Guitar assembly with no strings attached

DN Staff

August 6, 2001

7 Min Read
Guitar assembly with no strings attached

Even the best-sounding acoustic guitars can strike a sour note on the assembly floor. Designed for sound quality, rather than manufacturability, guitars traditionally consist of individually fabricated and glued wood components. Guitar makers at small shops and big factories alike still perform much of this assembly work by hand since it goes right to the heart of the instrument's sound quality and structural integrity. Now, a patented guitar-building technology from Garrison Guitars promises to eliminate some of the more time-consuming steps needed to make a wood guitar.

A masterpiece of parts consolidation, Garrison's Active Bracing System (ABS) replaces more than two dozen wood components with just two injection-molded parts made from a glass-reinforced thermoplastic polyurethane. Serving as an endoskeleton for the guitar body, the ABS takes the place of all the internal wood braces that help the instrument's top withstand the tension exerted by the strings-which comes to about 150 lbs for large guitars. The ABS also incorporates the "kerfing," a collection of interior wood blocks that provide gluing surfaces for the top, back, and sides. And the system integrates the guitar's external "binding," a cosmetic strip-like component that hides the glue joints where the guitar's sides meet the back and top. The ABS lastly provides molded-in features that allow the guitar's neck to be attached to its body.

Garrison's Active Bracing System creates a thermoplastic endoskeleton for the guitar's solid wood top, back and sides.

To put its guitars together, Garrison employs a custom-formulated cyanoacrylate adhesive to join the guitar's solid-wood tops, backs, and sides to the ABS. "We essentially build the guitar body around its binding, while traditional guitar makers do it the other way around, building the binding around the body," explains Chris Griffiths, ABS' inventor and Garrison's president. "And that difference fundamentally changes everything about the way our guitars are built."

Chief among the changes is how quickly Garrison can put a guitar together. A guitar's bracing has traditionally represented a double-whammy from a manufacturing standpoint: "Bracing is labor intensive, and it requires a high skill level," Griffiths notes.

Whether carved manually or with some automation, the two-dozen or more bracing components in a typical acoustic guitar take up a big chunk of manufacturing time.

Griffiths estimates that braces in a semiautomated manufacturing environment take three hours per guitar, plus more time for the kerfing and binding. The ABS components, which mold in a 45-second cycle, arrive at Garrison's plant ready for assembly-no carving required. And with Garrison now ramping up to produce as many as 16,000 guitars/year, shortened manufacturing cycles add up to more than just chump change.

The ABS also helps reduce the expertise needed to construct a guitar. In one of the more difficult construction tasks, luthiers have traditionally sought to carve and position the braces in a way that strikes a balance between structural integrity and the need for the top to vibrate freely enough to generate sound. Many makers even try to optimize the mass-to-stiffness ratio of individual braces by scalloping them where they meet the top, Griffiths notes. "Bracing is where a lot of the art comes into guitar making," he says. The ABS vests the skill needed to brace the guitar properly in the hands of a single designer.

For all its manufacturing benefits, the transition from craft to modern manufacturing did pose some technical challenges. "The net benefit is huge in terms of efficiency, but we have to address manufacturing issues not faced by traditional guitar makers," says Andy Fisher, a mechanical engineer who oversees Garrison's production plant.

Modern manufacturing methods, for example, have raised the bar on fit-and-finish standards. "Old-school" guitar makers can afford to build in a relative fashion-getting pieces to match each other rather than an absolute standard. And they can hide sloppy glue joints under the binding. Garrison can do neither because it must drop its wood components into a framework that barely varies and offers no chance for cosmetic cover-up. So while a traditional guitar maker might accommodate tolerances as loose as plus or minus 1/8 inch, Garrison must cut its tops and backs within plus or minus 0.005 inch, according to Fisher.

So tight are these tolerances that Garrison cuts its wood components on a laser CNC machine running "as-built" toolpaths. As Fisher explains, the company had to create toolpaths (in MasterCAM) that correspond to the as-molded part-rather than the CAD nominals. "By the time the ABS components went through design, moldmaking, and molding, they differed from the nominals just enough to make a difference in our process," he says. Once the wood has been cut, it has to be installed immediately to ward off any dimensional changes from temperature and humidity.

Fine-tuning the design. Taking the ABS from concept to production required more than two years of design iterations to improve the system's performance and moldability. "At first we overengineered the parts, which is a natural response for something that's so new," recalls Griffiths. His first-pass design weighed in 35% heavier than the version slated to go into production, and had wider cross sections. "As we went along, we became more intelligent about mass versus strength," Griffiths says, pointing out that the brace's stiffness derives more from its height than width.

To tweak part design for moldability, Garrison teamed up with the Industrial Research and Development Institute (IRDI) in Midland, ON. "Our ideal part would never have come out of the mold," recalls Griffiths. So he and Fisher worked with IRDI engineers to optimize draft angles and parting line configurations in a lengthy trial-and-error process. "Getting the draft angles right was the trickiest part," Fisher says. The 16 x 20-inch parts obviously needed some draft to come out of the tool, but they couldn't tolerate any where wood meets plastics since visible mating surfaces need to be flush for cosmetic reasons.

The IRDI and Garrison also worked together on materials selection. Together they tested dozens of thermoplastics to see which one best approximated the vibration behavior of spruce at a guitar's typical resonant frequencies. A 40% glass-filled TPU, IsoPlast from Dow Plastics (Midland, MI), won out not only the vibration tests but also because it had the right balance of mechanical properties, cost, availability, and UV-resistance, reports Paul Tichauer, IRDI's president.

As Garrison's design evolved, IRDI provided a variety of design services, creating prototypes on a Stratasys fused deposition modeling system and using PTC's Pro/ENGINEER to split the cores and cavities from a part model. Later on, the institute helped design and build Garrison's tooling, a single mold with a set of five inserts that enable it to turn out standard, left-handed, 12-string, and cutaway models. The institute also molds the ABS components on a 400-ton press.

Good vibrations. Manufacturing advantages may be music to an engineer's ears, but guitar players care about sound. The ABS, though intended to ease manufacturing, may just satisfy engineers and guitarists alike. "One thing we weren't expecting when we designed the system was its significant increase in acoustic response," says Griffiths.

He doesn't know for sure why the ABS helps improve the sound, but he theorizes that any sound improvements derive from its uninterrupted pathway for vibrations. "Our bracing system works as a single-stage vibrating unit," he says, explaining that traditional bracing systems have dozens of discrete vibration pathways. "Traditional braces do a good job passing vibration along their individual lengths-but there are dozen of lengths," he adds.

With his guitars about to go on the market, Griffiths says his company is conducting tests to measure the objective parts of a guitar's acoustic response-qualities such as sustain and frequency response. Guitarists, however, probably care too much about what happens in the lab. "To them," says Griffiths, "a guitar that sounds good is good."

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