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I am building a tall display case. Two boxes stacked and joined. Red oak ply for the box with solid wood banding, red oak for the balance (rail and stile doors with glass panel, crown, and base).

Parts for the box where all batch cut and the same size.

When trying to glue up the first box I could not get it square. I got three square corners and the fourth was out a quarter on a inch over the length of my 8 inch square. Diagonals where good be box not square.

I knocked it apart and cleaned up the glue and left it overnight.

Today I again verified that the parts where the same. Looked at the rabbets and saw that they were not all the same depth, and varied along the depth of the part (14"). Luckily, the saw was still set up. I added a featherboard above the dado stack and recut.

Then verified with caliper that the depth and width where all good.

Tried again. Same result.

Tried with the material from the second box. Same result.

Diagonals where basically the same but only one corner was square.

I really do not know where to go from here.

Thanks for any ideas.

Jerry
 

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You may be trying to do too much at one time.


Do you only have an 8" square. If so, go buy an accurate carpenters square.


Then set up only one corner. Be sure that it is square. Tack diagonal strips between the two boards to be sure angle is held. Then add third board and 2nd corner. Again tack the diagonal braces. After this the fourth board should fit exactly in the rabbets of the first and third board and the diagonals should be equal and the corners square.



Once you have verified this then glue.


George
 

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I thinks it's the rabbets ....

Use a tri square and set the depth so it just bottoms out in a rabbet. Then go up and down that rabbet and all the others to see if any are a different depth. If so, recut them again. If your board is warped and you don't press down firmly over the dado cutter, the rabbet won't be an even depth. This is what I suspect is going on here. They must all be the same depth.
:|
OR, if the board/panel wasn't held snug against the fence for the entire dado, it will show a slight curve. See if that's the case as well.
 

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Usually when you have a problem like that the miter angle is off slightly. It doesn't take but a 1/4 degree angle to add up to a large gap when you assemble a frame. I usually will miter a large scrap of wood and then dry fit and put a framing square on it to see if the angle is right
 

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There are no miters on this ....

Usually when you have a problem like that the miter angle is off slightly. It doesn't take but a 1/4 degree angle to add up to a large gap when you assemble a frame. I usually will miter a large scrap of wood and then dry fit and put a framing square on it to see if the angle is right

These are rabbet joints, no miters. See my post above.
 

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look at the CORNERS real close.
These corners are rabbet joints.
It's the way all four sides were drawn as if you are looking through the box. It shouls have been drawn on a standard perspective view. \The way it is drawn now, you eye is drawn to the center and it looks like a miter. just look at one corner wit just 2 pieces, not the whole thing.
It confused me tooo at first

Actually something is wrong with his verbage or his sketch.
The drawing does not indicate any diagonals.
I suggest you take the shading out of the sketch.
The drawing looks like a square or rectangle with rabbets.
 

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Have you checked your square? If the rabbets are of equal depths AND widths, and the sides are of equal lengths, then mathematically it has to be square.

How are you clamping it & what type of clamps?

I prefer to measure diagonals. When the diagonals are equal, it is square.

Clamp up with moderate pressure & measure diagonals. If not square you can either place a clamp diagonally on the longest dimension, or slightly skew the clamps. Gradually tighten clamps, each time re-measure diagonals.

Another option is to make a jig, or use corner brackets. This can be as simple as a piece of MDF with a couple holes for clamps. If there is a back, it can either be installed or temporarily inserted during clamping.

But checking diagonals is still the easiest, most accurate way.
 

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Cases like this is when cutting to length is more important than cutting to a dimension.

Check that the boards with the rabbits are exactly the same length and length of rabbets match up with the boards laying flat.

If they are:

Tip the boards up on edge and measure length of board required to achieve outside dimension, do the same for other end. This will overcome any variation in depth of rabbets, if outside measurements at both ends and both sides are equal the corners have to be square.
 

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Has Jerry (@Kuvasz Wook) checked the alignment of his table saw?

Following up on @FrankC's post, I wonder whether you can lay the two matching pairs back to back. Rotate the top piece 180 degrees and match again. Then take the pair of boards and flip them to match rabbet to rabbet, and again rotate 180 degrees and repeat the match. Sight down the lines to make sure they are all straight and matched, and the non-rabbeted sides are also straight and matched. That is four comparisons per pair of boards, a total of eight to do. Make sure they match perfectly in all directions.

If all those tests pass, then I would be looking at the depth consistency and squareness of the rabbets.

Off topic:
At first glance, the image did look like miter joints to me too. After I read the text and looked more closely, I saw that they were rabbet joints. I wondered whether it might be an optical illusion at that scale.
 

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Discussion Starter #15
Thank you all!

I tried or already tried all of the suggestions given. Still the same result.

I finally just glued up one of the carcasses and made sure that the diagonal measurements where the same.

After it dried, I checked and all corners were out of square. After I looked at it, I checked that the plywood was not bowed.

Guess what three of the four sides were. When I used a spreader to push the sides flat, the corners popped into square.

I am going to try getting these flat.

While this was originally going to be without a faceframe. I will now add one.

Thank you again for all of your help.
 

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"Guess what three of the four sides were. When I used a spreader to push the sides flat, the corners popped into square. "

I am guessing that you were using clamps that had been cranked too tight. With clamps you just use enough pressure to squeeze the excess glue out of the joints.

George
 

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Beginner / Intermediate Woodworker

I am building a tall display case. Two boxes stacked and joined. Red oak ply for the box with solid wood banding, red oak for the balance (rail and stile doors with glass panel, crown, and base).

Parts for the box where all batch cut and the same size.

When trying to glue up the first box I could not get it square. I got three square corners and the fourth was out a quarter on a inch over the length of my 8 inch square. Diagonals where good be box not square.

I knocked it apart and cleaned up the glue and left it overnight.

Today I again verified that the parts where the same. Looked at the rabbets and saw that they were not all the same depth, and varied along the depth of the part (14"). Luckily, the saw was still set up. I added a featherboard above the dado stack and recut.

Then verified with caliper that the depth and width where all good.

Tried again. Same result.

Tried with the material from the second box. Same result.

Diagonals where basically the same but only one corner was square.

I really do not know where to go from here.

Thanks for any ideas.

Jerry
Sounds to me that some of the edges may not be square to the face, As a result, all would appear to be fine until clamped. If the pieces are plywood I assume they were cut on a table saw. I would check that the blade is square to the table. Not just good enough. Use a good machinist square or one that you know is dead on accurate. Even one degree will compound itself and create those kind of issues. Out as much as yours seems to be requires a solution. If it it is only slightly out it is ok to pull them into square when clamping up. Cabinet and furniture shops have clamping jigs designed for just that purpose.
 

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Ah, that pesky unflat plywood I forgot about that.

Thats why I measure diagonals and forget about the square.
 
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