Radiusing the fingerboard

With a radiused fingerboard (5) a guitar is easier to play. Common radii are 7", 7.25", 9.5", 10", 12", 14", 16" or 20". With bigger radii, such as 12" or 16", you get a flatter fingerboard curve which makes it easier to pull strings without causing them to touch the frets, while smaller radii are better suited to the shape of the human hand and thus make fingering chords easier.

A cylindrical and a conical fingerboard surface are fundamentally different, and it is important to understand this difference. Take a tube, for example: when you place a straight edge on it so that it is parallel to the cylinder axis the straight edge will lie flat on the tube. But if you move it only slightly out of this parallel position, it will be left resting in the center only and you will be able to move it up and down at both ends. The two outer strings on a guitar are in a similar “non-parallel position”. By filing a fingerboard or the frets along the line of the strings a kind of cone, a so-called compound radius, is produced. It is, however, also possible to make a fingerboard that has different radii at the nut and at the end of the fingerboard. This is, for example, the case with all instruments of the violin family.

The difference between a cylindrically-shaped and a conicallyshaped fingerboard surface is illustrated, or be it slightly exaggerated, in the illustrations on the left which show the view from the nut towards the end of the fingerboard.

You can see that on the cylindrical fingerboard (a) the fingerboard edge is getting increasingly thinner towards the wider end of the fingerboard. In reality, the difference is less than 1mm, i.e. very small indeed. This also explains why it is possible to file a compound radius into the frets.

If the fingerboard has a compound radius (b), the edges of the fingerboard remain (more or less) equally thick from the nut to the end of the fingerboard. But the main purpose of a compound radius should be that the strings run parallel to the fingerboard surface.

Very often, though, the fingerboard surface is made cylindrical, and long concave sanding blocks can be used for leveling it. If, nevertheless, a small compound radius is desired, it can be made when dressing the frets by filing them accordingly.

a

R

b

Height of fingerboard edge remains more or less the same

Using a belt sander

The best way of making an accurate fingerboard radius is by using a jig mounted over a large belt sander, as shown in the picture above: the fingerboard, or the neck with gluedon fingerboard, is fastened between two arms and can be moved over and lowered onto the belt surface bit by bit until the radius is right.

The jig shown above is used by the acoustic-guitar manufacturer Martin, and the distance between the fingerboard surface and the pivot is exactly 12". By lowering the whole assambly onto the belt sander and rocking the fastened fingerboard a precise fingerboard radius is produced. (Other manufacturers use similar self-developed jigs which rely on the same principle).

Making such rather complicated jigs obviously only pays off if you build guitars in large quantities. If, however, you happen to own a long belt sander, you should seriously consider constructing such a jig for making the fingerboard radius.

With such a clever jig it is also very easy to make a compoundradiused fingerboard by simply using arms of different lengths so that, for instance, the front arm produces a 10" radius and the back arm a 16" radius.

Making a radius-sanding block

Long radius-sanding blocks for finish-sanding fingerboard surfaces are a great help. Necks with cylindrical fingerboard are moved over these blocks, which are fastened on the workbench in a lying position. As I am unfortunately not aware of any source of such (long) radius-sanding blocks, you will have to make them yourself.

The one I am going to describe in the following gets its radius from three pieces of bent 3mm (1/8")- thick plywood. With plywood thicker than that you won't get an even curve. Even better, because stiffer would be using five pieces of 2mm (0.08")-thick plywood.

The base block can be built from five boards. The necessary protrusion (h) of the two side boards can be calculated for any radius with the formula given. Spread glue between the plywood sheets and screw them down along the center line.

W

h = ( R + s ) - ( R + s ) 2 - 0,25 W 2

s = 9 mm, R = 12" = 304,8 mm, W = 150 mm

h= 9,09 mm

s = 0.375”, R = 12", W = 6”

h= 0.369”

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A cylindrical fingerboard will be produced if you consistently sand strictly parallel to the center line (b). For this purpose only may a radius-sanding block be used over the whole length of the fingerboard. Wooden or synthetic sanding blocks for the most common fingerboard radii are available via mail order from guitarmakers' suppliers (1).

Using a pencil, or white chalk for dark-colored woods, draw a center line on the fingerboard. After that radius the fingerboard surface by sanding in long, even lines, using 80-grit self-adhesive sandpaper on a radius-sanding block (2). Check the progress you are making at regular intervals - use a radius template, which you can make yourself, for this purpose. To prevent too much wood being sanded off on one side it is also advisable to change the direction in which you are working a few times by regularly turning the fingerboard by 180 degrees. How quickly you make progress will depend on the material used for the fingerboard - with ebony, for example (3), this can be quite a bit of hard work and take time. When there are sanding marks all over the fingerboard surface except for a narrow strip in the center, the radius is nearly finished and you can switch to finer 120-grit sandpaper. If the fingerboard is to have (thin) inlays, fit them now, before you start sanding with 120-grit paper.

On cylindrical fingerboards the flatness of the surface always has to be checked parallel to the center line (b). Place a straight edge parallel to the center line and check the gap against the light (4);

spots where light shines through are depressions in the fingerboard. Mark the spots on the fingerboard that are too high: starting at one end, place the straight edge repeatedly parallel to the center line across the width of the fingerboard and mark all the spots where no light shines through with pencil lines. This will give you a good indication of where material has to be removed. When all lines can be drawn all along the fingerboard it is finished and ready for finish-sanding (5). Use progressively finer grits of sandpaper for sanding, starting with 180-grit and continuing with finer grits until you are happy with the result. An ebony fingerboard can also be polished using a buffing wheel and a polishing compound.

A compound radius can be made with a plane. By always planing along the (imagined) line of the strings a radius that becomes increasingly flatter towards the end of the fingerboard will be created (like the fingerboard of a violin). Always work in the direction of an imagined point at which the sides of the fingerboard would concur (6,a). Fine-sanding with a flat sanding block also has to be done radially. A compound radius is certainly far more difficult to make than a cylindrical radius.

The technique of dressing the frets with a long, narrow sanding block also works for leveling compound-radiused fingerboards. For details please read the section on how to set up the guitar.

Can you make a radiussanding block with a cove cut?

Yes, in theory you can; in practice, however, you can't make radiussanding blocks by making cove cuts on the tablesaw. For a cove cut a block of wood is moved several times across the sawblade at an angle; the blade is set slightly higher for each pass until the channel is the right height and width.The channel thus cut will, however, not be circular but elliptical. This is fine as long as all sanding blocks you use have been made in this same way. One thing that might, however, be a problem is the size of the sawblade required: for making a 12" radius the blade radius has to be greater than 12" (305mm), which would equal a saw blade diameter of 25" (635mm) at the least. Large tablesaws suitable for cutting firewood could probably do the job, but they won't produce a fine-enough cut and are also far too dangerous.

Making your own neck support caul

A neck support caul is quite easy to make with a cove cut made on a tablesaw. For details on how to make a cove cut see above and/or consult any good book on joinery.

Using clamps and a plastic tube as a clamping caul a layer of cork is glued into the channel. Under pressure, the tube, which should be about 100mm (4") in diameter, will adopt the shape of the channel.

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