How do you troubleshoot a M&T joint?


Hi there-
I hand-sawed and -chopped a double tenon with haunches on a 17" wide glue-up for a bed footboard. It goes in most of the way into the mortised leg and then stops. Is there a trick for how to tell what the problem is? How can you tell exactly where along the 17" of joint the problem is when nothing's visible?
-Tom
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I'm assuming that the problem must be in the mortise, as any problem with the tenon should be instantly obvious. I'm guessing there's a spot, somewhere, that you didn't get chopped out deep enough.
Is there some reason you can't look into the mortise with a flashlight? That ought to show you where the shallow spots (if any) are.
If the problem is that the tenon is being pinched (i.e. tenon too thick, or mortise too narrow) somewhere along the way.. make a test fit, then take them apart, and examine the tenon on both sides for signs of wear or compressed wood fibers. That will tell you where the fit is tight.
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Doug Miller (alphageek at milmac dot com)
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I haven't tried this, so someone else please correct me if it won't work, but why not color the sides and end of your tenon with chalk or a soft-lead pencil, try it in the mortise, and see where the chalk rubbed off most inside the mortise? Seems to me like it should work. In the past I've always been able to figure out which sides need more chopping or paring by just wiggling the tenon piece and noting where it feels like it is stuck. Does that make sense? Good luck, Andy
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IMO it almost certainly would work to identify the problem area. I've never tried it, though, because I worry that the chalk or graphite may interfere with the glue bond. Maybe I'll try an experiment some day...

That's what works for me most of the time...
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Oooh, Andy was too fast on the gun. Yes, chalk on the tennons. To make it easiser to find the bad spot just chalk them close to where they start to bind.
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Its the part of the tenon that gets slightly squished and maybe shiny. You'll have to look *real* close. Is it bottoming out or just getting stuck? Try taking the edge off the edges, too.
Chris
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Use a set of calipers to mic the width and thickness of the tenon to see if it is consistent. If it gets thicker toward the shoulder you can reduce the thickness or width. If it the same all the way across the tenon the mortise is to small at the bottom or the tenon is too long. max

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"nireedmot" wrote in message

If you look real close you should see that the high side will generally be shiny on both parts from the friction of the trial fit. Those are obviously the areas to spend your time on.
Since it's a double, you also may have a situation where the mortises were not cut parallel, which may require some shimming once you get past the mating of the two components.
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I have an engineer's (metal machinist's) depth gauge. It's a chunk of thick steel with a thin wire that slides vertically through it, with a small clamp. Excellent gadget for probing how deep mortices are clean to.
Another useful gadget is an internal caliper with a screw thread adjuster. Hard to use as you need to hold it square, but it will often pick up a narrow spot that needs paring.
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If you suspect tha the mortise is the problem, take a narrower (2") piece of wood, and cut a tenon in it. Drag this through the mortise to see where it catches. If necessary, thin the tenon slightly so you won't be wearing the inner surfaces of the mortise.
If the tenon might be the problem, then, cut a mortise-sized dado in maybe a 3" length of stock and drag that along the length of the tenon to see where there's interference. If you must hand-chop, then chop a 3" mortise into a longer piece, then cut the ends off to turn it into a dado.
...just some ideas,
-Mike
nireedmot wrote:

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I suppose you can glue up a scrap making sure it's the "official" mortise dimensions. Might be easier than cutting a dado, if you can wait for the glue to dry.
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This sounds like a good candidate for loose tenon joints. Are you too far along and too committed to integral tenon?
Steve
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I'm not having the problem. Mike suggested cutting a dummy mortise to find why the real M&T wasn't fitting.
I'm just saying instead of cutting a dado, which will fit 2 sides, and may not be deep enough, to glue up a mock motise with p`4 pieces of plywood. A tablesaw can cut pieces squarely. You might be able to use a rubber band just to hold the pieces together.
Personally - I would first try the "look for the shiny bits" method. Or inside and outside calipers (if you have them). And only use the mock-up if you can't find the problem.
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On 19 Oct 2005 19:22:05 -0700, with neither quill nor qualm,

Ah, but it IS visible if you know what to look for. As you assemble a joint, the wood will burnish at any heavy contact point. Look for shiny spots on the tenon and in the mortise. Pare or sand those down and try again. Repeat until you have a snug joint which fully comes together.
You would also be better served by several tenons rather than a single long one. The footboard will probably crack on you as the wood in the panel dries. If I were doing this and had already cut a long tenon, I'd probably glue it only in the center of the mortise, about 2", then rely on the rails to provide the majority of the strength.
G'luck!
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