A Plane! A Plane!

I started making a frame for the mold to build the ribs on. It seems that most guitars are built on an outside mold. Mine's going to be built on an inside mold. Why? Because I'm doing the Grandfather Guadagnini guitar. Guadagnini used an inside mold. Besides that, I don't have enough material to build an outside mold! The mold will be double layered, and will be collapsable. That way I can glue the linings on, and even the back if I want, and the form will slide right out.


 But first, I have some progress on the belly. I built a convex scrub plane to rough it out. It is built out of cherry wood, and has a Lie Nielson Scrub plane blade as its main feature. The thing is awesome.

     




 The sole is convex in both directions, and matches the inside arch nicely. It is Surinam Ironwood, so it should hold up.

 I went a little deeper than I was planning on going. I should have adjusted the blade to take a little less bite. It cut really well, but the flame caught me one time off guard, and it took a little over a mm to straighten everything out.

 I used the smaller Lie Nielsen convex plane to do the light cuts, and finished up with my trusty curved scraper made from an old Craftsman plane blade. The steel on that one is quite good. Right now, the diagonal arches are right on, and the horizontal arches are as well; all the way until the center area. There I have arches perpendicular to the diagonal arches, that blend in from the neck block and into the horizontal cross arches. The cutaway side rises very quickly, the other side at the top, not as much. Now I'll have to rough in the outside some, and then I'll rough cut the outline.


 

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.

Guitar Basics

Ok, let's talk about design aspects. If you find something that I write here that is in error, or needs more thought, please don't hesitate to comment. I really know nothing. The first is how does a guitar even work. It has the same basic range as a cello, yet it has much smaller belly and back plate area; it has shorter strings; it is no wheres near as thick in the ribs, so the internal volume seems like it is less than half of a cello. How does it give any sound at all?

 When I read that a cello has the same range as a guitar I was surprised. It sounds MUCH deeper than a guitar. The guitar is written on the G clef, but it sounds an octave lower. It doesn't really sound an octave lower, does it? A low E on a clarinet (which is actually, low D) doesn't sound almost an octave higher than a low E on a guitar, does it? It does, but it doesn't seem like it. At least not acoustically. With amps, yes. Even when a cello is played high up on the fingerboard by the body, it has a much lower sound than something that I imagine coming from the seventh fret of a guitar.

 Volume wise, the cello drowns out an acoustic guitar. It isn't even close. I found that a cello can play at from 85-111 decibels. A guitar will top out at 80. Top out. The strings on a cello have about a 27 5/32" scale. About what a baritone guitar would be. Since a baritone goes down to B or C, that makes sense. The 4 strings on a cello give 120 to 130 pounds or so of pressure combined. The high bridge on a cello translates more of that tension to the belly. Using 125 as an average, and a 153 degree string angle, 58 pounds goes to the belly, leaving 67 pounds to be counteracted by the with the grain strength of the spruce top and the neck. Many of the arch tops I've looked at, (On line, I've never actually even LOOKED at an archtop in person, let alone held one) have a shorter 25 inch scale. Why? I don't know. The six strings have a wider range of tension; maybe 160-190 pounds. Let's call it 175. The angle I have drawn up for the archtop is 175 degrees. That gives us 46 pounds on the bridge, and 129 pounds pulling on the top and the neck. So the guitar has almost twice as much tension pulling on the neck. No wonder they want truss rods. But notice that the pressure going on the body is about 20% less than what is on the cello body. But also remember that the guitar does not have a soundest to help hold up that pressure. Does that mean that the guitar belly actually has to hold more? How much more? 50% or so?

 Now consider the body size. A cello is about the same width, but it is much longer. Deflecting it at the bridge seems like it would be easier. The smaller body of a guitar would make it inherently stiffer. The cello has very deep sides. That gives it a huge volume, and that is where it's deeper voice comes from. I can't find anywhere what the A0, or the breathing mode of the cello is. For an acoustic guitar I found that it can be from 80 to 150 or so. Steel strings at the lower numbers. Classicals were higher. Anyway, the guitar low E might, or might not have any support from the body itself. The main difference is that the guitar is strummed or plucked, and the cello is usually bowed. The cello gets continual energy from the bow, and the guitar has to save the string energy, and effectively transmit it to the body. The body of a guitar seems like it needs to be extremely resonant. At the same time it has to be quite strong.

 As far as to the tuning, free plate tuning, combined with weight, seems like it just gets you close to some general stiffness guide. I usually get the inside finished, and then get the outside close, and tune it after it is glued to the ribs. I noticed very little mention of actual notes in doing a lot of web surfing. But what I did find seems to be that the guitar plates are done pretty much the same as a cello. To me, that seem like they would have to be thinner. Wouldn't they? We'll find out. For a picture today I give you a Guadagnini guitar. It's labeled an Anacleto Guitar. What does that mean? He was the grandson of probably the third most famous violin maker G.B. Guadagnini. I will make this guitar as if G.B. was around, and saw what the boy was doing, and put his take on it. I'm still leaning toward the baroque neck joint at the headstock. Should be fun.

 

 I plan on using the parallel bracing design. That should work for the lengthwise string tension. Cross arching, combined with the cross catenary curves on the inside should give the cross arch stiffness. That will be the next post.

Build An Archtop Guitar

I want to build an archtop guitar. Why? I've built violins and violas. I have a cello started. I made a 1/2 scale 5 string guitar for the grandsons based on a 5 course Stradivarius baroque guitar. I don't play the violin. The guitar is with the boys. My old classical that I got at a garage sale 30 some years ago is in need of repair. The neck has always been bent. The saddle is coming unglued. The action is too high. The frets are pretty low. It's unplayable. I could just fit it up. But then I saw this:

It was on a Chicago guitar makers site: http://koentoppguitars.com/blog/the-hog-amati/

There is a whole lot of inspiration there. So I looked in my stock of wood, and the only thing I seemed to be lacking was a belly. I can use the top part of a board I bought to replace the back on the cello I have started. I hacked the first one one while sawing the wedge in half, and it will be not as high of an arch as I wanted. It isn't spectacular; but it is a one piece European Sycamore slab. That's maple for us.

I can use one of my rib blanks for the cello, and split it lengthwise for the sides. A piece of Birdseye maple can be cut off a piece that will be a viola back to make two sides of the neck, and the headstock. A piece of Padauk can go in the middle, and house the truss rod. I know NOTHING about those. A cool piece of curly Bubinga will work for the fingerboard. I found a piece of older curly redwood at Orcas Island Tonewoods in the specials page.

http://www.radiofreeolga.com/tonewoods/specials.html It looks very cool. It does seem like I'll have to be careful about the cross gain stiffness. When it gets thin you can snap it. Any tips about that? Once it is glued around the edge it should be better. Double it from the start? Line the f holes?

I drew it up 400 mm wide by 500 mm long. Just about 16 inches. I couldn't go much bigger because of the size of the maple. That also gave me enough wood on the Redwood before sawing it to book match to give me 2 parallel braces. I made a copy of the rough plan, and glued it on to a piece of 1/4" hardboard for a pattern. I still need to make a form. That will be the first real step. But I have done some planning.

(Yeah, I know that drawing has a REALLY DEEP dovetail. That was before I bought a book to give me the basics.)

 I make violins and violas from the inside out. A bit weird maybe, but I think it is the right way to do it. Why? Well think of the shape. It follows the outline, and the arching almost like a fabric made up of lines connecting each point to the next. When we get into it you will see how it works. I worked out the inside and outside arches today, not perfectly, just with sketches based on cross arches, catenary curves that I drew up. I bought the Benedetto book on making Archtops. It doesn't have the things I was looking for. I'm always looking the opposite way that everyone else does. But the arches I drew up today match the long arch profiles almost exactly.

 A catenary curve is what you get when you hold a chain on both ends, and let it hang. You see them all the time. Bridges are made with them going upwards. Electric lines show them going downwards. The steel trucks that come into our shop, (I'm a CNC machinist), have beds that are catenary curves. Loaded with bar steel they look almost flat. Empty they rise up maybe a foot. But that is just part of the curve, the rest, the outside edge, is the re curve. It can be thought of as a spring. It is a curate cycloid. It is what you would make with a Spirograph if you use the straight bar and any wheel. The smaller the wheel, the narrower the arch. The further from the center, the higher the arch. You have to decide where the low point will be. A lower, wider arch can go further in from the edge. A higher, narrower arch can go closer. On an archtop, it doesn't change dramatically; the narrow point is not that narrow, and the arch is not really that high.

 Check in and see how it goes.

 Ken