Tenon length versus joint strength - Woodworking Talk - Woodworkers Forum
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post #1 of 5 Old 07-21-2013, 07:44 PM Thread Starter
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Tenon length versus joint strength

Hi All,
I have a question on depth of the mortise versus joint strength. I read *somewhere* that the depth should be half the rail width. A rail that is 4" wide would have a mortise 2" deep. With the particular router bit I have the deepest cut is about 1.25". Would the 1.25" mortise be strong enough?

Additional data: stock is 26/32 thick, mortise is 3/8"wide, rail is 4" wide, stiles are 4 and 3" wide. Application is a loft bed frame.
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post #2 of 5 Old 07-21-2013, 08:04 PM
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The mortise will be plenty strong enough, it's the tenon that really needs the strength. You should be ok, but to be safe I would plug it with 2 dowels. At least you know the joint will be secure.
If you have room you could run the dowels from the backside and they won't be visiable.
Good luck!

I am by no means a pro, but there are plenty of pros here, and if one of them tells you otherwise, I would listen to them.
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post #3 of 5 Old 07-22-2013, 07:59 AM
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This looks like a good place to discuss just how a mortise and tenon joint adds strength over just a glue joint. I am a little pushed for time this morning so will let someone else start.

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post #4 of 5 Old 07-22-2013, 05:46 PM
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I am very interested in this discussion....I'm going to build a tv table within the next few weeks, and plan on using m&t joints.....
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post #5 of 5 Old 07-22-2013, 07:15 PM
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The question I usually hear is, "Why don't you use a 1/2" or at least a 3/8" tenon in 3/4" wood?" Or why not just 1/2" stock and shove the whole end into a 1/2" mortise? Well, that's not the way it's done! Oh wait, that was my father speaking...
But actually, we don't need overkill in this case and we also don't want to weaken the mortise walls either. The old rule of 3rd & long works for at least 99% of all case work I've ever done. A 1/4" tenon from the center of a 3/4" piece of stock, that penetrates at least 1/2 of the mortised stock is the formula I use. The way this equals the strength of the parent 3/4" stock is that the cheeks and shoulders are square or slightly undercut and drawn against the mortised stock with enough force that no racking (or rolling, for that matter) can occur. And the tenon is long enough that it offers a sufficient long grain gluing surface. I want to mention here that it is imperative that the tenon be composed of LONG grain. Any knots, swirly figure or anything other than LONG grain should not be used. Set it aside for a short piece...maybe. Remember that the glue is stronger than any wood, so you don't want a failure (breakage) to point to you! And funny grain is unpredictable.
Now, all that said, I know of no rules against 'rail fence' tenons and I've used them many times when I've plenty of mortise wall to work with and/or when I'm pretty sure that when the object is moved for the first time, it will be towards the dumpster!

Well, I've hogged the mic too long here, someone elses turn...
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