I am wondering what purpose two small double mortise and tenons serve rather than using a wider mortise and tenon. I notice in some furniture plans there is often a double mortise and tenon when joining an apron piece to a leg or a rail to make a draw cavity. I am assuming it has to do with wood movement in the joint. However, the joined rails are often not very wide and wood movement would be extremely small.
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Replies
The only reason for using them is on a very wide tenon. You'd make 2 in order to account for wood movement. But I've never used one on anything less than about 6 inches or so.
The drawer cavities you describe usually have rails above and below that are wider than they are high. A single tenon would have contact surfaces that were mostly (proportionally) end grain in the leg to flat grain in the stretcher.
Making two tenons increases the amount of flat grain surface contact for effective gluing... you wind up with 4 vertical surfaces instead of just the two on the outside of the single "horizontal" tenon, making for a stronger joint.
“[Deleted]”
From a FWW article on double tenons:
By doubling the surface within a wood joint, you can greatly improve the joint’s strength without increasing the size of its parts.
It is an interesting question though.
I get that the dogma is that the twin tenons are stronger but is the difference the equivalent of argon fill in triple glazing? Makes a difference, but would anyone other than an engineer notice?
Kinda similar to the argument of one glue is stronger than the other. Yellow glue is stronger than hide glue, but hide glued joints are still stronger than the wood itself, so why does it matter?
Consider the thickness of the tenons, would double tenons leave you with tenons so thin they could be sheared off under stress?
It is far more secure and superior to a single horizontal tenon in my experience. It effectively locks the two components together eliminating any racking.
I always use them.
Ah. In my response I was thinking table apron, not drawer divider. The divider shown is a different matter entirely.
But I would use a sliding dovetail on drawer dividers, not a mortise and tenon, double or otherwise,
There is no question, they are stronger. With a dual M&T joint you increase the mechanical connection which increase the ability to resist racking and twisting. You also increase the amount of glue surface. It is a high strength joint and while it doesn't always need to be used, there is no problem if you do.
A double tenon also leaves more material in the mortise part improving its strength.
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