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I started building a dining seating bench. This is the plan that I created in SketchUp:

bench

The first part is to build the carcass which supposed to look like this: structure

I am using construction grade 5x10cm lumber(pine). I've spent some time choosing the most straight pieces from the lumber yard but they are still not perfect.

At this point, I have finished building the carcass for the bench and, as I feared, it is not perfectly flat and is rocking along the diagonal while on flat surface.

I kind of expected that and initially thought that I could mitigate that by installing adjustable legs at the bottom but now I realize that I won't be able to fit the top plywood piece (the seating).

Are there any ways I could fix or mitigate that?

Thanks!

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    Pics of the actual piece, including measurements showing how far off it is may be helpful. Of course, the simple thing would be to saw/plane/sand the high spots, but that may depend on how bad it is.
    – FreeMan
    Aug 11, 2022 at 11:09
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    Is it a solid piece of sheet goods across the front and sides? If so, you might be able to force the whole box back to square. This depends on it being only a little bit out, though. Aug 11, 2022 at 13:42
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    My apologies that this comment does not help solve your problem, but it does point the importance of fully milling the members of the frame members to be consistent in in profile and dimension, and assembling them on a flat surface with the same precision and attention as you would use for any finished project. As a general rule, any defects in the sub frame will only be amplified when applying the panels ( I know from personal experience.)
    – Ashlar
    Aug 11, 2022 at 15:04

2 Answers 2

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I've spent some time choosing the most straight pieces from the lumber yard but they are still not perfect.

I was getting ready to to say be more selective then got to this sentence. So instead, be even more selective :-) A lesson I picked up somewhere very early on is if you must build decent stuff, especially furniture, from construction lumber you need to pick only the best stuff (leaving the rest for someone else who doesn't care as much) or expect to do A LOT of milling later.

Construction lumber is just that, it's made for construction, where many sins that are unforgivable in furniture work are acceptable. (It's still not as good as it should be frankly, but it is what it is.)

Anyway this also indicates that you didn't further process the wood before using it, which compounded the first error.

So, just a quick primer on buying SPF for anything other than rough carpentry:

  • Be hyper-selective what you get, because it's sure not likely to get any better once it gets into your home/workshop. 1
  • Do not expect to use the wood soon after you get it home. 2
  • Still expect to have to do at least some milling to get acceptable parts! 3

I kind of expected that and initially thought that I could mitigate that by installing adjustable legs at the bottom but now I realize that I won't be able to fit the top plywood piece (the seating).

I wouldn't bother using adjustable feet here, you could simply shim the raised corners. Shimming is a perfectly acceptable building practice even in fine carpentry to get square items to fit in non-square locations, or the opposite :-) If for any reason you must have something adjustable you can install lag screws or bolts as feet, for a very large range of adjustability at almost no cost.

Spend all your adjustment time getting just the top flat and level because this is really the only critical surface on your carcass. Unless the distortion is very bad the top should easily be brought to level with some hand planing.


1 Accept no twist (literally none), minimal bow (a lot of this can be taken out when cutting down to size) and zero cupping. Discount pieces with too many or too large knots while you're at it. Depending on where you live and how many big-box stores or wood suppliers you have within acceptable driving distance this might mean visiting more than one of them, or ALL of them, to find the wood for a single project. Or, visiting the same one or two multiple times over a period of weeks in order to build up stock of the best stuff on offer.

2 Dimensional lumber should be acclimated at home before use. Don't think days, think weeks. You want to wait at least a few weeks and ideally longer; remember this is the wait time after the last piece gets home, not the first one. Wait more than a month if you can....... it still won't be as dry as it should be, but it'll be better. Even though you aren't starting with green material this is still drying wood and the lumber should be treated accordingly — sticker and stack it, with heavy weight on top.

3 A slightly bowed 8' 2x4 is acceptable because it will yield 2-3 project pieces which are very much less bowed than the starting board. But don't forget that no furniture pieces should have any bow in them at all, so before use in the finished project what little remains should be milled out, typically with a jointer and thickness planer or by planing (with a hand plane or with a power planer). It's not ideal but if you had to you could also do this using a belt sander fitted with a very coarse belt; you want to use well under 80 grit here if you can get it.

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  • Thanks a lot for your elaborate answer. Very much appreciated! Unfortunately I do not have any milling tools and not even a planer. Apart of a couple of drills I only own a mitre saw and jobsite table saw(not a very good one either) so can't really do much with that wood. I probably made all the mistakes you've mentioned with wood selection and that's why I am where I am with this project. I did hope it would line up somehow judging from a dozen of youtube videos I watched. My last hope is the plywood sheathing which maybe will "pull" the structure in the right way.
    – Eugene S
    Aug 12, 2022 at 12:13
  • You may already have realised this for yourself but in case not, you really do need at least a few more tools to be able to 'finer' work. A no. 5 (or more ideally a 4 and a 5, or two 4s set up differently) will go a long way towards broadening the scope of what you can do without a major investment. Checked your profile and see you're in Oz, is Perth a part of the country where Falcon planes are relatively plentiful? Hard to go wrong with one of those, or an older Stanley or Record.
    – Graphus
    Aug 14, 2022 at 18:32
  • Yeah, I need to change my profile, used to be in Oz but not anymore. Now I am in Israel where unfortunately I have no space for pretty much anything. But yeah I definitely understand that what I have is far from what I need. Was still hoping this is an easy enough project even done with "as-is" construction grade lumber. Well, as said, I'll try to do the best I can with what I have. Thanks again for your suggestions!
    – Eugene S
    Aug 15, 2022 at 19:50
  • Darn it, I was hoping I was giving some really on-point suggestions there haha. And interesting that construction wood seems to be just as bad in Israel as it is in most other places!
    – Graphus
    Aug 16, 2022 at 16:46
  • Haha, sorry for the disappointment. Yeah it is, maybe even worse.
    – Eugene S
    Aug 18, 2022 at 5:45
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Depending on the severity of the warp, you may be able to use steel cables (1/8") with thimbles and screw eyes to pull the shape square.

Determine the larger diagonal to be "shortened" and screw an eye into each corner. Secure the cable through both thimbles and eyes. The routing is from the center of the rectangle, through one eye, then the other and back to the center, providing a complete loop. Ensure to thread on the swages during the process.

Securely clamp the loose ends together. Place a suitable tool in the center of the loop and spin it, twisting the cable, making it shorter, pulling the diagonal into position.

If this single panel correction solves the problem, crimp the swages, otherwise secure the twisting tool and perform the same process on the other diagonal(s). You don't want to crimp the swages until all the diagonals are squared.

You can do away with the nicopress swages and the cable if your sheet goods are strong enough to hold the forced distortion once the cables are released. In that case, split bolt clamps commonly used for ground wire attachment can be used and removed easily later.

Any method that can be used to pull or push opposite corners in the desired direction will work; cables are not a strict requirement. Pipe clamps of suitable length with appropriate fastening locations may also provide enough force to twist things back into shape.

I've used the cable method to warp an aircraft wing structure into true, but the cables remained in place, as the "sheet goods" were wing skins and not strong enough to hold the shape.

twisted box

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