Radiusing top and sides/rims

[seangil]

I'm looking for some builder advice. I'm working on a parlor-sized steel string. I have put a 25' radius into the lower bout, but was planning on leaving the top flat due to the fingerboard. I have talked with people who put a radius into the transverse brace and then plane the middle, but it seems tricky to get the dimensions just right. However, if I radius the lower bout braces and leave the upper braces flat, then I also understand that I need the rims flat above the soundhole.

I am curious to hear from others who use this technique and how you approach have rims/sides that have a radius on part of the guitar and no radius on the other. If you have any photos to post of jigs or technique, then it would be very welcome.

Thanks

[Bruce Sexauer]

The First 50 or so guitars that I built, I used a hand plane to fit the rims to the plates. Make sure you err, if you err, on the outside of the joining surface (making the inside edge have full contact) and the binding will largely correct the error.

[charles Tauber]

Originally Posted by seangil

if I radius the lower bout braces and leave the upper braces flat, then I also understand that I need the rims flat above the soundhole.

That is exactly what I do.

I am curious to hear from others who use this technique and how you approach have rims/sides that have a radius on part of the guitar and no radius on the other.

Like Bruce, I use a hand plane, mostly. You can also use a 2 x 2 x 18", or so, piece of wood that has the radius you want in one side with a piece of 80 grit sandpaper glued to that side. You then run the piece of wood back and forth over the lower bout. This gives the the angle of the linings, but not the contour of the sides.

There are numerous methods that can be used to determine the contour of the sides.

The easiest method is to have a guitar with the side contour/plate arches you want, and simply trace them to obtain the contour of the sides. You can then cut that contour on a bandsaw prior to bending, or hand plane them to contour after bending.

Another method, depending upon the sequence and jigs you use, is to suspend the top over the sides, maintaining the curvatures as you do, and trace on the sides the desired contour. This can be done with dividers or a compass with a pencil. Then hand plane to the line. For tracing, threaded rod, large washers and nuts can be used to suspend the top over the side assembly.

Another method is to level, flat, the top surface of the assembled sides and linings, then use a hand plane to plane the contour into the lower bout of the sides. This requires intimate knowledge of the what the contoured shape will look like. (It is quick and easy to do, if you know what you want to end up with before starting.)

I started making guitars in the Spanish style, assembled face-down on a work board. This is the method that I used for that. I started with a flat work board - plywood or MDF. I then use a heavy card stock of paper - the same that I use for making templates - as a base. It is cut out to the shape of the guitar. I then add strips of that card stock to the perimeter to build up the curvature to what you are using for your top. The strips of card stock can be glued in place or taped. Essentially, this creates an overlay to put on a flat board that is a "dish" the shape of your guitar. The upper bout is flat, and the curvature gradually builds from the waist to the width of the lower bout, then tapers back towards the end block. This gives you the 3D contour of your sides.

You can then hand plane the bent sides - or side assembly, depending upon your building sequence - to match that contour. (Place the sides on the contour and plane till they match.)