Night stand table – No longer square after glue-up
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Tagged: not-square after glue-up
- This topic has 6 replies, 3 voices, and was last updated 4 years, 10 months ago by Ed.
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I built the carcass of my night stand, and went forward with gluing/clamping. While glue was drying, I squared by comparing opposite diagonals. It apparently went out of square during drying (it got bumped or settled?), because my diagonal measurements now are about 1/4″ off.
Is there a way to rectify the squareness? I thought about trying to dampen the glue with water, but the glue is water resistent. I also thought about “forcing” it to square, then bracing the corners.
More ideas?
So here’s the thing … I went back and measured it again yesterday. The diagonal measurements were about 1/16 different. I double checked with a steel square, and it certainly looks square.
Is it possible the wood expanded or warped temporarily and threw off the square, or at least my measurements?
If its only 1/16 off, the diagonal is off by only 1/32 and for, say, a 2’x2′ carcass only amounts to a 0.1 degree deviation from 90. No one will notice it and, if needed, you can workaround it in the rest of the build.
Also see https://www.popularwoodworking.com/woodworking-blogs/your-carcase-is-not-square-now-what/
[quote quote=582312]If its only 1/16 off, the diagonal is off by only 1/32 and for, say, a 2’x2′ carcass only amounts to a 0.1 degree deviation from 90. No one will notice it and, if needed, you can workaround it in the rest of the build.
I’m not a math wiz, so I’m curious how you got to your calculations. I’m sure I can find them … somewhere …
Thanks for the article … I read it, and it was informative.
I glued and squared up my drawers. They slid right into the carcass like they belonged there.
I’m not a math wiz, so I’m curious how you got to your calculations. I’m sure I can find them … somewhere …
Its called the “law of cosines” which gives the included angle
C
of a triangle with sidesa
,b
,c
ascos(C) = (a^2+b^2-c^2)/(2*a*b)
where
C
is the angle included between sidesa
,b
. In our case we have a nearly right triangle withc
as the diagonal. What you measure as 1/16″ is the difference between diagonals of a parallelogram[Note 1] that is nearly a rectangle. For small deviations this difference will appear as half that much (1/32″) in each diagonal, one shorter and the other longer than ideal values.All that put together, and using the fact that deviations are small, and letting
a = b
for simplicity, gives a simple expression for the angular error in radians as[Note 2]angular error = 0.7 d/a
(or
0.5 c*d/(a*b)
in the more general case of differinga
andb
).where
d
is the measured error (1/16″ in our case). Multiply by 57 (180/pi to be precise) to get the angular error in degrees. For d = 1/16″, a = b = 24″, its about 0.1 degree.Notes
1. The carcass is a parallelogram provided the pieces were cut accurately to start with.2. To get that expression use
cos(x) = sin (pi/2 - x)
and some approximations:
(i) sine of a small angle is the angle itself in radians
(ii) for small deviationsd^2/c^2
is much smaller thand/c
and can be neglected3. For narrow and long boxes (
a << b
orb << a
), the angular error is close to0.5d/a
wherea
is the shorter side. Which could be much larger thand/b
whereb
is the longer side. This may come as a surprise to some.Here’s another way to get it if, like me, you’re too feeble to remember the law of cosines, but you remember Pythagoras’ Theorem and the definition of sine and cosine. You also need the trick of knowing that sin(x) is approximately equal to x when x is small and given in radians, not degrees.
The problem here, though, is what I mark as, “a.” For the example of 24 inches, “a” works out to about 1/16″. That means that, if you hang a door in this opening, the width of the gap between the door stile and the cabinet will change by 1/16″ over the length of the door. You can’t see the 0.15 degree angle, so things will look square, but you likely can see that 1/16″ develop over the length of the door. It’s not too bad as it is and, also, you can scribe the door to the opening to help remove the change in the gap.
- This reply was modified 4 years, 10 months ago by Ed.
- This reply was modified 4 years, 10 months ago by Ed.
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