Build Notes

The blender file for my clock model is here .

Gears

To make the gears I used gear.scad and gearlib.scad .

Some calculations from my notebook: 

- gears:       15T -> 45T -> 15T -> 45T -> 15T -> 75T
  reduction:   (45 / 15) * (45 / 15) * (75 / 15) = 45:1

- definition:
  [1] (diametral pitch) = (number of teeth) / (pitch diameter in inches) = (number of teeth) / ( (pitch diameter in millimeters) / 25.4 )

- or, rearranging:
  [2] (pitch diameter in millimeters) = (number of teeth) / ( (diametral pitch) / 25.4 )

- gears will only mesh properly if diametral pitches are equal

- for convenience, choose diametral pitch of 25.4mm

- then, using [2], gear sizes will be:

    gear    teeth       pitch diameter, mm
    ---     -------  -----------------------
  - small     15     15 / (25.4 / 25.4) = 15
  - medium    45     45 / (25.4 / 25.4) = 45
  - large     75     75 / (25.4 / 25.4) = 75

  - and center-center distances calculated by openscad will be:
    15T to 45T = 30mm
    15T to 75T = 45mm
To reduce gear mesh friction I rescaled the outside diameters of the medium and large gears by 0.985 using blender, leaving the small gears unchanged. Further changes to the profiles were done by trial and error as I learned more about how the gear train behaved under load.

The gears are run using 4mm screws as shafts. Each screw is supported by a pair of bearings, one at the front and one at the rear. Each gear is held in position with respect to its rear bearing by snugging down its screw. The front end of the screw is left free to "float" within the front bearing. This arrangement ensures that the gears will not bind up due to concentricity and alignment errors in gear faces, holes, and shafts.

Chain Sprockets

For the chain sprockets I used sprocket.scad with parameters obtained by trial and error to fit 2.4mm ball chain: 

  ball_dia   = 2.6; // diameter of the balls on the chain
  ball_space = 0.9; // distance between the balls on the chain
  chain_dia  = 1.0; // diameter of the chain between the balls
Then, to improve traction, I used blender to increase the ball hole depth by hand extruding the rim faces.

Ratchet Wheel

For the ratchet wheel I used ratchet.scad .
Then I shaved, cut, rescaled, etc using blender.

Bill Of Materials

I used these parts for my build: 

- (12x) 624ZZ 4x13x5 ball bearings
    - $10 "624ZZ Ball Bearing 4mm x 13mm x 5mm Double Shielded 624-2Z 80024 Deep Groove Ball Bearings (20 PCS)"
    - www.amazon.com/gp/product/B0932DQPGM
    - shields easily removed
    - nylon ball retainers prevent balls from falling out after shields are removed

- (4x) 4mm steel balls for ratchet
  - $5 "uxcell 4mm Bearing Balls 304 Stainless Steel G100 Precision Balls 50pcs"
  - www.amazon.com/gp/product/B07YKRT82C

- assorted M4 screws, washers, nuts

- ball chain
  - $4 2.4mm ball chain
  - www.hobbylobby.com/Beads-Jewelry/Bead-Stringing-Chains/Chains/Ball-Chain-Spool---2-4mm/p/154129

- gear motor
  - $8 "Antrader 30 RPM DC 6V Micro DC Geared Electric Motor GA12-N20"
  - www.amazon.com/gp/product/B07FYBQ7Z4

- motor controller
  - $12 "TB9051FTG Single Brushed DC Motor Driver Carrier"
  - www.pololu.com/product/2997

- limit switches
  - $12 "OctagonStar Endstop Mechanical Limit Switch for 3D Printer RAMPS 1.4(6pcs)"
  - www.amazon.com/gp/product/B01I1E84SG

- pendulum swing sensor
 - $6 "DAOKI 10PCS KY-003 Hall Effect Magnetic Sensor Module 3144 for Arduino"
 - www.amazon.com/DAOKI-KY-003-Effect-Magnetic-Arduino/dp/B07X97JXHT

- display
  - $20 "SparkFun PID 11442 7-Segment Serial Display - Blue"
  - www.amazon.com/SparkFun-11442-7-Segment-Serial-Display/dp/B01N5ORKQA

- case
  - (2x) 1 inch x 6 inch x 6 foot poplar board              [actual dimensions: 3/4" x 5-1/2"] (Lowes)
  - (2x) 1 inch x 8 inch x 6 foot primed finger joint board [actual dimensions: 3/4" x 7-1/4"] (Home Depot)
  - The poplar side boards are stained using Minwax "chestnut" and "red chestnut" gel stains (Lowes)
  - The front and rear boards are covered with textured flat black peel and stick wall paper (Amazon)

Tips

I haven't intended here to a provide a complete set of stl files and build instructions, but rather to share some ideas and experiences.

If you're thinking about building your own clock, here are some tips.

Start simple. Purchase and try out one of the JBV models to see if you can make it work. Do this before trying to build something more elaborate.

Typical ball bearings are shielded and packed with grease. This is good for skate boards but bad for clocks. Before installation, I removed the shields by prying them off with an Xacto knife and then soaked the bearings in a mixture of automotive brake cleaner and WD-40 until all grit and grease was gone and they spun freely.

The 3d printer must be carefully calibrated in x and y directions to ensure perfectly circular gears. I did this by printing a 100 x 100 millimeter rectanglar outline on the printer bed. Without removing it from the bed I carefully measured its dimensions with a set of calipers. I then adjusted the printer's "steps per millimeter" settings until I could print a perfect square.

Getting the entire gear train to turn without binding requires a fair amount of fiddling. I found that creating the gears with curved edge profiles allows their engagement to be fine tuned by sliding them fore and aft with respect to their neighbors. Clockmakers say "if it rattles it will run". This seems to be true.

Happy clockmaking!

--Derek

Last updated 02 April 2023