An exploration of the Mortise and Tenon joint used in making Tonfa. A de-constructed joint is used to reveal its construction, strengths and weaknesses.
Thanks to a combination of natural hand position and relative safety compared to other weapons, tonfa are one of the most commonly encountered martial arts weapons.
Interestingly, tonfa use possibly the strongest and the oldest known woodworking joint: the mortise and tenon. Understanding the strengths and weaknesses of this joint enables Kobudo practitioners to ensure their equipment stays in top condition.
The below image shows the similarity between joints used on a bed I recently built and my tonfa, both of which make use of the mortise and tenon. Due to its large size the bed post was made in three parts, which provided the opportunity to document its ‘inner workings’.
There are actually three parts to a Mortise and Tenon: Mortise (the hole), Tenon (goes in the hole), and wedges (lock everything in place).
The tenon is placed in the mortise and the wedges are hammered in, permanently securing the joint. Wood glue prevents the wedges from slipping out, however the strength of the joint comes from the dynamic tension created by the wedges, not from the glue.
With the top layer of wood removed, the inner arrangement is visible. The sides of the mortise (hole) are cut at an angle, so when the wedges are driven into the tenon, the sides splay out and it is then impossible for the piece to be removed.
This image shows how the wedges perfectly fit the empty space left by the angled mortise.
Glue up: it’s not easy, it’s not pretty, and you’ll probably be finding globs of glue on your hands/arms/neck for days. The important concept here is “squeeze out”, which (I’m sure you guessed) means an amount of glue should squeeze out once the joint is closed. There is an art to glue up, where only a bead of glue emerges, rather than a river. I have not yet mastered this art.
It is, however one of those rare cases where ‘too much’ is infinitely better than ‘too little’. Excess glue is wrung out of the joint like water from a sponge, leaving the perfect amount behind for a joint much stronger than one with dry surfaces.
So what makes a good mortise and tenon? In a word: precision. As mentioned earlier, the strength of the joint comes from the dynamic tension created by the tenon being pushed outwards against the sides, and the wedges pulling the tenon further into the joint, which is resisted by the sides of the mortise and shoulders of the tenon. As long as this tension is maintained the joint will stay strong.
A poor joint is usually the result of inaccurate cuts producing a loose fitting joint which lacks this tension. These joints may employ backup techniques to add reinforcement, such as pins or simply relying on glue. These joints will not last.
Kobudo weapons are subject to a great deal more stress than even the most enthusiastically used bed, so it is imperative that the mortise and tenons on our tonfa are as strong as possible. It is each martial artist’s responsibility to ensure their weapons are fit for training, and understanding the mechanics of this joint will enable you to do this effectively.