3D printed star tracker

[vlaiv]

I'm having idea to design low cost 3d printed star tracker.

Back in the day - barn door trackers where low cost solution to get into wide field astrophotography. I'd like to go about creating something similar - with use of 3d printer.

I have idea how to go about tracker itself, but now I'm at wedge stage and I'm wondering following:

- should I go for regular bearings or 3d printed alternatives?

At the moment, I think that I'll be needing something like 6 bearings in total to make wedge operation smooth.

2 needed for azimuth axis, 2 for "altitude swing" and 2 small ones to make azimuth worm smooth to use.

Some of them will be radial type and some axial.

So far I've figured that PLA on PLA does not have all that much friction. A bit of lube and it can be very smooth motion. None of these bearings will be particularly load bearing and won't turn with high speeds.

By the way, when I say 3d printed bearings - I don't mean ball or roller bearings - more like sleeve bearing type - where there is just smooth contact surface between two pieces.

Now, regular bearings that I would use are not really expensive - like ~1-2e per piece (depending on type). Question is - which one would you rather use and why?

[kbrown]

Suppose if you can just print everything then it would be easier and faster to put together? I'd probably use bearings though...

[vlaiv]

8 minutes ago, kbrown said:

Suppose if you can just print everything then it would be easier and faster to put together? I'd probably use bearings though...

I just finished printing some bearings.

608 for example is working OK, but being print in place design - tolerances are a bit loose and there is quite a bit of play.

Axial bearing is easy to print as it is not print in place design. Radial is much more tricky and I'm now looking into doing design that is assembled instead of printed in place.

This will make it much easier to lubricate and clearances can be tuned in so there is minimal play.

I guess that I can make 3d printed versions the same size as actual bearings and then make it matter of choice? I'll be using 3d printed versions at development phase (except for 608s as I have bunch of them on hand so I can try with real bearings and compare) and replace it for first complete prototype.

[Elp]

I've found with pla assemblies you generally need 0.5-1mm a side clearance otherwise you'll be filing a lot to get close to an interference fit. Also what temperature will be the usage scenario, pla isn't very heat or UV resistant and will bend very easily, put it near a heat source and you'll see how quickly it deforms. Petg/abs/nylon/poly-mixed type would be better for such applications if your printer can print those.

[vlaiv]

10 minutes ago, Elp said:

I've found with pla assemblies you generally need 0.5-1mm a side clearance otherwise you'll be filing a lot to get close to an interference fit. Also what temperature will be the usage scenario, pla isn't very heat or UV resistant and will bend very easily, put it near a heat source and you'll see how quickly it deforms. Petg/abs/nylon/poly-mixed type would be better for such applications if your printer can print those.

I think that for star tracker - cold is much more problematic than hot

I can see people using it at -10C but not at +40C.

I'm planning to make parametric models with clearance as a parameter for all important bits.

Btw, I just printed 51108 axial bearing (40x60x13)

In this format:

+ this

With 0.1mm of distance between groove and ridge.

Works fairly well, and rotates under quite a bit of load. Only issue I have is that I had seam on "aligned" and it now has one place where it binds up as seam comes against the seam on other part.

I really need to see how to adjust seams to that they don't end up on sliding parts.

[Elp]

What's the seam for?

[vlaiv]

1 minute ago, Elp said:

What's the seam for?

It's byproduct of printing layer by layer.

Each layer has to start somewhere. When stepping to next layer printer stops extruding plastic (retraction), moves to start of new layer, primes nozzle (moves filament forward) and continues extrusion.

This retraction / priming is not exact science (but linear / pressure advance helps) and a bit more plastic is often extruded which creates a bump in that place as there is a bit more material.

Here is picture of it (a bit exaggerated, but it is still there even if your printer is tuned properly):

[kbrown]

Maybe you could print housings for the bearings and then add thin teflon strips as the mating surfaces?

[Elp]

It's quite minimal on mine, but then I usually file out holes a little to relieve the assembly anyway. I find with threads in particular printing them sideways helps a lot with their formation though I can only usually print coarse thread pitches.

[pipnina]

For something that isn't bearing (ha) a lot of weight, maybe PTFE would be a good material? Very low coefficient of friction, but I don't know if it can work in a printer.

It would be great to get some designs for a printable tracker, but i do worry that 3d printing won't be very cheap given the high cost of printing larger objects. The bracket for my autofocuser cost 60 quid!

[vlaiv]

26 minutes ago, Elp said:

It's quite minimal on mine, but then I usually file out holes a little to relieve the assembly anyway. I find with threads in particular printing them sideways helps a lot with their formation though I can only usually print coarse thread pitches.

I was able to print fine metric pitch on mine without too much issues. I do print those in vertical orientation and use random seam position. I printed M72 x 0.75 (filter thread for my samyang lens) just fine. Layer height was 0.12mm

8 minutes ago, pipnina said:

For something that isn't bearing (ha) a lot of weight, maybe PTFE would be a good material? Very low coefficient of friction, but I don't know if it can work in a printer.

It would be great to get some designs for a printable tracker, but i do worry that 3d printing won't be very cheap given the high cost of printing larger objects. The bracket for my autofocuser cost 60 quid!

That is way too much. In material, something like that costs maybe few quid at most.

I think that many will find investing in low cost (but quite usable) 3d printer like Ender 3 (mine is V2) quite cheap in the end. Printing just few items can justify the expense if we go by those prices.

I'm having quite a bit of fun with mine.

So far, astronomy related, I've printed:

- bits for counter weight system for AzGTI (hand screws for adjusting position on M10 threaded rod - these hold M10 nut inside, system for centering and holding together some dumbbell weights of 0.5kg each with 28mm center bore)

- long dew / light shield for Samyang 85mm T1.5

- F/2.4 aperture stop for same lens (this one screws into filter thread)

- cable clips (work in progress) to hold in place power and USB cables on AzGTI + Samyang setup. I'm yet to design mounting parts for those clips - I need two. One for vixen dovetail and another to attach to AzGTI itself (maybe I'll use M10 threaded rod for that?).

Planned so far:

- Focus system for Samyang lens - motor bracket + gt2 gear to go on focus ring

- Motor brackets for auto focus system for my two other imaging scopes - RC8" and TS80 apo

- Lowspec spectrometer

- Spectroheliograph like Sol'ex (I've some other variants if I remember correctly, so I'll choose at that time)

- Star tracker

- different kind of bits and bobs - I want to make small refractor from surplus lens, and I'll need different adapters to connect everything together.

 

[tomato]

Vlaiv, Looks like the 3D printer potential has really captured your imagination.

I do have room for one in my warm room but I was raised in the realm of machine tools, and I would still prefer to machine a piece of billet rather than extrude plastic filaments.

[kbrown]

18 minutes ago, tomato said:

I do have room for one in my warm room but I was raised in the realm of machine tools, and I would still prefer to machine a piece of billet rather than extrude plastic filaments.

I'm by no means an experienced machinist but I started off with a desktop CNC router. I still have it and use it occasionally but since I got my 3D printer it has proven to be much more versatile for my purposes.

[vlaiv]

39 minutes ago, tomato said:

Vlaiv, Looks like the 3D printer potential has really captured your imagination.

I do have room for one in my warm room but I was raised in the realm of machine tools, and I would still prefer to machine a piece of billet rather than extrude plastic filaments.

To be honest, I'd be happy with either - small lathe, cnc machine and such are on my wish list.

It appears that I've been bitten by some sort of manufacturing bug

 

[pipnina]

5 minutes ago, vlaiv said:

To be honest, I'd be happy with either - small lathe, cnc machine and such are on my wish list.

It appears that I've been bitten by some sort of manufacturing bug

 

First you buy a 3d printer, before you know it you have a lithography machine and make Intel 486s for fun

[ONIKKINEN]

10 minutes ago, vlaiv said:

To be honest, I'd be happy with either - small lathe, cnc machine and such are on my wish list.

It appears that I've been bitten by some sort of manufacturing bug

 

3D print the lathe? 🤣

[Alien 13]

A star tracker (tiny one for my phone camera) is on my to-do list when I have finished all the "fun" projects I have on the go, as to bearings I use lots of those "skateboard" ones for anything that has to move and they are reasonably cheap too.

Alan

[Alien 13]

7 hours ago, ONIKKINEN said:

3D print the lathe? 🤣

Not such a daft idea as you may think..

Lots of examples out there..

Alan

Edited July 28, 2022 by Alien 13

[Elp]

21 hours ago, vlaiv said:

small lathe, cnc machine and such are on my wish list.

The Pocket NC machines look great. Wallet doesn't agree. But then again workshop CNC machines aren't cheap either especially if they're more than 3 axis and a decent bed size, spindle speed and no of tool changers.

Edited July 28, 2022 by Elp

[pipnina]

13 hours ago, Elp said:

The Pocket NC machines look great. Wallet doesn't agree. But then again workshop CNC machines aren't cheap either especially if they're more than 3 axis and a decent bed size, spindle speed and no of tool changers.

Indeed, a relatively small CNC mill at my college cost over £20'000. It was very cool to watch it run but sure was costly!

The CNC lathe next to it had a black and white CRT television for a camera-feedback it supported. Very strange as the machine was not that old looking.

I dread to think what the largest machine at my workplace cost, it's many times larger than the one at the college, I dare say you could put some furniture in there and live in it!

[Elp]

Over 100k for sure. They're wonderful things to watch when machining soft materials like aluminium, especially a nine axis lathe.

[pipnina]

Out of interest Vlaiv, what program are you using to design these parts? I've tried a few and had some difficulty with all of them.

Also with the bearings, you could try "bedding them in", where you effectively use tapered mating faces, fit them together, and then turn them on eachother until they wear themselves true? Maybe some fine lapping paste would be useful here. I don't know how well it will work but it might be worth a try to get the bearings to avoid sticking and sliding?

[vlaiv]

3 hours ago, pipnina said:

Out of interest Vlaiv, what program are you using to design these parts? I've tried a few and had some difficulty with all of them.

I'm using FreeCAD.

At first, I had some difficulty using it. I was using version 0.19. At that time I was mostly trying to print decent thread and was using thread work bench - which turned out to be flawed. Also I probably did not understand workflow in part design work bench very well.

After few days of using it - everything just clicked into place for some reason. I stopped using thread workbench as I figured out that I can create threads more accurately myself with a bit of help from wiki (I just looked up metric thread profile and everything is well explained on the page) and I did not need to mess with industry tolerances / clearances and just use clearances that my printer requires (like 0.2mm clearance from nominal tread diameter).

I did run into topology naming problem - but since I'm computer programmer - it immediately became clear what the issue was and how should it be addressed. In fact, if one uses parametric approach to modelling (using spreadsheet and calculations to produce model values) - then topological naming problem goes away.

At the moment, I'm rather happy with FreeCAD and what it can do. It is a bit crude UI wise, but quite capable piece of software.

3 hours ago, pipnina said:

Also with the bearings, you could try "bedding them in", where you effectively use tapered mating faces, fit them together, and then turn them on eachother until they wear themselves true? Maybe some fine lapping paste would be useful here. I don't know how well it will work but it might be worth a try to get the bearings to avoid sticking and sliding?

I ended up using printed bearings for prototyping purposes and will probably advise using regular things (not too expensive) in final product for smoothness of motion. While printed variants work - they lack the finesse / smoothness of regular items, even in very low speed conditions.

I have two more hurdles to overcome now.

First is - modelling worm wheel / worm gear assembly. Worm gear itself is easy - it is just trapezoid profile revolved, but worm wheel itself is another method.

Spur gear, for example - does not have simple profile:

It is curved instead of trapezoid shape - and this is because it is not "linear" but rather follows circular path (two circles - smaller and larger gear).

With worm wheel / worm gear - one is simple linear / trapezoid profile, but other must be similar to above but little less bent - it is shape that is "cut" by trapezoid as it revolves while wheel itself is turning.

I really need to consult some good machinist handbook in order to find proper profile for this.

Another thing is - clutch.

I need to design some sort of clutch that will be tightened when wedge is in position and secure it from moving. All plastic parts have low friction - they are not quite smooth, but are not suited for breaks either.

One solution that I've come up so far is to simply cut pieces of paper and glue them onto surfaces that will act as clutch surfaces. Paper on paper has very high friction coefficient and is very affordable solution.

[kbrown]

+1 for FreeCAD. It's not perfect and does have a bit of a learning curve but it's pretty capable once you get used to it.

[saac]

I use Fusion 360 which coincidently has a involute gear profile generator which may be of use to you vlaiv.

Jim 

Edited July 29, 2022 by saac