Thread: Need advice. Cannot square box.

It will possibly be the angle of the mitres. You only have to be out a fraction on each one and the last mitre joined will be out.

If the diagonals are both the same then your square is out.

The ultimate test for squareness is to measure the diagonals. Only other possibility is that one of your sides is not the correct length but you say that they are all correct.

Oh dear, what would I do? Apart from drink more tea. Check component lengths are as they should be using an accurate rule rather than a tape measure. Assemble the box without adhesive and measure the diagonals. Provided the top and bottom are the same size and the sides are the same size you should be able to get matching diagonal measurements. Then check with your "square". The corners should be square. If not, check the square. It wouldn't be the first time a "square" was anything but 90 degrees. Find a board with a straight edge, mark a line with the square, flip the square over and mark another line. They should be parallel. If not there's your problem. If they are then stuffed if I know what's wrong. But good luck anyway.

Tune the fourth side to fit rather than relying on it to do so.

Regards from Perth

Derek

How are you clamping it? Is it possible that one or more of the mitres is slipping, so that the ends of some of the adjacent sides aren't touching? I like to tape three corners together, fold them up and tape the fourth corner, then clamp them lightly using band clamps. Measure the diagonals, and if they're not equal, put a clamp from one short side to the other, aligned slightly off square towards the long diagonal. Adjust the pressure on this clamp until the diagonals are equal.
Measuring the corner angles with a square is not really accurate, as 1) the square may not be accurate, and 2) if you've clamped too tight, the sides will bow inwards.

1) the square may not be accurate, and 2) if you've clamped too tight, the sides will bow inwards.

^^^^This. The amount of times I’ve got everything dead on with the cut and diagonal measure to find the corners aren’t right with my square?!? Release a bit of clamp pressure and it springs back ok. With any sheetgood carcass nowadays I don’t even bother with the square provided the diagonals are the same.

I could be wrong, but I don’t think the corners are mitred, just butt jointed with a rabbet.

Seems to me the square isn't.

Which could mean both the sides are trapezoidal, or parallelogram, and 'square' corners aren't. If the sides are the same length, and the diagonals are the same, (exactly), then it is square, by definition, regardless of what your square says.
I found an out-of-square square a couple of weeks ago.

Originally Posted by cava

I could be wrong, but I don’t think the corners are mitred, just butt jointed with a rabbet.

According to the diagram, they are mitred.

Originally Posted by AlexS

According to the diagram, they are mitred.

If you look closely Alex you will see the sides are rabbeted (apologies to those who loathe this word ). I notice that the OP doesn't appear to have been back. Maybe he's solved his issue?

Originally Posted by aldav

If you look closely Alex you will see the sides are rabbeted (apologies to those who loathe this word ). I notice that the OP doesn't appear to have been back. Maybe he's solved his issue?

Sorry, I only looked at the thumbnail. I stand corrected.

It looks as though the rabbett cut is not parallel with the outside edge, when assembled if the fourth side is pulled in it will leave a gap. did I get that right?

Originally Posted by aldav

I notice that the OP doesn't appear to have been back. Maybe he's solved his issue?

According to their profile, the OP was back checking on replies between posts #9 and #10.

Issues I can suggest
1. the OP should check that their square is "square" -- using the known straight edge score, flip and score again technique. Being out by a thou or two over 2 inches could account for the box edges not meeting square.
2. having verified that the square is actually square, check that the ends of each piece of each box is square to the same reference edge of the sides. If necessary use blue tape to keep track of which edge is deemed to be the reference edge.
3. then check that the shoulder of each rabbat is square to the reference edge. It's relatively easy to very slightly rotate a long piece when cutting a rabbat across the short ends. Dust or other debris on the table saw sled or miter gauge would be sufficient to throw the cutting off just enough.


Then I start to run out of ideas.

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.