How to make a concave wooden circle?

[Wabbit]

Hello, I am fairly new to wood working.  For a project I need a circle of wood about half an inch thick and 28 inches across that is slightly convex.  

Not sure how to describe it otherwise, but it would need an even rise from the edge to the center that would put the center about 1 1/2 inches higher.

If it matters, there will be a hole in the center, about 6 inches across.

I'm not particular on the kind of wood, it will be painted, but something light and strong would be preferred and I need it to hold it's shape in the end.
Does anyone know how to do this?
Thanks!
 

[wdwerker]

Curved pair of guides for a router. Like a flattening jig but arched up. After each pass you rotate the blank disc slightly. The curve desired would need to be the starting point to draw the larger radius for the curved guides, allow for the bit to stick down and the guides strong enough to not flex in use.

[Wabbit]

Wow, you obviously know what you are doing, but I don't understand much of that.  Are you saying it would basically be carved out of a thicker piece?

 

I am very much a novice.  I impressed myself with the tree house I built for my kids.  I thought it might be something along the lines of using steam and pressure to slowly bend the circle down, but not the case?

 

 

And how did this site get that picture of me?

[wdwerker]

Didn't you upload a picture ?

So you would need to glue up a blank thick enough and possibly square, then cut it into a circle. Mount the circle to a base board (3/4 sheet should work) with a screw in the center. The curved rails would mount to the same base.

There is always paper mâché .......

[Wabbit]

I did not upload a picture.  I used this picture in one other place, a Harley Davidson site.  I wonder if they are connected?

 

Curved rails?
Paper mache wouldn't be historically accurate, and not strong enough. I wonder how they made concave wooden shields in ancient Greece and medieval Europe?

[wdwerker]

Hand planes and lots of time. Or pound a piece of sheet metal over a bag of sand, repeatedly !

[Coop]

Wow Steve, I'm impressed!

Good luck Wabbit

[wtnhighlander]

Steve's (wdwerker) answer is good. You description makes me think of a slightly domed wooden base for a table, lamp, etc... Are you actually thinking of a large dish shape, like a Captain America shield, relatively thin and light?

That also could be carved from a solid block or lamination. You would need the jig Steve described for the convex side, and a complimentary jig, perhaps a pendulum, for the concave side. Or a really big lathe.

You might also achieve the same effect with some steam bending and coopering, but it would be complicated.

I read that some ancient wooden shields were made by layering veneers like plywood, so it should be possible to reproduce such using bent lamination techniques. I believe Marc recently posted a short video on such a technique:

http://www.thewoodwhisperer.com/videos/bent-lamination/?as=lamination&mode=posts&ap=1

[Wabbit]

Yes, basically a shallow dish shaped shield, about 1/2 inch thick.

Thanks for the link, that is interesting, but it looks like it is made for bending one direction.  My plan requires a bend in all directions.

I am thinking of trying the steaming method using a piece of 1/2 inch plywood.
Can anyone recommend a good wood type for this?  I understand some woods steam better than others.

Should I consider using 1/4 inch plywood and steaming, then gluing them together?
Thanks!

 

[krtwood]

I did a video showing what Steve was talking about with the curved router jig, though in my case I'm making a donut shape rather than a single curve so the jig looks a little different.

 

 

You might be able to do it bending layers of 3mm Baltic Birch.  It's certainly flexible enough across the grain and probably with it too.

[Wabbit]

That is cool, though beyond my means and abilities with the handful of garage tools I have.

I think my best option is to find something of about the same shape and try to steam/bend the sheets over it.

Thanks!
 

[krtwood]

I don't think you necessarily need a true form or to use steam if you use the 3mm ply.  If you just have a circle of 3/4" ply the same diameter you want and put a block in the center the amount of bend you want and then just clamp around the edges.  I don't know if you'd be able to get 1.5" that way but it's very low effort and worth a shot.

[drzaius]

As I understand it, your trying to obtain a semi-spherical shape, like a very large, very shallow bowl? If so, I don't think your going to be able to do it by steaming & bending plywood short of having a massive hydraulic press. The problem is that the wood will have to stretch in the center and/or shrink around the circumference.

 

A really big lathe would be easiest if you have access to one. Otherwise wdwerker's suggestion is a good one

[Tom Cancelleri]

Sounds like you're making a radius dish for building an acoustic guitar. When I did it, I built a pendulum for my router and made passes over the mdf.

Sent from my 831C using Tapatalk

[Don Z.]

What exactly are you making?  Torturing ply may be difficult.  It doesn't like compound curves.  On the other hand, a jig, a bit of epoxy, and veneer and you can make your own ply in the shape you want.

 

If that's too difficult, the router sled is probably the easiest way to go.  It's not beyond your abilities.  A couple of 2X4s and any number of ways of making the curve, and you can ride your router over it.  It's really not that difficult.  

[Raefco]

I would consider fiberglass matting and resin, once the mold is made it is pretty simple, but a bit messy.

[Janello]

If I were doing this with limited tools, I would build a frame out of ply with a jig saw and pie cut plywood, then face nail that to the frame. You can use 2 layers of 1/4" ply and stagger the joints to make it easier to bend and achieve your 1/2" thickness. Think Pizza Pie slices, over an arched plywood frame.

[krtwood]

If you went with the multiple layers of ply you could make multiple relief cuts along the radius and offset them in the different layers to relieve the stresses but it would still be strong.  (Pretty similar to what Janello just posted as I was typing). 

[wdwerker]

If you are trying to glue up layers either use 3 or 5 layers. It's called balanced construction. Regular plywood is always an odd number of layers for stability.

Draw and cut out a cross section of the outside of the desired curve. Then you can copy that shape to make ribs to complete the circle.

Maybe you could use a hydraulic bottle jack and a 4x4 under a main beam of the house to press the layers.

Or you could make a framework of the inside of the shield if you had access to a vacuum veneer press. Still going to have to glue up pie shaped wedges and offset the seams at each layer.

I saw a guy laminate a strip canoe with a staple gun and glue. You would have to grind off any staples sticking out of the back and fill a lot of holes before you painted the shield . A air staple gun would let you adjust the force to keep from driving the staples too deep.

Circles of plywood layers are not going to stretch and stay that way. Hammering sheet metal or laying up fiberglass would be other avenues to explore.

[Wabbit]

Thanks all.  I am working on a project for my dad's birthday to make a fairly authentic round wooden shield based on designs from the "dark ages".  Most at the time were easy to make- just flat with a hole and boss in the center, but the ones that look good and were also more effective were shallow bowl shaped, about half an inch thick.  I'm not sure how they did it then, but steam was suggested as a likely way.  They also used wooden boards, rather than sheets of wood.

[wtnhighlander]

Don't try to steam plywood, it will just delaminate. The mist authentic in appearance will probably be the glued up layers of veneer. Using very thin veneers, cut pie-shaped sections as Steve said. Perhaps wet them to be more pliable, and lay them over a dish-shaped form of some sort, maybe one of those dish shaped snow sleds. Add glue (moisture cured polyurathane, like Gorrilka glue, if doing this wet) and another layer of veneer. Shift the layer so the seams don't align with the first layer. Pile sandbags in top, which will conform to the shape and clamp it together until the glue cures. Repeat until the desired thickness is achieved. Just like paper mache, but with wood.

Alternatively, cut rings of plywood that overlap when stacked to approximate the dome shape, then grind / sand it smooth. Use segmented rings of solid wood if ply doesn't appeal to you. Plenty of youtube instruction in making segmented bowls that would apply.

Any way you go, a good deal of time and effort is involved, but expensive tools don't have to be.

[h3nry]

If you want a "fairly authentic" dark-ages design - then I'm guessing that plywood and router-sleds weren't part of the manufacturing process. Probably hacked out of a thick slab with an adze.

 

Can you not ask a museum that holds an example how it was constructed?

[Beechwood Chip]

Viking round shields were flat, with simple construction. 

 

The Scots targe is a round shield described as concave, but I can't find a picture or description of to to make one.  The wooden ones seem to be flat, and the curved ones are covered with leather so I can't see the construction.  If I wanted to make a concave one from wood, I'd layer up planks to make a thick, flat shield, then use an adze or hatchet to rough out the inside and outside curves, and then smooth it.  Then maybe cover it in leather with lots of studs/nails.

 

The Spartan round shield was apparently made by turning a thick wooden blank on a lathe.

 

If you want a curved Captain America type shield , then to be authentic you should make it out of some super alloy in a high tech manufacturing facility.