Most stable 2440x2440mm ply configuration build

[niman]

I may need to create a surface of this size shortly and with thickness up to 40mm. It should be as rigid as possible.

It will be supported centrally with a large lazy susan bearing and at the outer perimeter with ball-bearing rollers.

I'd appreciate possible comment to also consider any possibility of warping due to the gluing together of whatever thickness individual panels may be considered optimal (building 3 layers at a time may be best to counter this?).

If the "solution" could be extended to create a surface of greater than 2440mm square that could also provide various options for my project.

I'm assuming the various layers should be oriented at 90° to each other?

Any comment really appreciated.

PS: if it could be of help I do have a biscuit jointer.

 

[Steve Maskery]

Hello niman and welcome to the forum.

You have set yourself a significant challenge, and I'm afraid you are going to need a bit more than a BJ.

However, let us try try to find a way to achieve this.

It seems to me that there are two approaches:

1 Start with 40mm stock and glue up sideways

2 Start with 8 x 4 sheets and laminate

In the first case, you could use 40mm kitchen worktop, and glue several boards together until you get your width. Clamping is the issue, but you could make 8' clamps which use wedges, or you could use pocket screws along each joint.

In the second case,, again the issue is clamping. 2 x 12mm and one layer of 16mm ply would give you your 40mm, but you would need a pretty big vac bag to press it

I shall be interested to see how you achieve this.
Steve

 

[niman]

In the second case,, again the issue is clamping. 2 x 12mm and one layer of 16mm ply would give you your 40mm, but you would need a pretty big vac bag to press it

I shall be interested to see how you achieve this.
Steve

Thanks Steve.

It matters not that I damage the surfaces so I had also assumed that high screw count should do the job?

I had also been thinking that 6 layers of 6mm (at 90°) would be optimal but am now too old to remember all those "Newtons Law" things from school days

 

[Steve Maskery]

OK, I'd not thought of screwing the layers together. If you do that, pre-drill first, buy in a load of beers and get some mates round to help screw. You will not have a lot of open time if you are using PVA.

I'd use thicker stuff than 6mm though.

 

[Phil Pascoe]

You need one thicker core to screw into if you were to use screws. Also you will not have much strength (if the end product is to be stressed) - I tried it with three half sheets and was surprised how poorly they pulled together, especially if the glue starts to cure as you're working.

 

[MickCheese]

I'm with phill on this. I tried it a while ago just laminating three 18mm lengths of MDF using glue and screws. It was not too successful. The glue went off and the screws just didn't pull the pieces together enough to cause squeeze out.

Mick

 

[Terry - Somerset]

A single 8x4 sheet of 18mm ply weighs about 30kg. Double that to get to 36mm and double again (approx) gives a weight of approx 100kg + bearings etc. The frame which I assume sits underneath will need to be robust as will the track on which the ball bearings run.

If your design allows you could consider reinforcing the underside with radial ribs.

Terry

 

[Mark A]

Would a torsion box design be possible? They're rigid, resist warping and weigh a lot less than a solid 8ft square slab.

 

[The Bear]

What are you actually trying to make, whats it for? They may be better ways if we know what its intended use is.

Mark

 

[niman]

You need one thicker core to screw into if you were to use screws.

Thanks – I had not considered that.

Also you will not have much strength (if the end product is to be stressed) - I tried it with three half sheets and was surprised how poorly they pulled together, especially if the glue starts to cure as you're working.

To be sure – do you mean not much strength only if PVA glue starts to cure prior to finish of screwing? i.e. if I had finished prior to curing do you think it could be ok? (i'm not particularly familiar with woodworking terms and it is a long time since I completed any serious work!)

A lazy newbie question – how long could I have between application / setting?

Could it be beneficial to glue up & do a single 8ftx4ft (on the top of the base 8ft x 8ft) to give more time (or do I really need to be more concerned with getting both sides of the sandwich done asap re moisture?)

Contact cement perhaps?

I have also read this elsewhere:

“Problem with glueing plywood faces together is that you introduce moisture to one side of the sheet and not the other. We all know what happens then.”

If that is a real problem – how long might I have between “flipping” the first 2 layers and getting the other side done.

If your design allows you could consider reinforcing the underside with radial ribs.Terry

Unfortunately not (see later). Your comment reminds me that this is non-trivial challenge.

Would a torsion box design be possible?

Again – unfortunately not.

What are you actually trying to make, whats it for? They may be better ways if we know what its intended use is.Mark

Hi Mark, the requirement is to move a set of (probably 12) points, perhaps each of 10Kg, in/out radially along routed channels with respect to the centre of the surface. The surface will need to bear the weight of 2/3 adults as they cross it towards the centre (which will be supported via the lazy susan) - i'm not sure if I should be particularly concerned about breakout at the closest point of the routed channels.

I had originally considered a Longworth chuck type mechanism but the points of intersection between the 2 plates are not constrained to exact radial motion. I now have another plan to use a “pulley” mechanism for exact radial motion but that could also change – so, for now, if the Lonwgorth gives you enough of an idea then, I think, the basic forces involved for the ply should not change too much.

Thanks again for all the comments & any further would be most welcome.

PS: a 3 metre diameter (square) would actually be preferred hence reference to that in the original comment - but baby steps first.

 

[Phil Pascoe]

Maybe the first part of my comment wasn't very well worded. Of course you'll have an amount of strength even if it's only stuck in places, but you won't have the strength of a perfect lamination. I suspect you'd do better avoiding PVA although I believe there are slow curing ones. Cascamite for one will give you a lot longer open time - do a bit of research. If you are building it up in multiple layers there's no reason why you shouldn't make it the size you want so long as the appearance is unimportant - just stagger all the joints.

 

[blackrodd]

I'm with phill on this. I tried it a while ago just laminating three 18mm lengths of MDF using glue and screws. It was not too successful. The glue went off and the screws just didn't pull the pieces together enough to cause squeeze out.

Mick

In out joint shop, When the glue press was being otherwise used, someone had devised props from the ceiling to squidge out the glue and flatten boards.
Rodders

 

[Phil Pascoe]

I was going to suggest that, but wondered if the OP actually had a flat surface 8' x 8' or larger to use.

 

[blackrodd]

I was going to suggest that, but wondered if the OP actually had a flat surface 8' x 8' or larger to use.

I hope hope the poster has the dozen or more friends to carry the sodding thing, when glued!
Rodders

 

[niman]

Tiled floor (on concrete/screed) will eventually accomodate the 3m fixed item routed in situ.

Prototype for now on screed/concrete floor. Something like multiple of this http://www.screwfix.com/p/forge-steel-extension-support-rod-pack-of-2/78272 against ceiling joists (or could that just be child's play...aside from raising the upper storey :roll: )

i.e. was something like full builder's Acro props being referred to?