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A question about the tongue-and-groove corner joint and why it is only suitable for sheet goods

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Forum topic by HarveyDunn posted 02-16-2014 01:02 AM 977 views 0 times favorited 9 replies Add to Favorites Watch
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HarveyDunn

286 posts in 483 days


02-16-2014 01:02 AM

I’m working my way through Gary Rogowski’s “Joinery” book. On Page 101 he discusses “the tongue-and-groove corner joint”, illustrated thusly:

The text says “remember that the joint described here is useable only for plywood or other sheet-good material”.

I’m wondering why that is – the answer is not supplied in the text.

I supposed the answer is “movement” (isn’t it always?) but I can’t work out why.


9 replies so far

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Oldtool

1921 posts in 943 days


#1 posted 02-16-2014 01:58 AM

Pure speculation on my part: since the outside of the joint – plywood to solid wood – is flush, any swelling of the wood sides would stand proud of the corner piece. My best guess. You should email Gary Rogowski for confirmation.

-- "I am a firm believer in the people. If given the truth, they can be depended upon to meet any national crisis. The point is to bring them the real facts." - Abraham Lincoln

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jdh122

421 posts in 1570 days


#2 posted 02-16-2014 12:55 PM

If you replace the plywood with a wide glued up piece of solid wood, it will expand and contract seasonally, while the corner block will be almost totally stable along its length. This will eventually cause the glue to fail or the wood to crack/buckle. With plywood, on the other hand, there is very little dimensional movement because the plys are alternated 90 degrees to each other. To do this kind of joint with solid wood you’d need to only glue the center and then pin the ends in elongated holes like you do for a breadboard table.

-- Jeremy, in the Acadian forests

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CharlesA

1929 posts in 550 days


#3 posted 02-16-2014 02:35 PM

How is that different than tongue and groove frame joint (for movement, that is)?

-- "Man is the only animal which devours his own, for I can apply no milder term to the general prey of the rich on the poor." ~Thomas Jefferson

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Smitty_Cabinetshop

10374 posts in 1370 days


#4 posted 02-16-2014 02:42 PM

Harvey, I’m with you.

Following along to see what comes of this post, it’s a good question.

-- Don't anthropomorphize your handplanes. They hate it when you do that. -- OldTools Archive

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a1Jim

112936 posts in 2329 days


#5 posted 02-16-2014 02:57 PM

IMO Jdh122 hit the nail on the head ,it’s all about wood movement.BTW I think Gary Rowgowski’s book “Joinery” is the best book out there on the subject of Joinery.

-- http://artisticwoodstudio.com Custom furniture

View PittsburghTim's profile

PittsburghTim

214 posts in 1074 days


#6 posted 02-16-2014 03:03 PM

Boards expand and contract across the width of the grain, not along their length. The wider the board, the more movement that takes place. Smaller frames may never see enough movement to cause a problem. A wider joint may cause problems is the temp and/or humidity varies seasonally.

Think of a wide joint, like the breadboard edge on a table. If you don’t install it to allow for movement, you will see cracks down the road. That is why they are not gled along the entire length.

Again, in a well-regulated environment, you may not see the problem, but let the AC off while you’re on summer vacation and you could have an unpleasant situation when you return home.

-- She asked me, "Who are you going to please with that?" I said, "Me."

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jdh122

421 posts in 1570 days


#7 posted 02-16-2014 03:09 PM

Charles,
I think it all depends on scale. It’s true that if you make a frame with tongue and groove (or a mortise and tenon or even a lap joint, for that matter), the pieces are in cross-grain orientation, but the glue joint is never more than a few inches wide, so movement is not a problem. But if you get beyond about 6 inches or so it could start to cause problem.
In terms of the diagram above, you could use solid wood as long as the grain was oriented in the same direction as the post (imagine a blanket chest with this joint: it would be fine as long as the sides all had grain that was running up and down rather than along the front and sides, although I’m not sure how you’d attach a bottom so it wouldn’t either crack or fall out). But in the picture the grain on the plywood is running perpendicular to the grain on the posts.
Just my 2 cents, house-bound on this snowy Sunday.

-- Jeremy, in the Acadian forests

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Loren

7826 posts in 2400 days


#8 posted 02-16-2014 03:22 PM

You can probably glue up something 6” wide that way
out of solid wood, but wider is asking for trouble. With
breadboard ends we don’t glue all the way across, we
use slotted screw holes or accept a little seasonal gapping
of the joint at the shoulder.

-- http://lawoodworking.com

View CharlesA's profile

CharlesA

1929 posts in 550 days


#9 posted 02-16-2014 03:35 PM

I guess I was taking the pick too literally and not imagining a joint over 4-6”.

-- "Man is the only animal which devours his own, for I can apply no milder term to the general prey of the rich on the poor." ~Thomas Jefferson

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