80mm long focal length refractor

[rwm]

More complex than I imagined. I will have to read more about this:
"In all, the focal length requires three optical stops to prevent grazing reflections from entering the eyepiece and I decided to machine them as joiners."
R

 

[savarin]

The front cell is finished

all the lens edges have been blackened but I still have to blacken the screws inside.
The gloss black doesnt look as glossy in the photos.
You can just see the pusher grub screw that allows the cell to tilt for collimating.

Finished threading and polishing two of the tubes, (only one in the photo) how tedious is polishing. The original surface is the top tube.

Still some ultra fine scratches to remove so its back to 1500 grit then rouge.
I cant cast the brass joiners yet due to the changeable weather, sudden showers that I dont want to have molten metal around.

 

[savarin]

The first brass lump has been cast as a lost foam project for the joiner.

Man thats ugly, runners and sprue to remove.
Then a bit of machining and a thread cut we get this, its difficult to see but there is a 1.25mm thread there.

I dont know if it was required but decided that discretion being the better part of valor I decided to make a threaded plug to fit the end so there was no chance of distorting the part when I clamped it in the chuck to part, turn and thread the other end.

Screwed on

Yep, thats a small inclusion hole you can see there, dang! but I'm not re-casting that (yet)
Its back in the 4 jaw and I've now parted it to length and hope to start on the thread today. (no pics of this bit yet)

 

[savarin]

And now its machined and threaded to size and successfully holds the two parts together as well as looking great (or so I think)

Only another two to cast and machine. At least I've finished polishing the tubes.

 

[turnitupper]

Looking great! , you're fooling yourself. That is magnificent work but I may be a little biased as I am an itinerant banana bender. Did you grind youy own lenses as I knew a painter friend in Rockhampton who ground his own but it was a PITA just watching him slave for hours over one facet.
John.

 

[savarin]

Thanks for the kind words.
The lens's came from surplus shed and were old Jaegers.
I forgot to mention I parted off a 3/4" section first before I turned the inner dia. It went very smoothly for such a large lump.

 

[savarin]

Had an awesome couple of days, so much done.
Cast the three joiners in brass in green sand.
Machined them to size, bored the baffle dia.
Then bored half way through to leave a thin section for the actual baffle and threaded the inside.



I then screwed a mandrel into the joiner, skimmed the bit thats protruding in the hope it would be more concentric when reversed to bore and thread the other side. This turned out to be the case but lets be honest, its not sub micron accuracy here.



Once the other side was finished it was impossible to remove it without marking the brass so I wrapped it in plastic tape and jury rigged a strap wrench from some steel banding



ID-10-T alert! The design was for 15mm of thread on the ends of the tubes and in the joiners.
Unfortunately after machining all the defects off the brass there was only sufficient material left for 10mm of thread so I had to either recast or shorten the tube threads. Yep, easier to shorten the threads.
I removed the bearings from my fixed steady and replaced them with oil soaked hard-wood shoes to keep the marring of the tubes to a minimum as I shortened the threaded ends



This worked really well and they all screwed up to make this. There is an "O" ring on each end of each joiner to seal the joints.



Tomorrow I'm off to get the 50mm tube for the focusser draw tube and then very little for the next 4-5 days as I have to go for an operation on my jaw. (Shameless plug for some sympathy)

 

[Franko]

Wow, Savarin. Outstanding work. I just found this thread, or I would have said it sooner.

 

[savarin]

Thank you kind sir for those words of encouragement.
This was meant to be a quick and simple project whilst I collected the bit for the giant bino.
I've got carried away.

 

[savarin]

Making the draw tube for the focusing unit was a bit of a pain.
The closest size tube I could obtain was a threads thickness too small so I had to bore and thread the inside of the tube then make an insert that was just that much thicker.

Just so the diagonal could screw on.

Still, it works and it all looks good.
Now I need a block of ally 70mm x 70mm x 50mm thick for the focusser body that will bolt to the brass end plate.
I've found a source for an 80x80x80 block for $26 but with postage I expect that will end up closer to $50.
Thats a lot of spondulics for what will be a lot of swarf. Maybe I will have to cast one.
Time to get some grain refiner and de-gassing tablets.