Hello to you All !
This may seam like a stupid question, but here goes anyway,
I am putting a weight on a cupped mahogany table top, the cup is upward and is about 3 or 4 inches, the table top is half mooned . Now I have put a wet towel over the area and install 24 weight’s on top of it.
My question is this ( if I put the weights over an area of 21 x 34 would it have the same effect as installing them in an area of 24x 10. each one is about 3 1/2 lbs
The 21×34 is spread out over the area of the table, the 24×10 would be just over the table top that is cupped.
Thanks
Jack
Replies
Disclaimer, I'm not a math student:
Strictly speaking, concentrating the weights over the smallest area will have the greatest average force per square inch. Lets do the math:
24 weights x 3.5 lbs each = 84lbs total weight
21 inches x 34 inches = 714 square inches for the entire table top
10 inches x 24 inches = 240 square inches for the cupped portion
Thus, applying weights to the entire table will yield an average pounds per square inch (psi) of:
84 lbs / 714 square inches = approximately .12 psi
Applying that same weight to the cupped portion of the table will yield an average psi of:
84 lbs / 240 square inches = .35 psi
----
In conclusion, applying the weights only to the cupped area will give almost 3 times as much force.
Don't forget that if you are spanning a distance between supports that the force increases with the square of the distance. ie. a 2 by 4 on edge is twice as strong as a 2 by 4 laid flat over a given distance.
Sparky,
The weights are not going to help unless you plan to leave them there forever. The wood has a memory and as soon as you remove the weights it will start returning to its original state. Now as to the damp towel. Again are you leaving it there forever? Because the same applies as for the weights. It could work for a temporary cure, but it sounds like you are applying it to the convex side of the cup. If so you are compounding rather than curing the cup, it would need to go on the concave side.
Now let's talk about a permanent cure for the top. Was it originally flat? If so, what causes 99.9 percent of cupping/warping is uneven moisture between top and bottom. The concave side will have the lowest moisture content and the convex side the highest. You need to equalize the moisture between the two sides.
Do the top and bottom have the same finish? For stability they should be the same. If they don't, I would advise sanding both top and bottom to bare wood, then let the top set for 5-6 weeks somewhere where there is good air circulation on both sides. It should flatten out by itself.
BTW we had a thread here a few weeks where a panel had cupped after lying on the floor for a few days. The bottom had absorbed more moisture than the top. After a week or so with air circulating on the both sides the panel straightened out.
Good Luck, George
You don't stop laughing because you grow old. You grow old because you stop laughing. - Michael Pritchard
Edited 1/22/2009 7:53 pm ET by DustyGeorge
Edited 1/22/2009 11:45 pm ET by DustyGeorge
Read the other posts but, if I understand your post . . .
Puting a wet paper towel on the convex side of the cup will do exactly the opposite of what you want -- it will cause it to cup more. If you plan to use moisture to fix cupping, the moisture source should be against the concave side (so it will swell a bit).
That said, your post really doesn't give enough information to evaluate if that will fix the problem permanently. This could be a wood grain issue or a simple moisture issue, depending on how the piece was constructed and finished.
Mike Hennessy
Pittsburgh, PA
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