Musicians usually have great fondness for their instruments, so protecting them from wear and tear is important. Bob Glass developed a unique guard to protect his acoustic guitar with the aid of rapid prototyping. Here’s his story.
Some time ago I decided to replace the pick guard on my vintage Guild acoustic guitar. I play it all the time, and it holds a lot of sentimental value for me. It was my very first guitar. I purchased it new back in 1982 in Northern California. After nearly 30 years of use, some minor maintenance and repairs were in order, including replacing the original pick guard.
I removed the old pick guard, cut out a new pattern matching the old guard (using new acrylic plastic) and applied the new one on to my guitar. That’s when I decided something needed to be done to prevent further damage to the lower portion of the sound-hole. I noticed over many years using a pick that this area had eroded away considerably. Bare wood from the soundboard was now exposed and growing downward from the sound-hole edges. Depending on the guitar manufacturer, there is usually a gap between the edge of the sound-hole and the beginning of the pick guard, which runs concentric to the sound hole. This arrangement exposes these sections to damage from even moderate use.
My initial concept was to develop a flat pattern that could be laid onto the soundboard with an overlapping piece that bent around to the back of the sound-hole. The development piece would need a pressure sensitive adhesive to adhere it in place. I used a 0.020 in. thick piece of Mylar for the original pattern, but soon afterward came up with a more permanent and stronger concept that works very well and is still on my guitar.
I designed a flat pattern out of 0.125 in. thick piece of polycarbonate, heat formed it around a fixture that duplicated the sound-hole/soundboard dimensions. This design acts as a clip that fits over the lower half of the sound-hole. I attached the prototype piece onto my guitar with some silicon rubber adhesive for it to stay in place. Although this prototype prevented further damage, I felt I needed to refine the design by reducing the wall thickness (the prototype appeared bulky) and to simplify the fit by eliminating the use of an adhesive.
I used SolidWorks to properly build and design 3D solid models. My first design incorporated negative draft on the two walls to act as a clip to squeeze onto the soundboard. I also designed three concentric ridge features on the inner wall to act as teeth to bite onto the soundboard, preventing movement and eliminating the need for the adhesive.
After converting to an .stl file, I was ready to shop for a prototype service that would build the part. After some searching, ZoomRP’s pricing seemed reasonable. Plus they offered fast turn-around times, and their on-line quoting system was convenient and almost immediate, within seconds after submitting the .stl file. I decided to go with the Poly Jet HD Blue process, which advertised the highest resolution, highest accuracy, and was specific to smaller prototypes.
When I received my first part, I was impressed by the accuracy and quality of the surface finish and the details of the very small teeth on the inner wall. My part would allow me to test for fit and function. The only problem was an interference issue, which I overlooked in the design process. The inside edge of the outer wall of the part was catching on the edge of the pick guard, preventing it from seating properly. The dimension of the outer wall of the part was too close to the location of the edge of the pick guard. So I went back to my SolidWorks design.
I also felt it necessary to play around with wall thickness and draft. Thickness of 0.100 in. still seemed too thick and the fit also seemed too tight. I did extend the front wall to fit over the pick guard and added a small radius extending the entire inner edge.
A couple of designs later I was able to fine tune all design concerns including the right amount of draft, wall thickness, overall length, and front wall length. This piece should fit and protect all acoustic guitars that have round sound-holes.
Bob Glass, Strum Hard