Picking up from Part 2, the third and final part of the wood click gears with motor drive build covers the stepper motor drive. The gears ended up driving this really beautiful clock. Note that I did not build the clock, just the gears driving the hands.

This part of the build was fairly simple since it was just a matter of driving a stepper motor at a constant speed. I also included a switch to be able to reverse direction.

Block Diagram

The drive system is extremely simple as shown in the block diagram. I did not create a schematic for the build and just used the block diagram for reference instead. The circuit was powered by a standard 5V regulated wall charger.

Stepper Motor and Driver

Since the project didn’t require a large motor, I used the 28BYJ-48 stepper stepper motor with the ULN2003 driver. This is an extremely popular stepper/driver combination with a great deal of documentation available online.

ATtiny85 Microcontroller

I opted to use the ATtiny85 microcontroller since I only needed a few pins to control the stepper motor driver. The ATtiny85 has 5 usable I/O pins — I used 4 for the stepper driver and 1 for the forward/reverse switch. Lastly, it is largely Arduino compatible so many library work without modification.

Code

The code (included below) was fairly straightforward — there is nothing fancy happening and I used the AccelStepper library. It is able to run the motor more efficiently than the default Arduino stepper library and includes a half-stepping mode, which is recommended for this motor.

Here is a great tutorial on using the Arduino Uno to program the ATtiny85. As a side note on the tutorial, you can get away with programming it without using the capacitor. 

// Simple program to drive a stepper at a constant speed
// using an Attiny85
// Paul Bupe Jr

#include <AccelStepper.h>;

// Defining some useful constants
#define HALFSTEP 8
#define MINUTE_STEPS 1.13777
#define SECOND_STEPS 68.266
#define SECOND_STEPS 115

// Motor pin definitions
#define motorPin1  0     // IN1 on the ULN2003 driver 1
#define motorPin2  1     // IN2 on the ULN2003 driver 1
#define motorPin3  2     // IN3 on the ULN2003 driver 1
#define motorPin4  3     // IN4 on the ULN2003 driver 1

#define dirPin     4     // Input for Clock Direction

bool last_dir = HIGH;
float step_speed = SECOND_STEPS;

// Initialize the stepper library in half-stepping mode
AccelStepper stepper(HALFSTEP, motorPin1, motorPin3, motorPin2, motorPin4);

void setup()
{  
   pinMode(dirPin, INPUT);
   stepper.setMaxSpeed(1000); // Arbitrary max speed 
   stepper.setSpeed(step_speed);	
}

void loop()
{  
  int dir = digitalRead(dirPin); // Get direction from switch
    // Only executes if the direction changed
    if (last_dir != dir) {
      if (dir) {
        stepper.setSpeed(step_speed); // Clockwise
        last_dir = HIGH;
      } else {
        stepper.setSpeed(-(step_speed)); // Counter Clockwise
        last_dir = LOW;
      }
  }
   stepper.runSpeed();
}

Putting Everything Together

With all the parts in and program written, I did some quick prototyping just to make sure everything worked as expected.

I then designed a quick mount in SolidWorks and printed it out using my 3D printer.

Then it was just a matter of crimping a few wires and soldering everything together.

After that I tested everything out then mounted it on to the back of the gear assembly.

Finally a quick test with everything assembled and it was good to go!

The post Wood Clock Gears with Motor Drive – Part 3 of 3 appeared first on Paul Bupe Jr, PhD.

Cutting the Gears

Since I designed the gears in SolidWorks, it was just a matter of exporting them as .DXF files and cutting them out on a laser cutter. I used wood approximately .2in thick which was sturdy enough for my needs.

Gluing Everything Together

As shown in the drawing from  Part 1, the 10 teeth gear is physically coupled to the 40 teeth gear while the 12 and 36 teeth gears share the same shaft but are free spinning. The 12 teeth gear controls the minute hand while the 36 teeth gear controls the hour hand.

Gears glued together with bearings

The 36 teeth gear has a hollow tube with a .25in inner diameter through which the shaft from the 12 teeth gear (the minute hand) goes through. I used quick setting epoxy to glue everything together.

The 12 teeth gear will be directly coupled to the motor so I fit it with a .25in shaft. I also made a D cut into the shaft to help with the coupling to the stepper motor.

Assembled Build

The post Wood Clock Gears with Motor Drive – Part 2 of 3 appeared first on Paul Bupe Jr, PhD.

I was asked to design and build the gear and drive mechanism for a 6 foot clock. The clock had an hour and a minute hand and needed to move in both the clockwise and anti-clockwise direction. This was not a “realtime clock” and needed to move visibly fast.

I decided to use wood clock gears with a motor drive for the sake of simplicity. This build was definitely more out of my comfort zone than usual but proved to be a fun and educational experience. Part 1  goes over the design of the gears. Part 2 will cover the physical build and Part 3 the electrical design and final build.

Preliminary Design

The design called for a minute and hour hand so the target gear ratio was 1:12 (since it’s a 12 hour clock). A clock requires the output shafts to be aligned, meaning I needed to create a compound gear configuration of four gears with two sharing a shaft each. The shaft spacing between each gearset needs to be the same. I chose to use spur gears since they are the most appropriate for my design and are also very easy to manufacture.

Calculating Gear Ratios

I set the minimum number of teeth to 10 since I only wanted to use four gears and based on the scale of the design. This made the gear ratio 10:120 (which is still the same). I then multiplied 10 by the square root of 12 (since that’s the gear ratio I’m targeting for each) to yield a new compound gear ratio of 10:35:120, which can be represented as 10:35 and 35:120.

Because I didn’t want to deal with a 120 teeth wooden gear, I needed to find a common denominator with which to divide the second set of gears. I bumped up the 35 to 36 and divided the right side by 3 to yield a final result of 10:36 and 12:40. This yields a gear ratio of 1:12

Gear Design

Up to this point I had the following information:

• The smallest gear has 10 teeth.
• Both gear pairs needed to have the same center distance (C).
• I wanted the pitch diameter (the effective diameter at which two gears mate) of the smallest gear to be around 1.5 inches.

The following image from Groschopp is a useful reference for gear terms and meanings.

The most important value I had to calculate was the module (m) of the gears. The module is the ratio of the pitch diameter (d) to the number of teeth (N):

Two gears must have the same module in order to properly mate. With a pitch diameter of 1.5 and 10 teeth, the module of the first gear is . Using the module formula, the pitch diameter of the second gear has to be . The center distance, C, is given by

which evaluates to . Using this information and the fact that the diametral pitch is the inverse of the module, the rest of the gears were calculated and designed in SolidWorks. In the final design I ended up increasing the pitch diameter to 1.52in.

Notes

At this point I’m fairly happy with the design. Since I’m designing wood clock gears I needed to make sure everything was scaled large enough to handle the stresses of the clock hands. Unfortunately I don’t know just how heavy the clock hands will be so the design assumes “reasonable worst case” scenarios.

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