Cupping with wide boards
I am planning on building a chest of drawers out of mahogany. The case will measure around 20 inches deep (front to back). I have access to some boards that measure in the range of 15 inches across.
My question is: At what point (width) should I be concerned about a board cupping across its width? Should I rip the 15 in boards down to 7 and then glue them back together, or am I ok to leave them un-cut at their current width?
If the boards are dry and flat you shouldn't have a problem. Make sure to do both sides at once when you finish them. That way the tension on both faces is the same.
As Mikaol says, flat and dry is good to go. Concentrate on matching show face joint lines with a hard eye to what will not show. For example, bookmatching 10" boards for the sides leaving 5" cutoffs to glue up for the bottom is a trade I'd make. Don't get caught up in using a full 15" width just because you have it if it doesn't help your finished project.
Flat and dry.
If you rip a wide board in half and glue it back, it is just as likely to warp as it was before you ripped it. You've done more work for nothing.
If the wood is dry and milled flat, it will stay flat with good construction techniques.
Thats what I always believed was true.
Mahogany is pretty stable once its already dry.
To the above, I will add only that it is prudent to mill the lumber in stages, leaving enough thickness and width to fix the issue if it does cup when milled.
A further point is that mahogany and walnut are two of the most stable woods. Stable in this situation means that they are least likely to cup, as their coefficients of expansion and contraction in the radial and tangential directions are almost equal. It is the difference between these two that causes cupping as the wood dries (if it is allowed to dry equally on both sides, which the above respondents address.) Oak has a ratio between the two directions of about 2 to 1, which is why flat sawn oak is so prone to cupping as it dries. You will still need to design the construction to accommodate seasonal expansion and contraction to prevent other problems.
Another point is that if you have 15" wide boards and rip them in half, you also devalued the lumber. There is a premium price for boards as wide as you have. If you don't have a "need" for a single solid board 15" wide for a panel or top, use something less expensive.
I would never rip a wide board into narrower parts! It ought to be a crime.
To me it makes sense here. He's going to have a glueline to get to 20 inches no matter what. Most wide boards have better & worse looking sections or sapwood to trim off anyway. In this case it will all get used. Saving this lumber waiting for a 15"-wide project somewhere down the line and buying more for this one makes less sense to me.
I got a 24" wide slab of chestnut at an estate sale a coupla years back for $3. I'm sure he was saving it for the perfect use.
If I had to achieve 20", I would probable try for 3 equal width boards. While ripping down a 15" board will yield two pieces, it's unlikely that they would be glued up back in alignment for aesthetic purposes, so what's the point of starting with expensive boards and ripping them to become less expensive boards as part of a glue-up?
Another option is to feature the wide board in the middle of the sides, by gluing ~2.5" wide pieces on each side, that have a different look (i.e., rift sawn instead of flat sawn as the wide boards might be.) This works better than adding a random 5" wide board to one side, unless you can get the grain to match so well that the two pieces blend into each other.
More on cupping. Not too applicable if assembling carcasses, but addresses unsupported wide boards..
Quartersawn vs. flatsawn—When viewed from the end of a board, the growth rings can tell you a lot about whether the board is likely to cup. If the rings meet the face at between 45° and 90°, the wood is considered quartersawn. The rings on flatsawn wood meet the face at less than 45°.
Quartersawn wood moves only about half as much as flatsawn and is much less likely to cup. So quartersawn wood often is a good choice for tabletops that cannot accept a mechanical support to help keep them flat.
When edge-gluing several flatsawn boards to create a tabletop, some woodworkers prefer to alternate the growth rings (concave toward the top, then concave toward the bottom, and so on), while others prefer to run them in the same direction. But after 40 years of gluing up hundreds of tabletops and thousands of panels, I find that grain orientation really makes little difference. My priority is to position sapwood and blemishes on the underside of the table, which usually means that the growth rings are concave toward the top.
From Fine Woodworking #183