Making a new worm gear for a camera tracker

I have challenged myself to make a worm gear from scratch.

To buy a decent worm gear set could set you back a few hundred dollars and for that amount of money you can almost buy a second hand telescope mount to do the job of tracking a camera for wide field long exposure astrophotography. My previous camera tracker used a stepper motor with a small worm and belt gearbox but it was not accurate enough and was causing up to 25 arc-seconds errors in tracking accuracy in RA (right ascention). Look at two of my previous posts to see what I have done before:

https://starsinphotos.wordpress.com/2012/03/29/camera-tracker/

 https://starsinphotos.wordpress.com/2014/03/23/diy-autoguider-for-long-exposure-astrophotography/

So, the challenge is to take a small slice of stock bar aluminium and machine a worm gear set that can be used directly with a stepper motor to drive the tracker. I aim to reduce the number of moving parts and to make the gearbox as solid as possible to prevent tracking errors. My aim is to get a tracking accuracy of plus/minus 5 arc-seconds.

I tried making a worm gear with a 65mm diameter aluminium disk to see if I will be able to do it. This was the result:

Raw aluminium disk (65mm diameter, 15mm thick)

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I then milled the flat surfaces smooth and drilled a centre hole (10mm). As the centre hole will never be exactly in the centre of the disk I had to turn the disk on a lathe. This was a bit of a problem because I don’t have a lathe, I only have a small mill. Well, I made a plan and used the mill as a lathe. It worked fine.

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I then used a M10 tap to cut the worm gear, this was quite exciting and took about an hour and a half.

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The end result was not bad at all. There are 132 teeth with a 1.5mm pitch. I am quite happy that this worm will work reasonably well. I just have to scale the process up to a larger aluminium disk.

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I have now obtained aluminium disks, 130mm diameter and 14mm thick and will try and repeat the process to get 270 teeth.  Using this gear with a stepper motor running 1/16 steps should work out well.  Watch this space for progress …..

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2 thoughts on “Making a new worm gear for a camera tracker

  1. Dasphys

    Hi,
    My enquiry is about when the thread approaches the begining aftr cutting around the disk. with your job, I worked out that 132 teeth at 1.5 pitch is 164 mm circumference = 2πr , at the inner most diameter , that is the inner most point of the centre of each thread. This would mean a radius of 164/ 2π. = 26.1014 mm or diameter of 52.2028 mm .
    A – How did you ensure the teeth lined up exactly ?
    B – it would be nice to see a video on the process .Are you happy with the small mills and lathes in terms of accuracy ? I’d like a lathe and mill but not sure how good the smaller units are.
    C- Is there a need to do the whole disk ? If you are taking shots less than 10 minutes would it not be possible to build either a disc with say 1 hours worth of thread (1/12) of curcumference ) or to cutup the whole fabricated disc in say quarters to get four times as many discs reducing cost an mounting it onto a round base ?
    D – could we simplify the design if the time of exposure only requires a small arc ?

    Reply
    1. Theo Post author

      Hi there,

      I think I can answer your questions…

      A: Its hard to get right. Sometimes the teeth are not perfect at first but as the threat gets cut deeper they align and everything is good. Sometimes they don’t align and then I have an area of the disk that is not perfect or usable. That’s ok, as I mark that section and just re-wind the tracker when I get close to it.

      B: I have a YouTube video, Look at: Making a worm gear pair for an astrophotography camera tracker
      I have just got the Sieg SC2 lathe a few days ago, so I’m not sure about it yet. Previously I used a Sieg SX2 mill and it was a bit of a learning curve. I was also using the mill like a lathe and that is not ideal. The lathe should improve my accuracy quite a bit. Obviously a larger machine will be better but they get quite big and expensive. You can also ask small industrial workshops to do some work for you when they are not busy and they normally would be able to do it for a low cost.

      C: I would not want to try that. What I have found is that it is critical to get the RA axis hole in the wheel and the outer edge perfectly concentric. That is only possible on a lathe (or a mill as I did). You cannot cut a section of a circle on a lathe, not easy anyway…
      You need a high level of accuracy between the worm wheel and the worm otherwise the two will jam and cause all kind of problems.

      D: I will think about that. Its true that you don’t need a large arc for a small tracker. I have moved on and on my current tracker I use the complete worm wheel to position the telescope or camera as it is now an an equatorial type mount. I don’t us a clutch. That’s a future project…
      A full circle is easier as you can let the wheel just keep on spinning and improving the thread.

      Look at my later posts on the subject…

      Good luck,
      Theo

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