My Arnfield Pendulum Clock

This is a 3d printed clock I built in early 2023.

It was inspired by a model I spotted on the website of a company called JBV Creative . It was a simple bare escapement designed to be wall mounted and hand wound. The owner sells stl files for his models so l purchased a set for the princely sum of $8 CAD and printed them out on my Creality Ender3 printer. To my surprise the little mechanism ran quite well using its simple drum and threaded weight. It had a run time of a few minutes.

I decided to see if I could extend the run time by adding a gear drive, ratcheted winding sprocket, and ball chain. One thing led to another and I eventually wound up building a post-modern "long case" style grandfather clock. It was intended as more of a kinetic sculpture rather than for telling time. A 48 inch pendulum yields a stately 2 second swing and the Arnfield escapement generates a distictive asymmetric tick tock.

When connected to the mains via a 5V usb power supply, a hidden gear motor and a pair of limit switches automatically rewind the drive weight as needed and the clock runs unattended. An ESP32 microcontroller monitors the limit switches and runs the motor. While this is massive overkill for such a simple job, it's also a ridiculously cheap ($15) and easy to use solution. As a side benefit the ESP32 comes with builtin wifi so I can update the software over-the-air from my armchair as the clock continues to run undisturbed.

I eventually decided to make a proper time telling clock by adding a swing sensor and small led display.

The initial time of day is set at power-on via wifi using an internet NTP server.

The pendulum then runs freely, measuring elapsed time by counting pendulum swings using a magnet and hall sensor. and outputting the result on the led display.

The intent here was to maintain the spirit of a gravity powered pendulum clock but with a convenient way to initialize the time without turning any dials or clock hands. Sort of a "Galileo meets the internet" approach.

The escapement is fascinating to watch.

The Arnfield mechanism allows the pendulum to swing freely regardless of irregularities in the power delivery from the drive weight through the gear drive to the star wheel and finally to the pendulum.

This is especially effective in the case of 3d printed plastic gears running on simple screw axles turning in cheap steel ball bearings. All of these components introduce variations in concentricity and meshing which can clearly be heard during the "tock" phase as the escapement turns.

Notice that the left arm only contacts the pendulum briefly when its latch is released and the arm drops to give the pendulum its push. This is the "tick" phase.

As the pendulum continues on its way, the left arm strikes the right arm, causing it to release the star wheel.

As the star wheel turns, it raises the left arm's latch back to its upper position, ready for the next pendulum swing. This is the "tock" phase.

Some clever geometry and timing ensures that the star wheel only advances 1/6 of a rotation for each pendulum swing.

The resulting accuracy of this type of escapement is very impressive, keeping time to with 2 or 3 seconds per day (this is a plastic clock!).

I designed everything with blender and openscad using the original JBV stl files as a starting point. This was by no means a "press the button to print then assemble" project. Getting a 3d printed clock to run reliably is difficult. Friction is the enemy and eliminating it enough to enable a clock mechanism to run is not easy.

Here are some notes describing my build.