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Can anyone recommend a machinist?

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#26 mrflib

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Posted 05 October 2024 - 11:36 AM

Just as an update, I mocked up an extremely rough spider vane from photos and knowledge that the ring is about 200mm in diameter.

 

2mm vanes (I do not know what the tolerance is here, and whether they could / should go thinner). I did 5mm radius' between the vanes and the secondary holder and outer ring.

 

Quotes in 7075 Aluminium with a glass-blast finish were coming in between £100 to £260 depending on supplier, which all in all, is pretty **** good. Discounts for multiples available also.

 

I've just made the vanes totally rectangular, not seeing any need to complicate them (some vanes I have seen are nearly diamond shape in side on view, which I do not understand).

 

If anyone has any feedback on design tolerances (I am absolutely not an engineer) I would appreciate it. Before I pull my beautifully collimated scope apart to take real measurements, I'd like to get this base mockup correct.

 

 

Thanks again for all the help.

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Edited by mrflib, 05 October 2024 - 11:38 AM.

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#27 triplemon

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Posted 05 October 2024 - 12:53 PM

I'd be willing to take the risk if I could find a good machinist to attempt it. I got the scope extremely cheap.

Its a bit a contradiction. Non trivival one-off custom precision machining is unlikely to improve the bang-for-the-buck of a cheapie ...

 

Also, even monster diffraction spikes will not cause square stars. The aspect ratio of the spikes is still super narrow - so you get brighter spikes, but not shorter/fatter ones. The brrightness falloff along the spike with distance isn't affected by vane thickness. It sounds to me its more likely there are all kinds of astigmatism (if that is at the center) at work when the stars aren't round.there. And maybe there is the reason for the pricing you got ...

 

You could make even thicker vanes by adding ones made from tape to convince yourself that the aberation you see is different from the ones caused by thick vanes.

 

On the vane thickness when manufacturing - it really depends on how you create this thing. Given the prices you mentioned this is likely just for a waterjet cut out of sheet. So you could not get differing heights across the object - which would be a separate manufacturing step on a lathe or CNC mill.

But 3D CNC machining and waterjet cutting would have a really hard time with vanes that are much narrower than tall, for reasons as mentioned before. Only EDM comes to mind if creating this from a single piece to make such narrow structures. The exact capabilities really depend on the machinery and process used to manufacture this - so only the particular shop offering to make it could answer such questions.

 

As for a vendoor, I have worked with https://www.aurorap.com/ a few times.


Edited by triplemon, 05 October 2024 - 01:19 PM.


#28 Peds

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Posted 05 October 2024 - 03:22 PM

One thing I’ve learned is that it’s always great to 3D print parts first (use the cheapest filament available and perhaps use Craftcloud if you don’t have a 3D printer). You may need to iterate a few versions to get things fitting just the way you want them.

Only after I’m happy with the way things look/fit, do I commit to a more expensive CNC version.



#29 mrflib

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Posted 06 October 2024 - 11:50 AM

Its a bit a contradiction. Non trivival one-off custom precision machining is unlikely to improve the bang-for-the-buck of a cheapie ...

Also, even monster diffraction spikes will not cause square stars. The aspect ratio of the spikes is still super narrow - so you get brighter spikes, but not shorter/fatter ones. The brrightness falloff along the spike with distance isn't affected by vane thickness. It sounds to me its more likely there are all kinds of astigmatism (if that is at the center) at work when the stars aren't round.there. And maybe there is the reason for the pricing you got ...

You could make even thicker vanes by adding ones made from tape to convince yourself that the aberation you see is different from the ones caused by thick vanes.

On the vane thickness when manufacturing - it really depends on how you create this thing. Given the prices you mentioned this is likely just for a waterjet cut out of sheet. So you could not get differing heights across the object - which would be a separate manufacturing step on a lathe or CNC mill.

But 3D CNC machining and waterjet cutting would have a really hard time with vanes that are much narrower than tall, for reasons as mentioned before. Only EDM comes to mind if creating this from a single piece to make such narrow structures. The exact capabilities really depend on the machinery and process used to manufacture this - so only the particular shop offering to make it could answer such questions.

As for a vendoor, I have worked with https://www.aurorap.com/ a few times.


There are quite a few examples of people having square stars being fixed by thinning the vanes, and perhaps reducing The radius that joins the vanes to the central obstruction.

It was the only change made and the stars went round.

I am going to 3d print some mockups. For the sake of £100 or so to get a replacement in 7075 Alu it seems like it is worth a try at effectively zero risk.

It does say CNC on the site, but I know nothing about this so I will certainly take heed of what you say.

Cheers

Edited by mrflib, 06 October 2024 - 11:51 AM.


#30 Jon Isaacs

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Posted 06 October 2024 - 05:24 PM

I'd use a 12" flat b@sta#d file. Configure good support for the vanes, work slowly and gently, and know when to quit. Be careful of the radius at the ends of the vanes. Files can be great precision tools for this kind of job.

 

The vanes are cast, more than like aluminum. They are brittle and not under tension. If they do not break under the loading from filing, they will be flexible and the secondary will not stay aligned.

 

Jon



#31 Jon Isaacs

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Posted 06 October 2024 - 05:34 PM

 

Just as an update, I mocked up an extremely rough spider vane from photos and knowledge that the ring is about 200mm in diameter.

 

Just a comment or two;

 

Thin vanes need to be in tension. 

 

Steel is a better material for spiders because it's about 3x more rigid than aluminum for the same dimensions 

 

For this, strength is not as important as stiffness. All aluminums are equally stiff with Young's moduli of about 70 GPa. Steel is about 200 GPA

 

If you're going to design a spider, use a modern design that's torsionally stable. There's a long thread in the ATM forum by Oberon where he tests various configurations.

 

Jon 


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#32 tony_spina

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Posted 07 October 2024 - 09:38 AM

Check out youtuber Astrobiscuit. He is in the UK

https://astrobiscuit...utm_content=web

 

He just recently started offering Newtonian front ends.  You can see his recent videos which showcases the new spider vane and secondary holder




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