Bowed cupped lid - is this fixable?

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Rorton

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I was messing about making a box using the 45 degree bevel on the table saw to make mitre corners, got carried away, and the box turned out quite nice, was just supposed to be a test of the mitre...

Anyway, I cut the top from the base, and I'm now left with a cupped/bowed top.

When the top rests on the base, its not 'that' bad, but if you press on one end of the lid, it exaggerates how bad it looks.

I tried sandpaper glued to a flat sheet of wood, (70 grit) and passing the lid over this for a while hoping to flatten and this didn't really made a dent in it.

Would it be worth securing this flat/square to a piece of ply, and sending it through the thicknesser? if I do get it flat now, will it just keep cupping?

Should I just cut my losses and make a different kind of lid?

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TheUnicorn

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Just a line of enquiry so to speak, but i would look into steaming to soften the wood, then clamp straight and leave for a good amount of time to dry and settle. two issues I can foresee are 1. the glue softening and the joints coming apart 2. The wood re-warping when the clamps are released. I would consider a piano hinge and couple (or more?) of clasps to give as much support from the base to the lid as possible which might do something to minimise future warping
 

MARK.B.

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Couple of small powerful magnets concealed just below the surface of the wood may help, if it was me I would wait a while and see what else it does as the wood adjusts to its new found freedom of movement :)
 

skeetstar

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Yep, leave it for a bit to settle down. I made an oak bread bin last year, the top bowed significantly, but after a while it went back to , almost, its original shape and I was able to sort it.
Give it some time, it might start to behave itself.
 

Sgian Dubh

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I was messing about making a box using the 45 degree bevel on the table saw to make mitre corners, got carried away, and the box turned out quite nice, was just supposed to be a test of the mitre...
Anyway, I cut the top from the base, and I'm now left with a cupped/bowed top.
Should I just cut my losses and make a different kind of lid?
Am I correct in reading that lid as having a solid wood panel (top) tongued into a groove worked in the sides and ends? If so, I suspect your problem is that the panel forming the top has cupped and is strong enough to pull the sides and ends up with it. Relatively thick tops can do that because that thickness confers strength meaning it may be strong enough to distort the whole structure, the lid in this case.

If I'm reading the circumstances correctly your best recourse is likely to be to wait and see if the lid will straighten itself out on its own. If it won't, then you'll have to live with the cupping, but you might be able to straighten the long edges of the lid where the upper section meets the lower section, i.e., you make the section near the middle thinner than both ends to create a match - you could, of course take a bit off at the middle of the long sides of the bottom section too.

There are potential problems with that fix:
  1. The difference in width of the vertical pieces might be unpleasantly noticeable.
  2. It's possible the cupping of the top panel could change over time and a gap would again open up, but not necessarily at the same place or places.

Anyway, all that being as it may, and assuming you have got quite a thick top panel, which I obviously don't know for sure, a traditional way to reduce the strength of a panel in that circumstance is to make the top thinner, whilst at the same time it appears to be thicker than it really is. Basically, this just means hollowing out the underside of the lid panel. Below is one version of this technique, which you (or maybe someone else) may find useful for the future. Don't get too excited about the somewhat out of whack proportions or scale of the parts in the sketch - it was drawn in probably 60 seconds, so little or no care taken to make it pretty and accurate. Slainte.

dbl-tong-lid-web.jpg
 
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Rorton

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thanks for the detailed reply.

Initially, I put a 6mm deep groove in the top and bottom of the sides , which was 6mm from the top and the bottom.

My base (birch ply), had a rebate cut into it 6mm in size, when assembled, the base was flush with the bottom of the sides.

I did the same with the top.

Once glued up I got carried away, and wanted a bevel on the lid, so I cut that with the table saw, wasn't happy with the first pass, and went a bit further, and at that point - not thinking - I ripped off the side of the groove meaning all I had left on the top was a rebate!

At the point, because I only applied a very small amount of glue to the top to stop it rattling about, it fell out.

I was therefore left with a box with a top that's come off, the edges of the top now about 3mm In thickness, with the centre part of the top the full thickness (12mm)

At this point, I glued the lid into the rebate with a 1mm gap all the way around, let that setup, and cut the top off the box after 24 hrs.
 

Sgian Dubh

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... not thinking - I ripped off the side of the groove meaning all I had left on the top was a rebate!

At the point, because I only applied a very small amount of glue to the top to stop it rattling about, it fell out.

I was therefore left with a box with a top that's come off, the edges of the top now about 3mm In thickness, with the centre part of the top the full thickness (12mm)

At this point, I glued the lid into the rebate with a 1mm gap all the way around, let that setup, and cut the top off the box after 24 hrs.
Hmm? You've got a rather badly compromised result caused, I'd say, by things going wrong. Gluing the top in all around a rebate allows nothing for expansion and contraction, which if the panel was set into a groove, as you planned initially, would have allowed the panel to expand and contract without also bending the sides. Now, with it glued in, every time the panel changes in width in response to RH changes, and also probably cup, it will either pull the side pieces with it thus bending them, break the glue joint, or eventually split, maybe eventually all three. In addition, the top panel is quite thick, probably about 12 mm thick from your description, therefore it has quite a bit of strength in it when it changes shape due to, as already mentioned, changes in RH.

My advice, based on all you've said above, is to make the best of what you've got. Get the edges of the box base and its lid married up as best you can, as I've already described, and live with it. It may survive, or it may not. If it eventually fails, as seems possible, it fails, and it's time to move on. I wouldn't bother making a new lid because it's very hard to get a new lid to match the existing base. Better, I think, would be to take on board the lessons you've learned and incorporate that knowledge into the next box you make. After all, as you said in your opening post, you were just messing about making mitres, so your box could be seen as a bit of a happy(ish?) accident. Slainte.
 

yetloh

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That's a very sad outcome - the box looks really good - I hope you manage to get it sorted or that time allows it to sort itself.

Jim
 

Rorton

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thanks, yep live and learn I recon!

With a new lid, I was thinking of doing something different, just a flat panel with some offcuts or similar guide inside and have it sitting on the top, a bit like this:
maple.walnut.keys.jpg


Make the new lid a push fit, or remove the offcuts, and just have it as a flat panel, slightly oversize, and hinge it?

Shame as I lined the inside with thin oak too!

Lesson learned though, would be nice to salvage what I can if possible,
 

Rorton

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Had some time to revisit this - the original lid is like a banana now, so I've scraped that...

I glued together some offcuts of oak and walnut - tried to find ones that were quarter sawn to reduce any cupping, flattened them, and then put a slight bevel on the top, and a 45 chamfer all around, added hinges, added some black felt rubber type material to the bottom to stop any scratching, and also on the inside of the bottom (neoprene its supposed to be) and called it done.

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