The construction of an acoustic guitar in a nutshell- Part 1
Thickness Sanding and Bending The Sides
This set of rosewood is very old. We were saving it for a special build, and when this project came along it just made sense to use it. This set had an amazing tap tone! We choose pieces of wood by holding them suspended by a "node", an area of the perimeter of the piece where vibration is minimally disturbed, and tapping on the piece to listen to the resonance and tonality it will produce. You can read a lot about the different wood varieties and their tones, but each piece is very unique. One advantage to a hand built guitar is that we can take this step to match pieces together that will give complementary sounds. Because of the richness of its resonance as well as its beauty and ease of bending, Rosewood is our favorite tonewood for acoustic guitar backs, sides, bridge patches and sometimes bridges and fretboards, but it is becoming a challenging material due to overharvesting and illegal logging, as well as the environmental cost of transportation of exotic woods. Because of this, we rarely utilize it. This project, however, was special and this set of wood has its own story, which deserved to be used here. We had a customer who bought this set for himself many years ago, intending to build a guitar. Because of a medical condition he was unable to handle the fine tool use, so he brought the pieces to us. We were able to build him a guitar and accepted the rosewood in partial trade, and in this way the wood was able to be used. We think it's very important to treat each piece as precious, remembering that our planet is sustained by its forests and honoring the long life of a tree that can produce wood worthy of a guitar.
Prepping The Sides For The Arm Bevel
After we bend the sides, it's time to establish the curve of the arm bevel along the side of the guitar. We start from our drawings and measure down a curve that compliments the body shape, then sand the preliminary shape by hand. This happens now, because the purfling that separates the binding from the guitar body needs to track along the bottom edge of the bevel. That line must be smooth and attractive, and it gets established at this step. Arm bevels are a multi-step process. It's most important to ensure that they don't disturb the performance of the guitar's soundboard. We prioritize the sound, comfort and playability of our guitars first and foremost. After sanding the arm bevel into the side, we glue the sides to the end and tail blocks, then clamp the sides into the mold. This will support the delicate sides until they are supported by the bracing, back and top.
Gluing In Kerf Liner And Arm Bevel Block
Guitar building requires a lot of clamps! The kerf lining is placed all around the upper and lower edges of the sides to give us a surface for gluing the back and top. Binding channels will eventually be cut into this material as well. We use reversed mahogany kerf lining. The reverse cut is a little harder to work with, but it stiffens the sides a bit more. The guitar's sides need to be strong without adding too much weight to the whole piece. Here you can also see the arm bevel block, which is still rough and uncarved in this photo.
Carving the Arm Bevel Block
A block of basswood is glued into the area where the arm bevel will be, instead of kerf lining, to give a surface for the bevel. This is where the carving can have a big effect on how much flexion we can get from the guitar's soundboard. The arm bevel needs support, and there needs to be a glue surface for the soundboard, but the block needs to be as minimal and smooth as possible. The more we can get that top to move, and the fewer sharp edges we introduce to the guitar's interior here, the more sound we can get from that lower bout! The lower bout of an acoustic guitar is the primary sound "pump". This is the section that needs to be strong enough not to swell or split, while moving as much air as possible.
A View Through The Side Sound Port
Side sound ports! Why are they important? To extract your picks when they go into the vacuum of an acoustic guitar body? To reach your pickup's batteries without recruiting a small child or twisting your hand into knots? All joking aside, we hear a big difference in how much a guitar projects when it has a side sound port. It's not just for the player's ear, although that is a great side effect! There may be something in the idea that the upper bout can allow more air to move when it is ported. Generally, the upper bout doesn't contribute as much to the sound of the guitar because it is under so much tension that its primary function is stiffness. We believe a side port lets this area "breathe" as the lower bout moves air, without jeopardizing the strength in this critical area. To our knowledge, there isn't much science on this yet. Sometimes you just have to trust your ear! In addition to the sound port, you can also see the kerf lining is now glued on both edges of the sides, and mahogany side struts are also in place. The sides are now sturdy enough to take them out of the mold for fun photo ops.
An Essential Lutherie Tool: The Go-Bar Deck
Here you see the spruce center reinforcement strip being glued over the seam in the guitar's back. The tool that we use to clamp pieces as they are glued is called a Go-Bar Deck. It's a centuries old way instrument builders have solved the challenging task of gluing bracing stock onto fragile and curved tops and backs. We made this deck to accommodate the large radius dishes that support the tops and backs until their bracing can hold them in the correct curved shape. Bracing stock is sanded to the radius we need, then glued to the top or back on a dish of the appropriate radius. There is a lot of pressure exerted by those fiberglass rods. Placing them so the parts don't "skate" and shoot across the room is a trick! You don't know fear until you have a Go-Bar Deck launch 25 fiberglass rods at your face!
Bracing The Back and Carving The Back Braces
The two pieces of bookmatched rosewood first need to be glued and sanded to thickness. We joint the pieces where they will be glued, then use hand planes to ensure a perfectly mated seam with no gaps. We apply glue and clamp the two pieces together into a special clamp that applies a great deal of force downward and inward across the entire piece, using ropes and wedges. When cured, we take the back out of the clamp and sand it to its final thickness. The soundboard pieces will eventually get glued together in the same way. A strip of spruce taken off the wood that we'll use for the soundboard becomes a center reinforcement strip which we glue over the seam of the guitar back. We then glue mahogany braces, which we have first sanded to match the back's radius, across the inner surface of the back. Each brace gets carved to give the best amount of stiffness and flexion. We want our guitar backs to provide some movement and tonality to the performance of the guitar, so as we carve, we tap on the back from top to bottom, listening for a good progression of tones. Each piece is unique. When it sounds right, we stop carving and finish sand the braces and inner surfaces of the back.
Trimming The Rosette To The Final Shape
We love making rosettes. We save almost all our cut-offs from previous builds. We love to use these precious woods to add little details that are pleasing to the eye. Not only that, but every piece of wood we use has a special story linked to the builds they come from. It's important to us that these woods came from living trees and should not be wasted. The rosette might be based on a sketch of some geometric pattern, or it might be pieced in a free-form way to best use the most attractive facets of our offcuts. We glue the rosette together on a piece of heavy paper where the final curve has been drawn. Ultimately the paper gets stuck down to a stiff board so we can send the rosette through our thickness sander. Lastly, it gets cut out using this router jig to cut the inner and outer perimeters. This same jig gets used to cut the sound hole out of the guitar's top, and a shallow channel where the rosette will be inset.
Bending Maple Binding For The Sound Hole
Each surface of a guitar affects the final sound. The binding of the sound hole can have a surprisingly noticeable effect. Here we are hand bending maple binding. The radius of Sophie's sound hole happens to be the same as a can of the tomatoes Rachel uses for pasta sauces, so those cans make great molds to clamp the cooling binding down! Before this step, we had already glued the soundboard pieces together. The rosette and binding are glued into the top, and the whole top gets thickness sanded to its final dimension.
Attaching The X-brace To The Soundboard
It's critical to correctly judge how thick the top should be. The top is where all the flexion needs to come from. Stiffness varies from piece to piece, so we shake and tap on the top as we sand until we know it's loose enough. Bracing will reinforce it in the areas where it must be strong, while allowing for movement in the areas where it must move. The X-brace is the most important brace on the whole guitar. The stress this wooden assembly is able to handle is impressive. A steel string acoustic guitar has about 180 pounds of string tension trying to fold the guitar in half! The placement of this brace also plays in to how the top will move. It is related to the location of the bridge and bridge patch, which provide the pumping action that moves the whole top. As you can imagine, it's the exact placement of this leverage point which determines the largest aspect of the guitar's voice.
Shaving down the sound hole binding.
This hard edge around the sound hole not only protects the softer top wood, but helps to improve tone. Think of it this way. In an inflatable pool, when a wave is created and then hits the soft edge of the pool some of that wave's energy is spilled away. In an acoustic guitar we want as much energy as possible moving that top. We know that it's these small gains that add up to an excellent sounding instrument.
Shaping Braces On The Soundboard
The term "voicing a top" is a bit misleading. The "voice" of the guitar is determined by many factors of its design, some of which you have already seen. When we carve braces, we are shaping the voice of the top. When all the braces are glued down, we begin the process of carving, tapping, listening to the sound it is making, and judging exactly when each area of the top has achieved its best balance of strength and flexion. This part is exciting because it begins to introduce us to the subtle resonances we will hear in the final guitar!
A View Of The Interior Of The Guitar
We try to build as clean as possible. When we glue down the guitar's back, we have a chance to clean the seams and ensure that there is no squeeze out. But when the top gets glued on, the box is officially "closed" and it's impossible to reach everywhere! We do this task with four hands and under a considerable time crunch to spread and clean glue, attach clamps and tighten them down uniformly. In order to make this possible we first carve channels in the guitar's sides where the bracing will sit. The centerline must match on the top, sides and back and all parts must glue down perfectly aligned without any gaps. Before the top goes on, we attach our label inside the guitar with our signatures. "The closing of the box" is an important day for us! That's when the guitar begins to vibrate as a unit. We can always feel it moving to the music in the shop. It comes alive!
The construction of an acoustic guitar in a nutshell- Part 2
Carving The Soundboard Bracing
Here's another view of the inside of the soundboard with all braces attached, while we're in the process of carving them to final specifications. We brace the soundboard with spruce. This guitar has a torrefied Adirondack spruce top. Torrefaction involves heating the wood under pressure, without oxygen, drying it out and removing some of the volatile compounds within, and then rehydrating it under controlled circumstances. Torrefied wood is well on its way to that aged sound you want, with a bigger and richer voice and more flexion than when it was raw. Because the moisture and oil level has already balanced to a large extent, there is less danger of a top splitting when exposed to varied environments when it is still new. This is not to say that you don't need to take good care of your guitars! Only that less shrinking and stretching are likely in the early phases of the guitar's lifetime. Torrefaction also produces a Maillard reaction, which is what makes the proteins and sugars brown and tasty in foods, and Rachel swears torrefied spruce smells like caramel. She doesn't promise she's never tried a piece.
A Luthier's Eye View Of The World
Building a guitar requires accuracy to fractions of millimeters in so many areas. One small inaccuracy leads to downstream effects that can make or break a guitar. Through the view here, you can see us cutting the channel for the X-brace to fit into when we glue the top down. That particular brace ties into the back and sides to unite the piece with a solid "skeleton". Eventually the binding channel will run around this edge and it will glue into those braces too where they penetrate the side.
Cutting The Binding Channels On The Top
This process requires a multi stepped channel in order to accommodate the binding and purfling lines. If you think about the shapes involved, there's a curved top, then a purfling strip, then the binding, and another strip of purfling between the binding and the side. That strip runs around the underside of the arm bevel. Each piece needs its own channel. This is a picture of the top binding channel being cut for the first stage of the process. The purfling and binding needs to be glued in so it sits tightly, without gaps, and without bulging away from the guitar's body, because when we do the final sand it's important not to have binding that gets thin and then thick as it flows around the guitar. This requires measuring each piece with a micrometer-nothing is ever uniform-and cutting the channels to match, then using tape and bands to laboriously brace the pieces while they're glued. When we make binding decisions there are also intersections to think about. For instance where the binding crosses the end wedge, the binding and/or purfling might "T" off, or might run around that area with visual continuity, requiring a careful inlay to get the angles perfect. There are very few right angles on a guitar!
Carving The Arm Bevel (Some More)
Like most operations in guitar building, it is preparing what is underneath a thing that make a the biggest difference. Here you can see the top purfling and binding has been glued in place. You can see how the arm bevel looks at this phase. The edge of the block needs to be carved smooth and inlaid with a piece of veneer to match the binding. You can also see the tape-covered sponges we stuff into the guitar throughout the remaining steps. We will be carving, sanding and spraying, so we need to protect the interior of the guitar.
Test Fitting the Guitar's Neck Joint
This guitar uses a bolt on mortise and tenon neck joint. We feel this is the strongest neck joint. Some people prefer a dovetail neck joint and we have built guitars with that attachment method, but this is stronger. A perfect mate between neck and guitar ensures transfer of vibration from the strings throughout the body. The neck angle corresponds to the angle of the strings between nut and saddle. This has a lot of effect on playability and how effectively the strings can drive the bridge/top.
Cutting Fret Slots On The Fingerboard
Before we cut the fret slots, we radius the fretboard using a homemade jig (not shown) that holds a router. This is an area that can affect the feel quite a lot if it's not uniform. When it comes to cutting fret slots by hand, this jig is nice because it takes all the guesswork out of getting the frets in the just the right spot. This is a must for the guitar's intonation. Fret slots have to be cut at a uniform depth and width. Keeping the saw level and correct is the biggest challenge. Rachel is not a fan of this part and usually finds something else she "needs" to do when it's fret slotting time.
Carving The Guitar's Neck And Heel
Lots of builders say this is their favorite part of the building process. Carving a neck takes time and a light touch. Again, feel is of primary concern here. The human hand is very good at picking out irregularities. We want our guitars to be a joy to play! Some test "air guitar" playing tells us what we need to know! Here you can see we have already routed a channel for the truss rod, installed the truss rod, and glued down the fingerboard after binding it. Binding the fingerboard means extra steps when we shape the frets. Fret wire has a tang which fits into the fret slot. You can see that tang on the edge of unbound fingerboards. On bound fingerboards, we file the tang off each side of the fret so it can float over the binding. Fret wire must be bent to the radius of the fingerboard, cut to size, tangs are filed to match the width of the fret slot, and each wire is pounded down. This step is done after the neck is already glued on the guitar, so when it comes to the highest frets that are past the neck joint, a special tool protects the guitar's body and gives support through the sound hole. The fret job and setup happens much later, at the very end of the project after finishing.
Carving The Neck To The Body Joint
We are sticklers for a smooth transition where the neck and body meet on a cutaway guitar. We find that it will feel jarring when the neck does not smoothly join the body, for instance if there is an irregular step down in this area with a Spanish style heel. We prefer to draft a flowing and not a disjointed shape here. In small ways like these, each luthier makes their instruments their own. This is why we often recommend that our customers speak to many luthiers before placing an order, and try to play examples of their instruments. Frequently it is difficult to express what goes into the overall feel a player is looking for.
Scraping Bindings And Sanding The Guitar Body
Surface preparation is key to a good finish. We are proud that we do our own finishing. Rosewood is oily and it likes to stain neighboring blond wood! We use shellac as a barrier as needed. Typically our guitars have an epoxy pore fill. A few purists appreciate open pore guitars, but most don't hear a difference and would prefer a nice smooth surface. This is mainly a cosmetic choice.
Applying Epoxy Pore Filler To The Body
We use a marine grade epoxy to pore fill our guitars. To ensure the thinnest possible finish on our guitars, we brush on the epoxy to the surface to fill the pores and then sand it all the way off leaving just the smallest amount of material to level the low spots in the wood. Plenty of feel goes into these steps! A thick coating will smother the wood, while sanding through leaves a noticeable mark.
Level Sanding The Pore Filler With The Aid Of Power Tools!
Using a combination of power sanders and a lot of hand sanding, this process takes a lot of time to get right. Finishing is the hardest part of the building process.
Spraying Water Based Lacquer And Checking Ones Reflection
We are very happy with the waterbased lacquer that we use. Originally we wanted to offer only French Polish, as this is the most environmentally sound option. Our testing ultimately gave us mixed feelings about French Polish. For the average hobbyist guitar player, a natural shellac finish of that type is surprisingly durable and beautiful. Under heavy professional usage, though, it doesn't seem to be durable enough to make us feel that we are offering the best to our customers. We made the switch to a very clean waterbased lacquer that has excellent acoustic properties. We have a chance to measure the thickness of the finish we have applied at the time that we attach the bridge, so we are able to visually ensure that the finish is thinner than any factory guitar. The result has been holding up to vigorous professional use as well as any product we have ever seen. The last steps of the build process include the most important placement and gluing of the bridge. This must be perfect for the guitar to intonate correctly. Finally we bevel the frets, then test and spot-level the frets on a jig that simulates string tension, then sand them smooth. Setup is finished by shaping a nut and saddle and adjusting each to achieve correct string height. We do this over a period of time so the guitar can adjust to being under string tension. When possible, after a month or so of playing in the new owner's typical environment, we like to take another look at the setup and make some final adjustments.