Roughing the re-curve

The central part of the cross arches can be accurately defined by the catenary arch on the inside. How close you make it to the edge will determine how close you come to a curate cycloid as your outside shape. The recurve starts out by figuring out where the line of inflection, and the low point is going to be. The low point is easy; look at the plans; or in my case where I'm making it up from scratch; come up with a number. I'm using 7 mm in the c bouts, and 9 or so for the rest. I'm putting double purling in; since I'm making it somewhat like a Maggini shape, so those numbers should work. Amati models may go in further than that, an other models may get closer to the edge. Now we have to find the inflection line, where the concave curve changes to convex. Turns out it is easy.

You know how a cycloid is formed. A disc is cut out with a circumference that will cover the distance form low point to low point in one revolution. A hole is placed 1/2 the height of the actual outside arch (height of the arch minus the thickness at the low point), a pencil is inserted, and the disc is rolled on the circumference. We don't need to make all that, we just use some simple trigonometry. Figure out how far the disc rolls in 15 degrees, find the sine and cosine of 15 degrees, and use those numbers to find where the pencil would mark at 15 degrees. Do it for 30, 45, 60 and 75 degrees, and you will have 4 points that define the recurve exactly.

I had to edit this part. I was trying to find the point of inflection, and forgot to add the width it rolled with the disc. DAH! So I did it over. For a normal Strad type arch it is about 66 degrees of rotation at the c bouts. For a slightly wider and shorter del Gesu arch, it is about 72 degrees of rotation. So, a Strad arch is about 75% convex, and a del Gesu about 70% convex. The upper and lower bouts will have even more rotation to get to the point of inflection. I draw out some of them, I picked 7 on this one, and I can pick and choose what sort of tool I will use to cut them. Here are my choices for concave curves:

And the recurves and the ones that will cut them:

Turns out, the recurve, even in the c bouts of this 5 string, use my three widest tools. The bottom of the lower bout even needs to be somewhat flatter. This is what I came up with doing one side of the belly and the back.

These are not finished. This is only to rough out the recurve for preliminary tuning. I also only cut the recurve part first, and didn't blend the central part of the arch in to it. I cut the recurve from the low point to the inflection line, or a little past it, and then use the thickness punch to put pin pricks on the inside to finalize the inside arch. It is only after that that I use the punch to do the outside.

This is what the inside looks like:

Well, those photos don't want to load. I'll put them in another time.

You have to be aware of how much the concave arch goes down from where the platform that you have is set. This one goes down just less than a millimeter in the c bout area, and less than 1/2 of that everywhere else. This means that for this one, I will end up having to cut the central area, without being able to use a full arch like I roughed it out with. What the maker does when this happens is up to individual style. They can just keep the thicknesses really thick, or cut them out with smaller cutters. They can make a hump, or a tiny radius, or any number of things.

This may explain, or confuse!

5 String Roughed Out

I had the plates roughed out. Now I filed and cut them close to the finish outline. I double checked the outside long arch, and inside long arch, and then the inside cross arches. Cut the edges down to about 5 mm thick, and blended the outside in some. They are still quite thick from 1/2 way to the edge. The center may be somewhat thick, but I've never done this before; nobody has; so I want to start thick and try to find out what works.

I bought a wall cabinet to put stuff in. Mostly varnish stuff, and that kind of thing; not tools. It works good.

These photos show the size of the beast I'm making. It is more of a 5 string viola with a violin scale than a 5 string violin. Hopefully it will roar like a beast. It isn't long, but it does seem quite wide.

The 5 string

Like I said, I'm doing a 5 string. I drew up a Maggini cello from 1600 that I think looks great. Turns out in 1/2 scale the stop length is the same as a violin. My brain went crazy and thought 5 string; so that's what it is. I had to change it some. The actual stop is right on center. Bruce Carlson said that is fairly typical for Brescian instruments. But they had the bridge low. Here it is:

It is a sweet thing isn't it? I lowered the f holes so they matched where the bridge was. Then I had to lower the lower corner because the cool f holes somehow lost some of their length. Not really, but it just looked that way. Then I had to lower the centerline of the lower bout. I ended up with this:

Once I get the outline on the roughed out plates, it won't take too long to finish up.