Ok. Let's talk about the arching on the back. First things first. I've never held, or looked at an archtop. So I don't have any idea how it really looks! Why am I making one without seeing one? Having faith that it will work out is a very valuable thing. It is essential in life. In making an arch top you have to have an idea as well.
Here's the premise: GB Guadagnini's grandson comes over, and shows him his latest guitar. The old man looks at it, admires its outline, strums the strings a little, and notices how it sounds. He wonders about the flat plates, and looks inside. Seeing all the braces he says to his grandson, "Whatsamatter you guy? You forgot everything I taught you? Get me some wood! I'll show you how it's done."
Yeah old man Guadagnini had no more idea about archtop guitars than I do. But he was still going to make one. How would he approach it? Probably like building a cello. They have just about the same range. But the guitar has a much smaller footprint. More strings. More pressure wanting to budge the belly up, but less pressure pushing it down. How do you bring out the low notes with such a small area, and with such a puny volume to support them? The archtop addresses the pressure problem somewhat. The larger string angle brings more downforce on the bridge, and reduces the squishing force of the strings on the body and neck. A carved belly will counter the downforce, and TWO large 'bass' bars will take up the contracting force of the strings on the belly.
He was intrigued by the laminated neck, and thought that would work just fine. We'll add a truss rod, because everyone wants one. So let's see what I've come up with so far. I used a 1100 mm radius for the outside long arch. Why? I have a gauge for one, and it gives an arching of the right height. Maybe GB had one? Two diagonals on the inside generate the arch on the inside, but they are not solid arches. They rise up quickly from both ends, but only go 3/4 of the way or so. It leaves it thicker in the middle. The cross arches are the same way. I have the central area on the outside pretty much the way I want it, except for the upper area. the recurve area of the bottom isn't there yet. But NONE of the recurve area is close, Especially at each number 3 in the middle.
Yesterday I took over the dining room table:
The area in the middle is the area that is not part of the cycloid that forms the outside cross arch. The arch rises too quickly on the edges to make that work. So the central area continues the arch, matching the width of the outline. The line outside of that marks the area that is pretty much, "to print." This is basically how I do violins as well. The central area is defined first.
To determine what I had, and see what I needed, I had to use my plastic profile gauge. But it was too short. So I went to Rockler Hardware, and picked up another to make it 20" long. It was more of an ordeal than I expected. They are identical, so to put them together, one of them has to have the hundred or more red plastic pieces pulled out, and reversed. If you have one, you'll know what I mean. They are angled on the back and the angles will not match. It's all set now, and I'm not taking it apart.
Taking patterns inside and out 60 mm apart, I drew them on an 18 X 24 sheet of drawing paper, and examined them to see what I needed to do. I drew cycloids on the ends of the outside, and determined how much of the center would just be extension. Turned out to be 50-80 mm. Then I turned the paper upside down, and figured out where the ends of the cross arches should be, and how deep they have to be at the center. Then I had to check and make sure that the diagonal arches still fit with the deeper, wider cross arches.
I did the long arch as well, and I saw that the gauge does its job. Now I have carving to do.
Here's the premise: GB Guadagnini's grandson comes over, and shows him his latest guitar. The old man looks at it, admires its outline, strums the strings a little, and notices how it sounds. He wonders about the flat plates, and looks inside. Seeing all the braces he says to his grandson, "Whatsamatter you guy? You forgot everything I taught you? Get me some wood! I'll show you how it's done."
Yeah old man Guadagnini had no more idea about archtop guitars than I do. But he was still going to make one. How would he approach it? Probably like building a cello. They have just about the same range. But the guitar has a much smaller footprint. More strings. More pressure wanting to budge the belly up, but less pressure pushing it down. How do you bring out the low notes with such a small area, and with such a puny volume to support them? The archtop addresses the pressure problem somewhat. The larger string angle brings more downforce on the bridge, and reduces the squishing force of the strings on the body and neck. A carved belly will counter the downforce, and TWO large 'bass' bars will take up the contracting force of the strings on the belly.
He was intrigued by the laminated neck, and thought that would work just fine. We'll add a truss rod, because everyone wants one. So let's see what I've come up with so far. I used a 1100 mm radius for the outside long arch. Why? I have a gauge for one, and it gives an arching of the right height. Maybe GB had one? Two diagonals on the inside generate the arch on the inside, but they are not solid arches. They rise up quickly from both ends, but only go 3/4 of the way or so. It leaves it thicker in the middle. The cross arches are the same way. I have the central area on the outside pretty much the way I want it, except for the upper area. the recurve area of the bottom isn't there yet. But NONE of the recurve area is close, Especially at each number 3 in the middle.
Yesterday I took over the dining room table:
To determine what I had, and see what I needed, I had to use my plastic profile gauge. But it was too short. So I went to Rockler Hardware, and picked up another to make it 20" long. It was more of an ordeal than I expected. They are identical, so to put them together, one of them has to have the hundred or more red plastic pieces pulled out, and reversed. If you have one, you'll know what I mean. They are angled on the back and the angles will not match. It's all set now, and I'm not taking it apart.
Taking patterns inside and out 60 mm apart, I drew them on an 18 X 24 sheet of drawing paper, and examined them to see what I needed to do. I drew cycloids on the ends of the outside, and determined how much of the center would just be extension. Turned out to be 50-80 mm. Then I turned the paper upside down, and figured out where the ends of the cross arches should be, and how deep they have to be at the center. Then I had to check and make sure that the diagonal arches still fit with the deeper, wider cross arches.
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