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High reduction ratio Crayford

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#1 Crayfordjon

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Posted 14 November 2012 - 08:37 AM

Experience with trying to get a sharp focus with an imaging camera prompted me to design a high reduction ratio system. It has been in the back of my mind to do this for some time. Using spur gears or an epicyclic gear box built into the focusing knob is not easy, as it is not an option to cut spur gears without specialized equipment that is not available to hand crafting. But cutting a worm and wheel set is. I set up simple jig to hob a worm wheel on the lathe, using a 3/8" X 11 TPI Whitworth tap as a hobbing tool. A description of this technique will be posted at a later date. I built this new system into a 2" focuser for a client who was enthusiastic about the idea. The system works very well and one turn of the fine focus wheel will advance the eyepiece tube 0.34mm at the ratio of 55:1, the wormwheel having 55 teeth. I used "Delrin" or "Acetal" for the worm wheel.

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#2 Crayfordjon

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Posted 14 November 2012 - 08:44 AM

This is the technical specification of the worm reducer, the worm wheel rotates freely on the focusing shaft until the clanp nut is tightend, forcing it against the pressure disc fixed to the shaft.

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#3 Crayfordjon

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Posted 14 November 2012 - 08:48 AM

The actual hardware as built into the focuser. The worm was cut onto a 3/8 inch diameter turned in brass, with a piece of 6.0mm silver steel bonded into a hole drilled and reamed through it.

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#4 Crayfordjon

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Posted 14 November 2012 - 08:51 AM

Refering to the diagram ,the ratio quoted is 52:1 it should be 55;1 :foreheadslap: :p

#5 tcmzodiac

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Posted 14 November 2012 - 11:15 AM

JEEZ! Incredible. I am not going to post about my silly little gadgets anymore, but I will not take my ball and go home...yet!
Simply amazing work!

#6 dan_h

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Posted 14 November 2012 - 11:28 AM

Very nicely done. I have some 20:1 worm/wheel sets that I had often thought about putting onto a focuser shaft but I never figured out the clutching mechanism. Your solution is simple but elegant.

dan

#7 Crayfordjon

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Posted 14 November 2012 - 01:29 PM

TCM please dont be put off by my CF,s I am a builder of many years experience, you go ahead and post your efforts, you have something to contribute. I will certainly take notice of what you post, and give you encouragment.

#8 Crayfordjon

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Posted 14 November 2012 - 01:37 PM

A further innovation is the fixing of the focuser shaft or pinion bracket, I decided on a floating dynamically located assembly. The bracket is held in place by the pressure of the pinion against the eyepiece tube and it locks in place. A pressure screw is used to vary this pressure. Also there is a locking screw to jam the tube when under heavy loading.The pic shews the pinion bracket partially removed from the CF body.

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#9 Mark Harry

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Posted 14 November 2012 - 01:38 PM

Nice work, John!!!
M.

#10 Crayfordjon

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Posted 14 November 2012 - 01:39 PM

And here is a diagram of the principle.

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#11 Crayfordjon

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Posted 14 November 2012 - 01:40 PM

Some final pics

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#12 PrestonE

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Posted 14 November 2012 - 01:40 PM

Beautiful design and execution, I will definitely be remembering this for future work.

Thank you so much for sharing this brilliant work. :bow: :bow: :bow:

Best Regards,

Preston

#13 Crayfordjon

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Posted 14 November 2012 - 01:41 PM

And

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#14 Crayfordjon

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Posted 14 November 2012 - 01:42 PM

More

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#15 Crayfordjon

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Posted 14 November 2012 - 01:45 PM

Thanks Mark, thanks Preston, my pleasure entirely.

#16 tcmzodiac

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Posted 14 November 2012 - 04:14 PM

TCM please dont be put off by my CF,s I am a builder of many years experience, you go ahead and post your efforts, you have something to contribute. I will certainly take notice of what you post, and give you encouragment.


You are the kind of people who made it possible for me to complete my first scope recently...the encouragement from you-and-yours is splendid!

#17 kfrederick

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Posted 15 November 2012 - 09:15 AM

Very nice I love all your work. Cloudy nights is a huge amount of great ideas . Glad you share when we are gone our ideas will still live here I hope .

#18 glennnnnnn

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Posted 16 November 2012 - 05:08 PM

Your designs are an inspiration!

#19 Crayfordjon

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Posted 17 November 2012 - 01:56 AM

Sixty years of experience dedicated to ATM has taught me a thing or two, it is only rightI should pass it on, or it dies with me.

#20 Lightning

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Posted 17 November 2012 - 02:50 AM

Very nice, very clever clutch mechanism for the ultra-fine focus. Thanks for posting John.

-Cam






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