Shipwright's compass

[charangohabsburg]

Nothing really new here. Instead of using stretchy ropes or heavy beams as a compass for drawing a curve with a huge radius, or being dependent on the radius of someone else's template a shipwright's compass may be the way to go.

This is the sketch which shows how it works:

The angle ACB is always the same.

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In any case the radius is kind of an odd measurement to use when designing a dished workboard. But the setting parameters of a shipwright's compass are exactly the data points we are interested in! (Lower bout width and dish depth respectively doming height).
However, here goes the math part ready to use:

radius.xls
radius.ods (the same in OpenOffice format)

(For those who own the Gore/Gilet book: I used a different approach than Trevor did to calculate the same thing, so the formula in his book looks different than the one I ended up with in my spreadsheet. But the formulas do exactly the same!)

And this is the tool itself:

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I glued on a piece of 80 grit sandpaper to increase friction when tightening the screw.
This is important because otherwise the compass would change its opening too easily while drawing the curve!

2014_D70_8062.JPG (61.81 KiB) Viewed 13194 times

And here even more pictures about how to use the compass.

[wadkin]

great post I'm sure to use this handy method in the future cheers
Alf