Tuesday 5 April 2016

The Making: Ukulele fingerboard using CNC router



A ukulele fretboard (also known as fingerboard) is usually specified by it's scale length, the width at the nut (0th fret position) and the width at the 12th fret position. For a standard tenor ukulele they are 431.8mm, 35.7mm and 46.0mm respectively. A typical fretboard normally has 18 or 19 fret slots for inserting the corresponding fret wire. The position of each slot (d) as measured from the nut is calculated using the following formula:


d = s - (s/(2^(n/12)))

where s is the scale length, and n is the fret number. Note that the 12th fret position is half of that scale length. We could easily customize a slightly different scale lengths for a similar body design.

The top surface of the ukulele fretboard may have a radius as normally found in many acoustic and electric guitars. The radius facilitates a more comfortable bar fingering when playing the instrument as it follows natural human fingers in a relax situation. However the radius fretboard renders additional work on the part of the builder.

Other normal features include fret markers, side binding to hide the slot's end, and other optional adornments.
The overall thickness of the fretboard varies between 4mm to 6mm. It is popularly constructed from woods that are stable and strong such as ebony or rosewood. Other woods such as maple may also be used for the construction of a fretboard.

Constructing a complete fretboard is a bit tedious process.
Starting from a fretboard blank, people have successfully employed CNC router to help construct the fretboard. The product will be more precise and reproducible. The method we are employing here is to write CNC codes namely the g-code thus avoiding the use of CAD/CAM software for the design and g-code generation. To ease the development, rather than doing it as a continuous process, we break the process into several small tasks. Each task may be applied independently. For example, fret radius may be used for fretboard that is already been previously slotted (with a fret saw). This will help in the construction as each module may require a tool change with different type and size of router cutting bit.




1. Fretboard radius


For the radius part, we normally have to construct a purpose built jig that we could slide hand held router along a guide. This jig enable us to cut the radius as if the fretboard is sitting along the surface of a cylindrical of specific radius. For ukulele it is typically 304.8mm (12in). The adjustable jig could also be used to make a compound radius. The radii vary as we move along from the nut position towards the last fret. If we are constructing a compound radius fretboard, it is like placing the fretboard along the face of a cone with the smaller radius at the nut. However both these techniques may be simulated using CNC machine.






2. Fretboard slotting


Slot positions are calculated using formula given above. Cutting the slot is not the most efficient process as it needs to be cut with a small router bit requiring a slow feed rate. The recommended cutter is 0.584mm (0.023in.) in diameter. Cutting the slot with CNC enables us to make a "blind slot" i.e. the slot falls short of the edge of the fretboard. In this way slots ends are hidden without additional fret binding work. In addition, the slot cuts follow the curvature of the fretboard instead of across the board horizontally.



3. Fretboard marker


Their positions are calculated and 4mm round pockets are cut to 1mm deep to accommodate round fret markers.


4. Fretboard cut


This is a short g-code program that cut along the final shape of the fretboard.


5.Fret chamfering


Optionally it may be possible to properly insert the fret wires in situ while the fretboard is still clamped on the CNC table and let the CNC do the chamfering. This should be executed before step 4. Certainly it is more convenient and safer to install the fret wire away from our CNC. Afterwards, place the fretted board back on the CNC, with the same zero reference, and do the chamfer.
 

Here is a sample cut for a 19-inch (482.6mm) scale fretboard.






 


    

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