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This is a method that boat builders use to make large arches without using gigantic compasses or trammels.

It is really rather easy to do and quite accurate.

You do not need a center point.

Let's take a practical guitar building example -
For a 24 inch radius dish with a 25 foot radius, the run would be 12" and the rise would be .240".
For this example you'd need to made the jig taller to make it stiff enough. Just add another inch or 2 to the bottom of the jig while maintaining the shape of the upper portion.

Drive nails into the the template at the center and the ends of the arch.
Position the jig with the hypotenuse against the end nail and the flat against the center nail.
Hold a pencil in the notch and advance the fig while bearing against the nails.
Flip the jig to complete the other side.

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

Steve, nice work. This has been on the net for a while. Mario shared it once with us. Time passes though and new folks can certainly benefit.

I Keep it simple. By tacking a nail in my deck and hooking a tape on it, then walk down the deck 25 feet. At that point I tack my template stock, and make an arc with a pencil held at 25 feet. Then off to the bandsaw. Once you have an arch it can be transferred to things like braces to shave them to a proud fit, then hit it in the sanding dish for completion.

I've see others come up with a formula for the height of the arc, then with two points bow a slat in between and draw the arc. Probably not perfect. It all gets close.

I saw this come up a few weeks (months?) ago. I searched my files and could not find my notes. I searched the net and could not find this. I searched here and could not find it. I remember the thread and was frustrated because the person asking did not get, to the best of my memory, a good answer. Then I stumbled across my notes and found this. Either I'm not good at searching or it isn't that easy to find. Maybe a little of both.

I'll always be able to find it here now.

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

Thanks for sharing Steve. I did not know this.

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Joe Breault
Merrimack, NH
Perpetual novice

Thank you very much for sharing this Steve! File saved!