Wood bending question

[DirkM]

I have been building a chair of bent oak, glued 9 planks together to create the shape of the chair (similar to the IKEA chairs with the s shape) After testing it, I found that the curve of the wood is not quite enough and I’d like to bend the oak a little bit further. However since all the planks are glued together I can no longer steam the wood (As that would soften the PVA glue as well) and was wondering if someone out there knows how I could bend the wood a bit more. I thought of simply soaking that part of the chair and then clamping it tighter, but I am not sure at what temperature the wood lignin will soften, as soaking it in boiling water may be difficult (wrt the size of the chair).
Any advice welcome,

Thanks

 

[Trigs]

have a search for heat blankets that luthiers sometimes use to bend guitar sides.

 

[marcros]

how much time and money do you have tied up in this?
how much extra do you need to bend it?

 

[profchris]

I bend guitar sides.

I don't think a heat blanket will help. Your chair is much, much thicker than a guitar side.

However, although PVA softens with heat, it hardens again when it cools. It loses some strength, but it might still hold well enough to keep your laminations together.

The risk, if you steam it, is that the wood will swell and then shrink as it dries, opening up the laminations. So I'd try dry heat - I've found oak to be quite an easy wood to bend, and I'm assuming your individual laminations are maybe 3mm thick?

I have a mental picture of a chair back with a hooped top, where the legs aren't quite close enough together. If that's right, a rope Spanish windlass across the bottom to apply tension, then heating with a heat gun until you are confident the wood is heat soaked and too hot to touch comfortably. Most heat where you want it to bend most.Wind the legs together, probably a little past where you want them to end up, and let it cool.

If this fails, you will have separated your laminations and can bend them all individually then re-glue.

The other risk is scorching the wood, so don't do that!

 

[DirkM]

All,
Many thanks for the quick replies. I may try the heat gun approach, as that seems to interfere least with the glue. The main issue I have is that each individual plank is not laminated but solid oak (about 12mm thick) and they are glued together on the thin side so with nine of them I have a 450mm wide bent panel (see attached picture).
I was originally thinking I could bent it a bit more by simply soaking part of the chair in warm/hot water but I don’t think that will work as the temperature won’t be high enough.
Thanks again
Dirk

 

[profchris]

Ah, laminated edge to edge! This is trickier. I tried this with some ukulele sides, and they came apart during the bending.

This is what I suggest if you try the heat gun, recognising that it might fail.

Work on one section at a time. If the bend behind the knees want to be tighter, start with that one and only heat the bend where you want it to move. Put it in tension to where you want it, and also put some clamps across the full width before you start heating. I'd be tempted to add battens top and bottom at intervals to stop the planks moving apart in that direction too. Then heat where you want the bend until the tension disappears, then over-tighten a little to allow for springback.

If that works and you want the chair back more upright, fix up some kind of jig to get the tension on it and repeat the process at that bend.

Good luck!

 

[peter-harrison]

I hate to say it, but I would cut my losses. I can't see you being able to get much to change here. I would make a new one and think of ways to repurpose your first attempt.

 

[samhay]

If you want to make it more upright, could you put a block/wedge under the bottom at the back to pitch the whole thing forward a little?

 

[DirkM]

Thanks for all your suggestions, I had already planned to put two wedges at the bottom, and now I will simply make the front one thicker, so the chair tilts a bit further back. As for 'Peter Harrisons' suggestion, this is my first bending project, and I have put many many hours into this, so discarding this attempt is not going to happen....it's a chair for myself anyway, and I'm sure I'll get it in an ok-ish state
Thanks again, much appreciated.

 

[Inspector]

You you could always make a new one and use this one on its side with a sheet of glass on top. Matching table.

Pete

 

[SteveW1000]

Not sure whether this is any use but the Youtube videoer Engels Coachworks has a video of him bending timber using a blow torch. Haven't watch that particular video but I find his videos interesting watching him practice a dying trade, he doesn't do the product placements or any of the other nonsense that some do.