two (5 to 10 degrees) is not huge, but worthy of thought. If you are comfortable shooting for the lower 5 degree angle, go for it. Don't forget that the wedge mortice needs to chopped out parallel to, and about 2-1/16" past the stretcher's shoulder line.

A final point, pernickety as it may seem to others, but I like to use proper technical terms for reasons of clarity. You correctly identify the stretcher, but the rails at the top of the leg structure joining the two end frames are just that, rails. In the US these rails are sometimes called aprons.

Here's a good technical book. It's worth every penny. Ernest Joyce, The Technique (US Encyclopedia) of Furniture Making. ISBN, 0 7134 0217 2, UK, or in US, 0 8069 6440 5.

pnbrown's picture

(post #106258, reply #2 of 3)

The mortise thru the stretcher for the wedge will be no problem as long as one excavates from both sides and meet in the middle. I generally remove the waste first on the drill press with a good chip clearing bit and finish out with a chisel. One would then cut the angle at the end with a mortising chisel.

Steven_P._Thomas's picture

(post #106258, reply #3 of 3)

I used tusk tenons on my kitchen table and went with four degrees on the tenon and mortise. I really cheated in the way that I did it for accuracy, but hey, it worked (and still is).

I sliced the stretcher vertically into three pieces on the bandsaw. I took the middle slice and cut out the mortise on the table saw on each end, and then reglued the pieces as they were to begin with minus the mortise waste of course. In order to keep the tusk tenon from pushing out the end (that had been sawed and then reglued) I drilled holes and drove pins through the ends after the glue dried. I was able to get the 4 degrees exactly, and the tenons only back out a little after scooting the table (twice a day) for about 6 months. It's easy enough to disassemble too. Since I sliced the same piece of wood in three pieces when it was reglued, it is very difficult to tell that it was ever ripped to begin with.