Introduction and motivation:
OK, this project might appear to push the envelope for some folks, especially those without a workshop and some machine tools, but I am putting up this Instructable mainly because it represents the belief that you should never NOT build something just because there's a big risk of failure.
After all, there are centuries of research and best practice, including highly sophisticated designs, world famous builders, critical selections of materials, glues, and finishes that go into building a fine musical instrument, and there are very exclusive guilds that carefully protect these secrets and techniques. So what in the world would any commoner in their right mind be thinking to even imagine that a good sounding, perfectly playable instrument could be designed and built by a rank -- well, lets just leave it at that.
Step 1: Design
The most important part of an acoustic instrument is the sound board, such as the top of a guitar. A traditional bass has a top, or sound board, constructed from a large, thick slab of spruce from one of just a few forests on the planet (which of course are running out of trees). The design of the top (and back) of the instrument are carved in such a way that the arch shape, important for strength, is shaped from the thick block of wood and given a somewhat uniform thickness -- a huge challenge but one that carves away most of the original slab. Arch-top guitars, cellos, violins, and some mandolins also are made this way. Nowadays large CNC milling machines are used by some folks to do this critical carving.
This design was based on the idea that a sound board and back could be created from a uniform thickness piece of wood that would simply be bent or curved to provide the necessary strength to support the massive downward pressure of the strings on the bridge, but at the same time have good acoustic qualities. The big job of carving would be eliminated, and much less wood would be needed.
It also seemed like a good idea to make the sound board bigger. After all, this is a bass, and a bigger sound board should help emphasize lower frequencies, right? This is the reason for the "teardrop" shape of this instrument. The music most frequently played at our house is bluegrass, so I wasn't worried about being able to bow the instrument, however it is possible even with the wide body. I do expect comments that in the construction photos it looks like a small boat.
Step 2: Materials
Finding wood for a musical instrument is a big challenge and occupies a great deal of the time and effort professional luthiers must put into their work. Woods must be properly cut, properly seasoned, and free of defects -- and certain kinds of wood are literally disappearing from the planet.
There are several sites on the web that supply spruce and various hardwoods for instrument making, but most only have materials for guitars. The International Violin Company of Baltimore MD sells wood blanks for traditional basses, but this design does not require the big thick slabs and can be fabricated with thin boards properly glued to for adequate size.
In fact, although I was able to scrounge some amazingly good wood for the project as described below, I believe if you bought some top grade 1/4 inch mahogany marine plywood, you could build this instrument without all the fuss of re-sawing, sanding and gluing that I went through. Less expensive basses are made with essentially plywood tops and backs that are formed into the arch shape when glued, but the added density from the glue reduces the resonance and ultimate sound quality.
In my case, when going WAY out on a limb with an untested project, it seemed prudent to scrounge around for materials, rather than spend a lot of money on expensive new stock.
I was looking at the end grain on a pile of old lumber when I noticed some perfectly quarter-sawn one-by-twelves. Quarter-sawn means that the way the original logs were sliced resulted in planks that have grain that runs relatively straight across (the thin way) through the board. This gives maximum strength, and is what luthiers look for in selecting stock for instrument tops, etc.
The wood was some very old, dry California Redwood that had once been shelves in a closet. Redwood is remarkably light and strong and is also used in making guitar tops as an alternate to spruce. There were a few nail holes and paint on the boards, but this was a very lucky find.
For the back, I selected a few pieces of walnut that had been found as fallen trees on a property we had years ago in NJ. I had taken the trees to a sawmill and have made several projects with this lovely wood. With careful re-sawing I had just enough walnut left to make a back. I also found some thicker cuts that I decided to use for the neck.
For the sides, I was looking for a straight-grained hardwood that I hoped would bend well. I found a thick piece of rough-sawn wood that appeared to be something quite hard, and I initially thought it might be ash or white oak. But when I started re-sawing it I detected a slight yellow-greenish color, and realized it was an old piece of locust wood that I had also scrounged from the forest floor in New Jersey. But it was very dry and passed the "thump" test with flying colors. What is the "thump" test? You hold a large, thin slice of the wood gently by one corner and thump it with your knuckle. You hope it will ring, or resonate with a nice tone, rather than just go "thud."
Step 3: Construction
My band saw will only re-saw wood up to about 8 inches, so I knew the back and front would have to be glued from four slices of wood. I intended for the top, back, and sides to all end up in the range of 1/4 to 5/16 inches thick, so I set my re-sawing guide for a thickness of just under 3/8 inch. The edges had to be perfectly square and straight for gluing, so the jointer is necessary for this step. I used a large piece of plywood covered with wax paper as a flat gluing table. For clamping, I nailed two parallel pieces of wood to the plywood just slightly wider than four pieces to be glued, with just enough space to also permit a long wedge to be tapped in on one side to apply the squeeze. I used Titebond II for all of the gluing on this project.
The neck, tuning head and integral upper support block were next to be glued up and cut out. Since I didn't know what I was doing, I left a large amount of wood on which to glue the sides and eventually the top and back. The back view shows how far the neck piece extends into the body for strength at this critical junction point. Of course, the finger board has not been made or installed on the neck yet.
The sides and bottom were re-sawn and rough sanded but left full width. I put the boards into the hot tub for about 24 hours to soften them up, and then bent them while wet using the hot pipe method. This is where you take a maybe foot-long piece of maybe two-inch iron pipe, clamp it very tight in the vise, and aim a propane torch right down the inside of the pipe. Using heavy gloves, you force a bend in the wood while rubbing it over the hot pipe, and soon the wood begins to retain the desired shape. I then clamped the pieces over curved forms until the wood was thoroughly dry again.
The next step was to glue the sides and bottom together with the neck piece. On the inside bottom view you can see that there are big gluing blocks In the bottom corners. I cut these out of thick lumber in what could be called "cross grain" fashion. This way the grain of both the sides, bottom, and gluing blocks all run the same direction, allowing all of the wood to expand and contract in the same way over the years. The same is true of the large block in the middle of the bottom that will carry the whole weight of the instrument when standing on it's peg (which was also glued-in at this time).
OK, now we get into some real home-spun goofball engineering. The top and back must be curved for strength, so the bottom and sides must be cut to give the necessary shape. These curves are well shown in the photo of the finished bottom of the instrument. These cuts could be very complicated to figure out, so the diagram called "Laser" shows how to make this shape without screwing it up. One side of a block of scrap wood is cut to the curved shape as per the drawing. A laser pointer is attached to a small try-square. The block and bass are carefully aligned, and as the square-with-laser is moved over the curved edge, the laser points to the exact line of the needed cut.
After cutting, kerfing strips need to be glued on the the front and back edges of the sides. I made my own because I only found very small ones for sale, and I wanted to start out with strips about 3/4 inch wide and deep. Making them means ripping wood (clear pine in this case) and then making saw cuts every perhaps 3/4 inch that are just deep enough to leave about 1/16 inch of wood holding the strip together. This, of course, allows the kerfing to easily follow curves. The strips must be glued on with enough material sticking up to allow for shaping them (as well as the edges themselves) to match the overall curve as needed.
Next, the top was glued on. It was cut extra large with the intention of trimming the edges later.
Since this is such an untraditional design, bracing the top to support the enormous stress from the downward push of the bridge, was a subject of considerable, um, consideration. Traditional basses do have a peg inside that attaches the front to the back, and I believed this would be very important both to help with the force of the bridge, and to transfer vibrations and make the back a big part of the acoustic resonance. On traditional basses the precise placement of the internal peg is a very big deal with respect to tone, etc. But this bass would not have the superior strength that comes from the carved arch of a normal viola and I felt something in the way of bracing would be critical.
First, to deal with the pressure directly under the feet of the bridge, I glued on two 1/4 inch thick round redwood pads. Then two ribs were installed with cuts carefully matched to the pads that run the full length of the top. Since the pressure at the center of these ribs could eventually cause them to break away from the top at the ends, blocks were added to back-up the ends of the two long ribs. I then added a redwood cross piece to support the post. The position of this piece is of course a huge guess but intended to be close enough to the bridge pads to add strength, but far enough away from the bridge location so that it does not prevent too much of the vibrations from the bridge being transferred to the top. From the ultimate sound quality POV, this was probably the most important decision to be made, and it came down to a big fat guess.
To make the post strong but light (again, so it wouldn't unnecessarily dampen the sound vibrations) I used the lightweight redwood and drilled holes to remove unnecessary material. A pad was added to help make a strong transition to the back.
After designing and cutting out the sound holes, I added some little braces because of the very weak cross grain left on the remaining wood in these areas.
Eventually everything inside was cleaned-up a bit, sanded a bit, and sealed with shellac.
For reasons unknown, after the top was glued on there was a tendency for the sides to warp outward. My solution was to instant glue four little drilled wood blocks to the edges, as seen in the photos taken just before the back was glued on. I used some fine wire to pull the sides in to the proper position. After the back was on, I was able to snip and remove the wires through the sound holes.
I turned a nice block of walnut on the lathe to support the foot peg, and made the post itself out of a very strong piece of rosewood that also had to be turned to size on the lathe. I have no idea how this piece of rosewood ended-up in my shop, and a piece of 5/8 inch maple dowel would have done just fine. I have a hand-me-down set of 5/8 inch tap and die for wood projects so I used those to make the peg screw into the bottom and thus be removable (but not adjustable).
I used a router jig to trim the edges of the top and back, and bought some wide white binding plastic from Stewart McDonald to finish the edges.
I couldn't find anything in my workshop suitable for a fingerboard and tailpiece, so I drove over to Orlando for a visit to the wood room at WoodCraft. There I selected a beautiful piece of African Lacewood, which is very hard and oily. But the millions of beautiful curly figures in the wood make it hell to work with. A trip across the jointer results in little divots where the curly grain chips out. So it needed to be cut to roughly the right shape and then sanded, which was a lot of work.
And with respect to the fingerboard, the shape is very critical. I suggest that if you are going to build one of these, find a decent traditional bass and make a series of templates of the fingerboard shape.
A bass requires machine tuning heads, and a full set can cost several hundred dollars. Here is a link for some tuners, which are in the $100 range for a set of four.
The strings and an inexpensive "student" bow came from Shar Music.
I had a little bit of good hard dry maple lying around, so I decided to make my own bridge. I somehow made one that was a little short, and had to repeat this step.
For the nut at the top of the fingerboard, I used a few pieces of pre-shrunk type G plexiglas, which is nice and hard. A piece of bone is traditional, but the hard acrylic works fine.
Step 4: The rest of the story
By the time I had the major construction complete I decided to do a trial tune-up prior to finishing, because giving a piece this size a fine lacquer finish is a big job in itself, and if this thing was going to sound like crap or implode on stringing, I might as well know about it before getting out the spray equipment. There is a photo of the tuned-up bass leaning against my workshop door.
Amazingly, it didn't fold-up, and the sound was strong and clear. Frankly, I was shocked. I decided to proceed and ordered lacquer sealer and instrument grade gloss lacquer from Stewart McDonald.
Finishing is a big challenge in itself. I made a jig using the holes in the tuning head and a threaded dowel in the bottom peg-rest hole so that the instrument could be suspended horizontally and rotated lengthwise so all surfaces would be accessible for spraying. We were having damp, cool weather at the time, and even with retarder added I was having trouble with coats turning milky. Eventually I closed my workshop, ran a de-humidifier for a few days, and sprayed the bass while wearing a SCUBA outfit.
Since building the bass I have had two honest-to-goodness bass players try it out and their reaction has been very complementary. Another fellow, who is a recording studio engineer with lots of experience recording acoustic instruments, has given the sound quality very high marks. In any case, it makes a pretty addition in front of our living room window.