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Tuning Forks

   Tuning forks are useful for generating a reference tone, and can be quite loud depending on where the handle is laid after being struck. With a common Yamaha fork as a model, and some titanium gifted to me by a friend, I drew up some designs for a set of four different sizes.

   Each tuning fork would vibrate at a different frequency: E, A, D, and G, chosen by their appearance on the open guitar strings. With the length of the tines being the primary controlling parameter for the vibration frequency, preliminary calculations on the tine dimensions were important. The strategy was simple: determine the theoretical length for the tines, add 1/4" extra, and then slowly remove it later as necessary.

   The real challenge was to make the tuning fork handles attractive by joining sections of stainless steel, brass, and titanium. Each handle would then be attached to its own "U" shaped stainless steel tine. As a final touch, the titanium components were to be anodized.



   Since each tuning fork handle was to be made of three sections, I chose to sandwich Ti between different combinations of SS and brass, and set to work. Tapping 6-32 holes down the center of each section allowed everything to be joined on a small and hidden piece of threaded rod.

   Having only turned titanium previous to this, I learned that Ti gets angry when being drilled or tapped. As a result, the Ti sections were made to be a a lot thinner than the other sections.


   The sections closest to the tines were given a concave shape to accept the tines during soldering. As I was threading everything together, I regrettably broke one the 6-32 rods off inside of a brass section. Some say that 6-32's have the poorest diameter-to-strength ratio, and they may be right! Down to three forks...


   Now the handles could be turned down into their final shape. This part was cool because any continuous cut along the length of the handle would pass through three different metals.


   Polishing was next. I didn't want to spend too much time on this with the prospect of the subsequent soldering operations fouling everything up.


   Turned around, I polished the handle ends to a shine.


   These are the finalized handles. The next step was to bend the tines in preparation for brazing.


   The handles were bent in a table vice using two pieces of 1/2" steel tubing as human operated levers and a little heat from a torch at the bend. To get the bend really nice and tight I used the table vice itself. It was a fairly crude setup but produced acceptable results.


   Here is a picture of my brazing setup. This was technically silver brazing (not soldering) because the melting point of my solder was 1300 F. As a rule, everything below 800 F is considered soldering. Because the base materials are not being melted, this is not welding.

   With some flux between the handle and tines, the parts were visually aligned as best as possible before heating. Approximately when the metal began to glow, the flux turned into a clear liquid, and I dabbed in some silver wire. Having done some practice pieces, these tuning forks turned out OK. One was great, another was satisfactory, and a third was OK but I felt that any attempt at improvement would be too risky.


   Sadly, the only remaining turning fork which incorporated brass had some kind of air trapped inside from the 6-32 threading stage. While brazing, the heat caused the section to expand like a little balloon. It also had some issues during brazing, resulting in a joint which was questionable at best. I continued, thinking it might be salvageable...or at least useful later on.

  It was now time to tune the forks by shortening the length of the tines slowly on a grinder. The approximate target lengths were determined by calculation previously.


 
   An ordinary guitar tuner (shown) can but used for accurate tuning. I used that, in conjunction with an online tone generator to verify that I was at the correct octave, and later for generating a 'beat frequency' as the tuning fork approached its target tone. A beat frequency can be observed when two very similar (but unequal) simultaneously played frequencies seem to slowly fall in and out of phase with each other. I used my wristwatch as a timer to estimate the beat frequency. Keeping track of these estimates, I continued grinding until the beat frequency was sufficiently close to zero. Of course, I only had to go through this trouble for the notes which my digital guitar tuner had trouble with.


   So here is the tuning process which is occurring simultaneously with the grinding. I settled on making forks with extraordinarily long tines and which generate a lower tone. I also think that the longer tines look nicer.

   The aforementioned fork with the poorly brazed joint and the inflated 'bubble' actually struck a note so sour that it came apart. This one was already borderline usable, and so it was nice to have the verdict come in. Down to two forks...

 
  Using a dremel-like air grinder, various soft wheels, and some polishing compound, these forks were done. Polishing took a long time.


   The final step was to anozide the small sections of titanium. I had done this before on some titanium rings and created brown, blue and purple colors.

   Voltage determines the anodizing color. The positive lead from the power supply is connected to the forks. When immersed in cola (used for the phosphoric acid), a current between it and the cathode (a piece of aluminum with a negative lead attached) sets up the conditions for anodization.


   Bubbling occurs in the soda just above the cathode, and in a few seconds, the whole process is finished.


   Before anodizing...


   After anodizing...

   The bronze colored section was produced at approximately 15V, and the purplish section required something more like 35 V. My power supply was limited to 30V and I ended up using a series connection of 18 AA batteries. If I had more batteries I could have hit blue. I believe green appears somewhere around 100V.


   These tuning forks are done. They turned out better than expected, despite being the only two out of the original four that made it through manufacturing.

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