Concept: Take two threaded rods, lay them side-by-side, and intermesh their teeth together. A third threaded rod will also intermesh with the first two rods. If the first two rods are bend to form a circle, the rods will become the teeth of a worm gear. The third rod becomes the worm. I used Nylon threaded rod because they are flexible yet tough.
Construction: I used a 1/2-inch plywood between two sheets of 1/4-inch plywood. I used my router and a circle maker attachment. The two 1/4-inch sheets are 1/2 inch larger in diameter than the 1/2-inch sheet. This forms a natural groove for the threaded rods. I use #8-32 bolts to clamp the boards together in case I need to replace the Nylon threaded rods some day.
New Construction Note: My latest gears are made from two boards instead of three. I eliminated one of the outer boards. I still two different size boards so I can have a lip to secure the rods. The rods are attached with wood screws and nails. See the fourth picture.
How to attach the rods to the boards: One method that can be use is a steel rod as as a pin. I use a 1/16-inch steel rod from a hobby store to make the. I drilled 1/16-inch holes across the groove to hold the rods together. The other method is to use nails or wood screws at an angle to attach the threaded rods directly to the boards.
Recommendations: Use bigger rods for larger scopes. My smaller scopes use 1/4-20 rods and my 10 inch uses 3/8-16 rods.
P i c t u r e s !
Click on them for a larger image.
My new prefer way of making the gears and attaching the rods to the gear.
How big is my Az gear and is its gear ratio? The gear is just over 22-1/2 inches in diameter. The gear ratio is 1409:1.
How about accuracy? I did the laser pointer test. This test is simple. Take a laser point and attached it to the gear. Mark the spot at where the laser beam is pointing. I use a sheet paper taped to my garage door which was 19 feet from the gear. I use Mel's software to command the motor to move the gear 120 degrees. I repeated the command two more times to get a total of 360 degrees. The laser beam returned to the pencil mark on my garage door. I then repeated the commands but in the opposite direction. Again, the laser point returned to the pencil mark. I think the test was a total success!