Part 1: Body Assembly
Take the back plate and remove the structural supports, along with any extraneous artifacts created during printing. Make sure it's more or less flat when placed inside-side down on a table. If it's not, use a hairdryer to heat the plate and bend it back into shape. Be careful to focus only on the plate edge and not flatten out the arch itself.
The next optimal step is to start attaching the sides to the back. First, glue the top block of the sides - which includes the neck slot - to the back. Apply cyanoacrylate glue liberally to both of the surfaces to be joined, then carefully put the block in place in the marked joint area of the back, being careful to align the parts precisely so that the neck slot is centered with the rest of the instrument. Clamp it in place with standard C-clamps or spool clamps and allow a few hours for the glue to dry.
Think of the top block as an anchor: once it's in place, it's far easier to finish the remainder of the assembly accurately. Cyanoacrylate glue sets quickly, and it's possible that you might find yourself wishing you'd moved the top block higher or lower. If you do, don't worry: it's easy to work around this with the rest of the assembly; the more critical aspect of the process is that the top block is on-center.
Try to optimize the placement of the bottom block so that the sides all fit into place correctly around the instrument. You can accomplish this by moving the block north and south along the joint area with the back. Clamp it in place without glue. Do the sides fall where they should along the gluing surfaces of the back? If so, glue the bottom block in place and, once it's dry, begin to glue and clamp the sides in place. The gluing surfaces are wide enough that if you find yourself with an odd fit or a slightly irregular overhang it won't be a problem.
With the sides in place, attaching the neck is an ideal next step. Depending on the dimensional accuracy of your printer, you might need to smooth out the heel of the neck where it fits into the top block. As with other parts, the neck should be attached firmly into the body using liberally-applied cyanoacrylate glue, being careful to ensure that it firmly meets both gluing surfaces. Clamp the neck in place using two C-clamps: one to hold it against the long edge of the neck block, and another to hold it from top to bottom against the button of the back.
Once the sides and neck are in place, you can start to glue the soundpost in place. Violin and viola soundposts are best made from spruce dowels 6mm in diameter. Cut one end flat and apply cyanoacrylate glue liberally to both surfaces, then put the soundpost in place, holding it firm. You can also use a hair dryer (on a low or no heat setting) to expedite drying.
Note that we don't specify a generic soundpost height: this will vary depending on warpage and minute variations in arch height that occur due to support placement and aspects of printer setup, in addition to the actual size of the instrument. Even a minor variation in X-axis belt tension, for example, can create changes to arch height.
The best way for you to establish the necessary soundpost height is to estimate it visually, add a few millimeters, and then cut the top end. Flatten the top surface of the cut end with a blade or with sandpaper, and then test out the height by putting the top in place and holding it in place at the sides of the f-holes. When held in place tightly, an optimal fit will generate enough pressure that the upper f-hole wing on the treble side of the instrument rises about 0.5mm from the rest of the top. Once that fit is established as you gradually reduce the soundpost height, glue the top end of the soundpost to the gluing surface on the inside of the top, and clamp it in place to dry with C-clamps or spool clamps once again.
When the soundpost is dried in place, it's time to start affixing the top to the sides. This may take a bit of work, as the contraction that takes place as the model is printed and solidifies might have caused some of the parts to be slightly out of spec in size, and as the parts are all assembled the cumulative movement of tolerances may make assembly more complicated. If everything doesn't seem exactly lined up as you prepare to close the body, don't despair. Once again, starting from the blocks is the right approach: but rather than trying to pull the top (and the small soundpost glue joint with it) toward one of the blocks, push the blocks inwards so that they meet the glue joints at the top. The reason to do this is that the typical contraction pattern of the back will actually make its arch flatter than the model specifies, making the length of the back slightly longer than spec in most cases. By pushing the blocks inward, you're actually raising the arching of the back again and keeping the torsion on the soundpost constant. Once the blocks are in place, glue the sides themselves along the joint surface.
Part 2: Final Assembly
When you've fully assembled and closed the body, attaching the fingerboard is the next step. Once again, apply cyanoacrylate glue to both gluing surfaces, and clamp the fingerboard in place using three C-clamps. Ensure that the fingerboard does not slide off center during the clamping process; alternating the position of each clamp should help to accomplish this.
The next step is to install the endpin. While the endpin hole is a part of the model, its diameter is smaller than the diameter of the endpin shaft. You'll need to drill out the endpin hole and glue the endpin inside the lower block.
Finally, you'll need to smooth out the top between the f-holes so that the bridge feet will make uniform contact with the top. Sandpaper will do this nicely, and you can restore the shiny, black surface of the top afterwards by brushing it sparingly with acetone.
Part 3: Final Setup
There are a few key components to the final setup of the instrument that we'll address in this section:
First is the string spacing at the nut. For 4/4 violin and viola, the string spacing between the top and bottom strings should be between 16 and 16.5mm. Mark that dimension on the nut, centering it equally from both edges. To the inside of each marking, use either a blade to make a shallow, V-shaped groove roughly 0.25mm deep for the string, or a needle file to do the same - and then lubricate it liberally with graphite from a pencil. Make two additional grooves for the inner strings as well, so that the spacing between the upper three strings remains equidistant.
The bridge should be addressed in much the same manner. We provide a relatively generic bridge template; the final height, the final fit of the feet, and the final marking of string grooves is something you'll need to adjust - because the necessary height of the bridge will depend mostly on your accuracy in setting the neck.
Fitting the bridge feet, assuming you've smoothed out any irregularities in the top, shouldn't be too difficult. For beginners, the easiest way to accomplish this is to take a small, thin piece of paper and fully cover it with pencil graphite, then, with the paper between the top and the bridge blank, rub the bridge up and down along the surface so that the graphite rubs off on the high surfaces of the bridge feet. Then, use sandpaper or a blade to reduce those areas and repeat the process until the feet of the bridge make full contact with the top.
Once the proper fit of the feet has been established, fitting the string heights of the bridge is the next step. This is done by establishing the correct height of each string above the end of the fingerboard: generally, for violin, a string height of 5.5mm above the fingerboard on the G string and 3.5mm above the fingerboard on the E string is considered a standard concert setup. For viola, 6.5mm on the C string and 4mm on the A is generally considered standard, with the heights above the fingerboard tapered between each other. Keep in mind that the bridge should still have the same general arc shape as the fingerboard despite the slight taper: these heights only represent the height at which a string should hang above the end of the fingerboard once the bridge is in place.
Once you've established the string heights and cut and sanded the bridge to the correct shape, it's time to set up the string grooves in much the same way as with the nut. For 4/4 violin, the string spacing between the top and bottom strings should be between 34 and 35mm. Mark that dimension on the bridge, centering it equally from both edges. To the inside of each marking, use either a blade to make a shallow, V-shaped groove roughly 0.25mm deep for the string, or a needle file to do the same - and then lubricate it liberally with graphite from a pencil. Make two additional grooves for the inner strings as well, so that the spacing between the upper three strings remains equidistant. For viola, the correct spacing is generally 35-37mm.
Once the bridge is ready, it's time to put the strings on. Center the bridge between the f-holes using the bridge as a guide, and set it just above the f-hole nicks. Attach the tailpiece over the endpin using the tailgut (tailpiece adjuster) and wind the strings on the tuners (or pegs, if you decided to go that route) starting with the outer strings. Once the strings are on, you can experiment with moving the bridge north and south to optimize tone. Generally, we've had good results with the bridge 3-4mm ahead of the f-hole nicks, but your results might very depending on dimensional creep during assembly and other variables.
Part 4: Resources