3d printed star tracker

[vlaiv]

I just wanted to share few ideas and not complete solution. I still don't own a 3d printer (hopefully that will change some time soon), but I do from time to time think about what can be 3d printed astronomy related.

Star tracker + DSLR and lens is very cheap way and easy way to get into astrophotography. Most people, when thinking about DIY star trackers - think barn door tracker as a cheap solution - but I thought - what about 3d printed options?

There are several full fledged 3d printed EQ type mounts, but I had in mind something simpler. Worm and worm gear are not really 3d printable. Sure you can make them, but they are not very smooth.

While watching youtube videos and prompted by recent thread on a new harmonic drive available - I found 3 rather interesting and 3d printable gear reduction ideas.

First a bit of math:

360 degrees x 60 arc minutes x 60 arc seconds = 1296000 arc seconds in full circle

360 / 0.9° * 64 micro steps = 25600 micro steps in full circle if we get 0.9° per step stepper motor and do microstepping with 64 micro steps.

1296000 / 25600 = 50.625

We need x50.625 reduction to get 1" / step resolution. We can use more than that - say double to get to resolution of AzGTI which has 0.625"/step.

How to easily get that sort of reduction? It turns that there are several 3d printable solutions - which can get that and much larger resolution in single assembly (no multiple stages required).

1. Strain gear drive / harmonic drive

- very low / no backlash

- can't be back driven (like worm / worm gear arrangement)

https://www.youtube.com/watch?v=-l7FbsJSteg

https://www.youtube.com/watch?v=fGI-4dHYz9U

and there are of course several more interesting designs that you can check out on youtube

2. Cycloidal drive

https://www.youtube.com/watch?v=tgEOpl880KM

https://www.youtube.com/watch?v=7uXN_y7JdyM

This is probably a bit more expensive as it involves quite a bit of bearings and shafts to be added - but I suspect it can be made real smooth even 3d printed

3. Ring split compound epicyclic gear

https://www.youtube.com/watch?v=-VtbSvVxaFA

https://www.youtube.com/watch?v=66MlWxoQE1s

I love this one because of crazy reductions available - but it does suffer from backlash as most gear based systems. Good thing about this design is that there could be multiple teeth in contact at the same time which helps with backlash and smooths the transmission. If we use helical gears instead of spur gears things get better as well.

It is also very simple design in terms of bearings and shafts / pins needed.

In the end - it is fair to mention that x50 is really not difficult reduction ratio to achieve with synchronous belts - all we need is two 1:7 stages so even that design should be working well.

 

[Chriske]

Nice, very nice indeed...! Going to make me one, a strain wave gear...👍

Edited January 17, 2022 by Chriske

[Mognet]

There are some interesting ideas in there. I am especially intrigued about the harmonic drive with a compliant mechanism. I've had a vague plan for a while to print a star tracker, possibly with own software too, so might use some of this as a starting point

[Chriske]

Busy printing this one this very moment...
Its a three stage drive.

The holes on which the 'planets' rotate, I'll fill them with brass rods instead of printed versions.
I also will redraw the part on the right, to allow a stepper instead of a round DC motor.
The outer diameter of that thing is about 95mm, small enough for me.

[vlaiv]

29 minutes ago, Chriske said:

Busy printing this one this very moment...

What will be reduction of that assembly, and what sort of backlash and precision do you expect from it?

[Alien 13]

Watching with interest as my Son has recently acquired a 3D printer and asks me if I want anything printed? 

Have the DSLR/Lens combo covered with my EQ3-2 but would like something much more compact without going the SA route. I have even thought about a tiny Star Tracker for a mobile phone as these are beginning to get reasonable Milky Way shots even now and will only get better.

Alan

[Chriske]

3 hours ago, vlaiv said:

What will be reduction of that assembly, and what sort of backlash and precision do you expect from it?

I'll tell you all about when finished and assembled. Should be very soon...;-)
Planning to use it on my equat. platform. Observing visually will not be a problem, nor taking pictures with a widefield lens on a tripod on top of that platform.

[vlaiv]

4 hours ago, Alien 13 said:

Watching with interest as my Son has recently acquired a 3D printer and asks me if I want anything printed? 

Have the DSLR/Lens combo covered with my EQ3-2 but would like something much more compact without going the SA route. I have even thought about a tiny Star Tracker for a mobile phone as these are beginning to get reasonable Milky Way shots even now and will only get better.

Alan

Not sure if compactness will be feasible.

3d printed parts are not that tough and require quite a bit of bulk to provide sufficient stiffness. They will however be lighter than metal counterparts.

As far as miniature platform for mobile phone tracking (even DSLR) - there is this:

https://www.teleskop-express.de/shop/product_info.php/info/p6540_TS-Optics-NanoTracker---compact-camera-tracking-mount-for-astrophotography.html

Now that is compact

I really doubt that anything 3d printed will be that small.

[vlaiv]

When we are at it, does anyone have sensible idea of how to provide

3/8" connection on bottom and top side of 3d printed part that is load bearing?

Would simple 3/8" insert be enough to attach tracker to tripod on bottom and carry ball head and camera on top?

[Alien 13]

10 hours ago, vlaiv said:

When we are at it, does anyone have sensible idea of how to provide

3/8" connection on bottom and top side of 3d printed part that is load bearing?

Would simple 3/8" insert be enough to attach tracker to tripod on bottom and carry ball head and camera on top?

I would be tempted to bond/screw the base and top plate to something like this lots of similar designs available too.

Alan

Edited January 21, 2022 by Alien 13

[Chriske]

Depending on the weight I make my own connection. I usually print a few holes If it's a larger part. But in a case like the one I'm printing right now they're already there.
These 'special' inserts I do not use them. They have to be applied correctly to be save.
Very often I use these wood inserts. I have different sizes, I do insert them at the back of the printed part so the inserts can not be accidentally pulled out because you'd be pulling toward the part instead of pulling away from the part like these special (hot insertable) inserts.
These inserts I use, you need to screw them in with an Allen key. When done you're left with a metric thread inside. In my case I have M6, M8 and M10 inserts. Very strong, but most of all very handy, and safe, I very often use these inserts in wood, for which they were intended to be used for. But I also use them very frequently in printed parts. When applying I warm them up first a little bit first.

[Chriske]

The main body was not printable at all, especially in the lower part. It needed lots of support, so I change a few things there.
Stronger main body.
New holes for a Nema17 stepper.
Widening the foot. Holes were hidden under the main body. No way to reach the screws or bolts to fasten the unit.

[Chriske]

Next time I'll be posting this planetary gear system in my 'Equatorial platform' thread.
Planning to integrate this unit in that EQU-platform.

[Alien 13]

Now I have a 3D printer I might give a Harmonic drive a go, wondering if even the flexible ring can be printed too for a small low torque system for use in a camera tracker or remote controlled normal camera mount.  I have had good results so far with carbon fibre filament for the stiff strong parts and thinking maybe PLA + for the semi flexible stuff.

Alan

[vlaiv]

6 minutes ago, Alien 13 said:

Now I have a 3D printer I might give a Harmonic drive a go, wondering if even the flexible ring can be printed too for a small low torque system for use in a camera tracker or remote controlled normal camera mount.  I have had good results so far with carbon fibre filament for the stiff strong parts and thinking maybe PLA + for the semi flexible stuff.

Alan

For 3d printed stuff - I like cycloidal drive the best. Depending on design - it has the most points of contact which means the least backlash (although it is back driveable).

I'm hoping to make one myself at some point.

[Alien 13]

1 minute ago, vlaiv said:

For 3d printed stuff - I like cycloidal drive the best. Depending on design - it has the most points of contact which means the least backlash (although it is back driveable).

I'm hoping to make one myself at some point.

The good thing about 3D prints is that you can experiment with real parts and its so easy to modify or re-design them.

Alan

[vlaiv]

6 minutes ago, Alien 13 said:

The good thing about 3D prints is that you can experiment with real parts and its so easy to modify or re-design them.

Alan

Good point.

If motor bracket, wedge and belt transmission parts are designed with care - they can be reused on multiple iterations of reduction gear.

Btw, do you think 3d printed wedge would be usable for small loads (or maybe "hybrid" version with both plastic parts and metal bits - like bolts and nuts for adjustment)?

[Alien 13]

14 minutes ago, vlaiv said:

Good point.

If motor bracket, wedge and belt transmission parts are designed with care - they can be reused on multiple iterations of reduction gear.

Btw, do you think 3d printed wedge would be usable for small loads (or maybe "hybrid" version with both plastic parts and metal bits - like bolts and nuts for adjustment)?

Think any wedge design would need to be a hybrid, PLA especially the carbon loaded one is very stiff so would be OK as the main frame and support structure but would not survive any large clamping loads so would need some metal inserts. 

I found this video regarding 3D printed harmonic drives and the basic design will probably be what I use for my experiments.

Alan 

Edited May 17, 2022 by Alien 13