This weekend 's project

[mr]

Or at least it will be if I manage to escape the office. (If I spent less time looking at woodworking sites during the working day praps I'd escape earlier - just a thought).
Anyway the small boy has comissioned a toy chest which has to look like a pirates treasure chest, I know where Im going with the chest bit but does anyone have any advice as to how to work out the angles for the sides of the pieces that will make up a coopered lid ? Maths never was my strong point. I could just go for a planked coopered lid if you follow but I'd like to have a go at making a smooth lid.

Cheers Mike

 

[Scrit]

Hi Mike

Have you thought about drawing it up in a CAD program?

Scrit

 

[ByronBlack]

HI Mike

I was thinking of doing something similar a little while ago but didn't do it in the end. Here are some links that might help

Wood Central Article

Or

Ozzy Forum Discussion

Or

DIY Network article - nice instructions.

Hope that helps a bit Mike.

 

[Anonymous]

I had to make something just like this last year.

I solved the problem by making the coopered lid first - just guessed at a suitable angle and ripped a load of pieces. I glued them up into the lid, planed & sanded it into a smooth curve, and then made the chest to suit it.

It worked a treat - the client's son was delighted!

Cheers
Brad

 

[mr]

Hi All
Thanks for youre replies.
Scrit - I can see the benefits of a cad app but only have access to Sketch Up and I just cant get my head round it. I keep saying "just one more try" and still get it sorted.
Byron thanks for the links Ill have a look through, and Brad I hadnt thought of working "backwards" that might be the way to go, Im sure a bit of guesswork will get me into the right sort of area

Thanks again all.
Cheers Mike

 

[Chris Knight]

Mike,

The necessary angles depend on the thickness of the stock you will be working with as well as the dimensions of the coopered lid. The easiest way to do it without maths or CAD is perhaps with a full scale drawing as follows:-

1. On a large piece of paper, draw the end view of your lid, showing both upper and lower surfaces (ie representing the true thickness of the lid).
2. Cut out several pieces of paper the same size as the end view of the boards you will use.
3. Overlay and overlap the pieces on top of the drawing of the lid so that you create in effect a segmented curve that will be the shape of the glued up boards from which you will finally shape the lid. Glue the bits of paper together at the overlaps to retain the shape.
4. By eye, bisect the angles at the overlaps with a pencil line - this is what you will need to set your saw at. It doesn't matter if the angles is not precise as long as you have enough wood thickness from which to make the final result. If you glue up one section at a time, you can check as you go along with reference to your original drawing.

 

[mr]

Thanks Chris thats a very similar method to one suggested in one of the links ByronB posted above. Certainly looks to be the simplest way without having acces to Cad
Mike

 

[Shultzy]

This was a tricky problem but I think this is the maths.

If the chord length(c) is the width of the treasure chest and height(h) is the height of the curve at the mid point then the radius of the curve is

r= (c squared + 4 x h squared)/(8h)

if c= 20" and h=2" then
r= (20squared + 16) / 16
r= 416/16
r= 26"

You could then lay this out on a template, divide the arc angle into how many staves you want which will give you the stave angles required.

If you are a bit more adventurous the equation for the arc angle is

theta = 2 x arcsine(c/[2r])

which will give you

theta = 2 x arcsine(20/[2x26])
theta = 2 x arcsine(0.3846)
theta = 2 x 22.62degs
theta = 45.24degs

For ease of calcs lets say the angle is 45degs and you want 15 staves, that's 3degs per stave
which means the saw needs to be set at 1.5degs from vertical.

Hope this helps.