Arching, Revealed

Checking Top Arch

Over the last few years I’ve been messing with a contractor’s laser level to show violin arching more clearly. It’s a variation of the maker’s idea of using a ruler and light to cast a shadow on the arch while shaping it, as pictured above, and initially I used a series of photos, and then went to movies for the same purpose. There’s more from the laser, and a movie, at this post.

Finally, I got the idea of a way to mix both together, so that the entire arch would be mapped in one shot, of higher resolution and sharpness.

laser one-shot

If you have an outline of the violin so that you can scale out the widths, and just this one photo, you can make accurate templates. It’s simpler than the old way of spending all afternoon cutting templates off the real violin with thin slips of wood, or using a carpenter’s contour copy gauge. Now, if I have ten or fifteen minutes with a violin, I can extract all the data I need to make a copy of it, from just a set of drawings and photos.

The final step in this process for me, since I’m only copying Cremonese models, is to redraw the arch without 300 years of distortion. This requires a bit of reverse engineering to figure out what they were thinking then, and what their originals must have looked like when they weren’t so bent out of shape as they are now, through three centuries of strings tugging the parts of the violin in different directions.

In conjunction with Canadian violin maker Quentin Playfair, who originally outlined the role of curtate cycloids in Cremonese violin making in a STRAD magazine article some years ago, Stephen Mann developed free computer software to draw the appropriate curves without any fuss. All that’s necessary to generate the cycloid shape is the distance between the low spots of the scoop around the plate and the height of the arching, at the location for where you want to make a template.

Even if you’re not making a violin, the software is fun to play with: as you change parameters, you can watch the curve change in real time. It’s an interesting shape which appears to change radically as you approach extremes, yet all of the apparently different versions are mathematically related.

There are links on the same page as the software if you’ll like to learn more about these interesting shapes and their associated math. Those of us who once were children may remember the spirograph, or may even have generated cycloidal curves using buttons and a pencil

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