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Designing a New Bridge

by Robert McNally October 12, 2017

Designing a New Bridge

Simplicity has always been a hallmark of the design of the Strumstick, with the economy and elegance simplicity brings. But we do bring decorative touches in here and there, like the Rosette soundholes for example.

I have wanted to design a bridge for the Strumstick (the little piece that holds the strings up over the soundboard) with more style to to it. The existing bridge is simple to the point of plainness. There was no problem designing bridges that were more elaborate and stylish; the sticking point was getting more flair without adding significantly to the price of the Strumstick. Here I'll describe the process by which we arrived at a new bridge design that improves the sound of the instrument, looks really great with the Strumstick shape and is cost effective.

The first things I thought of were adding detail to the existing bridge, by having the top swing down to the bottom with a concave curve instead of a convex one. Somewhat like the feet of a banjo bridge.


Top: Original Bridge   Bottom: Banjo Style Cutaway

 

The problem with the "banjo-cutaway" design was two-fold;
• Sanding concave curves in tiny pieces of wood is time consuming, hazardous, and tedious. 
• It was not a very original design for the instrument.
(Many the shapes I came up with had those at least one of those two problems.)

Lets think about everything a bridge has to do:
Bridge Functions:

It lifts the strings a measured distance above the fretboard for easy fretting without buzzing.
It transfers vibration to the soundboard efficiently.
It needs notches to space the strings evenly from one another.
it has to be stable (not prone to tipping over).
If it's too wide, the sound will be weakened, ditto if it is too broad and heavy.
If it is too thin it will be fragile and prone to tipping over, and if too fat it will look clunky and diminish the sound. 
The angle at which the strings cross the bridge make it want to tip over forward (toward the soundhole) more than rearward, toward the big end of the instrument.
The Strumstick is modestly priced, making the bridge cannot add substantially to the cost.


Old Bridge

The original Strumstick bridge has a trapezoidal cross section that slants down on the front side, to create a stable base, and is no wider than it needs to be, is thin at the top, thicker at the bottom (stability), and notches are hand filed with a gang file to space the strings. 

How the bridge would be made is part of the design process. We considered CNC machines, injection molding with appropriate resins, 3-D printing, and laser cutting.
I had access to a laser cutter so I bent my efforts in that direction. Laser cutters are 2-axis cutters, not 3-axis. That means they can move back and forth and left and right, but they only cut straight down, not at an inclined angle to the material they are cutting. That posed a challenge for tapered cutting.

Stability was a key consideration. Gluing two pieces of wood together to make an "upside down T" makes a nicely stable bridge with a wide base, but is costly to do. The cut taper I had been using (which is rather like an "upside down T" in function, but neater looking) works well, but a laser cutter laser could not cut that shape with the slanted face.

So the mechanical problem became, how to have the wide base stability of an upside-down-T without either gluing or tapering?

I figured the laser could cut a third axis if I cut a piece from above (a top view), laid it on its side, and cut it again. Tapers were not possible, but I could make shapes that were not just 2-D outlines that way.

What if I made the bridge too thick and bulky, but cut away the excess on most of it? In the sketch below, the uppermost outline is the top view, and lowermost are the views from the front and side. The wide ends give tip-over stability, the narrow center section gives lightness and a thin base of support for the strings, and the front outline shape is more elegant than our original bridge. I thought of this as the Dogbone design.


The Dogbone Bridge design

 

This picture below shows a crude bandsawn test of the concept. It worked!

The Dogbone Bridge design rough cut

This was a start, and gave a physically working design, but needed to be more styled to be worth the trouble. I looked at various end shapes


Various Dogbone shapes

I liked the design with curves at the end. To lighten it more I brought back in the banjo bridge cutaway ends. In the picture below, red is top view, black the front outline. The second pair have some shape lines drawn, and also show the notches being added. And a little "M" as a logo.

bridge design

Various Dogbone shapes refined

 

This yielded an interesting sculptural shape; it has intersecting curves in two planes, so it makes a third curve at the intersection; It has shape variation, without departing from the essence of it's functions. But still, even with the curve cuts, I felt it lacked the whimsical feel the strumstick shape embodies.

Then, as it happens when you spend time with an idea and start to dig into it's nooks and crannies, I realized something; I might not really need the wide ends on the back side of the bridge, only on the front. Thats because the angle of the strings over the bridge makes it more inclined to tip over forward (toward soundhole). I made a crude mockup from an regular bridge to test the concept.

bridge
regular bridge cut for "forward" feet

 

And that shape looked like a "U" from the top, which made me think of a curve compared to a blocky shape. That opened up a world of possibilities. I made sketches and then a third rough prototype.


"crescent" bridge rough cut

Lo! It worked great! It was stable, light thin but sturdy. The crescent curve echoed the curve at the end of the Strumstick. I started wondering if an S curve might work (only one support at the front?). As I pondered, my daughter the artist came over, glanced at my sketches and without any discussion at all said "here's what you should do" and dashed off an S curve above my crescents. 

Of course I had to try it. Another trip to the bandsaw, and...it worked great, looked great, and sounded great. Now I went to the computer and drew a whole series of S shapes, forward and backwards "S", on three spaced lines to represent the string crossings.


many "S" variations

 

Of course, another hybrid crept in, mixing curve shape and the wide end approach. This picture shows the shape, and how the strings would align with it. One Drawback was that it looks just like the Bass Clef sign from music notation, only backwards.

 


"Eyebrow" Crescent shapes

 More sketches of "S" Curves and Crescents followed. 

More shape refinements

 

From these I chose my favorite candidate, a forward reading "S" or "Wave" shape. Using the "mountain" shape for the profile made the most sense with the "S" shape; if the ends were cutaway, they would be exceedingly fragile points, just where strength was needed. The whole design was so slim the weight was fine. The line of the string contact points has to slant across the top, it is not perpendicular to the line of the strings. That is for correct intonation. I was able to work it out nicely that the width of the S and the slant of the middle section cross the string lines at exactly the slant across the strings I wanted. A reverse "S" would slant the opposite way. A lot goes into designing anything, as you go deep.

The Chosen Design!

Now I had to figure out exactly how to laser cut it, because I was not going to bandsaw thousands of tiny bridges!

I decided to cut the front view profile including the notches. That would create the several different height bridges we use, with notches pre-cut. Then I would turn it on it's side, as cut (onto the bottom of the final bridge) and cut the "S" curve.

I had to carefully align the cut profile piece for the second cut so the laser would cut the S without going outside the top of the profile. Using the laser cut a series of rectangular openings that exactly fit the profile in another piece of wood created a tray to place the profiles in, precisely located so the "S"s would be in the right places. This all worked, thanks to the accuracy of the laser cutters locating system. I wound up with a piece that looks like this:

 

which separates into:

 

Here below is our new Strumstick bridge, which now comes standard on most Strumsticks, at no additional cost! It is also available if you would like to add one to an older Strumstick (Wave Bridge). We are really pleased with the design, I believe it sounds better, looks better, is more stable and distributes the strings vibration over a broader area of the soundboard. 






Robert McNally
Robert McNally

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