Fix for warping ply?

[ian maybury]

Building a wall cabinet for a hand tools using what's sold locally (i'm in Ireland) in 18mm (used to be 3/4in) marine ply. I didn't dare to buy the stock grade, and the WBP/construction grade initially ordered and delivered went back immediately as it was obviously rubbish.

This Eastern 'marine' ply was a lot more expensive (heaven help anybody that uses it for a boat), and is only a shade better - the reddish (luan?) top veneers are sanded through/thin in places at the edges, there's lots of internal defects (voids/overlaps rolled together etc), some of the inner layers are mush, it messes saw blades up in no time with glue/resin, and it's warping as soon it's cut free of the panel. As luck would have it the pieces used so far for the back and sides were nothing like as bad, and the rebates straightened it fairly well.

Trouble is i've just cut the pieces for five shelves, and all are warped. (bent crosswise rather than twisted) I could cut another set from another sheet, but would prefer to send it back/there's no guarantee it'll be any better.

1. Does anybody know of any fixes that might straighten the shelf strips - at least for long enough to get them glued in? ( each is 112 x 900 x 18mm - the ends are glued into rebates, and the back edge to the full thickness back panel)

2. Does anybody make decent quality birch and standard WBP and marine grades of ply any more on a scale that's likely to be sold in Europe?

Thanks

[scott vroom]

Hi Ian,

To flatten shelves you can attach a strip of hardwood to the front edge (and back edge as well if necessary). I typically do this with 3/4" x 1 inch or greater, with the longer side perpendicular to the shelf face, like this:

[ian maybury]

Ta Scott, nifty book case. My shelves are fixed and located in rebates (rabbets?) at the ends - so the situation is easier in one way than was yours. Which means I can straighten the back edge by clamping a straight piece to it - and fix it with glue and screws or maybe dowels in that condition to the back panel. Then remove the straight piece. What I'm not so sure about is whether this will or will not result in a straight front edge - it may well do so. I could add a front stiffener, but was hoping to leave the ply laminations exposed as is the situation with the outer panels (sides, top and bottom) of the case.

I guess it'd for that reason be nice if there was some means by which i could persuade each shelf blank to settle out flat before it's fitted - using heat, a water spray, storage under weights or something like that...

[fRED mCnEILL]

I straighten shhets of plywood quite often. Put the pieces concave side down on the lawn in direct sunlight. The ply on the bottom will absorb moisture from the grass causing it to expand while the sun will cause moisture reduction on the upper side causing it to shrink. It can happen quite rapidly (like within an hour.)

[scott vroom]

Ian, you can try flattening by securing the rear edge to a ledge. In any case, you should install the shelving with the high side facing up ("sad face") and allow gravity + the weight of books etc to further flatten it over time. It really just depends on how much warping you have. How long are the shelves and if you lay them on the ground, how high does the crown rise above the floor?

[ian maybury]

The five shelves are close to 900mm (36in) long and 112mm wide Scott (4.5in) - the cabinet is to store hand planes etc so it's very shallow. The intended shelves vary a bit in terms of straightness. Laid on a flat surface they are not twisted, but are bowed up by about 3mm. One or two make a fairly regular curve, the rest are pretty straight, but with a quite sharp kink about 12in from one end which seems to correspond with a discontinuity in the laminations - possibly where two overlap and were rolled together. I'm not awfully optimistic about getting these kinks out, but with a bit of luck the screwing and gluing will hold them straight. Good quality birch ply next time i think….

I fitted two shelves after posting tonight - after clamping them to a straight strip on edge roughly down the centreline. It actually seems to be working well so far - the narrow shelves mean that the whole lot is coming straight (i was a bit cautious that the front edge might not even if the back was held straight) which permits the drilling of pilot holes for the screws attaching them to the back panel in the correct locations. Once screwed and glued they seem to be staying straight…

I'm going to try your grass and sun move in the morning Fred - it'd be useful to have the technique available for other jobs. Luckily the weather is decent here at the moment...

This is a not very clear photo of the basic cabinet before fitment of the shelves. The corners between the sides and top/bottom are lock mitred on a shaper, the purposely fractionally oversize 18mm ply back panel is half rebated and glued into the opening. (the plan is to flush trim and round over the part of it that's visible in the photo) You may just about be able to see the slots/rebates cut in the inner face of the opposite/further away side for the shelves:

lock mitred cab assembled corner 5-7-14.jpg