wood expansion

[fred]

I splashed out and got a Sjobergs workbench complete with storage. The storage has two doors manufactured from laminated strips. Within a few days thy bowed across the width of the door. I assume this is caused by expansion due to humidity. If I leave them in the heated workshop they flatten out. I screwed, 3 screws,two battens on each one but am worried about future wood movement. Would cutting a slot where the two outer screws are fitted be the answer. ( like a bread board end solution ) I just cant understand why they bowed in the first instance as there was no restriction to the expansion movement. Any answers ?

Incidentally I started building my own work bench about 5 years ago and so far only got as far as making the trestle kegs. I reckon at this rate I could be dead before it is finished, and besides there are no pockets in a shroud.

[michael adams]

Incidentally I started building my own work bench about 5 years ago and so far only got as far as making the trestle kegs. I reckon at this rate I could be dead before it is finished, and besides there are no pockets in a shroud.

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Bowing has nothing to do with expansion per se but results from the growth rings in the finished wood trying to straighten out. As they grow successive circular growth rings are in tension otherwise the tree would topple over. When timber is machined the circle is broken and growth rings will try and straighten out which causes cupping and bowing. Which is aggravated by changes in temperature and humidity as the light and dark parts of a ring expand and contract at different rates, If the rings run more or less parallel to the machined surfaces as with premium quarter sawn timber then this won't be a problem. Your solution using slots at either side should prevent bowing ( but it may cause the joints to open at one side ?).

However your problem only exists because inferior grade timber was used and I'd be sorely tempted to unscrew the doors - which aren't critical to the use of the bench and send them back for replacements.

michael adams

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[tabbypurr]

There are 2 causes of bowing, both prompted by expansion due to increasing RH.

1. Wood bows/cups on its own due to how it's cut & lack of stability
2. 2 bits of wood fixed together, one expands more than the other, result bowing of the assembly.

NT

[nothanks]

Given the cost of Sjobergs benches I'd be talking to the vendor about replacement parts. Having said that, I've occasionally compared pictures of these benches to my own DIY bench and wondered whether to splash out. What do you think to it (ignoring the current problem)? Is it heavy and rigid enough to avoid wandering about and racking when hand planing? Any good/bad points?

[Brian Gaff (Sofa 2)]

Yes I remember this kind of thing from model aircraft. Balsa wood did this all the time distorting carefully aligned parts. Brian

[Thomas Prufer]

Changes in humidity... One side is more humid, or the outside is more humid than the inside. This causes part of the wood to expand, but not the other, and that causes warping.

Leave it for a while, a week or so. If it's OK after that, ignore the problem. (Oil, varnish, paint etc. would also alleviate the problem.)

If its not -- I personally wouldn't slot the holes, the wood will adapt IMO. (I first thought "contact the supplier", but after five years that might be more trouble than its worth.)

Thomas Prufer

[fred]

Yes apart from the doors its a helluva of a bench. The base was easily assembled but it took 3 of us, well 2 plus a crumbly, to lift the top in place. The vices are essentially rack free. It comes with 4 heavy steel bench dogs but because of the thickness of the top normal hold downs like the ones Festool use won't work.

Having taken the doors off and screwed battens to them I have let them sit for a week and they appear to be ok now.

I did contemplate complaining to Axminster but getting it to start with was a long convoluted saga.

One final word of warning The dimensions given on the Axminster site art over the vices, not the actual size of the top which is a lot smaller