Square deal

[PerranOak]

So. I glued-up the drawer and the diagonals were 1mm different. Close enough.

Now I measure the squareness of the drawer, the side is out by about 3mm!

Is this to be expected? Does the difference multiply?

I thinks it's out-of-square because the lap-dovetails are hand cut and, I guess, the base of the sockets are not quite true.

Second question: when glueing-up, if diagonals are not equal, what do you do? No one (not even the mighty Norm :norm: ) tells you how to correct it. It's not even in books!

 

[jasonB]

Diaginals can measure the same if you have a parallelogram (sp?) In other words the back could be 6mm shorter than the front which would give 3mm on a tri square yet coner to corner would be the same in either direction.

Yoy can adjust the diagonals by giving the two long corners a pull together or push the short ones apart, sometimes you can do it by hand other times you will need a clamp across the corners. Also ensure all your clamps run parallel to the sides or front/back as if they are at an angle they can pull the box out of square

Jason

 

[xy mosian]

jasonB, sorry I think you have the wrong shape. A Parallelogram has opposite sides parallel. Therefore opposite sides are of equal length, but the included angles are not square. If they are you have a Rectangle. I think the shape you are thinking of is the Trapezium. They have one pair of parallel sides, which are not equal in length, the others may be of equal length but are not parallel.
As you said the Trapezium may well have equal diagonals but no square corner, whereas the Parallelogram will only have equal diagonals if the included corners are square.
The length of diagonal test for squareness should only be used when opposite sides have been checked for equality of length.
As you say long diagonals can be adjusted by using a cramp to shorten them.
I hope this helps
xy

 

[Digit]

With drawers I find it preferable to square front an sides with a suitable square and not worry about whether the back is parallel to the front.
If the sides and front are not true right angles it may prove impossible to close the draw with the front sitting in line with casing.
The back doesn't really matter.

Roy.

 

[jasonB]

Must have been off school that day and missed the geometry lesson, Yes you are correct.

Jason

 

[9fingers]

The error depends on the drawer size but lets assume a drawer 300mm square and the resulting drawer is your figure of 3mm out of square.
This is the equivalent of 0.57 degrees out with one pair of opposite corners being 89.43 degree and the other pair 90.57 degrees. Solving the triangles with the cosine rule we get a difference in diagonals of 4.22 mm.

A similar drawer with a diagonal error of 1mm would have an angular error of 0.135 degrees equivalent to 0.7mm.

So i guess something else is wrong. The most likely candidate is that the opposite pairs of side are not of equal length.

If you do end up with a cock up and the glue is on then the most important thing is to get the two drawer sides parallel and at right angles to the front.
The back of the drawer is rarely seen and can be out of square and tolerate gaps at the corners reinforced with glue blocks is necessary.

The moral is:- do a dry fit including fitting the clamps and check everything not just the diagonals before getting the glue bottle out.

Bob

 

[xy mosian]

Jason, one of the very few things that stuck, well nearly, I had to check my memory
xy

 

[steviem]

With drawers I find it preferable to square front an sides with a suitable square and not worry about whether the back is parallel to the front.
If the sides and front are not true right angles it may prove impossible to close the draw with the front sitting in line with casing.
The back doesn't really matter.

Roy.

Spot on - as long as the drawers sit flush at the front - it doesnt really matter whats happening at the back

 

[yetloh]

What no one seems to have answered is how you correct it. If you have glued it up with animal glue it is simple. You just heat the corner joints with a hot air gun and clamp it on the diagonal with just enough pressure to bring it into square, then leave to set. This is why I always use animal glue for drawers. If you have used any other sort of glue, I'm afraid there is not much you can do except, if the front is out of square with the sides but they are parallel, and the error is not too great, you might be able to taper the thickness of the front to bring it into square. You would need to do this with a hand plane and how much you can correct will depend on the thickness of the lap on your dovetails. It will look odd when you open the drawer, but at least the front will sit parallel with the opening.

Jim

 

[PerranOak]

Cheers all.

I think I panic a bit on glue-up: it's all those dovetails!

The drawer does still fit and work ... thankfully and the out-of-squareness is not obvious.

I bet that putting a clamp on the diagonal is tricky?

 

[yetloh]

Not really. Only a relatively small amount of pressure is needed and those rubber faced Urwin squeasy clamps or similar work a treat.

Jim

 

[jasonB]

Or if you have reversible heads it can be easier to spread the short diagonal from the inside.

Jason

 

[PerranOak]

I'll try both ways!