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Gina's Giant Wall Clock

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Apart from the size, being 3D printed and powered by Arduino and stepper motor, the main difference with this clock is the gear train.  This uses "epicyclic gearing".  I discovered it when I was doing an online search for wooden clocks which also fascinate me.

The principle involved is that when a pinion is moved round two spur gears on the same axle, with just one tooth difference and one gear is fixed, the moving gear rotates by one tooth for each revolution of the axis of the pinion. So if the fixed gear has 59 teeth and the moving gear 60 teeth the gear ratio between the rotation of the pinion axle and the moving gear is 60:1. This is the ratio required for seconds to minutes in a clock.

When the gears are different by two teeth the moving gear turns by two teeth for each revolution of the pinion axle thus if the gears have 22 and 24 teeth the gear ratio becomes 24:2 = 12:1. This is the minutes to hours ratio.

Close up of the gears.




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This photo shows an early version of the clock.  The dial and hands looked overpowering in reality so I changed to a more streamlined design as in the second photo.



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This is not the end of the story - there were a couple of difficulties.  Firstly, setting the time.  Although the clock could be advanced by turning the seconds wheel, this would have taken a long time so a slipping clutch was needed between the gear drive and the minute hand and gear.  Secondly, a more technical little problem.  The stepper motor takes 200 pulses per revolution whereas the RealTimeClock module used to make the clock keep time, worked in powers of 2.  Initially, I used a pair of printed spur gears but these were noisy so I abandoned that idea and put my mind to a programming solution.

I will continue to explain how I designed and produced the slipping clutch and maybe post the code for the Arduino and circuit diagram.

To be continued...

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  • 6 months later...


There were two things I mentioned earlier that needed sorting out.  A slipping clutch system for setting the time and getting rid of the gears.  Also, the stepper motor was quite noisy when fed from the A4988 stepper driver so I changed this for the TMC2100 which makes stepper motors very quiet when used in a special mode called "stealthChop".


Now to the gears :-  The 25:32 gears that provided the 1 second drive to the seconds wheel were far from quiet in spite of being accurately printed.  There are possible ways of improving the gears such as helical teeth but this mainly applies to machined gears.  Printed helical gears will be rough due to the layers.  Consequently, I looked into the possibility of just using code in the Arduino sketch.


The RTC module is capable of producing square waves of 32kHz, 8192Hz, 4096Hz, 1024Hz and 1Hz.  The stepper motor has 200 steps per revolution or 3200 steps/rev with 16x microstepping.  The TMC2100 only does "quiet" in 16x or 4x microstepping.


One thought was to apply a time correction every so often to keep the clock in time.  ie. using the 32KHz (actually 32768Hz) gives a ratio of 10.24 for 3200 microsteps per second so a correction of 24 counts per 1000 or 3 in 25. 


Another thought was to generate pulses controlled by the Arduino clock such that a number of them finish just short of the fixed time.  eg. produce 3200 pulses in just short of a second and use the 1Hz square wave to restart the sequence.  The 1Hz is a symmetrical square wave so timing can be taken from both edges resulting in two smaller gaps in the pulse train to the motor rather than one larger one each second.

Edited by Gina
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Final sketch.  Had to use a screenshot to keep the colour coding as this forum doesn't support code mode.  Click on it to expand.

Edited by Gina
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Gradually finding parts of this project.  Sorry this thread is rather disjointed.


This is the block diagram.  The A4988 stepper driver was replaced with the TMC2100 quiet driver.  Oh and the power is 9v at 200mA.609231992_Screenshotfrom2018-11-1113-26-49.png.5dbc8ad84760e8507d0d2def51641ceb.png

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Found an early example of the slipping clutch. 


Part way through the build with clock parts on the floor.  A version of the slipping clutch can be seen (if you know where to look 😁, see below). 



Here's a blow up of the relevant part of the image.  The light green printed part will have the minute hand attached, while the white inner part is spring loaded and rubs on the inside of the blue gear.  This means that the minute hand can be manually moved with respect to the blue gear that normally drives it.



The other end of the light green part has the (purple) pinion that drives the hours, as shown below.  So the hours follow the minute hand whether driven by the clock or changed by hand.



Minute hand attached.  This minute hand was later replaced by a more elegant slimline version.


Edited by Gina
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