Editor: Our Technical Chief takes a close look at the Bulova tuning fork as an interesting engineering feat to create a different kind of watch movement. Inspired by a vintage Spaceview she picked up recently, follow her as she delves into the mysteries of this amazing watch.
A bit of history
“It doesn’t tick. It hums.” That was the advertising line in the 1960s. When the Accutron was first released, the Spaceview was simply a marketing display design to give customers a glimpse into the working concept of a tuning fork movement. But it garnered so much interest that Bulova quickly decided to turn that design into a production piece, and named it the Spaceview.
The name could perhaps be traced back to its earlier applications on Spacecrafts. Even before the first consumer wrist watch was introduced, Bulova has been serving NASA programs since 1958, providing the same Accutron movement as a timer aboard communication satellites. The most notable was the use of 3 Accutron devices on the Gemini II mission; and the Accutron timer on the seismometer on Apollo 11 which marked man’s first landing on the moon. The timer marked man’s first step on the moon as 2:56:19am GMT, July 21, 1969. While clearly a choice as an astronaut’s wrist watches, it surprisingly did not pass all the tests to outdo the Omega Speedmaster which eventually became the Moon Watch that we know today. However, there have been evidence that a Bulova chronograph (not Accutron) wrist watch did make a landing on the moon during Apollo 15 and on subsequent missions, an interesting story that you can read more from here.
Tuning fork movement started development as early as 1940s, focusing on adapting it for space use, before introducing the movement as a consumer wrist watch. It was marketed as Accutron by Bulova, meaning Accurate Electronic. As its name would suggest, it was indeed the most accurate watch of its time, boasting a guaranteed accuracy of 1 minute per month and usually achieving accuracies less than 5 seconds a month. This was of course overtaken by the quartz when Seiko unveiled the Astron in 1969.
The movement is fantastic. To appreciate how the tuning fork and the copper coils interact, we have to go back to some high school physics on the topic of electromagnetic induction. We have to recall Faraday’s Law of Induction, which states that when the magnetic flux linking a circuit changes, an electromotive force is induced in the circuit proportional to the rate of change of the flux linkage. The force that is induced would oppose the direction of motion that caused the initial change in magnetic flux. We also have to know the basic functions of transistors, capacitors and resistors.
The battery drives a current through the 2 main coil assemblies as well as the rest of the electronic circuit. When the coils have a current going through them, it causes a magnetic flux to build up and is amplified by the core within. Faraday’s Law and Lenz’s Law would say that this changing magnetic flux would create an electromagnetic force to move the tuning fork’s cups. The moving prongs will cause a change in the magnetic flux which then drives a change in direction of current in a secondary copper coil that’s housed within one of the two main coils. The secondary coil’s “reverse” current direction would then tells the transistor to switch off. As the transistor switches off, the fork springs back to its original relaxed position. This “relaxation” again causes a change in direction of current in the feedback coil, which signals the transistor to switch on again, re-starting the motion of the fork. This alternating current in the feedback drives the to-and-fro motion of the tuning fork, and the speed at which it springs back and forth is highly dependent on the fork’s stiffness properties.
The description seems like there is a long chain of events and a slow movement going on, but all this happens 360 times per second. Most balance wheels work at 4 cycles per second, putting the tuning fork 90 times faster than most other mechanical watches. The time-keeping element of the movement is also the only electrical component in the movement. The rest of the movement is entirely mechanical, and has shown to be extremely impressive even by today’s advanced watch-making standards.
The Tuning Fork
A tuning fork is almost always used for musical purposes. When struck, the prongs vibrate at a specific frequency to emit a pure pitch. That’s also why this watch hums, it doesn’t tick. On the spaceview, all but the core can be seen. The fork is also made of Ni-Span-C, which is a Nickel-Iron-Chromium alloy that has near-zero thermal expansion coefficient and very stable stiffness from about 0°C to 100°C. This keeps the vibration going at the same speed over most of watch wearing conditions. For a slow motion video of the vibration of the fork, check out this video on YouTube, by Taofledermaus. The video also gives a detailed explanation of the gearwork and components in the video.
Pawl and Index Fingers
To transfer the linear vibrations to a rotary motion to turn the hands, a ratchet and pawl system is used. One long and ultra fine index finger is attached to one of the prongs, another equally fine pawl finger is attached to the main plate and stays stationary, both fingers with micro-miniature jewels attached to each end. The vibrations then move the index pawl to push the index wheel one tooth at a time, and the pawl finger is there to stop the Index Wheel from going backwards when the index finger retracts. The jewels on these fingers are the size of the Index Wheel teeth, no more than 10 to 20 microns big. I am not entirely clear how it is possible to use traditional jewel making techniques to produce such a small and accurately sized jewel.
The Index Wheel
To me, this is the most impressive part of the watch. Measuring just 0.095” or 2.41mm in diameter, with 360 teeth cut into its circumference, this is an insanely small and precise piece of work! I’ve always marvelled at how watchmaking can miniaturize components and gear, but this is taking it to a whole new level. Each tooth is no more than 20 microns (0.02mm) wide. For comparison, a human hair is about 100μm wide. High end CNC machines of today boast an accuracy of 20-25μm, which is certainly not enough for the Index Wheel. Precision manufacturing methods that could produce parts with accuracy of a few microns are limited to electroforming and perhaps very good laser cutting. The size of the Pawl jewel and Index Wheel really put our modern watchmaking technologies to utter shame, and to think that they were manufactured in the 1960s. Until now, the manufacturing process of the Index Wheels and Pawl fingers and jewels are not disclosed, leaving us to make wild guesses of how they were made.
The two coils prominently displayed near the 11 and 1 o’clock. Each coil has 8100 turns, or about 100m of insulated copper wires to form a solenoid. The copper wires used here are yet another example of precision engineering. Typical copper wires used in electronics only go down to 0.08mm, and these are not commonly produced as wires for household appliances are usually 1-2mm thick. These wires are just 0.013mm diameter thick and Bulova had to draw these ultra-fine copper wires in-house with a series of special diamond dies.
The Case, Hands and Dial
This example of Spaceview uses the Accutron Cal 214. The case measures 34mm, a great size for my wrist, though I can imagine most men preferring a larger size. The main difference between the 214 and other Accutron calibers would be the case back and crown. In the 214, the crown is hidden at the back, by flipping up a D-ring. The battery hatch can also be opened without opening the case back. Later calibers such as Cal 218 usually has the crown at 4 o’clock and entire case back needs to be opened to access the battery.
There isn’t much of a dial to speak of on the Spaceview. In fact, the lack of a dial is the highlight of a Spaceview. Even today, there are watch shops providing “Space Backs” to replace the case back of dress Accutrons with a clear one, to easily admire the unique movement. The underside of the crystal has a screen printed logo. In some Spaceview examples, the hour markers were printed onto the underside of the crystal. Because the dial movement has got so many colours, the hands on the Spaceview were designed white and broad for better legibility. The white lume-filled hands are also one of the sure ways to tell if the Spaceview was an original Spaceview or a modified one, as most other dress Accutrons had gold or silver hands with a dressier style.
In the late 1960s, accutron’s designer Max Hetzel left Bulova for Ebauches SA and went on to design the ESA 9162 which was largely similar to the Accutron 214. This movement was sold to many Swiss brands including Omega (Speedsonic) and Longines (Ultronic). Almost all of these watches were designed as dress watches and came to close competition with Bulova Accutron. It was still quite a far competition as the Swiss brands carried a premium for their branding. Omega went on to develop their own tuning fork movement for the MegaSonic f720Hz which would become the only one to use an asymmetrical tuning fork with a single coil, and a new magnetic clutch train to replace the ratchet and pawls to eliminate the wear and tear problems. These watches were barely considered as competitors due to the stark difference in price. Bulova Accutrons were selling in the low hundreds, while the Omega MegaSonic f720Hz retailed for a few thousands. Despite all these tuning fork watches that joined the market, the real competitor appeared in the 1970s, when quartz watches proved to be cheaper and more robust.
In today’s market, the Accutron is a vintage collectible, although some may find it undesirable as Bulova has long stopped production of spare parts. The recent release of the Electrostatic Accutron Concept Movement certainly displays large similarities with the Tuning Fork Accutron from the 1960s. The construction and movement also uses electromagnetism to drive the movement, but the working principles is quite different. Stay tuned for a detailed review once we have our hands on it!
This is an irreplaceable and very unique piece of time-keeping technology. It is an engineering marvel, and has set some seriously high standards of precision and miniaturization engineering.
Even decades after its production, we don’t see many watchmakers who can make parts to such resolution, accuracy and robustness. It is a fantastic time piece, at an affordable price point, and I’m glad I have at least one such hummer.