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Replacing Tuners
I made the decision to replace the tuning machines, because my current machines were not meeting my performance expectations. On one particular key, the machining had been worn down over time and would not hold the pitch. This "bad spot" on the key effected a half-step of pitch. To adjust for this, I would have to raise or lower my bridge. This problem had a bad habit of arising especially when I was traveling. Making these adjustments while traveling was terrible, becasuse it effected the setup of my instrument and impacted auditions I was trying to take. The most recent example was when I went to Chicago to audition for the Lyric Opera, and the night before my audition I had this problem. 
I have been curious in the mechanics of these machines for quite a while, so part of this project was to simply learn. While I understood the principle of how they worked, it is a very different thing to have hands-on experience.
1.Challenge No
  1. Title 1
The first challenge I had to overcome was dealing with the differences in the shafts of the old and new tuners. My old German tuners were fit with a tapered shaft and the Sloane tuners come as a straight shaft. To replace the tuning machines, the standard practice for most bass luthiers is to remove all of the existing hardware, plug the existing holes in the scroll, and redrill to meet the demands of the shaft. However, I few factors kept me from being excited by that option. 
Reason #1: I love the aestetic of my traditional German plates and was reluctant to remove them.
        Reason #2: I am not a luthier, and had no interest in drilling holes in my bass, or doing all the cosmetic work that was hidden        under the plates.
        Reason #3: Though it is the standard practice, I think plugging holes and redrilling is compromising to the strength of the scroll, and should not be undertaken lightly.
        Reason #4: I have access to an incredible metal lathe through Carnegie Mellon University, and believed it would be better to make adjustments to the metal instead of wood.

The challenge of choosing this method was that I needed to do precise metal turning on a machine I did not have much experience with prior to this project. In an effort to conserve the integrity of the new aluminum shafts, I chose not to make a full conversion to a tapered shaft. Instead, I measured out how much of the shaft would need to enter the wood at the end and only cut a taper for that particular section. This allowed me to leave a majority of the shaft intact and when inserted in the instrument, it essentially reads a straight shaft.

Tapered Shaft

Straight Sloane Shaft

Because I decided to leave the old plate, I also had to take away a ring of material away so that the gear mechanicism would protrude the proper amount. this ended up beginning an important feature, because it displaces pressure away from mortise and moves some onto the plate.

The shafts also all come in a default size, and must be cut to length. This photo is me trim the end. As you can see, the product is only brass coated with an aluminum core. This is designed to reduce excess weight.

2 .Challenge No
The second substantial challenge was how to make the connection between the instrument and the new machines. Because I had opted to keep the plates, I now could not use the wood screws provided to fasten the machine. After discussing with my shop mentor, Jon Holmes, we decided that it would be worth testing a machine screw connection between my old plate and the new machines. This would involve buying machine screws, and "screw tapping" the old plate for the connection. Before doing this on my instrument, I bought scrap brass of a comparable thickness and tapped it for the machine. I then had my friends tug on it to test its mechanical strength and no one could get it to budge.

The difference between these connections is that with a wood screw you make a pilot hole and then use the screw to displace the remaining material and that holds it in place. The distances of the threads allows for displacement with ease. However with a connection between two pieces of metal, both pieces are of more or less equal hardness and there is no room for displacement. This is what dictates "screw tapping", the hole must be prepared to receive the threaded shaft.

Because this screw was going into such a thin sheet of brass, it was crucial to get as many threads inside the sheet as possible. I chose to keep the thread count consistent with what they had sent, which was a 4-40 size. This size is part of a standardized system for threading.  

Wood Screw

Machine Screw

Ex. of threaded hole

Challenge #3

The final major challenge of this project was actually connecting the machine to the old plate. In the old plate, the key mechanicism was fastened in a way that was not intended to be taken apart. The pieces holding the key in place were made of steel and had fixed into place by heating the metal and  "mushrooming" the end into place. To remove the old keys, I made a jig to hold the plate, and then drill pressed the mushroomed metal off. From there, I was able to position the new machines, make markings and pilot holes, and finally "tap" my threaded holes for my machine screws. Once all of the connections were tested, the last step was assembly.