[cfullen]

I have a curved banquette to build. Everything is pretty straight forward, but the client wants the back to have a slope to it. I have no idea how I am going to cover this surface. Would bendable plywood flex enough to achieve the shape I want? I have never built anything with this form and I am stumped. Any light you guys could shed would be awesome!

 

[cfullen]

 

[Steve Neul]

Yes the bendable plywood would work. It's available in 8'x4' sheets and 3/8" thick so when you laminate two sheets together it becomes rigid. The bendable plywood is very flexable. It's almost like handing a sheet of rubber. I used it to make the center section and drawer fronts of this store display.

 

[ed_h]

What's the finish material?

 

[GeorgeC]

Bendable plywood will work for any curved section that does not also bend/lean back.

However, it is not going to bend/lean and go around a curve at the same time. You need stretchable plywood for that. The distance at the top and bottom of the curve is different.

George

 

[cfullen]

It will be finished with lacquer, and then covered with padding. It will probably never be seen, but the perfectionist in me just won't take mediocre work.

 

[ed_h]

It's not a compound curve, so it can be covered with flat material without stretching. There is a graphical way to make a flat pattern for the piece that will fit the curve.

 

[cfullen]

Ed, please explain. I'm a little lost in the theory of a flat piece (bendable) will conform to the "conical section" of this piece.

I broke down and ordered "Circular Work in Carpentry and Joinery," by Roger Holmes. I've been told there is a nice section about these curves.

 

[ed_h]

The graphical technique involves dividing the bent surface into a series of triangles around the curve and transferring the triangles to the flat material. It's much easier to do than to explain. Hopefully your book will show the method.

If you had the piece already build without the skin. you might be able to take the triangle measurements directly from the piece and skip the drafting step.

 

[rtutsky]

Was the 3D model you show done in Sketchup? If so there is a plug in for Sketchup that will accurately flatten the shape out for you.

 

[cfullen]

Rusty, any chance you know what that plug in is called? That would be killer.

 

[cfullen]

Rtusky, sorry for the "rusty"

 

[rtutsky]

Rusty, any chance you know what that plug in is called? That would be killer.

The plugin for Skype for what you need is called Flattery. Sorry for not getting back to you until now but I haven't been online for a while.

 

[cfullen]

Finished

 

[GeorgeC]

It's not a compound curve, so it can be covered with flat material without stretching. There is a graphical way to make a flat pattern for the piece that will fit the curve.

The original poster said it was a curve that leans back at the same time. I would say that was a compound curve.

Why do you say it is not?

George

 

[GeorgeC]

The graphical technique involves dividing the bent surface into a series of triangles around the curve and transferring the triangles to the flat material. It's much easier to do than to explain. Hopefully your book will show the method.

If you had the piece already build without the skin. you might be able to take the triangle measurements directly from the piece and skip the drafting step.

It could be done that way, but it would take a large number of small triangles.

George

 

[cfullen]

we actually spent a lot of time planning to do something way more complicated than we ended up doing. We used two pieces of bendy plywood and because the angle along the back wall of the seat was so slight, it worked well. If it had been any steeper of an angle, it would have taken 10x as long.

 

[ed_h]

The original poster said it was a curve that leans back at the same time. I would say that was a compound curve.

Why do you say it is not?

George

George--

Horizontal slices would be curved lines, vertical slices would be straight lines. There is a single axis of rotation. It's a simple curved surface.

 

[ed_h]

It could be done that way, but it would take a large number of small triangles.

George

George--

Well, the triangles are an approximation, so it depends on the error you can tolerate. That's one good aspect of the approach--you can choose the accuracy you need and design accordingly.

For something like a sheet metal transition (for duct work) a dozen or fewer triangles may work. For finite stress analysis, hundreds or thousands may not be enough. If I were doing this project with a triangulation method, I'd probably go for 20-30, knowing that the material is somewhat forgiving.

 

[Tony B]

Nice work.
For those of you that think this is a compound curve you will have to practice thinking 3-D. This is a simple cone.

There are lots of ways to do this depending on your skill level and available tools. Drawing programs are also considered in the 'tools' category.

If you didn't have the knowledge and skill level of the computer program which I don't. I would have taken a different approach.
I would have built the back rest section first with a top and bottom and no front. Then mounted it on top of some crates or whatever to raise it off the floor and have the front edge overhanging the crates. then get the curves plywood and tack it temporarily across the front. Due to the conical shape, the ends of the plywood would be overhanging on the end below the bottom surface of the backrest and level with the bottom of the backrest at the center of the 'cone'. Take a marking gauge and mark along the top and bottom of the backrest and the ends of the backrest. Now remove the curved plywood and cut/trim to the lines. In reality, this make take two or 3 sections of plywood but that is how I would tackle it.