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Making a telescope worm wheel and gear set on the mini-lathe for astrophotography . Step by Step.

This is my step by step guide to making a matched telescope worm gear and wheel set.

One of the most impressive things you can make on a mini-lathe is a matched worm wheel and worm gear set for a telescope mount.

It is critical to machine all parts concentrically to minimise periodic error, and the wheel and worm gear need to be lapped together to make the tracking as smooth as possible.

Commercial worm gears are very expensive, so making our own is a huge cost saving, as well as a lot of fun.

How accurate does a telescope worm wheel and gear set need to be?

The important factors are as follows

• Tracks the sky at the correct rate. Although this is largely dependent on motors etc, the teeth on the circumference of the worm wheel need to be equally spaced, to a high degree of precision.
• Tracks the sky smoothly. The worm gear must not have scratches and dings which make the tracking jumpy. How smooth is smooth? Depends what you are doing with your telescope. If astrophotography is important, then the answer depends on the image scale you are using. A 50mm SLR lens on a camera is more forgiving than a 2500mm focal length telescope with the same camera. It all comes down to image scale - arc seconds per pixel at the camera end. I often like to image in the 1 to 2 arc seconds per pixel range, so any interruptions greater than 3-4 arc seconds is going to cause me major problems.
  On a seven inch nominal worm wheel an imperfection of 0.0004mm or 0.000016 inches will give an error of 1 arc second in the pointing. 5 arc seconds error corresponds to 0.002mm or 0.00008 inches. And yes this is getting pretty close to the wavelength of light. However, just like we can polish a telescope mirror, we can polish a worm gear.
• The worm thread is concentric to the shaft to reduce periodic error to an acceptable level? What is acceptable? So long as it is smooth, autoguiding will guide out a half arc minute of periodic error quite easily.

This section is still a work in progress, but I have completed the first section of my series on making a 7 inch or 6 inch worm wheel and gear set on a mini-lathe.

Part 1: marking the center of the worm wheel blank

Part 2: Laying out the bolt holes for mounting the worm wheel blank on the spindle of the mini-lathe

Part 3: Drilling the bolt holes in the worm wheel blank

Part 4: Facing the worm wheel blank

Part 5: Calculating and turning the outer diameter of the worm wheel blank

Part 6: Gashing the worm wheel blank gear teeth

Part 7: Planning the worm gear

Part 8: Mounting and starting the worm gear cutting on the mini-lathe

Part 9: Setting up the lathe for threading the worm gear

Part 10: Cutting the worm thread

Part 11: Preparing the worm hob

Part 12: Hobbing the worm wheel

Part 13: Lapping the worm wheel and gear together

Part 14: Making a shaft clamp clutch for the worm wheel

Success lies in taking time and care at every step. With many mini-lathe jobs your can measure and check your work at different stages, and know if you have made a mistake. With worm gears you genuinely have no idea if you've made a small mistake until the work gears are on a telescope and tracking the stars. Nothing the amateur can rig up is able to replicate the smooth motion of the night time stars - the only way to test is to get out there any put them on a telescope.

Testing of periodic error can only be done using a webcam and popular software such as k3ccdtools to draw a chart of the motion. Peak to peak periodic error of 30-60 arc seconds is acceptable - so long as it is smooth. If it is smooth, then it can be corrected with PEC and guiding. If there is a lot of high frequency noise, then it will be difficult to correct and not suitable for long exposure astrophotography.

Take care and time at each stage. Don't try and do it all in one weekend, spread the task out, and have a good rest / think / beer between stages.