Thread: Worm & wheel gears

My friend Tony wants to find someone to make a custom worm and wheel set for his telescope mount.
Is there anyone in Australia who does this nowadays?
It needs to be high-grade, good accuracy.

Jordan

Specialist gear manufacturers (like Hercus in SA) will make worm/ worm wheel sets and commercially available worm drive gear boxes are available. Worms are relatively easy to cut on a lathe (basically single point thread cutting with a slightly odd feed rate). Worm wheels are typically hobbed, but not insurmountably tricky*.

How easy to get what he wants will depend on what ratio + size/ shape he is chasing. (By accurate, I assume you mean minimal backlash?)

Michael

* //metalworkforums.com/f65/t1708...78#post1661078

You can also use a backlash compensating split worm wheel from things like 4th axis units from CNC mills. You would have to find one used though as new cost is extreme.

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Worth noting too that a wormgear is made the shape it is to handle the transmission of power (that is, maximising the tooth on tooth contact area). If the main purpose is to position, a worm driving a helical gear will work well enough for that.

Michael

Don't don't think backlash is a big issue for this application as a power drive for a telescope is only ever in the one direction. Also if the equitorial mount has a counter balance then it can be set up so that either the motor feed or the weight of the telescope is always pushing or pulling on the worm drive.
Something in brass would be fine.

There's also not a huge amount of power transmision* so a semi enveloping worm drive would be fine.

*assuming it's not a 1M reflector or something massive!

Simon

Originally Posted by Michael G

How easy to get what he wants will depend on what ratio + size/ shape he is chasing. (By accurate, I assume you mean minimal backlash?)

The problem for telescopes is constant angular velocity. There are a multitude of ways that can creep in.

Originally Posted by lamestllama

The problem for telescopes is constant angular velocity. There are a multitude of ways that can creep in.

It really depends on what he wants to do. If he just wants to track the stars/planets for convenience while viewing so as not to have to constantly turn a wheel then near enough will be good enough. But, if he wants to use it to do photography with long exposures and high magnification then it will need to be alot more accurate. With regards to ratio, it's not a massive issue because you can set up a small stepper motor to accurately move through a range of rpm's so the final worm ratio is not that critical.

The biggest stuff around is when you set the telescope up to make sure the equitorial mount is facing polar north and that the base is level to the earth otherwise it will still wander off.

Simon

Very interesting comments, which I'll pass on to Tony.
Thanks folks,
Jordan

PS: Any more are still welcome.

these mounts are commercially available from telescope suppliers

Originally Posted by simonl

Don't don't think backlash is a big issue for this application as a power drive for a telescope is only ever in the one direction.
Simon

Not quite true. When you are searching for an object you slew all over the place. Tracking during viewing or imaging is in one direction until you need to do a meridian Flip. I had to do a lot of MFs for this one: https://www.astrobin.com/full/80259/0/

Charles

Originally Posted by cfranks

Not quite true. When you are searching for an object you slew all over the place. Tracking during viewing or imaging is in one direction until you need to do a meridian Flip. I had to do a lot of MFs for this one: https://www.astrobin.com/full/80259/0/

Charles

I see. So why do you have to do a meridian flip during an exposure?

Nice pic BTW.

Simon

Originally Posted by lamestllama

The problem for telescopes is constant angular velocity. There are a multitude of ways that can creep in.

Even with a helical gear and worm, backlash will not be an issue provided that the axis to axis measurement is correct for the gear combination. I worked on a bit of optical equipment once that required precise synchronization and the set up was not much more than has been suggested here.

As the contact point moves as the involute surfaces roll on each other, there might be some slight variation in speed (flutter basically) but that can be minimised with small teeth. For example, a 180t gear of 0.5 module has a depth of cut of 1.07mm and a diameter of 90mm. If the contact point moves over the full surface of the tooth, the variation in radius will be 1.07mm over 45mm - so plus or minus 1.1%. That of course assumes that only one tooth is in contact at any one moment. In reality, there would be multiple tooth contacts so I would expect that to be far less as the potential speed variations would average themselves out.

A worm gear is after all a helical gear that has more wrap on the driving gear (the worm) than a helical gear would using the same worm, but provided the helix angles match I can't think why a properly set up gear pair would not track well enough.

Michael

In an old post Steamingbill found this too -
Homemade Gears

Michael

I recently did a batch of 74T Mod 1.0 worm wheels at work that were made to 2 thou radial clearance, the backlash is barely detectable. We do one off jobs as well if you do want something custom made.

I passed on the extra info, thanks.

By the way, just for my own curiosity, would it not be possible to arrange for the worm to be able to move towards the wheel, in order to remove backlash?

Jordan