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How to Build a Guitar

When a guitarist plucks a guitar string, it starts to quiver back and forth — setting off a cascade of vibrations along the length of the instrument. The vibrations travel down the strings to the top of the guitar, called the soundboard, and then displace the air inside the body. The air bounces off the stiff wooden back and sides and bursts out of the sound hole: a musical note.

Thought to have originated in Spain some 500 years ago, early guitars were actually pretty quiet. But over time, their bodies grew larger, and today they can compete with other loud instruments like violins and banjos, even without electric amplification. ‘‘It’s just the wood and the string and the shape of that guitar giving you all of that sound,’’ says Jeff Allen, who oversees manufacturing for the C. F. Martin & Co. guitar brand in Nazareth, Pa. ‘‘It’s almost like a speaker.’’

For hundreds of years, the craftspeople who built guitars, called luthiers, worked entirely by hand. Now, robots, computer programs and big machines help make each instrument as perfect as it can be (though people still do a lot of the work). At Martin, it can take between three and six weeks to build each instrument; more complex guitars can take up to six months. And during peak production, the factory can put out more than 250 guitars each day. But there’s still a bit of uncertainty — trees are living things, so each piece of wood is different. Here’s how it all comes together at the Martin factory.


Guitars start as piles of wood. The backs and sides are usually made from sturdy hardwoods like mahogany, rosewood or maple. The tops are typically from a species of spruce tree, like Sitka or Adirondack, which is lightweight and flexible enough to make the vibrations that produce the guitar’s sound. (Spruce is also used in pianos and violins.) The Martin factory in Nazareth uses around 250,000 board feet (a unit of measurement for lumber) each year.


When lumber arrives at the factory, it still contains some moisture from when the tree was alive. Before it can be cut into shapes, it has to be dried. This happens over the course of months (or more than a year, in some cases), through a combination of sitting in climate-controlled storage and cooking in a kiln — a huge metal oven.

Once it’s dry, the wood is ready to be cut into the smaller pieces that compose a guitar’s body, including the top, back and sides. (About 115 different parts go into each instrument.) The top and back of a guitar are each made of a panel of wood that’s split in two. The two pieces are matched up based on their color and grain in a process called ‘‘bookmatching.’’ A technician might also use a plexiglass form to check that the pieces are the right size for a specific model and that there are no visible flaws in the wood.


Even if you look really closely at the top of a guitar, you might not be able to see that it’s two pieces glued together. That’s because the sides where the pieces join are shaved flat and tightly clamped together on a Ferris-wheel-like machine called a glue reel. Once they’re dry, the light-colored spruce tops can be held up to a light to check for defects, like knots or tracks from worms. This process is called candling, because luthiers used to hold the wood up to a candle in a darkened room.

The bookmatched tops and backs are loaded one at a time onto a laser cutter, which cuts out a model’s precise shape. (Here, it’s a Dreadnought guitar, which has a big, boxy body.) The laser also cuts panels for the sides. Compared with a saw, the laser is faster, more precise and wastes less wood; it’s also safer for employees to operate.


Some of the earliest guitars were decorated with a floral pattern around their sound hole, which made the guitar more attractive and was thought to strengthen the opening. Today most guitars are still adorned with these designs, called rosettes. A fly cutter, a tool that can create circles, carves a track where a luthier inlays the rosette by hand; this trim can be plastic, wood or mother-of-pearl.


Next, a person glues braces, thin pieces of spruce, onto the underside of the top. The bracing has one of the most important jobs, even though you won’t see it once the guitar is finished: It holds the guitar’s shape. ‘‘If I were to take all the bracing off,’’ Allen says, ‘‘the first thing that would happen is the top would start to collapse.’’ That’s because tightened and tuned strings can exert between 150 and 190 pounds of pressure. This strong X-shaped bracing supports bigger acoustic guitars that use tough steel strings.


It’s here that luthiers really work some magic: They bend wood to make the swooping sides of the guitar. Matching side panels are loaded into a ‘‘shoe,’’ a press that uses heat and steam to soften the wood and shape it into a curve. Then, the two halves are glued together and clamped into place using a form, to ensure the angles are just right. At this point, it starts to look like the outline of a guitar — which is shaped like a figure eight so it fits comfortably in your lap, with your arm over the top.


What holds an entire guitar together? Glue. This wood glue, Allen admits, isn’t so different from the white craft glue you probably have among your school supplies. A ribbon of mahogany, with grooves cut into it to allow it to bend, is glued along the side edges. This creates a bigger surface area that someone can glue the tops and backs onto, making those joints sturdier and more secure.


Most guitars have a contrasting trim around their edges that protects the raw ends of the wood from bumps, scrapes and changes in temperature and humidity. This binding is usually made of plastic or wood. A technician cuts a channel into the edges of the guitar — a process called ‘‘friezing’’ — that the binding will fit into. She also cuts grooves next to the binding to fit decorative trims like mother-of-pearl.


The entire outside of the body is sanded using an increasingly fine grain to get rid of any bumps and grooves. Some of the woods used for the backs and sides have wide ridges in their grain that can’t be sanded down. As shown here, a person fills these in using a paste to get the surface superflat.

The guitars are then hung on hooks in rows like sides of beef: The filler must fully dry out before the body is covered in lacquer, a liquid that dries into a hard, protective coat. When the surfaces are smooth, less lacquer is needed, which is important because a too-thick coat would dampen the guitar’s vibrations and muffle its sound. Afterward, there might still be tiny bumps in the surface of the lacquer — a texture called ‘‘orange peel.’’ These are sanded down.


The body is polished in three stages to get a glassy shine. First, a robotic arm picks the guitar up with suction and gently presses it into a big buffing wheel covered with wax-loaded cotton cloth. The arm and wheel both have sensors that can detect how much pressure they’re applying, so they don’t press too hard. Then a person uses a smaller buffing wheel and, eventually, soft lamb’s wool to perfect the polish by hand.


Back in the machine room, a computer-operated router, shown here, carves out the neck and the fretboard. The fretboard (also known as a fingerboard) is made of ebony, a wood that won’t show much wear, even after years of pressing steel strings into its surface. It’s lined with nickel bars called frets. Guitarists press on the gap between two frets to change the note the string sounds out. The spacing is set by a mathematical formula — it has to be just right to create the correct notes.


Once the narrow slots are sliced into the fretboard, it’s time to add frets. Most models have 20. A technician places segments of fret wire in each of those grooves, partly securing them with a hammer. Then, the machine here stamps them into place with enough pressure to wedge them in at the correct height.


The neck and fretboard are glued together, with a steel truss rod sandwiched inside the two components. Like the bracing, you’ll never see this rod once the guitar is finished, but its job is still important: It helps counteract the stress the tightened strings place on the guitar, keeping the neck straight. Once all these components have been put together, the neck is sanded and polished.


The guitar neck and body have traveled through the factory separately, so it’s finally time to put them together. They attach through a tight joint called a compound dovetail and are glued into place. The better the neck fits the body, the better the guitar’s vibrations travel across the instrument, and the clearer the sound.


It’s time to select the bridge, which is where the strings attach to the body, just below the sound hole. A technician uses an instrument called a gauge that shows where it should go and how high it should be. It’s important to get this placement just right: Too high, and it becomes really hard to press down the strings; too low, and the strings might not clear the frets below the one you mean to press on, creating a nasty buzzing sound.

Once the guitar is fully assembled, technicians turn their attention back to the frets. The whole guitar is placed in a chamber called a Plek, which simulates the amount of tension the strings will put on the body and neck and creates a 3-D model of the frets. If it senses they’re too high or too low in certain spots, it will sand them down to the correct height, which also helps eliminate buzzing.


At this point, the guitar is ready to be strung. Someone in final inspection stretches each of the six steel strings from the bridge all the way up to the metal pegs at the head. Each guitar takes about 15 feet of steel string, and the factory uses more than 500,000 feet of wire each year, estimates Frank Untermyer, its director of supply-chain management. Then the guitar is tuned and stored, letting it settle.


After roughly four days, the inspectors, many of whom are musicians, pull the guitar out again and play it. Occasionally, something goes wrong, like a big crack appearing, but this is uncommon. Inspectors check that the guitar sounds good and is comfortable to play. They play up and down the entire fretboard, using each fret and string and a combination of fingerpicking (playing individual notes) and strumming (playing a chord, several notes at the same time). Sometimes someone will break into a familiar tune, strumming along to the radio.


This article was originally published in the November issue of The New York Times for Kids. Find the section in your paper this Sunday, Nov. 29 (and the last Sunday of every month).


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