LUTHIERS: Where is the center point?

[Steven Bollman]

Originally Posted by charles Tauber

Steven,

One of the things that is learned from this and related discussions is that makers use a variety of geometries for the arching of the top (and back). In some cases, the method of construction (jigs, fixtures, hand tools) determine the geometry. In other cases, one uses construction methods to achieve and facilitate a chosen geometry.

For example, some guitar makers make dead-flat tops - no arch of any kind. Others leave the upper bout flat and only dome the lower bout. Others, still, use a spherical dome for the entire top.

Some contour the gluing surface of each brace and glue them with a backing to match that contour. Others only contour the X braces and glue the other braces, with their flat gluing surfaces, while in a flat state, adding the arch as a result of gluing the X braces.

There isn't one "right" method and geometry. You can either choose your method and let that dictate the geometry, or choose your geometry and use methods to achieve that.

I'm not entirely clear of either which methods you are using or which geometry you are trying to achieve. Hence, I can't offer much in the way of suggestion other than to be clear in your own mind of what you want to achieve and what methods you want to use to achieve it. I do understand that investing a lot of money in methods while you are uncertain about making another guitar is one of the driving factors. One possible option that reduces "tooling" costs while eliminating a whole bunch of grey area is to simply make the top flat. Many very fine instruments have been made with dead-flat tops.

Arching a ladder-braced back is a simple matter and can require little, if any, specialized tools/jigs/fixtures. As others have mentioned, John in particular, the longitudinal arch is forced onto the back at the time of gluing. This is done by contouring the sides to produce the longitudinal arch, then the back glued to it. If you are interested, I can outline a simple method for contouring the sides using hand tools.

Thanks, Charles. The design I am sort of following, at least trying to adhere to, is a Stahl Style #6 which calls for a 12' radius top and a 10' radius back. I would appreciate any advise on methodology/technique. I am trying to execute as much as I can with old school handtools. The only power tools I've used so far is a small router and a screw gun. I'm trying to keep the saw dust down and I'm also interested to see how far I can go without more power tools. Thanks for you help!

[arie]

Originally Posted by Steven Bollman

Thanks, Charles. The design I am sort of following, at least trying to adhere to, is a Stahl Style #6 which calls for a 12' radius top and a 10' radius back. I would appreciate any advise on methodology/technique. I am trying to execute as much as I can with old school handtools. The only power tools I've used so far is a small router and a screw gun. I'm trying to keep the saw dust down and I'm also interested to see how far I can go without more power tools. Thanks for you help!

http://www.hoffmanguitars.com/building_a_guitar.htm


he might be a bit "tooled up" for you but none the less he's a world class luthier and has step by step instructions with pictures.

do you have any books?

[Howard Klepper]

Originally Posted by Steven Bollman

Where is the center point of a braced top in regard to a radius dish? Is it the center of the X-brace? Thx.

Midway between the ends of the guitar. Also midway between its sides.

[charles Tauber]

Originally Posted by Steven Bollman

a Stahl Style #6 which calls for a 12' radius top and a 10' radius back. I would appreciate any advise on methodology/technique.

As we've discussed before, this doesn't tell you/me much. A spherical dome is a relatively new "invention" for guitars, coming about with "radius" dishes. Very doubtful that your Stahl was originally made with a spherical curvature to the entire top.

Are you trying to make a top and back with spherical curvature? If so, by far, the easiest way to do that is with radius dishes - all of the geometries take care of themselves and the shapes you are building are clearly defined. You can make your own for little cost, a router and a whole lot of dust. (I don't use radius dishes.)

If the top isn't spherical, what shape is it? Is it flat in the upper bout, curved in the lower bout? Is it cylindrical, the axis along the longitudinal joint in the center of the top? The advantage to first-time builders to follow a particular method of building is that these things are presented to you as a package - the method and the geometry.


If the back is not spherical in shape, and ladder-braced, contour the gluing surface of the braces to your 10' radius back. Once glued to the back, you have a cylindrical curvature. The trick is to determine the shape of the sides so that when the back is glued to it you have the desired longitudinal curvature. This is easy enough to do. One method is to do it by eye with a hand plane. With experience this works well and is quick. A method that relies less on prior experience and training is as follows.

Somehow or another - outside mold, solera, work board, whatever - you are holding the sides in place. Take a 1x2" stick slightly longer than the length of your back. Get a number of thicknesses of poster board or card stock. Stack them until you get the necessary thickness that represents how much the ends of the back dip below the center of the curvature of the back - essentially the chord height of the arc.

Put these between the back and the 1x2 at the ends of the 1x2. Using two small, light-weight clamps or heavy rubber bands, align the 1x2 along the longitudinal centerline of the back and fasten the 1x2 to the back at the waist of the back. Doing so, with the stacked spacers, will force a curvature along the length of the back, the highest point of which is at about the waist. You can adjust how much curvature there is by the thickness of the stack of card stock/poster paper. You now have a back arched across its width - due to the curved braces glued to it - and along its length, giving some sort of compound radius.

Suspend the back, with its 1x2 clamped to it, over the sides with the centerline of the back correctly aligned with the centerline of the guitar body/sides. It can be suspended in any convenient way. I use 4 threaded rods and a few nuts and washers - on each rod, two washers and two nuts sandwich and hold the back, each clamping a small area of the back.

The threaded rod allows you to adjust the distance that the back is suspended above the sides to be adjusted for taper in the body thickness from end block to heel, or side to side, for wedge-shaped guitars.

Using a divider - or compass with a colored pencil in it - mark a line onto the outside of the sides. One leg of the dividers/compass follows the inside surface of the suspended back, the other marks a fixed offset onto the sides. Adjust the opening of the dividers/compass to suit how far you have the back suspended above the sides.

Cut to the line with a coping saw, chisel, or plane. This is done prior to attaching the linings. Once the linings are attached, a little proud of the edges of the sides, the linings can be contoured to suit the arch. This can be done by hand plane or, as others have previously mentioned, sandpaper attached to a radiused 2x2.

I am trying to execute as much as I can with old school handtools.

Good for you! You obviously have the handtool skills to do it.

If you wish, you can build a guitar with only handtools. My first number were made that way, but for a router. I've since made furniture using nothing but handtools, not even sandpaper was used. Very interesting exercise, but not financially very viable for anyone trying to make a living at it.

[Steven Bollman]

Thanks, Charles. That's generous of you to detail a process. I think I understand your description. I know I'm being greedy... but would happen to have an image of the process you're describing? I'm a visual thinker. No biggie if you don't. I appreciate your help either way.

[charles Tauber]

Sorry, I should have just posted the pictures, which are self-explanatory.

These two pictures are from Jim Williams' book A Guitar Maker's Manual, ISBN 0-95890-750-1, 1986. He was a student at Charles Fox's school either just before or just after I was there. His book details exactly what I learned from Charles Fox about a decade prior to the publishing of the book. (The guitar making world, and what Charles Fox teaches these days, has changed significantly since the book was published, though the method are still viable.)

The book is a spiral-bound soft cover book, black and white, 104 pages, including full-size plans and some measured drawings for various jigs and fixtures in use by Charles Fox at the time. Looks like it is still for sale at various locations, such as Amazon.com: http://www.amazon.com/Guitar-Makers-.../dp/0958907501

[charles Tauber]

This photo, from the same book, shows a 2x3 with the desired arch cut into one face. Sandpaper is attached to that face and then run back and forth over the linings/sides to final surfacing them prior to gluing the back.

[Steven Bollman]

Originally Posted by charles Tauber

Sorry, I should have just posted the pictures, which are self-explanatory.

These two pictures are from Jim Williams' book A Guitar Maker's Manual, ISBN 0-95890-750-1, 1986. He was a student at Charles Fox's school either just before or just after I was there. His book details exactly what I learned from Charles Fox about a decade prior to the publishing of the book. (The guitar making world, and what Charles Fox teaches these days, has changed significantly since the book was published, though the method are still viable.)

The book is a spiral-bound soft cover book, black and white, 104 pages, including full-size plans and some measured drawings for various jigs and fixtures in use by Charles Fox at the time. Looks like it is still for sale at various locations, such as Amazon.com: http://www.amazon.com/Guitar-Makers-.../dp/0958907501

Originally Posted by charles Tauber

This photo, from the same book, shows a 2x3 with the desired arch cut into one face. Sandpaper is attached to that face and then run back and forth over the linings/sides to final surfacing them prior to gluing the back.

Charles! That's brilliant! Thank you! There is a treasure trove of info in those photos. It answers some questions I had. You made my day!

[charles Tauber]

Originally Posted by Steven Bollman

You made my day!

Great! Glad to be of help.

This is but one of numerous methods to accomplish this.

The guitar in the photos is being built face-down with a Spanish heel construction. Underneath the top, between the top and the work board is a piece of heavy card stock cut to the exact shape of the guitar body. In the lower bout, layers of card stock have been carefully stacked and cut to the contour of the body shape and taped to the guitar-body-shaped sheet of card stock. The stack of card stock supports the top and sides in the correct curvature of the top's arch. These layers extend from the edge of the guitar body about 1/2", all around the circumference of the lower bout. The upper bout is flat and requires no stacking of card stock to support the edges. Stacking the layers in the lower bout is tedious as the number of layers required changes along the circumference as one goes from the flat upper bout to a maximum near the widest part of the lower bout, back to a minimum again at the centreline at the end block. Of course, the contour is symmetric about the guitar's centreline.

The top, itself, is used as the template for determining how many layers of card stock to stack and where the transitions in the height of the stack occur. The top is clamped about the soundhole face-down on a flat surface and card stock is empirically stacked between the flat surface and the top until the stack just touches the top. This is done around the circumference of the lower bout.

Support for any curved surface can be built up in a similar manner, including as a support against which to clamp braces, either with a vacuum system or go bars. Charles fox used such an approach for x braces, effectively the precursor to radius dishes, decades before the use of cnc machines in guitar making. (With cnc machines you can cut dishes of any surface contour you can model. Many different ways - and levels of technology - to skin this cat.)

[Steven Bollman]

Originally Posted by Bruce Sexauer

This seems like over thinking to me. No harm in that of course! I currently use dish technology and the geometry takes care of itself, but my first few dozen guitars were build on Spanish plantia forms, and the back was fit by hand planing the sides until is seemed right. I always tried to make any error favor the inside edge of the linings as the binding would then take up the slack. 45 years later there has been no propensity to failure. Let's hear it for Elmer's glue-all! I do believe that better joinery is one of the major keys to guitar performance, but even the best we are capable of is nowhere near 100%.

Thanks, Bruce. BTW, I have had the great pleasure of playing some of your guitars that Eric Schoenberg has shown me. Gorgeous.