14 replies to this topic

[Rob Willett]

I have a Borg 76mm and am trying to find out the thread size of a the thread on the external draw tube. I want to make an adaptor to fit a focusor.  The picture below shows the area I mean.

 

The internal thread size on the drawtube fits the helical focusor and is M68.8x0.75 (from memory). The outside thread doesn't seem to appear on any Borg literature, I've checked the 2004 Borg manuals as well as a Borg thread list, e.g.

 

http://www.aokswiss..../borg_parts.pdf

and

https://www.sciencec...org/threads.htm

 

The thread is an external thread and measure 72.65mm on my digital calipers.

 

So I thought I'd use http://theoreticalma...icMProfile.aspx to work out what the threads might be. I'm now up to my sixth ring that I've test printed to see if I can get it to work. I'm hoping that it's M72.75x0.75. If not then M73 beckons

 

If it helps anybody I've setup an Excel spreadsheet to create the Fusion 360 thread XML for the ISOMetricprofile.xml, as I check a thread I need to generate the right XML and I use

 

http://theoreticalma...icMProfile.aspx

 

to get the six numbers, bang it in Excel and generate the XML string. I would generate it from the formula but I can't get a decent version of the ISO standard I can understand

<Designation>
      <ThreadDesignation>M72.75x0.75</ThreadDesignation>
      <CTD>M72.75x0.75</CTD>
      <Pitch>0.75</Pitch>
      <Thread>
        <Gender>external</Gender>
        <Class>6g</Class>
        <MajorDia>72.658</MajorDia>
        <PitchDia>72.1795</PitchDia>
        <MinorDia>71.786</MinorDia>
      </Thread>
      <Thread>
        <Gender>internal</Gender>
        <Class>6H</Class>
        <MajorDia>72.9075</MajorDia>
        <PitchDia>72.3665</PitchDia>
        <MinorDia>72.055</MinorDia>
      </Thread>
    </Designation>

Any guesses on the thread welcomed.

Rob

 

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[PrestonE]

I can say with Great Experience, that 3D printing threads that fine

is Rarely accurate enough to get a Good Fit...

 

I have tried many times with a very good 3D printer and though one

can get a kind of thread, they are Not the Proper shape for threads that fine.

 

Also, as you likely know...Focuser Manufactures Rarely us standard Metric Thread sizes!!!

 

Good luck,

 

Preston

[Rob Willett]

@preston

 

I have both an Ender 3 Pro and a new Prusa. The Ender 3 Pro was rubbish at any threads. It was a good printer but when you use a Prusa, you can see the difference. Mind you it's nearly 4x the price.

 

The Prusa is very good, if you know the sizes. As a test I printed the M68.8x0.75 internal thread, worked first time at 0.10 detail. I have done it again and got the same result. I didn't do any scaling on the model at all.

 

I don't know it's better at external threads or internal threads, or I could be making mistakes in Fusion 360. But I have a lovely Starlight Instruments focuser and I'd like to try it on my Borg. I'd have to get ian adaptor from the US, it would need to go through customs and tax and this and that, so trying to print it first makes sense. It would be solely for AP and would be carrying virtually no load apart from the focusor and a small CCD camera.

 

26 mins to go on the printer for the next ring. We'll see if that works.

Rob

Edited by Rob Willett, 31 October 2021 - 11:57 AM.

[sc285]

Not if it helps any...but here is a page which explains threads and has a listing of some of the common sizes and where used.

https://agenaastro.c...-explained.html

Go down into the section with focusers. Also to be sure of the correct thread pitch, there is a link to print out (1:1) a paper thread pitch gauge.

 

Rob

[Rob Willett]

Rob,

 

Thanks for this. I have already seen the page and checked it out. None of the threads are quite right. The M72 is too small amd the M74 is too large. The pitch looks very like 0.75, I've counted the threads and there's 8 per 6mm which is around 0.75,

 

I'll find the right thread eventually.

 

Rob

[don clement]

There ain't  real standards for threads used on telescopes. I have found it best for me when threading on the lathe or thread milling on the mill to match the thread to the part for telescope parts. Otherwise I will make the thread like 0.01" over/under size  to have ample clearance. There are precision methods for measuring thread PD. i.e. three wire method or if the thread diameter is less than 1" (25.4mm) I have a thread micrometer. BTW when measuring the OD with a caliper that is not necessarily the PD. 

 

Screw pitch micrometer

 

[Rob Willett]

Don,

 

Thats sounds very complicated I think I have now found the thread, I think it's M73 x 0.75. I've scaled up a thread with Prusa Slicer and got it to fit nicely. I am now going back and trying to do the job properly.

 

I will investigate what you say though.

 

many thanks

Rob

[Dale Eason]

There was a discussion in the 3D printed topic that discusses printing fine threads.   There you will find that I and others got normal filter threads to print using SCAD and my Prusa using PETG

 

https://www.cloudyni...omy/?p=11263255

 

Dale

[Rob Willett]

Thank all

 

I printed an M73x0.75 internal threaded part at 0.1mm resolution yesterday and left it to run overnight. Came back this morning and tried it and it fits very well. I think it needs a tiny chamfer on the leading edge to help the tube seat easily for the first threads but it threads nicely and evenly. I've just added in the mounting flange for the focuser to the model and am printing out a first draft now.

 

There are three M3 holes for threaded inserts, these are set in from the inside using a dedicated point on a soldering iron and push against an internal ridge inside the ring. Unsure how much load they will take, but we'll find out soon. The biggest limitation is the distance from the outside edge to the three holes, thats determined by the focuser and is not a lot for 3D printing. I'd much prefer another few mm, but can't happen

 

I also looked at the three wire method and micrometers and saw this, a bargain at £3,500 ($4.2K) and that doesn't include a micrometer.

 

https://shop.mitutoy...F37C7B08B82B0BF

 

After cleaning up the coffee from my keyboard, I could see that Mitutoyo are very high end equipment manufacturers and that if I wanted to do this, I could find cheaper kit. However I'm wary of cheap stuff unless it comes recommended. My garage is full of cheap junk that doesn't really work very well.

Thanks

Rob

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[don clement]

 

 

I also looked at the three wire method and micrometers and saw this, a bargain at £3,500 ($4.2K) and that doesn't include a micrometer.

 

https://shop.mitutoy...F37C7B08B82B0BF

 

 

I just use Pee-Dee thread wires for ~ $36. https://www.amazon.c...s/dp/B07F5Y5SFW and optionally Flexbar threadwire attachments for $22 https://www.amazon.c...001CTIATQ&psc=1

 

BTW I turn my Camera lens adapters out of aluminum on the lathe.

 

 

 

 

 

Edited by don clement, 01 November 2021 - 07:10 AM.

[Rob Willett]

Thanks, I'll look at those.

 

Rob

[Rob Willett]

I love the idea of having a lathe, sadly t'other half will not allow that. 

 

I have two 3d printers instead, an Ender 3 and a Prusa. Mk3+ The Ender 3 is now for knocking out big pieces of work and the Prusa for the more intricate work, though it's capable of big stuff.

 

I have now printed the adaptor and it works very well. Yet to take any pictures through it but it feels secure. It has M2 screws to secure the focusor, a brass hreaded insert to secure the screws rather than using the PLA adaptor.

 

The M73x0.75 screw fitted first time.

 

I'm delighted with the fit and look. I might print again overnight at 0.05mm as thats an option on the Prusa and see if I can make it look super sleek.

 

 

 

 

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[Rob Willett]

And here's it on the Borg 76mm.

 

 

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[Rob Willett]

Printing at the highest resolution (0,05mm) on my Prusa doesn't actually make the end result much nicer or smoother.

 

Just compared the 0.1mm version against the 0.05mm version and it looks the same. It did thread a tiny bit easier but that's it. As the 0.05mm takes 3x as long to print, thats a useful saving in NOT doing it.

 

I have a roll of PETG coming tomorrow, be interested if that's any more robust than the PLA version. PETG should be stronger.

 

Rob

[555aaa]

+1 on the thread wire method.