Archings

I do my arching from the inside first. It is somewhat unusual. Most people get the outside archings the way that they want first, and then they start cutting the inside, and thicknessing. I will explain how it works. The way that it works is simple, and can be adjusted by the maker to do whatever he decides to do. Let's look at the belly archings first. The belly archings on an archtop are very similar to a cello or violin. On the outside, the center long arch is flatter than a radius would give, and it rises faster at the ends than a radius would. The outside cross arching is basically a curate cycloid. But what about on the inside? I found a way that gives a long arch of the right shape. If you place diagonal arches from either side of center to a line set at the top of the blocks, and then make cross arches through them; you will end up with he right shaped arch. Where you place the diagonals is important. I use the upper f hole to help place them on violin based instruments. It won't work on an archtop because of its shape, but you can easily place the diagonals to go through the little arms that the cutaway makes. The back outside long arch IS a radius. You can pick and choose what radius you like using some geometry. You have to decide where you want it end. You have to decide where, and how much recurve you want. But the inside long arch is NOT a radius, the center area is not just 1.5 or more thicker than the edges; it is about 2.5 times thicker. On violins it is easy. You have 2 catenary long arches. One from the top going to a line through the corners or the lower bout, and one up from the lower block going to a line through the upper corner or upper bout. this give a thinner "lung" in the bouts, and the thick center, which may even have to be thinned some later if desired. It won't work this simply on an archtop. You have to use double diagonals because of its shape. Other than that, it is pretty much the same as for a violin or cello.

I get the inside arch correct, and a little short of the mark, .5 mm or so, and then rough the outside down. I get the edges and the central third or so, .5 - 1 mm thicker, and keep the transition area from just before the recurve to the ends thicker. At that point you can get a feel for what is going on. On a first time build, I would at least double these numbers; it is easy to thin it more at this time, and you may want more or less stiffness, so this way you have more options.