The Uncle Bob Guitar Project

Design Issues

The challenge in this project is to produce a fully functional musical instrument without expensive tools or materials and without sacrificing accuracy or tone. The elements of the design are discussed here;

Strings

The strings are what the whole design is about. The string, when plucked, vibrates at a fixed frequency, determined by it's length, tension and mass. This vibration is constrained by the contact points at either end; one being the bridge, the other being either the nut (if open) or the fret (if fretted).

The pitch of the string, when being played is altered by fretting which allows the player to select one of a sequence of pitches known as an even-temprement, chromatic scale. The open (unfretted) pitch of the string is adjusted by the tuners in a process known as tuning. The relative pitch of the differnet strings on the instrument is largly determined by the use of different masses of strings using a combination of different materials, constructions and thicknesses. The intervals to which the different strings is set is known as the tuning of the instrument. For six string instruments, common tunings include "standard" or "spanish" tuning (also known as "arbitary bizaro tuning"); being (high to low) E B G D A E and "open G" tuning (prefered by Bob) being (high to low) D B G D G D.

In designing the instruments we basically have two options for strings; metal and plastic. The option chosen influences the design of the tuner and is determined by availability. The "classic" Uncle Bob is designed for standard Acoustic Steel strings which are a combination of single strand steel strings (the high pitched ones) and steel core wrapped with bronze (the low pitched ones). The thichnesses of the strings range from about 10 to 50 thousands of an inch. This string set was chosen for this design to make it attractive to urban-based, relatively wealthy musicians who are the target market for the first release of the instruments. In this configuration, the strings cost more that all the rest of the materials combined.

Frets

The fretting system is one of the major inovations in the Uncle Bob design. Traditional frets require a specially manufactured wire to be fixed into slots cut into the fret board. The spacing of the frets is paramount to the accuracy of the instrument and the eveness of the tops of the frets must be maintained to prevent "fret buzz". Poorly spaced and leveled frets is the main cause of low tech instruments (such as those found in third world import shops) being practically unplayable.

The wrapped frets of the Uncle Bob design hark back to the "tied" frets used on the Viol which was a predecesor of the Violin. This instrument had frets of gut tied 'round the neck which could be moved to accommodate playing in different keys.

The Uncle Bob design requires that the frets be located with sub-millimetre accuracy at intervals determined by the proportion of the "12th root of 2" or 1.0595. For practical manufacturing purposes, the position of the frets can be determined using a special ruler with holes marked in the fret positions, or even a paper fret guide which is glued or taped to the neck during the notching process. The neck, or it's binding is notched and the fret material is wrapped around the neck following the notches. I must admit to some surprise when I made the first neck, that it worked so well.

The nylon "fishing line" used for the frets will undoubtedly wear out but it is acutally easier and quicker to replace the frets on an Uncle Bob than it is to replace the strings.

Bridge and Resonator

The bridge is the structure in the centre of the resonator where the strings effectively end. Bridge designs on instruments vary widely from the complex devices on electric guitar tremolo bridges to the one-piece straight bridge of the violin.

The correct positioning of the strings above the fretboard (the action) is controlled by the height of the bridge. The correct pitching of the fretted string, reletive to the open string is controlled by the position and shape of the bridge. This is known as intonation.

The volume and tone of the instrument is largely determined by the size, shape and material of the resonator (body). When you knock on a wooden table with your knuckles, it sounds different then when you knock on a stone floor. The table is acting as a resonator. When you tap a glass the resonance has a particular pitch and more pure tone. When you tap a drum (another resonator) the sound is "unpitched". This is what we're looking for in an instrument body.

The resonator also needs the structural integrity and strength to maintain it's shape under the tension of the strings. A standard acoustic "spanish" guitar has a bridge glued to the centre of the sound board. The tension of the strings acts to lift and twist the bridge off the sound board and the guitar has hidden struts to resist this force. It's an important feature of the Uncle Bob design that the resonator supports a "floating" bridge and that the resultant force of the strings is a simple straight vertical force. The bridge is of a harder, stronger material than the body dome (effectively the sound board) and so the fit between the bridge and body must be close and accurate so that the focused force of the strings is spread and reduced.

As the string vibrates, the vibration is transfered through the bridge to the body dome and into the body as a whole.

The body also has a sound hole cut in it to allow low frequency (bass) sounds to be reinforced. The size, shape and positioning of the sound hole effects the overall tone of the instrument and is determined by trial-and-error and convention.

Tuners

The design of the tuner depends on the type of string to be used. The goal of tuner design is that a full turn of the tuner will adjust the string by about a semi-tone.

The two designs used at this stage are a friction fit peg for nylon strings and a bolt-and-wing-nut design for steel strings. Nylon strings need about 5 times as much stretching as steel strings do. The nylon tuner is really just an adaption of the classic violin tuning peg using available materials. The steel string tuner could be seen as an adaption of the violin fine-tuner although similar designs have been seen on innovative electric guitar designs since the 1970's.

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The Uncle Bob is a CyberFeral Project