Tips on figuring out exactly where is keeping M&T joint from closing?

[Peter Stahley]

Hey all, I've got a breadboard end underway that I've jumped in to & its a touch snug... It's got a "typical" groove with spaced mortises on the breadboard and 5 tenon's with little stubs between them on the table side.

I can tell that the outer two tenons on either side of table fit pretty well, but something is hanging me up somewhere on the center 3.... However, being unable to see anything & since all the joints "go together at once" I can't troubleshoot it like a normal M&T.

Any tips on how to figure out exactly where it's hanging up? I don't want to indiscriminantly shave all 3 center tenons in case I make them looser than the should be if only one of them is the offending party.

I've used the trick of looking for the "glazed"/burnished-looking wood that has some compression on it from test fitting, but the hickory I'm working in doesn't show any sign of rubbing to me.... As you can see it gets close, but not quite all the way home.

274923

[Roy Harding]

Try some chalk on the tenons. It makes it easier to see where any excessive rubbing may be taking place.

[Jerry Miner]

If you can cut a groove in a block that matches your mortise width, you can "test fit" the block in various places. (won't help if the mortises are off, but will show fat tenons)

[Keith Weber]

Just to add to Jerry's good idea... you could cut a tenon in another block and use it to find a tight spot in your mortice.

[Brian W Smith]

Pencil or soot here.......but like the chalk idea,will try that,thanks.

[Bob Lang]

If you can get that far, it isn't likely an issue with the cheeks or edges of the tenons, it will be either the groove or the bottom of one or more mortises. Set your combination square from the end of the tenons to the shoulder, then set the end of the square in each mortise. A little extra depth in the mortises is OK. Check the depth of the groove by setting the square on the tongue.

Bob Lang

[brian boggs]

This is where a good dial or electronic caliper can save you a lot of time. What you are looking for is too small to measure with typical woodworking fractional measuring tools. A good caliper will allow you to measure in thousanths in a way that is easy to see. By simply measuring the dimensions of both the mortise and the tenons you can quickly find out where your tenons are too thick and by exactly how much. Mortise spacing is less critical so your fractional rulers can get you through that part. Flatness is another thing to check.
Adding dusty coloring agents like chalk seem messy not necessary if you can get a good accurate measurement.

[Mike Wilkins]

Rub some pencil lead onto the corners of the mortise, do a trial fit, and the dark areas on the tenons should tell you where the fit is tight.

[pat warner]

Of course, Brian has it.
Like plastics to Dustin Hoffman (The Graduate), Metrology is to the sleuth.

[glenn bradley]

+2 on calipers. If I am seeing the picture right, that is quite a gap. Do I understand that the breadboard is hard to slip on or that it is bottoming out?

[Peter Stahley]

+2 on calipers. If I am seeing the picture right, that is quite a gap. Do I understand that the breadboard is hard to slip on or that it is bottoming out?

Hard to slip on... Either end M&T closes pretty good on its own (pushing on diagonally) so I think I'm good at those two spots, but when I square it up and push all 5 tongues in square it gets tight before it closes.

I've checked mortise depth using my square as a depth gauge & am pretty certain I'm good all around, similarly with the haunch. So I'm guessing I've either got a thickness or an alignment problem (given that a slight misalignment will throw it off a decent amount over the 40" length).

I've avoided calipers up to this point, wanting to avoid the temptation to be that precise, but maybe it's the only way to spot check each interior M&T at this point and avoid uniformly thinning down in the center.

[glenn bradley]

I can understand not wanting to over-precision the thing. Have you laid a straight edge across the tongue to be sure that the panel has not arc'd? The top and bottom to tongue measurements could be exact but, if there is a bit of cup to the larger panel, you would get trouble.

[Sam Stephens]

good advice all around. Just to add, I had an issue like this after cutting the mortises for BB ends blind on my router table. After much head scratching, I realized that my router fence was not quite square (90 degrees) to the table.

[Brett Bobo]

Using your calipers, I'd check the width and thickness at the start and end of each tenon to confirm uniformity. In one or both planes of each tenon, you could have inadvertently created a flared tenon ever so slightly, which could be compounded having multiple tenons along the top.

[Charles Lent]

Is your tenon board flat? A slight curve across the grain could cause binding when the straight mortise board is pressed on. It doesn't take much.

Charley

[Chris Friesen]

Test that the mortise walls are all square. If one of them tapers inward slightly at the bottom it could cause that sort of problem. (And you probably want a bit of wiggle room in the outer mortises anyway to allow for expansion/contraction with humidity anyways...)

[Peter Stahley]

Ok, turns out the table had picked up a minor cup (likely due to some water that leaked on it into the garage - which nearly gave me a heart attack when I saw it, btw).

So, the cup seems to have 99% worked its way out by the time I came home from work today. I picked up calipers and everything looked good except one tenon that was dead nuts on so I trimmed it a little... and still had the problem.

I almost don't want to admit what the problem was, but I decided to recheck if my shoulder down below either end tenon was square. Turns out i'd only checked to the tenon depth, and not the underside on the two ends (in the thicker mahogany rails). Turns out both were out of square and I'd forgotten to pare that endgrain down underneath after sawing.

1/16" trimmed off and boom - closed it goes. Now to attack the other side (with my first ever breadboard end in the books, save doweling).

Thanks for everyone jumping in with all the suggestions.

[Jim Matthews]

So, the "bottom" shoulder of the breadboard was out of square?

[Peter Stahley]

Table top side, below the tenon; it was angled out proud of what would have been "in plane" and pushed the breadboard away about a 1/16".

It was an artifact of the way I cut the excess material off and forgot to trim that little "hidden part". Lesson definitely learned.