Warping

[Mono11]

A bit of help if you could

Why are these warping after gluing?

They are 2x2 (44x44) cut into angled pieces no longer than around 40mm and cut at various degrees ( using mitre saw ) then glued onto 6mm ply with evostick pva ( blue bottle exterior type ).
Its not a new idea so i presumend my take on the idea was simple enough until this hurdle.
This was just 1 of a few trial pieces i made to so see how angles and dimension worked/looked etc before i start proper and its my intention to start the first with 704 pieces or 16 x 16 so i need to find out what im doing wrong before hand.

Thanks in advance

Owen

 

[TheUnicorn]

the ply might be a bit thin, so i'd try going thicker, and then i'm wondering how you are clamping, and for how long? I'm wondering, and other's may disagree, whether the moisture contents of the pva is an issue. if you tried a two part epoxy that would potentially set quite quickly and might make the clamping less vital. also the temperature of your workshop could be a factor too

 

[Sgian Dubh]

The basic problem you have is that you've created an imbalanced structure. The plywood isn't rigid enough to remain flat after you've attached the wood blocks. It's like taking a piece of paper, laying it flat on a surface, and wiping the top face with a damp cloth - the paper curls up towards the dampened side, unless it was first taped down along each edge to the flat surface, which is a technique water colourists use. You get much the same effect if you veneer just one side of a board, and generally, the thinner the base board the greater is the cupping. The cupping of a board veneered on just one side is to form a concavity on the veneered side unless there is some additional structure to prevent the warping. For example, traditional drawer fronts were (and are) typically veneered on just the show side, but it's essentially largely restrained to prevent warping by the joints at either end into the drawer sides, although even here, and in other cases of single sided veneering (carcase sides, tops, etc), the additional joinery doesn't always succeed in keeping panels flat.

Your solution will be found in somehow making your base much more rigid so that it is able to resist cupping after all your wood blocks are attached. Thicker plywood might suffice, or you may have to add strengthening bearers of some sort to the underside of the plywood - no guarantee that either will work satisfactorily though so you'll need to spend some time experimenting to find out what works.

On a side note, gluing the end grain of all your blocks to the long grain of the plywood isn't the soundest of structural choices. You might need to consider some additional structure holding each block in place to prevent the blocks popping off the plywood base at some future point. Slainte.

 

[Cabinetman]

Why do you want to do it is a question that comes to mind, what is the end use?
Does it have to be on a piece of plywood, you could just glue them all together resting on something that they wouldn’t stick to, clingfilm? Piece of glass?
Another way would be to do it as if you were making end grain cutting boards, glue each row together, clean up the sides and then glue the rows together to form the block. Ian

 

[Mono11]

the ply might be a bit thin, so i'd try going thicker, and then i'm wondering how you are clamping, and for how long? I'm wondering, and other's may disagree, whether the moisture contents of the pva is an issue. if you tried a two part epoxy that would potentially set quite quickly and might make the clamping less vital. also the temperature of your workshop could be a factor too

Thanks Unicorn yes i think ultimately ill try 9mm or even 12mm to make sure but then i need to concider the weight as it will be hung on a wall - the pictures below repsonding to another reply will make sence of wat im trying to achieve

 

[clogs]

always remember, what you do to one side of the board / ply Mdf u have to do on the other side as well to stop this.....
when finishing stain / varnish etc I always do both side....
perhaps gluing up over a few days instead of just in one go.....
a bit dif with the lumpy bits on just one side....no easy way unfortuneately....

Sgian is comp right......

 

[Mono11]

The basic problem you have is that you've created an imbalanced structure. The plywood isn't rigid enough to remain flat after you've attached the wood blocks. It's like taking a piece of paper, laying it flat on a surface, and wiping the top face with a damp cloth - the paper curls up towards the dampened side, unless it was first taped down along each edge to the flat surface, which is a technique water colourists use. You get much the same effect if you veneer just one side of a board, and generally, the thinner the base board the greater is the cupping. The cupping of a board veneered on just one side is to form a concavity on the veneered side unless there is some additional structure to prevent the warping. For example, traditional drawer fronts were (and are) typically veneered on just the show side, but it's essentially largely restrained to prevent warping by the joints at either end into the drawer sides, although even here, and in other cases of single sided veneering (carcase sides, tops, etc), the additional joinery doesn't always succeed in keeping panels flat.

Your solution will be found in somehow making your base much more rigid so that it is able to resist cupping after all your wood blocks are attached. Thicker plywood might suffice, or you may have to add strengthening bearers of some sort to the underside of the plywood - no guarantee that either will work satisfactorily though so you'll need to spend some time experimenting to find out what works.

On a side note, gluing the end grain of all your blocks to the long grain of the plywood isn't the soundest of structural choices. You might need to consider some additional structure holding each block in place to prevent the blocks popping off the plywood base at some future point. Slainte.

Thankyou Sgian Dubh. I had a suspicon it had something to do with what you explained but didnt know the technicalities of it. I wonder what would happen if i screwed it down first ( behind one of the pieces to then later be glued back ) let it dry then unscrewed it? . I cant get around gluing the end grain to long grain unfortunately but did have in mind putting a brad through the backing into each block after drying.

 

[Mono11]

Why do you want to do it is a question that comes to mind, what is the end use?
Does it have to be on a piece of plywood, you could just glue them all together resting on something that they wouldn’t stick to, clingfilm? Piece of glass?
Another way would be to do it as if you were making end grain cutting boards, glue each row together, clean up the sides and then glue the rows together to form the block. Ian

Thanks ian - some context of what im trying to achieve

 

[Mono11]

always remember, what you do to one side of the board / ply Mdf u have to do on the other side as well to stop this.....
when finishing stain / varnish etc I always do both side....
perhaps gluing up over a few days instead of just in one go.....
a bit dif with the lumpy bits on just one side....no easy way unfortuneately....

Sgian is comp right......

Ah ok thats interesting, makes sence and worth a go

 

[Cabinetman]

Oh I see, yes quite attractive. And no plywood needed, jobs a goodun as they say around here. Ian

 

[recipio]

Its due to tension in the glue line - even 18 mm ply would probably warp a little. You can salvage that piece by running a saw kerf down the middle of the ply to a depth of 6 mm, glueing in a lath ( must be a snug fit ) and clamping the whole piece down on a flat surface to set. Endgrain pieces like cutting boards are inherently unstable. Its best to align the grain 'back to back' to minimise movement.

 

[Cabinetman]

Its due to tension in the glue line - even 18 mm ply would probably warp a little. You can salvage that piece by running a saw kerf down the middle of the ply to a depth of 6 mm, glueing in a lath ( must be a snug fit ) and clamping the whole piece down on a flat surface to set. Endgrain pieces like cutting boards are inherently unstable. Its best to align the grain 'back to back' to minimise movement.

Be interested to hear what you mean by back-to-back Recipio, maybe a sketch? Cheers Ian

 

[JoshD]

My 2.5p worth

1) you need to start with well seasoned wood. But even if well seasoned there will still be shrinkage if you bring wood in from a dampish workshop into a drier house. Bring the wood indoors and allow it to equilibrate with indoor humidity for a while. I use moisture meter and ideally compare with readings from furniture of the same species of wood that's been indoors for while. Equilibration can take weeks or months but should be quick if it's just short sections (ie after you've cut your random angles). This reduces your warp problem but doesn't eliminate it because there will still be changed in interior humidity.

2) use epoxy rather than pva as suggested above.

3) I would have thought you don't really need ply backing sheet at all, as others have said. You could just glue the random blocks side by side. This will massively reduce effect of humidity changes.

4) look at the ring patterns in your end grain. Wood shrinks much more along the rings (tangentially) than across the rings (radially). Try and keep a consistent orientation pattern so that your blocks don't pull away from each other after gluing.

5) choose a stable wood species. In general softwood is more stable than hardwood, particularly in the radial direction. Of the commonly used softwoods Western Red Cedar is particularly good (and will put less strain on your mounting system because it's very light).

6) if you really want backing plate there are things you can do. For example would you be happy if backing plate was glued just to one row of blocks, perhaps bottom one or middle one? Make sure that the grain on this row is all running up/down.

Good luck!

Josh

 

[stevek]

Lovely panels but dont you have to paint them first? Anyway, I would go with chopping them all up and with the plywood lay them all out in the place where they are going to be for at least a few days,,small bits shouldnt take too long to start to settle down if they are dry to start with, then glue them altogether, a little dab of glue should be enough clamping them together whilst they sit on a flat surface, and afterwards screw on your sealed both sides plywood back. Be a little careful screwing into endgrain,,,check it out if you go down this route,, but its simple enough.

 

[Ollie78]

Just glue them together using the side grain, you don't need a backing board. Once glued into one piece you should be able to put a keyhole router into the back for hanging it.

Ollie

 

[thetyreman]

I've made acoustic diffusers like this before and had not problems with warpage, I used much thicker plywood than this, which may have stopped it moving, it was 18mm thick and went right to the edges of the blocks, you can put a chamfer on the back edges to make it look thinner.

 

[Mono11]

Lovely panels but dont you have to paint them first? Anyway, I would go with chopping them all up and with the plywood lay them all out in the place where they are going to be for at least a few days,,small bits shouldnt take too long to start to settle down if they are dry to start with, then glue them altogether, a little dab of glue should be enough clamping them together whilst they sit on a flat surface, and afterwards screw on your sealed both sides plywood back. Be a little careful screwing into endgrain,,,check it out if you go down this route,, but its simple enough.

Yes i have to paint them all first so with that im not able to glue with PVA side by side and hence why i am using a backing of ply so i think rather than cut the required pieces to the job ill cut a job lot in advance and let them rest.

 

[Mono11]

My 2.5p worth

1) you need to start with well seasoned wood. But even if well seasoned there will still be shrinkage if you bring wood in from a dampish workshop into a drier house. Bring the wood indoors and allow it to equilibrate with indoor humidity for a while. I use moisture meter and ideally compare with readings from furniture of the same species of wood that's been indoors for while. Equilibration can take weeks or months but should be quick if it's just short sections (ie after you've cut your random angles). This reduces your warp problem but doesn't eliminate it because there will still be changed in interior humidity.

2) use epoxy rather than pva as suggested above.

3) I would have thought you don't really need ply backing sheet at all, as others have said. You could just glue the random blocks side by side. This will massively reduce effect of humidity changes.

4) look at the ring patterns in your end grain. Wood shrinks much more along the rings (tangentially) than across the rings (radially). Try and keep a consistent orientation pattern so that your blocks don't pull away from each other after gluing.

5) choose a stable wood species. In general softwood is more stable than hardwood, particularly in the radial direction. Of the commonly used softwoods Western Red Cedar is particularly good (and will put less strain on your mounting system because it's very light).

6) if you really want backing plate there are things you can do. For example would you be happy if backing plate was glued just to one row of blocks, perhaps bottom one or middle one? Make sure that the grain on this row is all running up/down.

Good luck!

Josh

Thanks Josh

The wood is what ever is available for the timber merchant at time so im buying 44x44 in lengths. The current moisture content ( toy arrived today ) is around 13% so i guess its still a little wet??

Although it had crossed my mind, i hadnt gone down the line of epoxy as i always assumend is was cost probibitive - ill look into

 

[toolsntat]

In my experience with modern day 44x44 planed stock it can dry and shrink to a rhombus shape.

Cutting all your bits and leaving them to dry/shrink could present a problem when trying to group them together.

I'd be looking at cutting into short machinable lengths and stacking in the house to acclimatise. Afterwards re-machine down to best possible size and take it from there.
Cheers Andy

 

[JoshD]

I hadn't realised that you were going to paint them first, so side by side gluing won't work.

Just a thought: buy a load of M4 mild steel threaded bar---about 50p per metre I think---- then drill horizontal and vertical holes through the middle of each block and string them along the threaded rod. One rod per row and say one rod for every 4th or 5th column. Use nuts on the two ends to tension up the structure and make it rigid, ideally hidden in the edge row/column ...

Best

Josh