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I had been watching The Garage Woodworker for several years, and I’d seen him build several pieces with loose tenons using his horizontal mortiser. The appearance of his machine was so intimidating I just kept putting off getting the plans and building my own—until a few months ago. Building it wasn’t easy, and I still have a few adjustments to make so it works more smoothly, but I can already tell I’m going to love it. However, you don’t need an expensive tool or a complicated home-made machine to use loose tenon joinery. In the past, I simply used a router with a mortising jig (hundreds of options on YouTube) or a rail guide to cut both mortises. I then cut my tenon stock to size and rounded over the edges on the router table to fit perfectly into the router cut mortises.
If the loose tenon is so great and easy to use, the question becomes whether it is strong enough to replace the traditional mortise and tenon. Ronald L. Aman and others published an article in a trade magazine called Forest Products Journal in 2008 discussing this very topic. According to Aman, the loose tenon falls between dowel joinery and the traditional mortise and tenon with regard to strength. This might not sound too good until you think about the convenience it provides. “The added strength [over dowel joinery] combined with increased wood utilization makes this joint an attractive alternative to mortise and tenon construction” not to mention the speed and ease of using loose tenon joinery. (Aman). Fine Woodworking also did a study in 2001 and discovered essentially the same thing—the traditional joint is stronger. What’s important to note, however, is the fact that the stress required to break any of the joints was in the excess of 3,000 lbs. “[A]ll of the joints tested were stronger than the most severe load that they would ever likely be subjected to in normal use or even abuse” (Gray 79). Additionally, “[i]n all cases, the bond at the glue line failed first, and then the mortise cheek failed. The tenons never failed or broke” (Gray 78).
One of the most important discoveries in both studies was the significance of the fit of the tenon, loose or traditional. “[D]imensional tolerances on the loose tenon stock must be carefully maintained to ensure a suitable fit between the mating pieces” (Aman). The tenon must fit tightly in the mortise for the joint to be strong—a friction fit—tight enough to resist gravity.
With all that said, there are still going to be times when a traditional mortise and tenon is going to be required, especially when a “super-strong” joint is needed. The loose tenon just adds another strong, reliable joinery option to your playbook.
Works Cited
Aman, Ronald L., et al. “An Evaluation of Loose Tenon Joint Strength.” Forest Products Journal, 28 Mar. 2008, www.freepatentsonline.com/article/Forest-Products-Journal/177361978.html.
Gray, Bruce, “Testing Joints to the Breaking Point.” Fine Woodworking Magazine, #128 Mar/Apr. 2001.
"Quick and Easy Loose Tenons." Woodsmith. April 2017 Issue 39 Vol 230. https://www.woodsmithlibrary.com/view/issue/230/