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Crayfordjon
Vendor - Zerochromat
*****

Reged: 06/17/09

Loc: UK
A new design, the R&P Crayford.
      #6280490 - 12/30/13 05:26 AM Attachment (183 downloads)

I have recently given thought to the best option for the Rack and Pinion Crayford focuser, and have come up with a very good design. This Hybrid focuser has been the result of complaints about commercial focusers slipping under heavy camera loads. I built the focuser and it is unbeleavably sensitive and smooth in action. The essential kinematic quality is retained to one degree of freedom as before, by preloading the tube separately to the rack and pinon drive which plays no part in the loading at all. It is the most free and smooth R&P focuser I have ever encountered. The pics shew the focuser and one view is of the focuser holding a steel block weighing 7lbs, (3.1kg).

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Crayfordjon
Vendor - Zerochromat
*****

Reged: 06/17/09

Loc: UK
Re: A new design, the R&P Crayford. new [Re: Crayfordjon]
      #6280492 - 12/30/13 05:27 AM Attachment (145 downloads)

And.

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Crayfordjon
Vendor - Zerochromat
*****

Reged: 06/17/09

Loc: UK
Re: A new design, the R&P Crayford. new [Re: Crayfordjon]
      #6280493 - 12/30/13 05:28 AM Attachment (133 downloads)

Finally a view shewing the rack fully extended.

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Ed D
Carpal Tunnel
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Reged: 04/30/10

Loc: Sunny South Florida
Re: A new design, the R&P Crayford. new [Re: Crayfordjon]
      #6280712 - 12/30/13 09:38 AM

John, I think a new revolution in focusers has just started. The best of both worlds - how ingenious is that! The beauty of it is that existing Crayford designs can be simply modified to include the R&P without a complete redesign of existing focusers.

Ed D


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Geo31
Carpal Tunnel
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Reged: 01/28/13

Loc: Kingwood, TX
Re: A new design, the R&P Crayford. new [Re: Ed D]
      #6280749 - 12/30/13 10:04 AM

Rack and Pinion and Crayford are mutually exclusive. I agree that it's a hybrid and the execution is quote nice. Why not put your own name on it because a Crayford it is not.

[edit] Realized I wrote ring and pinion instead of rack and pinion - got my head in the car world. Sorry.

Edited by Geo31 (12/30/13 10:44 AM)


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tim53
Postmaster
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Reged: 12/17/04

Loc: Highland Park, CA
Re: A new design, the R&P Crayford. new [Re: Geo31]
      #6280762 - 12/30/13 10:13 AM

How is this not just a rack and pinion focuser?

-Tim.


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Geo31
Carpal Tunnel
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Reged: 01/28/13

Loc: Kingwood, TX
Re: A new design, the R&P Crayford. new [Re: tim53]
      #6280831 - 12/30/13 10:49 AM

The focusing barrel slides along bearings as in a Crayford. The goal here appears to be to not use the R&P to set the tension. But yes, it's a R&P not a Crayford.

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Crayfordjon
Vendor - Zerochromat
*****

Reged: 06/17/09

Loc: UK
Re: A new design, the R&P Crayford. new [Re: Geo31]
      #6280972 - 12/30/13 12:07 PM

OK lets call it the WALL FOCUSER, it is an R&P using a kinematics focusing tube suspension, instead of having the tube sliding in a sleeve. The focuser is nearly frictionless, so even with the tube fully loaded from the track onto the rollers killing all side movement the tube will still roll out under gravity. When the rack and pinion engages, the right amount of slack is introduced to give free movement, and still the focuser is extremely light in action and the tube will still roll out, rotating the pinion when the tube is held downward. What R&P unit has ever been this sensitive!. A friction device on the pinion shaft is included to give resistance to "out rolling" under load.

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tim53
Postmaster
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Reged: 12/17/04

Loc: Highland Park, CA
Re: A new design, the R&P Crayford. new [Re: Geo31]
      #6280990 - 12/30/13 12:15 PM

I have a R&P on my 6" f/5 Jaegers refractor, built by a late friend (Dick Zanteson) about 15-20 years ago, that this focuser reminds me of (if I'm seeing it "right", that is). He used a focuser R&P assembly from an old stereo microscope that has a magnificient dovetail slide for the drawtube to move back and forth on. Very smooth, no backlash, and LONG travel range. The drawtube is about 2 1/2" or 3" ID, so I need a stepdown reducer for 2" eyepieces.

-Tim.


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bremms
Carpal Tunnel
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Reged: 08/31/12

Loc: SC
Re: A new design, the R&P Crayford. new [Re: tim53]
      #6281085 - 12/30/13 01:08 PM

I've built four focusers using sliding stages with an R&P.
two are dovetails two were linear bearing slides. They perform better than an standard R&P and way better than a Crayford. I'll post a pic of the one on my 6" F8 newt.


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orlyandico
Postmaster
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Reged: 08/10/09

Loc: Singapore
Re: A new design, the R&P Crayford. new [Re: bremms]
      #6282694 - 12/31/13 02:43 AM

So.. it looks like a normal crayford where the motion is constrained by the rail.. but instead of using a friction roller on the rail for movement, it uses a rack and toothed pinion.

Did I get that right? So it is a neat but logical evolution of the track type crayford...


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Crayfordjon
Vendor - Zerochromat
*****

Reged: 06/17/09

Loc: UK
Re: A new design, the R&P Crayford. new [Re: orlyandico]
      #6282714 - 12/31/13 03:19 AM

True, but the rack and pinion does not apply pressure to the tube as the smooth pinion does in a Crayford, this is done separetly by another roller. Re dovetail rack and pinion drives, Yes these are very good and I culled the R&P from just a microscope focuser, but these cannot be made by the average amateur without machine shop facilities, as they are precison engineering, but a Crayford can be made from wood and perform as well as a precision one. Also it has been mentioned that the slide focuser is better han a Crayford, I disagree, the crayford performs just as well and sometimes better than a R&P that is why it was invented in the first place. Perhaps the critisizer has fallen victim to one of the appallingly bad CF's now on the market made in the far east.

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don clement
Vendor (Clement Focuser)


Reged: 02/02/11

Loc: Running Springs, California
Re: A new design, the R&P Crayford. new [Re: Crayfordjon]
      #6282982 - 12/31/13 09:09 AM

Why not use a leadscrew?

Don


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Crayfordjon
Vendor - Zerochromat
*****

Reged: 06/17/09

Loc: UK
Re: A new design, the R&P Crayford. new [Re: don clement]
      #6283173 - 12/31/13 10:45 AM

Why not, there are many ways to skin a cat. You can get a higher ratio reduction gear that way.

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sonny.barile
professor emeritus


Reged: 10/19/10

Loc: In the middle of the glow.
Re: A new design, the R&P Crayford. new [Re: Crayfordjon]
      #6283216 - 12/31/13 11:10 AM

...but arent you creating an engineering faux-pas by preloading the tube to the rack and pinion?

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orlyandico
Postmaster
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Reged: 08/10/09

Loc: Singapore
Re: A new design, the R&P Crayford. [Re: sonny.barile]
      #6283508 - 12/31/13 01:34 PM

backlash free lead screws can be pretty expensive.. but they are available off the shelf..

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don clement
Vendor (Clement Focuser)


Reged: 02/02/11

Loc: Running Springs, California
Re: A new design, the R&P Crayford. [Re: orlyandico]
      #6283826 - 12/31/13 03:52 PM

Quote:

backlash free lead screws can be pretty expensive.. but they are available off the shelf..




Any good off-the-shelf micrometer head is backlash free.

Don

Edited by don clement (01/01/14 06:28 AM)


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bremms
Carpal Tunnel
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Reged: 08/31/12

Loc: SC
Re: A new design, the R&P Crayford. [Re: don clement]
      #6284487 - 12/31/13 10:06 PM

Don has a good point. One of my linear bearing units used a micrometer head for focusing. On an F15 it was a bit ssslllloooowwww it was a 50 thread per inch screw.

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Roy McCoy
Pooh-Bah
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Reged: 10/13/05

Loc: Glendale, AZ
Re: A new design, the R&P Crayford. [Re: don clement]
      #6284609 - 12/31/13 11:31 PM

Quote:

Why not use a leadscrew?




Wouldn't the lead screw get in way of the eyepiece? Don the lead screw on your focuser is off to the side.


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tim53
Postmaster
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Reged: 12/17/04

Loc: Highland Park, CA
Re: A new design, the R&P Crayford. [Re: Roy McCoy]
      #6284664 - 01/01/14 12:25 AM

I like the idea of a lead screw driven focuser. How 'bout a sled focuser with a lead screw, including half-nuts like on a lathe's cross slide? You'd have to put the screw on one side of the slide, of course, so you could get the eyepiece in there.

-Tim.


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