Oops All Tenon

A design pattern which uses CNC to cut many mortises and 'tenons' made of roundover beams.

Overview

I have a proclivity for the mortise and tenon joint. It's strong and relies on mechanical contact for loads in shear and compression. The glue is only put under load in tension, and most designs won't call for substantial loading of such a joint under tension.

The big drawback, of course, is that the mortise and tenon takes quite a bit of effort. Even with specialized tools (like a pantorouter) creating the tenons is relatively costly in terms of time. Accuracy can be pretty critical – small errors in tenon-depth create ugly gaps in the final assembly. The mortises are tricky to make as well, but they can be done by CNC with perfect accuracy.

I've come up with this design pattern in response to these issues: remove the tenon entirely (or make the entire beam a tenon). It's extremely easy to run rectangular beams down a table router and round off all four long edges. The result is one very long tenon. Mortises are cut by CNC and the whole project may be assembled easily with no signs of poor accuracy.

The advantages to this method are numerous:

• Damage to the edges of beams (the second most vulnerable part of the geometry) is filleted away at the end, removing it.
• Damage to the ends of beams (the most vulnerable part) is hidden inside of the mortises.
• Tight tolerances are only required on features for which it is easy to hold high tolerance (planed thickness and CNC-cut mortises).
• Glue-up step requires few if any clamps (still testing this one).

But of course there are a couple drawbacks:

• Finishing is a little trickier as the glue-mating surface of the 'tenon' must be protected.
• A flush surface between mortise and 'tenon' cannot be created as the mortise 'sets back' the edges.

Notable Specifics

Tenon Tolerances

Mortise cuts are done by CNC machine and will be effectively perfect. However, tenons must mate with these cuts and so themselves must have reasonably tight tolerances. Tablesaw cuts on even a nice tablesaw result in ±0.01" tolerances, which is pretty bad. Furthermore tablesaw cuts leave marks on the material which require additional sanding steps to remove. It's best to give tenon-beams a post-saw pass on the planar if geometry allows.

Next time using this number, check and adjust. There must, naturally, be some amount of gap between the tenon and the mortise surfaces. This gap is the minimum neccessary between the theoretical largest-possible tenon and the mortise. Experience suggests it should be around 0.005"