how flat must boards be?

[billfrommich]

Durango Dudes earlier post about flattening pallet wood prompted me to ask a question I have long pondered but never posted to any woodworking forum. Basically, how flat do you feel boards should be before they are assembled into furniture, cabinetry, etc. I recently obtained some already milled 3/4" x 3" x 7 ft. oak and maple boards from the local Habitat for Humanity Resale store. I was careful to select the straightest, flattest, etc. boards I could find, but some still had a slight amount of bow or twist. When I say slight, I could easily flex the boards back into flatness with only moderate force, using my bare hands (for example, there might be 1/4" of bow or 1/8" of twist over the total 7 ft. length). I believe that in many applications I could use clamps to force any project made with such boards into "square", but I wonder about the long time stability of any project in which that approach is taken. I know some will say always start with perfectly flat boards, but is that really possible, given that wood moves?

[jackellis]

I've asked myself the same question many times. The answer is, flatter is better. yes, wood moves, but the errors add up and they can make your life miserable.

I built cabinets for my shop out of Arauco softwood plywood. The stuff can warp and twist slightly and when it does, assembling simple boxes becomes a chore.

Get lumber as flat, true and square as you can.

[LCHIEN]

i would say it depends a lot on what you are doing with it and the type pf joinery you use.
Say you are making a clipboard of 1/4" mat'l then it has to lie flat on its own.
But, make a 5-sided box with finger joints then a bit of warpage will all be straightened out when you fasten all the sides together and the right-angle joinery will make it quite stiff.

similarly, a single board used for a bookshelf will be straightened up if you butt it to a vertical backboard 3-4 inches tall.

[JimD]

I think my message is similar to what has been said. Any solid wood will change shape with humidity. Do even if it were perfectly flat when you bought it, it would not stay that way. The lack of straightness you describe is very good for solid wood. When you cut the board into smaller pieces, do not be surprised if it changes some. Cutting can release tension in the board that causes it to move some.

For an application like the top of a blanket chest that is basically one big board with hinges on one edge I wouldn't trust a single board as the top even it I could find one big enough. Several boards glued up would be safer but could still warp. A bread board edge can help keep things flat. But the safest way to do it would be good plywood (hardwood core) or hardwood veneered MDF. MDF is nasty to work with (the dust) and not very strong but it is stable.

I built the base of a chest of drawers I am working on for my daughter right now out of a pretty badly warped cherry board. I should have used my jointer to flaten a face and edge but I just used my planner to get the board to the right thickness. I had plenty of wood but I was lazy. The board had much more warp on a narrow board. But the base is almost 4 inch tall picture frame type piece with miters at each edge. Joining the boards this way with miters reinforced with biscuits on each corner pulls a lot of the warp out of the board. What is left has been reduced buy fastening it to the cabinet. The miter joints are not as good as I prefer because of the warped board and corresponding difficulty in cutting them square. But it will be not so obvious down against the floor.

Finding a way to use each board is one of the fun parts of making furniture for me. I try to consider the color, figure, straightness and flatness in deciding where to use pieces. The question of how straight and flat it needs to be prompts another question: "For what". In other words, it depends on what you intend to do with the board.

Jim