Multi-Scale Fretboard Design

This article explains how to design and lay out a multi-scale (fanned fret) instrument neck. It will feature the patent pending TOONE & TOWNSEND intonation adjustable nut for its simple installation, adjustments and precise intonation.

Identical geometric and math principles apply, even if you are using a conventional nut or zero fret.

The most important principle to remember is every dimension is referenced from the nut intonation point on the treble string. That intonation point determines the width of the neck, the offset between treble and bass string scale lengths, as well as the location of the bridge.

Basic strategy:

1. Work from a centerline
2. Create a full size template including all strings from nut to bridge
3. Reference every dimension from treble string intonation point at nut
4. Transfer dimensions from template to neck blank
5. Be precise and compare work to template as you progress

TREBLE STRING INTONATION POINT

There are two reasons why this reference point is so important, both of which have to do with limits.

First, the offset between treble and bass string scale lengths at the nut will determine the ergonomic comfort of the instrument. As the difference between the bass and treble strings increases, our human wrist must rotate outward to compensate. This effect is particularly noticeable with barre chords.

Demonstrate by forming a barre chord on an imaginary guitar, up near the nut (F or G), then visualize the bass string end of the frets shifting backwards, beyond the nut. Notice how your elbow pivots in toward your body and your index finger points out away from your shoulder to compensate.

Wrist and elbow rotation comfort will vary considerably between musicians. If you are building for yourself or for a client, test "play" the offset using a template before starting construction.

I am currently in love with combined 24.5" & 26" scale for guitar. The 24.5" treble string bends easily, providing warmer tones commonly associated with Gibson scale lengths. The 26" (or even 26.5") has deep, tight, well-defined bass. An offset of 3/8" (9.525 mm) at the nut is proving very comfortable.

Second, hardware has offset and intonation adjustment limits. As you template the string scale length offsets at both the nut and the bridge, remain within the adjustment range of the hardware. Conservative adjustments will usually provide better hardware performance.

NOTE: TOONE & TOWNSEND nut permits 1/2" (12.7 mm) offset between treble and bass strings. Within that offset range, allow up to approximately 1/8" (3.175 mm) of intonation adjustment per individual string.

TEMPLATE

Paper works great. It allows you to work actual size and make changes easily. Once you've developed a neck layout you like, then transfer to CAD or to a more permanent material. Click image to enlarge.

PHOTO: Paper template with TOONE & TOWNSEND intonation adjustable nut and aircraft grade aluminum Neck Core. Committing dimensions and locations to paper will be useful during the neck shaping process.

LOCATE INTONATION POINTS

Our example neck specifications:

• 25" (635 mm) vs. 25 9/16" (650 mm) scale lengths
• 1/4" (6.35 mm) offset @ nut
• 1 13/32" (35.7 mm) string spacing @ nut (string center to center)
• 2 1/8" (53.9 mm) string spacing @ bridge (string center to center)

Instrument designers and luthiers are comfortable calculating string spacing at the bridge. However, we tend to refer to nut dimensions by overall width, rather than by string spacing at the nut. This difference will cause mistakes when calculating multi-scale (fanned fret) fret locations, because strings are not located at the outer edges of the nut.

Demonstrate this principle for yourself by leaving string spacing unchanged, but imagine stretching the width of the neck by several inches. If fret locations were determined by nut width — incorrect — then the spacing between the frets would constantly change as the neck stretched...frets would become more parallel.

This would also change the string scale lengths because, for example, a 25" scale length would increase or decrease as the neck width changed. Mathematical chaos.

In contrast, if we reference fret locations using string spacing, the fret locations under the strings remain unchanged as the neck width changes. Luthiers can add a comfortable distance between the outer strings and neck edges, without affecting the measurements between the frets.

Always reference string spacing at the nut.

STEP-BY-STEP

Begin by drawing a centerline on your paper template, long enough to display the full length of the strings from headstock to bridge. At the location of the nut, draw a line perpendicular (90 degrees) to the centerline. That perpendicular line would become the nut or zero fret on a conventional (parallel fret) instrument.

Because the string spacing of our intonation adjustable nut is 1 13/32" (35.7 mm) we divide that number in half to determine the distance from the centerline to the treble string intonation point. Half the string spacing distance is 23/32" (17.5 mm). Measure that distance from the centerline, on the perpendicular nut line, to locate the treble string intonation point.

The bass string will be located an equal distance from the centerline, centering the nut automatically. Click images to enlarge.

PHOTOS: In image above the nut is located on the neck blank, which is under construction. Fret slots have been cut. TOONE & TOWNSEND intonation adjustable nut will be located in the same position as a conventional nut or zero fret. It is a direct replacement.

Same principles apply for bridge location. Draw a perpendicular line from the centerline of your template approximately where the treble string bridge intonation point will be located. Approximate distance is acceptable, because we are going to create two lines parallel to the centerline, at bridge string spacing distance.

Using our example, string spacing at the bridge is 2 1/8" (53.9 mm). Divide that in half to center the strings. Measure 1 1/16" (27 mm) both directions from the centerline, then draw two parallel lines.

As we measure string scale lengths from the nut, these two parallel lines will confirm our bridge string spacing remains constant.

From the treble string nut intonation point measure the specified 25" scale length, and draw a straight line where it intersects with the parallel bridge treble string spacing line. You have just drawn the exact location of a tuned treble string. On a completed instrument, the string will stretch from nut to bridge along that line.

Repeat for the bass string. Draw a line from the bass string nut intonation point to the bass string bridge intonation point. The outline of the neck should now become visible.

By drawing a line through the treble and bass bridge intonation points, you create a reference intonation line used to locate the bridge hardware. Click to enlarge.

PHOTO: The wide angle camera lens causes distortion, but note the parallel lines at the approximate bridge location. The parallel lines are drawn parallel to the centerline of the neck, determined by bridge string spacing width: 2 1/8" (in our example).

FRET LOCATIONS

Once intonation points have been established at the nut and the bridge — based on the treble and bass string scale lengths — and lines drawn where the strings will be located, next step is to mark fret positions.

Calculate fret locations using Liutaio Mottola's fret position calculator. If your scale lengths are traditional to guitar, Stewart-MacDonald's fret scale rulers will save you time. If using an unusual scale length, one technique I often use is to reference the nut from a 1st or 2nd fret position on a longer scale length fret ruler.

Mark fret positions for the treble string, using treble string scale length. Then for the bass string, using bass string scale length. Connect these lines. They are fret locations for the fingerboard.

Using the template techniques outlined above, repeat these steps on the fretboard blank. The process will move much faster, working from the paper template as reference. Constantly compare the paper template to the neck-in-progress to ensure accuracy. Click to enlarge.

PHOTOS: Sequence of construction photos beginning with neck blank. Note importance of centerline throughout the process. The unusual neck shape is patented Trapezoid Neck Profile.

Comments

[Stanislav Passov]

Awesome! You added another dimension to the guitar intonation. How does it affect the sound?

[Marc]

Rick,

Excellent info and accompanying pics!

My Novax AX-6 guitar has an "extended range" scale, which is 25"-27"; the sound is huge and quite easy to play.

Marc

[Lon Rago]

Rick,

Here is a link to a useful tool for calculating multi-scale fret placement. You can also specify the fret at which there is no angle.

http://www.liutaiomottola.com/formulae/fret.htm#mozTocId525252

[Scott Summers]

I've been using
http://www.fretfind.ekips.org/2d/

I like how it is laid out, but I can't say how it intonates yet. I'm still working on it.

I have also emailed back and forth with
http://www.liutaiomottola.com/formulae/fret.htm#mozTocId525252 and he was very helpful.

[ALEXANDRE CHENAOUY]

Incredible stuff. Congratulations. pls contact me. Thx

[Clifford Wilkes]

Very clear and helpful. I just want to mention a typo. In the "Step-by-step" second paragraph is the following sentence. "Half the string spacing distance is 23/32" (1.75 mm)." The number in the parentheses should be 17.5 mm.

Thanks.

[Rick Toone]

Clifford — Corrected! Thanks for the catch.