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1972 Cave custom built 6" Cassegrainian/Newtonian

classic mount equipment
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#26 DAVIDG

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Posted 30 March 2020 - 12:47 PM

 I happen to have a  selection of gasket material  in 8" x 10" sheets that came in a pack   I got at the local auto parts store. The material I used is like a dense paper with a rubber filler  and about 1/8" thick but cork should work just fine.

 

                - Dave 


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#27 macdonjh

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Posted 30 March 2020 - 01:52 PM

 The Teflon strip is not the friction pad in a Cave drive. It is thin strip a few thousands thick that goes between the ID of the 100 tooth drive gear and the raised rim of the plate it is riding one. In these drives it is a  metal to metal clutch surface. In the drives I have rebuilt I added a thin gasket of material so it wasn't metal on metal.. This greatly improved the feel of the movement and made it much easier to adjust the clutch pressure.  First of all the metal on metal surfaces are not dead smooth so the contact area is not even and with metal on metal there is  a very limited pressure range  that has the right feel to move the scope  yet stiff enough to track. Adding  the clutch material gives a much larger contact area and by doing so increases the range of pressure you can set to have the scope track and  have the right feel when you move it  ie  not too easy or too stiff.

 

                     - Dave 

Thank you from me, too DAVIDG.  So the plastic/ PTFE strips in the Cave mount (was PTFE even invented with allenawilson's mount was built?) are there to allow the worm wheel to rotate freely against the fixed housing when the clutch is disengaged?

 

Perhaps it still doesn't matter too much how thick the plastic "pads" are; don't the screws which adjust the friction in the clutch have quite a bit of "travel", at least compared to the thickness of plastic sheet available?

 

From the second half of your description of the Cave clutch it sounds quite similar to the design Losmandy uses.  My mounts also have a plastic disc between the clutch surfaces, so there is no metal-to-metal contact.  Losmandy used to use PTFE, I think, but now use UHMW (ultra-high molecular weight poly-ethylene, they always forget to put the "PE" at the end of the acronym).  I've heard of some people, years ago, who switched out worn PTFE clutch pads with cardboard because they found it had just the right amount of friction.  I never did that, I simply keep my clutch surfaces grease-free and sand the plastic discs whenever I have my mounts taken apart.


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#28 bjkaras

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Posted 01 April 2020 - 03:04 AM

I thought the smallest combination scope he made was 8”.


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#29 allenawilson

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Posted 01 April 2020 - 01:50 PM

I thought the smallest combination scope he made was 8".

I think that's right, but it's my understanding this was a custom build.  Allen.



#30 allenawilson

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Posted 08 April 2020 - 02:32 PM

I would like to add a declination drive motor to this mount.  I have an old Digitrak VE330D (that works fine for the RA drive) that has a 1/8 inch two-conductor jack on the rear of the box for a declination motor.  The manual says most people prefer to use a 1 RPM motor with an O ring drive.  Any suggestions as to a source of such motors?  



#31 DAVIDG

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Posted 09 April 2020 - 09:49 AM

 The original Cave mounts came with two types of Dec drives. One used a tangent arm assembly and the other was spur gear drive like the RA drive. Both of these drives used reversible AC motor that were coupled with mechanical linkage. Both the DEC assembly on a Cave mount  and  size of scope on these mounts require a far amount of torque to move them. They got this from the gear down in the motors and the addition gear down of the tangent arm or spur gear on the Dec shaft.

   The suggestion to use a O ring belt  drive  is more for use with a classic Celestron fork mount on their Dec tangent arm which doesn't require much torque. 

   The output from your drive corrector is most likely 12 volts DC and it is reversing the polarity to the motor to change the direction. On the Cave mounts as I said they used an reversible AC motor and to change direction, the AC neutral was switched  between which end of a capacitor it was feed into. 

  For motor sources look on Ebay.  Just be sure that any one you consider has a high enough torque. Here is listing for 2 rpm unit.

 

    https://www.ebay.com...2.c100677.m4598

    

      - Dave 


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#32 allenawilson

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Posted 09 April 2020 - 12:40 PM

 The original Cave mounts came with two types of Dec drives. One used a tangent arm assembly and the other was spur gear drive like the RA drive. Both of these drives used reversible AC motor that were coupled with mechanical linkage. Both the DEC assembly on a Cave mount  and  size of scope on these mounts require a far amount of torque to move them. They got this from the gear down in the motors and the addition gear down of the tangent arm or spur gear on the Dec shaft.

   The suggestion to use a O ring belt  drive  is more for use with a classic Celestron fork mount on their Dec tangent arm which doesn't require much torque. 

   The output from your drive corrector is most likely 12 volts DC and it is reversing the polarity to the motor to change the direction. On the Cave mounts as I said they used an reversible AC motor and to change direction, the AC neutral was switched  between which end of a capacitor it was feed into. 

  For motor sources look on Ebay.  Just be sure that any one you consider has a high enough torque. Here is listing for 2 rpm unit.

 

    https://www.ebay.com...2.c100677.m4598

    

      - Dave 

Thanks for the suggestion and especially for the link to the motor.  I see that this motor has a load torque of 10KG.CM.  Is that enough?  Also, in the description it says "The worm gear motor with self-locking, ie, in the case in the absence of electric motors, the output shaft is not to move, that is self-locking".  (I don't think it translated to English very well.  Ha!)It sounds like you couldn't move the declination shaft manually.  Is that correct?  Would this motor also work for an electric focuser?  My old JMI Motofocus motor gave up the ghost!


Edited by allenawilson, 09 April 2020 - 01:05 PM.


#33 DAVIDG

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Posted 09 April 2020 - 02:19 PM

 Does your Cave mount have a Dec slow motion assembly ? From the picture in the  sales ad it doesn't look like it does. The Cave mounts that I restored did not have the option to manually turn the DEC slow motion. but when the amount of force to turn the shaft  was such that you could do it by hand. My comments about having  to use  a motor with enough torque is many DC motors don't have much and you can stall them by just holding the shaft. So you don't want a "toy" type motor. 

   I think what they mean by "self locking" is that it would take a fair amount of torque to move the shaft when the motor is not running so it would hold it's position. 

   The JMI Focuser used Hankscraft DC motors which you can still get and would be a direct replacement. They were trying to not allow the user to figure what the motor was by melting the numbers and name on them https://www.hankscra...c-gear-3440.php

 

            - Dave 


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#34 allenawilson

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Posted 09 April 2020 - 08:34 PM

Declination .jpg  Does your Cave mount have a Dec slow motion assembly ? From the picture in the  sales ad it doesn't look like it does. The Cave mounts that I restored did not have the option to manually turn the DEC slow motion. but when the amount of force to turn the shaft  was such that you could do it by hand. My comments about having  to use  a motor with enough torque is many DC motors don't have much and you can stall them by just holding the shaft. So you don't want a "toy" type motor. 

   I think what they mean by "self locking" is that it would take a fair amount of torque to move the shaft when the motor is not running so it would hold it's position. 

   The JMI Focuser used Hankscraft DC motors which you can still get and would be a direct replacement. They were trying to not allow the user to figure what the motor was by melting the numbers and name on them https://www.hankscra...c-gear-3440.php

 

            - Dave 

     Dave, I can't thank you enough for all the information.  I don't know why, but I hadn't even looked at the back plate on the JMI Motofocus.  It's an Autotrol PX-200.  A quick google search leads me to believe I can find an exact replacement.  I'll keep this post updated about that.  

     Now to the declination.  I'll post a picture and you tell me what you think.  Sorry it's sideways.  Thanks again, Allen.


Edited by allenawilson, 09 April 2020 - 08:38 PM.


#35 DAVIDG

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Posted 10 April 2020 - 02:39 PM

 It looks like  Cave parts were used to make the manual Dec slow motion drive.  You would need to make a mounting  plate for the motor. The ones with gear like yours used a reversible AC Hurst motor. I believe they wer 1 rpm. If you want  to use your drive corrector then I would use 12 DC motor. 

 

             - Dave 


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#36 PiSigma

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Posted 11 April 2020 - 01:23 PM

As Dave said, the clutch and drive shown are Cave parts. The drive motor and mount are missing and replaced by a manual shaft, something I have not seen on a Cave before. Here are a set of pictures that show what the entire assembly looks like. There is a small internally toothed rubber coupler that joins the motor drive gear with the gear on the end of the worm gear shaft. In my pictures the adapter plate for the motor is already bolted to it.

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Edited by PiSigma, 11 April 2020 - 01:23 PM.

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#37 PiSigma

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Posted 11 April 2020 - 01:24 PM

Then there was a brass cover and end cap.

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#38 PiSigma

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Posted 11 April 2020 - 01:54 PM

Installed.

 

 

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#39 PiSigma

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Posted 11 April 2020 - 02:01 PM

Here is the clutch disassembled (top), cleaned up (middle) and reassembled (shown with RA clutch).

 

 

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#40 allenawilson

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Posted 11 April 2020 - 06:59 PM

Mr. Miles, thank you so much for the photos.  Now I know what to do.  Your restoration is beautiful.  I would like to do something similar but will probably wait.  Mine has been a workhorse in the past as I used it to guide a Schmidt camera, and I want to start doing that again.  Actually, the only part that probably won't get worked on is the optical tube.  I have drilled a few holes to allow the camera to be mounted to it.  Another question: how firmly should the primary mirror be held in place?  I have had difficulty in collimating it and suspect that there is not enough movement due to the side clips (with the cork gasket on them) being too tight.  Any suggestions?  Thanks again, Allen.



#41 PiSigma

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Posted 11 April 2020 - 08:04 PM

You're welcome Allen. If you want to try and reproduce any of the fabricated parts I did draw them for someone else on this forum from mine. I could email you the drawing if you'd like it. Finding the motor, gears and coupling are the difficult parts of course.

 

I always left a gap the thickness of a sheet of paper between the mirror and the retaining clips. A dollar bill works well. And same thing for the Delrin threaded slugs that center the mirror in the cell. I tighten them equally so they are just snug then back them off so they are not putting any pressure on the mirror.


Edited by PiSigma, 11 April 2020 - 08:04 PM.


#42 TestnDoc

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Posted 14 April 2020 - 02:20 PM

What a great looking scope. Reminds me a lot of my childhood, when my father owned a Cave 12.5" f/7 reflector.


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