Kerf cutting/bendy plywood

[dvddvd]

I want to bend a piece of 9mm/12mm plywood into a cylinder shape.

The piece I have is 900mm x 150mm x 9mm.

I wanted it to be a 300mm diameter circlè/ cylinder.

I found a kerf calculator online but due to my Festool blade having 3.2mm kerf, when you put that in the calculator to get the cuts to close up when bent it came out looking like a 50 pence.

I did try it on a piece of ply and cut each slot 7mm apart and at a depth of 8mm. Some parts bent easily and some parts were stiff as a board. It was just wickes 9mm plywood I tried it on. I did try wetting the exterior and that helped a little but then I had to glue it into a circular end piece so it was not ideal.

The problem is that since the slots are 3.2mm wide they don't close up. So trying to fill the slots with adhesive before bending isn't doing much.
I know I need a thinner kerf on my chop saw ( which has a trench feature,)
Or is there another way to do it?
I do have a band saw, which has a lot thinner kerf and maybe a jig could be made with a end stop?
Thanks any suggestions?

 

[Terry - Somerset]

I want to bend a piece of 9mm/12mm plywood into a cylinder shape.

The piece I have is 900mm x 150mm x 9mm.

I wanted it to be a 300mm diameter circlè/ cylinder.

I found a kerf calculator online but due to my Festool blade having 3.2mm kerf, when you put that in the calculator to get the cuts to close up when bent it came out looking like a 50 pence.

I did try it on a piece of ply and cut each slot 7mm apart and at a depth of 8mm. Some parts bent easily and some parts were stiff as a board. It was just wickes 9mm plywood I tried it on. I did try wetting the exterior and that helped a little but then I had to glue it into a circular end piece so it was not ideal.

The problem is that since the slots are 3.2mm wide they don't close up. So trying to fill the slots with adhesive before bending isn't doing much.
I know I need a thinner kerf on my chop saw ( which has a trench feature,)
Or is there another way to do it?
I do have a band saw, which has a lot thinner kerf and maybe a jig could be made with a end stop?
Thanks any suggestions?

Calculating the diameter from a 900mm length gives 286mm - 900 / 3.142 (Pi) - slightly less than expectation.

The inside diameter would be 286mm less18mm (2 x the ply thickness) - 268mm. Using the formula (3.142 x 268) gives the inner circumference at 843mm.

You need to remove 57mm (900-843) in saw kerf to have the slots close up completely. This could be done with (say) 57 slots of 1mm each - would need to be a very thin kerf - not sure any are available at this size. Using a 1.6mm kerf would require 36 slots.

With 36 slots, each would be 25mm from the next so may not deliver a smooth round finish without sanding.

The pragmatic approach to construction would be:

• make two jigs - rectangles with a 286mm holes cut in them
• cut slots
• check it fits your jig properly - adjust as required, before -
• generously filling slots with glue (it will otherwise leave voids)
• insert into jig
• let glue completely cure before removing

 

[Cabinetman]

Suppose it depends what its use and destination are going to be. You could resaw a 2mm slice from a long 150mm board on your Bandsaw then find a former and wrap the “veneer “ around till it’s the correct thickness.

 

[ChaiLatte]

A hollow cylinder has an inside, an outside and to end faces. It would be helpful to say which of those four you intend to be a finished surface as that will open up some manufacturing possibilities and close off others.

The more slots you cut, the easier it will be to bend and the less faceted it will look when bent.

If the slots do not close up, but the exterior looks good, that is simply a matter of choosing the correct gap-filling adhesive (e.g. epoxy resin or a gripfil-type).

The idea above about veneering a smaller diameter cylinder is good. If you measure correctly, even if it looks faceted to begin with, it could be sanded to a cylindrical shape. The finished thickness of the veneer would vary slightly, being thinner where the points of the base polygon occur. The challenge with sanding a raw plywood item is the outer (face) ply is often very thin.

 

[Fergie 307]

I would be inclined to just use much thinner ply and build it up. You could probably bend thin stuff directly around a suitable former, especially if you can steam it first.

 

[pip1954]

You can buy it think it is ply that bends not sure if it bends far enough for what you want but have a search about.
You may need to make some sort of forma to help with making it round ,not an easy shape to make .good luck

 

[dvddvd]

A hollow cylinder has an inside, an outside and to end faces. It would be helpful to say which of those four you intend to be a finished surface as that will open up some manufacturing possibilities and close off others.

The more slots you cut, the easier it will be to bend and the less faceted it will look when bent.

If the slots do not close up, but the exterior looks good, that is simply a matter of choosing the correct gap-filling adhesive (e.g. epoxy resin or a gripfil-type).

The idea above about veneering a smaller diameter cylinder is good. If you measure correctly, even if it looks faceted to begin with, it could be sanded to a cylindrical shape. The finished thickness of the veneer would vary slightly, being thinner where the points of the base polygon occur. The challenge with sanding a raw plywood item is the outer (face) ply is often very thin.

Hi thanks its the exterior of the cylinder which will been seen

 

[dvddvd]

I have machined 2 doughnut shaped end pieces with a 9mm wide circular rebate, which is 4mm deep to put on each end. Which holds it in place while the glue sets.

 

[Distinterior]

Does it have to be Plywood....?
Coz you can get MDF in various thicknesses that is pre kerf cut already....

 

[dvddvd]

Hi thanks, I would of prefered plywood, but even with the pre kerfed mdf depending on the radius of the kerf its not for certain the slots will close up. Im not sure what the width of the kerf on the pre kerfed is ? 1mm? 2 mm ?

Ive seen the 6mm bendy mdf stuff, can you get thicker stuff? thanks

 

[niall Y]

I would go with using veneer and wrapping it round to get the thickness, As has been suggested.
One can buy Flexiply in sheets that will bend along or across the grain. But how tight these will bend is most likely related to their thickness

 

[dvddvd]

Calculating the diameter from a 900mm length gives 286mm - 900 / 3.142 (Pi) - slightly less than expectation.

The inside diameter would be 286mm less18mm (2 x the ply thickness) - 268mm. Using the formula (3.142 x 268) gives the inner circumference at 843mm.

You need to remove 57mm (900-843) in saw kerf to have the slots close up completely. This could be done with (say) 57 slots of 1mm each - would need to be a very thin kerf - not sure any are available at this size. Using a 1.6mm kerf would require 36 slots.

With 36 slots, each would be 25mm from the next so may not deliver a smooth round finish without sanding.

The pragmatic approach to construction would be:

• make two jigs - rectangles with a 286mm holes cut in them
• cut slots
• check it fits your jig properly - adjust as required, before -
• generously filling slots with glue (it will otherwise leave voids)
• insert into jig
• let glue completely cure before removing

Many thanks

 

[Yojevol]

If your final wall thickness is not an issue you could use the double kerfed sheet method. You apply an inner layer, with the kerfs facing outwards, to a couple of circular formers . Then apply an outer layer with kerfs facing inwards. This way you don't have worry about kerf width at all. If you make the whole assembly overlong you can saw off the formers to leave a hollow cylinder with smooth inner and outer surfaces. Strap clamps for gluing up are best.
Brian