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I am only starting woodworking and in several books I read that woodworking is all about prevent final product from damage of season temperature and humidity changes. So it means that to glue the bottom of the box is “the biggest evil in the world”. In the same time some famous woodworkers, like Paul Sellers, glue up the bottom of the box. I something misunderstanding or why in some cases bottom of the box can be glue up?

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  • Hi, welcome to Woodworking. If you need a little more of a primer on wood movement and where you need to account for it and where you don't, there's quite a bit on this here in previous Q&As. But if you have any further questions about apparently contradictory things in this area do feel free to ask. Just to mention, there is truly contradictory stuff too, unfortunately; woodworking is not in the least immune to competing and sometime utterly contradictory advice [in many areas]! – Graphus Nov 14 '20 at 9:39
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This is one of those things where it's partly a matter of usage — the scale of the piece, the weight it needs to hold, how rough-and-tumble the expected end use will be — but the most important issue is probably whether the bottom is solid wood or a manmade board of some kind. So the specifics matter.

As you know, all solid-wood elements expand and contract naturally with changes in humidity, and a bottom in solid wood can expand in width during the wettest part of the year which will try to force the box apart. In the worst-case scenario it could succeed by the way, especially if the box uses unreinforced mitre joints which are the weakest joint in woodworking1.

But expansion and contraction is related to the cut of the board (flat-sawn versus rift-sawn or quarter-sawn), the species used and a couple of other factors. Plus, as it's a fixed percentage the width of the piece matters. So, just like with smaller tabletops, you can often get away with ignoring wood movement if the piece is small enough.

With bottoms made of ply, MDF or HDF though, they are dimensionally stable enough that no movement needs to be allowed for at almost any scale. So in short, it's always safe to glue the bottom if it's any of those materials (or if you'd used plastic or metal, in case that's ever something you choose to do).

So, why glue the bottom of a box?
To add strength, if needed. That's it in a nutshell.

Where the bottom is not needed as a structural element, as in most small boxes made for a semi-decorative purpose2, you can easily get away with leaving the bottom to float in a groove. Obviously the box would be stronger if the bottom were glued in place, ditto if it's a mitred box having the corner joints reinforced in some way, however in real-world use neither is needed in many cases. Which is why so many boxes are built this way.


1 Especially as commonly done these days, without sizing the end-grain surfaces prior to the final glue-up.

2 I'd include many jewellery or watch boxes in this description, although obviously they are made to be used they're very much intended as beautiful decorative objects too in a lot of cases.

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  • Thank you, it is a perfect answer. But "get away with ignoring wood movement if the piece is small enough" - may be you have any sized or measurements? – Sergey Di Nov 14 '20 at 12:16
  • Welcome. only very loose guidance can be provided here because it depends so much on the wood, both the species and cut. Quarter-sawn wood has approximately half the expansion of flat-sawn wood across species, just as a very rough guideline (but note it varies a lot species to species). The age of the wood matters too, since old wood tends to expand and contract less than freshly processed wood; this is one reason wood recycled from old furniture, beams and flooring is valued by some woodworkers. [contd] – Graphus Nov 15 '20 at 8:18
  • Anyway, for pieces under ~25cm width it's probably always safe to ignore movement, above approx. 30-40cm you want to begin to account for it. Remember, the wider the individual board (or the glued-up panel built from narrower pieces) the greater the seasonal movement, so more and more allowance has to be made as you go up in size. – Graphus Nov 15 '20 at 8:21
  • Second weakest after end-to-end butt joints? Though I guess that's never really done so your statement is still true – SaSSafraS1232 Nov 16 '20 at 22:30

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