Gear Making

worm spur-gear bevel gear (1)

There are several types of gears that have their own benefits in various situations. For my telescope, I required a gear system that

  • Would greatly improve the precision of the stepper motor I was using. The telescope can be said to be very precise in positioning, if the least count of the motor’s motion is very small. The motor used without a gear, has 200 steps per revolution, ie a least count of 1.8 degrees. However I needed a least count less than 1 min (ie 1/60th of a degree). Thus I needed a gear with a large gear ratio(min. 108:1)
  • Would prevent the optical tube from rotating due to its own weight. When the motor is not powered up, the Optical tube should be held fixed in the same position because of the way the gear is attached. It should not constantly require power to hold it in place.
  • Does not have significant backlash error. Spur gears for example have a significant backlash error. So those are out of the question.
  • Is easily to install and easy to disengage from the motor. This eliminates belt-pulley systems and friction drives. I knew that I would not always use the telescope as an automated telescope, so I needed to make arrangements for its manual use. This point clashes with point#2 as manual movement is the same as unpowered rotation of the telescope (under gravity).

Keeping this in mind, I decided to use a worm-gear system with a large gear-ratio (~250:1) for both axes.

I searched for the appropriate worm-gears in hardware stores, industrial supplies stores, auto-parts shops, and even scrap stores. I even consulted various people in this line of business, asking them where I might find the right kind of gears, but in vain. I took this up as a challenge and decided to make my own worm-gears.

Mr. Abhay Deshmukh, CEO of Logical Solutions, permitted me to use the machines in his factory to make the gears. I had a vague idea of how I wanted to make the worm-wheel. A worm-wheel was like a disc, with threads on its circumference.  After looking at video tutorials online about tap-drills and how threads are made, I realized that I could make the gear on a milling machine.

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I would attach a tap-drill bit on the drill chuck of the milling machine, mount the gear blank on a shaft, hold the shaft in a vice using ball-bearing in such a way that it is free to rotate, and using the tap drill make threads on its circumference. This is like trying to make threads in a hole, which was sliced open along the axis, a kind of one-sided thread making. For the worm, I would use a threaded bolt with pitch matching that of the tap-drill.

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Using this method, I made 2 worm-gears, one for the altitude axis and one for the azimuth axis. The altitude axis gear has a gear-ratio of 290:1 and the azimuth gear has a ratio of 256:1.

Lessons Learnt:

This process of making gears was very informative and enjoyable. I learned how to operate a lathe machine and a milling machine. I learned about tap-drilling, hobbing machines for making spur gears, and worm-gear making. Working with the engineers in the factory, I learned many new and useful things, and tips and tricks of the trade. I also started appreciating the importance of proper planning of work.

 

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