How much metal?

[Vanessa]

Hi all I have a calculation query...

I want to make a dome for the insect I am making for my project. I followed the following method, draw out side view of sphere using compass and draw a horizonalnline through diameter then draw a line perpendicular to it at the bottom curve of the sphere. Then take compass and measure from centre where the curve meets the bottom line and draw an arc from the corner of the hemisphere to the horizonalnline do this for both sides of drawing. Then measure the distance between where the arc meets the line to the opposite side. This method should tell you have big a sheet of metal but when I did itbit went hideously wrong and is far too big.
Is there another calculation I couldb use that would be more accurate?

Thanks

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[Goldsmith]

Vanessa,
If you are making a half sphere, then the calculation is diameter multiplied by Pi (3.14) divided by 2, when I was an apprentice the old silversmiths didn't use maths, when they wanted to work out the size of a bowl disc they just added the diameter of the bowl to the depth of the bowl, this does give a quite a close accurate size when you account for the metal stretching when being shaped into a bowl with a hammer.
One last little tip, when I have been making odd shaped domes for animal and insect bodies, I have used a bit of binding wire, bent to shape on each curve of the design then straightened out and measured to give me my metal sizes, this works well when making oval shaped bodies.

James

[Vanessa]

Thanks James I'll try that ... I've been so confused and my tutor didnt understand how I got it so wrong but it was the end of the day so didn't have time to go through it with me. I'm sad i've wasted time on a bowl that's too big but I have other plans for it and will just try again to get it right. My tutor suggested that I keep reducing it down using the doming blocks until I get the correct size and then cut the waste metal off... but I really want to know how I calculate it correctly for next time otherwise I've learnt very little and will have wasted metal! its only copper but still to me any metal is valuable!

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