Gifted a case which the top cupped, going to make one myself. How do I prevent this?

Was gifted this gun case which is made of walnut. Sides are 1/2” and top is 1/4”. Going to be making my own as a gift for my father and was going to follow suit.

I’d like to know what I should do differently to prevent the cupping that is happening to this one. I’m not sure if maybe the moisture content from the walnut was too high when it was milled and assembled, or if it’s due to the way it’s built. The walnut I will be using has acclimated to my shop for over a year and is fine.


I know the grain is perpendicular for the side (since it’s dovetailed and dovetails don’t turn out so hot on non-end grain) compared to the top. But I figured since the top is only 1/4” that it wouldn’t have a ton of force when it expands since it’s thin. But I could be wrong?

Any thoughts or suggestions? I’m going to be dovetailing the sides together and will be sticking to the same board thicknesses for the sides and top.

Pics:
https://s3-us-west-2.amazonaws.com/isco/case/wood1.jpg

https://s3-us-west-2.amazonaws.com/isco/case/wood2.jpg

https://s3-us-west-2.amazonaws.com/isco/case/wood3.jpg

Can you post a pic?

Without knowing many of the build details (post a pic as Jerry says would be great), one thing that comes to mind is the finish. Make sure you finish all sides of the completed project with the same amount (coats) of finish on all surfaces.

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Originally Posted by Jerry Miner

Can you post a pic?

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Completely spaced on adding pictures

It appears to me that the material the original maker used for the top-of-the-top was seriously unstable...it takes a bit of effort to warp a "box" like that! If you work with quality material that is at proper moisture content, you shouldn't have a similar issue. I agree with David that you should be sure to use proper finishing techniques. Finishing the inside with shellac or a water borne product to seal it to help maintain moisture stability will be a good idea.

It’s best not to lock-in solid wood, weird things like this happen.

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Originally Posted by Brian Holcombe

It’s best not to lock-in solid wood, weird things like this happen.

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I would agree to this statement, however I don't see a way around it in this scenario. I can't flip the sides to expand/contract with the top because then the dovetails blow out (even with back/front boards clamped around since it's not endgrain). Do you have an alternative?

Use plywood and veneer for the top.

Attachment 373450What is the top made of, solid wood or ply? If solid wood, the top and bottom should not be glued directly to the sides, but should be fit like a raised panel into stopped dados cut into the sides (panel can be flat or raised). The dados are a little deeper than the top dimensions, so that the top is able to expand and contract seasonally. The four sides, top, and bottom are all assembled as one unit, only glued at the dovetails so that the top and bottom are free to move, being careful not to get any glue into the dados.

The top portion is then cut free on the table saw. Most suggest that the blade is set to not cut completely through the sides, leaving just enough to continue to hold the box together, then after all four cuts are made, the remainder is cut with a utility knife or fine saw. Another method is to cut through, but tape splines equal to the width of the kerf to keep the box stable as you go. Here is a pic of a partially exploded box to illustrate.

The stopped dados are cut with a router, which prevents the dado going all the way through and leaving a visible gap in the finished box. Ideally, the dovetails are laid out in such a way that the cut line falls symmetrically within the pattern.

You might check your local library to see if they have any basic box making books, which should cover this.

Dan

I did a case for a flintlock a long time ago. I did the top as a floating panel and glued about a third of each side into the groove and a couple inches, centred on each end. That made it strong but still allowed the ends to expand and contract and the long sides just bend a little, making them either slightly barrel shaped or hourglass like.

The problem is two fold. First the closed box creates a moisture differential. The second is the design. Make the panels on both sides floating or use plywood.

Another vote for a floating top or a stable substrate and veneer. Making the panel from a glue-up of narrow strips can lessen the effect of a poor construction choice but, will not cure it. If you are really wanting that appearance veneered plywood would be my choice.

Thanks for the advice everyone. Looks like I'll be cutting some walnut veneer and gluing it to plywood to give the same look without the gaps.

remember to veneer both sides of the panel

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Originally Posted by Steve Schoene

remember to veneer both sides of the panel

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Just out of curiosity, why do you say this? If plywood is dimensionally stable and won't be affected by the change in humidity, applying veneer to the inside piece seems irrelevant at that point. I know you apply finish to both sides of a piece of hardwood so the moisture change is equal on both sides, but this isn't hardwood.