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Many online sources, such as this guide from woodcraft.com recommend making the tenons 1/16 inch shorter and narrower than the space they are to occupy.

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Where is the benefit?

I tried to find an explanation, but the closest I've got was some forum mentioning that it is to prevent hydraulic lock. What is a hydraulic lock and why would it be bad? Are there some other reasons?

Martin Drozdik
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    Other than the excellent points in @FreeMan's Answer, another reason to leave loose tenons short (and actually to leave standard tenons short too) is to give a small pocket for excess glue to go. With loose tenons you can glue them into one side of the joint first, and in those mortises the tenon should bottom out for max strength (although it's debatable how much you actually gain), but on the mortises of the mating piece a small amount of space should be left, same as with normal tenons. – Graphus Jan 18 '24 at 08:58
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    P.S. Re leaving them narrower, there may be a secondary reason — to prevent any tendency they have to split the ends of boards should their seasonal expansion exceed that of the main wood species (which is certainly possible with beech floating tenons and numerous other furniture species). It's likely only an issue with wider floating tenons (as pictured) and is clearly not considered a problem with e.g. Dominos, which have exact-fit mortises for at least some of them; others can have wiggle room side to side to aid adjustment, and ease assembly. – Graphus Jan 18 '24 at 09:41

2 Answers2

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As a practical experiment, try making the mortise the exact size of the tenon and see if you can push the tenon into the mortise.

Now, add glue.

Without glue, you might get the tenon in place because the wood surfaces are rough enough that there are tiny channels through which the air can escape the mortise. Adding glue, though, provides a sealant to those tiny channels, preventing the air (and excess glue) from escaping. You're now creating pressure in the bottom of the mortise, and, when you let go of the tenon, that trapped air pressure will push the tenon some or all the way back out.

If it pushes the tenon out by just 1/16" and you made your mortises the exact length of your tenon, you may not notice that it popped out a bit, but, when the glue's dry and you're trying to put the other piece on, you'll discover that they don't go all the way in.

You might be able to work that way by gluing both sides at the same time and using a clamp to pull them together, but you may end up with a lot of glue squeeze out and you might starve the joint of glue.

This is the same reason that purchased dowels have flutes on them - gives air & glue a way to escape and not create pressure (or hydraulic lock) inside the hole.

FreeMan
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I have not heard of sizing tenons narrower (across the smaller dimension) than the mortise, as that goes against the most deeply held truth that a good glue joint requires good wood-to-wood contact. This idea was reinforced when attending a fine furniture woodworking school where we painstakingly trimmed our tenons by hand only until they just fitted the mortises perfectly. A properly sized mortice (no more than 1/3 width of the stock) will not split form any seasonal moisture variances.

There is the idea of making a tenon “shorter” (across the wider dimension, not depth) when there is the possibility of seasonal movement causing issues, but this is typically in long joints like headboards or tabletop breadboard ends, but I think that goes beyond the basic topic at hand.

As for the depth, the hydraulic resistance is a real thing. But also, the end of the tenon is end-grain and therefore offers little strength when compared with all of the side-grain contact.

Finally trying to fit a tenon to the exact depth of the mortice could lead to an unintentional bottoming out before the joint is closed, so leaving the tenon just short prevents that issue.

(Sorry for any misspellings, as my coffee hasn’t kicked in yet, or has kicked in too much, I’m not really sure!!)

Rob
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    True for traditional mortise & tenon joinery, but not so much for loose tenon joinery (as popularized, it seems, by the Festool Domino). – FreeMan Jan 19 '24 at 15:01
  • Hi, welcome to StackExchange. The key thing here may be that the floating tenons are commonly a different species to the primary furniture wood. Although they don't have to be by any means naturally, but where they are, as I mention in a Comment above, differences in seasonal expansion may be a concern for wider ones. But beyond that, especially when used as alignment aids similar to how biscuits are employed, deliberately doing this some or all of the time can make assembly much easier because of all the literal and metaphorical wiggle room :-) – Graphus Jan 20 '24 at 08:25