TOONE & TOWNSEND

Thermal Expansion of Neck Core

One topic several folks have written about is coefficient of thermal expansion (CTE) of the TOONE & TOWNSEND Neck Core:

"I am going to start building a 6 string electric and recently stumbled across your website.

"I was curious if you could shed some light on how your neck core reacts to temperature changes.

"I am aware that the thermal expansion coefficient of aluminum is generally greater than that of hardwoods so I was wondering how your necks with the aluminum neck core reacted to mild and moderate temperature changes in regards to flex and relief."

Curious myself, I have tested neck core CTE extensively, subjecting cores to extended time in the freezer then the oven, to measure movement.

From below 0 Fahrenheit (-18 Celsius) to about 220 Fahrenheit (105 Celsius) there is zero measurable difference using a ruler and viewing with human eye.

Dimensional stability is unchanged.

PLEASE NOTE: 220 Fahrenheit is above the boiling point of water, and almost 100 degrees hotter than the inside of a car parked in the sun.

At 400 Fahrenheit, an 18" neck core will extend by almost 1/16" (1.6 mm) in length. However, wood will easily flex that much, without any concern. Just watch trees moving and bending in a wind storm.

Of course, if your guitar has heated to 400 Fahrenheit then the guitar is probably the least of your worries. Because at 400 Fahrenheit, the plastic in your guitar, case, car, and clothing will have melted, leaving you with a gooey mess. You could cook a beef roast at those temperatures.

Instruments I've built with neck cores always surprise me because — unlike a conventional truss rod — I often remove the guitar from a case months later and discover it still plays perfectly in tune. Seasonal changes do not affect the neck.

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.

Neck Core Installation

This article explains in detail how to build an instrument neck using the TOONE & TOWNSEND Neck Core which has been engineered from aircraft aluminum.

I've been using neck core lengths from about mid 2nd fret to the body joint. It does not need to extend the full neck length. You can cut and file it as needed.

The basic steps are very simple:

1. Work from a centerline
2. Route a groove using a 0.75" (19.05 mm) diameter bit
3. Square the ends of the groove with a chisel
4. Glue the neck core and fretboard at the same time
5. Shape the neck

I've built many necks using this method and learned through experimentation the neck core is the most important factor determining tone and stability, for reasons outlined in neck design theory. Note the Trapezoid Neck Profile was specifically designed to accommodate an oversized neck core.

MATERIALS

Steel (even stainless) is not suitable due to oxidation, and other metals such as bronze do not have sufficient strength-to-weight ratio. Aircraft grade aluminum and titanium have very high strength-to-weight ratios, with musical tonality. TOONE & TOWNSEND neck core is available in titanium, by request. Titanium pricing is substantially more than aluminum.

VIDEO: Starfish claro walnut six string electric guitar. Stacked laminate neck with aircraft grade aluminum neck core. This instrument was my first test of this neck construction method, and has proven exceptionally stable since 1993. TOONE & TOWNSEND neck core is a refined version of the truss structure used here.

CALCULATE FORCES

Guitar, bass, and emerging wide octave or extended range hybrids will be subject to variable string tension forces, depending upon scale length, string gauge and number of strings. Luthier Graham McDonald freely offers his Universal String Tension Calculator (MPUSTC) and it allows you to input your own variables.

NOTE: TOONE & TOWNSEND neck core is designed to reinforce acoustic and electric guitars, including baritone, most extended range hybrids and bass.

I prefer to experiment with actual materials combined with full scale drawings. Others may prefer CAD and Young's modulus engineering charts. The same design and construction principles apply even to CNC operations. If you have expertise in contemporary machining methods please consider sharing your knowledge with others here. I will present the construction techniques using hand tools, for those who do not have access to elaborate technology.

DETERMINE DIMENSIONS

Once you've approximated string tension forces, evaluate outside dimension requirements for the neck profile at the smallest cross section of neck (nut) including width and thickness including fretboard. I have found musician's preferences vary widely and are often associated with finger length.

Once you make an accurate scale drawing of the neck's smallest cross section, measure the thickness excluding fingerboard. Allow at least .125" of wood as a safety margin between neck core channel and outer surface of neck. Click image to enlarge.

NECK CONSTRUCTION

I always begin construction from an oversized square or rectangular block of (wood) material, regardless of the subsequent profile or shape of the completed neck. The reason for this is it provides one perfect (true) surface to route a channel into and attach a fingerboard onto. That true plane surface will serve as a necessary orientation guide throughout the neck shaping process.

It is important to work from a centerline — as well as from the neck material/fingerboard joint — in order to prevent accidentally cutting too much material away and exposing the neck core. When squaring the ends of the routed channel, remove only necessary wood. Route channel 1/64" deeper than neck core. This allows a slight gap permitting glue to move freely during clamping.

Be precise.

Neck core should fit perfectly, end-to-end, slipping into the routed channel without force or play. Finger press effort. We've allowed clearance for glue to move freely around the top, bottom and sides of neck core, as needed, to hold it in place. A precise end fit turns the inherent strengths of wood and metal to our advantage, without need to rely on a chemical bond. Note the aluminum neck core can be cut, filed or drilled as necessary.

Because the neck is subject to longitudinal compression, solid neck core ends distribute the compression load. Draw lines (or take notes) where the neck core ends to avoid exposure during neck profile shaping.

PHOTOS: Testing a prototype TOONE & TOWNSEND Neck Core. Curly quarter sawn red oak and hickory neck. Note neck centerline in router photo.

ASSEMBLY

It seems simple and intuitive, but this advice took me awhile to accumulate. With an open gluing time of less than ten minutes (warm weather) and a complex series of steps involved in assembling a neck, it will save you time and frustration if you follow these steps:

• clear your work space
• gather all materials and clamps
• do a dry test assembly without glue pre-setting clamp positions
• adjust any problem spots
• repeat dry test assembly
• clean aluminum neck core with denatured alcohol to remove oils
• glue + clamp neck core to seat fully in channel immediately remove clamps
• immediately glue + clamp fretboard
• allow 24 hours drying time before removing clamps

I've been on a quest for many years to find the perfect glue, a series of ongoing experiments.

Titebond II (aliphatic resin) has served me faithfully for about 15 years, as it is very stable throughout temperature changes and highly resistant to moisture, including sweat. It clamps nicely, sands smoothly when dry, and is quite strong.

Titebond III displays the same properties as Titebond II except I find the dried glue more "rubbery" during sanding, and the glue line more pronounced. For this reason I prefer Titebond II.

Hide glue is on my list to try, but I seldom need to disassemble what I've built.

Lately I've been exploring Gorilla Glue (polyurethane). Different from Gorilla Wood Glue, which I have not yet tested. Foaming action during bonding requires some getting used to but is not an obstacle to success. To avoid over-sensitization I use gloves, eye protection, ventilation and a respirator — the fumes affect my eyes.

I am very pleased with the strength and tightness of the joints. Glue lines are invisible.

During testing I submerged a cutoff walnut & maple scrap from the neck structure above. Even after 36 hours submersion and subsequent return to room humidity, the glue line was never apparent. Just as impressively, I could not break the joint. Immediately after removing the scrap from water, I bridged it (with the grain) and bounced on it using my full 185 lbs.

No damage.

PHOTO: Use a straight (true) clamping bar combined with as many hand tightened clamps as you can fit. Seek strong, even, pressure. Nadine Steinhard, marketing manager for Gorilla Glue, confirmed this method in a series of emails we exchanged about their polyurethane product.

PHOTO: Masking tape is invaluable for precisely positioning the fretboard on the centerline prior to clamping. Fingerboards are lightweight, thin, and really slippery when wet with glue. Tape them down firmly. I've found triangular wood strips effectively distribute clamping force on curved (or compound) radius fingerboards, particularly along the outer edges.

SHAPING

Cut the neck to desired taper, from width at nut to width at heel.

On the side of the neck mark the depth of the truss structure channel, confirm the neck thickness, then remove excess material to within .125" of desired finished dimension. Because the instruments I build invariably have a thicker body than neck, there is an exponential taper in neck thickness from nut to body, based on personal preference.

Most importantly, do not risk exposing the neck core unless you are seeking that deliberate effect. Transfer the centerline to the back of the neck and use it for continual reference.

Determine and mark the width of the neck core channel, measuring from the centerline. Those lines, combined with the taper of the neck width, determine the minimum possible dimensions of the profile. In fact, I use the glue joint of the fretboard to neck as my reference — I seldom cut into the fingerboard itself, once the neck width taper has been established, preferring to leave as much fingerboard material as possible.

PHOTO: Marking the intended neck thickness on the blank. Neck has already been tapered to width. Note luthier's headstock joint in the cherry. Curly maple fretboard.

PHOTO: Trapeziod Neck Profile is clearly visible in this construction shot of Dove six string electric guitar. Note unusual asymmetrical slotted headstock.

SETUP & ACTION

Leveling and contouring the fingerboard pre-fret installation is critical to controlling action outcomes. Once the neck has settled in for several days and the glue fully cured, evaluate the fingerboard using a precise straight edge. Now is the time to make adjustments.

Typically a perfectly straight fingerboard is desirable. String tension will pull relief into the neck. On "soft" necks, you may choose to build in a back bow. With experience you can predict the amount of neck relief based on the materials chosen. It is not an easily quantified formula because so many variables are involved, including fret compression. Always string and test instruments EARLY in the construction process, before sanding and finishing.

I've successfully used the methods described above since 1993. The instruments I've built using "conventional" adjustable truss rods are not nearly as mechanically stable nor sound as good, in comparison. What I've explained in this article is exactly how I would build an instrument for my personal use.

Please extend this discussion by sharing your experiences in the comments below.

Neck Design Theory

Orchid bass has been described as having "piano-like" sound. Each note within a chord rings with distinctive clarity. These differences are clearly audible as Monster plays (1:58 to 6:32) in the video below.

Characteristics consistently apply to other instruments I've constructed using our Toone & Townsend Neck Core so it is not a function of Orchid, but instead of the neck design itself.

Neck core installation methods I've been using are designed to facilitate string vibration transfer by minimizing twisting and bending of the neck. Our Toone & Townsend half round ("D" cross section) neck core maximizes gluing surface and fretboard contact, yet allows conventional or slim neck profiles.

PHYSICS OF VIBRATION

In theory, a perfect neck would have zero flex, instead transferring all string vibration directly to the soundboard.

The reason for this is found in Newton's law of reciprocal actions: "For a force there is always an equal and opposite reaction: or the forces of two bodies on each other are always equal and are directed in opposite directions."

A string in motion creates force, in the form of vibration.

According to Newton, the neck and body of the instrument will react equal and opposite to the force of the string. This is important — and good — because it is the movement of the bridge against the body that amplifies string vibration, creating audible sound.

When I write "body" what I specifically mean is soundboard, because the body of an instrument serves two functions: a) an ergonomic device for musician comfort, and b) to amplify string vibration.

We often think of soundboards only in relation to acoustic instruments, but even a solid body acts as a soundboard. The same physics are involved in a solid body, archtop or acoustic guitar, for example, but each design has been optimized to amplify acoustic sound to varying degrees.

Kettering University's modal analysis of an Ephiphone Coronet solid body electric guitar clearly illustrates the movement of the neck and body in (equal and opposite) reaction to string vibration. Each animation is a different frequency, so an accurate model would combine all of these frequencies plus those not analyzed.

MAXIMIZING STRING VIBRATION TRANSFER

Because the neck does not act as a soundboard, the longitudinal and torsional neck flex apparent in the Kettering animation indicates lost efficiency. As the neck reacts equal and opposite to string vibration forces — by twisting and bending — it is not transferring string energy to the soundboard for amplification.

Additionally, these neck movements are acting to dampen string vibrations, as explained by Newton's law of inertia: "An object at rest remains at rest and an object in motion will remain in motion unless acted on by an unbalanced force."

Those of us fascinated by instrument physics can continue to learn from Isaac Newton — even three hundred years later — by applying his principles to contemporary design.