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Crayfordjon
Vendor - Zerochromat
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Re: A new design, the R&P Crayford. new [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. new [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
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Reged: 08/31/12

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Re: A new design, the R&P Crayford. new [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
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Re: A new design, the R&P Crayford. new [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
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Re: A new design, the R&P Crayford. new [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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tim53
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Re: A new design, the R&P Crayford. new [Re: tim53]
      #6284670 - 01/01/14 12:34 AM

It took me a while to find pics of the dovetail R&P focuser on the Zanteson 6" f/5 Jaegers:

Racked in:


Racked out:


-Tim.


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Crayfordjon
Vendor - Zerochromat
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Reged: 06/17/09

Loc: UK
Re: A new design, the R&P Crayford. new [Re: tim53]
      #6284757 - 01/01/14 02:03 AM

Nice idea about half nuts,you can disengage and push the slide up or down for rapid transit, re engage and fine up the focus.The arrangement can work without poking your eye out on the focusing knob. Dovetail slides are great of course but you have to be an expert machinist to make one to slide evenly, also the pro slides have adjustment slips built in, and the whole shebang is smeared with goop to make it move smoothly. The R&P CF moves unusually freely for an R&P focuser, there is no goop just mechnical freedom as everything runs on ball races.

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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: tim53]
      #6284915 - 01/01/14 06:35 AM

Quote:

It took me a while to find pics of the dovetail R&P focuser on the Zanteson 6" f/5 Jaegers:
-Tim.




Tim,

That looks similar to the helical R&P dovetail slide from an old American Optical Cycloptic microscope like mine as shown below. Backlash on the R&P and play in the dovetail slide are adjusted at the same time by rotating both knobs against each other.

Don



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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: bremms]
      #6284936 - 01/01/14 07:03 AM

Quote:

One of my linear bearing units used a micrometer head for focusing.




As does almost every linear stage in the Newport catalog unless direct driven by a linear motor: http://www.newport.com/Linear-Translation-Stage-Guide/1006111/1033/content.aspx

I really like the idea of direct drive. At one time I used an Electroglas 1034X wafer prober. The platen was floated on an air bearing above a Sawyer XY linear stepper drive- no contact, stiction free, backlash free precision movement. http://machinedesign.com/site-files/machinedesign.com/files/archive/motionsys... The Electoglas 1034X was a vast improvement over their original model that used rotary steppers to drive leadscrews to an ball bearing XY stage. I know because I actually have an original electroglas leadscrew wafer prober bought to work as a caustic tester.


I am thinking of a similar direct single axis drive to replace the leadscrew in my focuser, perhaps servo voice coil or Sawyer linear stepper. The drawback is power must be applied to hold position.

Don


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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: don clement]
      #6284959 - 01/01/14 07:48 AM

Speaking of rack and pinion: I believe the helical R&P is smoother than a straight spur R&P because contact is at more than just a single point or line. BTW Just lately I rebuilt the double overhead cam head on my ’97 Tacoma truck. I found it interesting that the two cams, exhaust and intake, were synchronized using two helical spur gears. One of the helical gears consisted of a split pair of coaxial helical gears. Backlash was minimized by spring loading the coaxial split helical gear against the other. In fact to install the exhaust cam which had the coaxial split gear, I had to wind up the split gear and insert a SHCS to hold the split gear in place before installing, then after all the exhaust cam bearings were torqued in place, remove the SHCS spring loading the gears. I wonder if backlash could be minimized with a helical R&P by using a coaxial pair of pinions spring loaded against each other?

Don


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Ed Holland
Carpal Tunnel


Reged: 06/16/10

Loc: San Jose, CA and Oxford, UK
Re: A new design, the R&P Crayford. new [Re: don clement]
      #6285395 - 01/01/14 11:59 AM

Good idea Don,

Good quality tuning mechanisms in old radio sets used a similar method where reduction gears were employed. Which makes me think... One could also use a tensioned cord to drive the focus mechanism.

I like the leadscrew & half-nut idea too, and was about to suggest it, but was pre-empted by Tim There are spring loading mechanisms for "full" lead screw nuts that eliminate backlash. These are certainly within the capability of an ingenious home machinist.


Alternatively, one could conceive of a 2 speed arrangement where the rack and pinion is really a worm drive with a long worm. For course adjustment the wheel can be rotated, then locked. Fine adjustment would be made by rotation of the long worm. Just a thought - and it should be highly resistant to slip, and backlash adjustable like mount drives. Again, just mental ramblings

Ed


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tim53
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Re: A new design, the R&P Crayford. new [Re: Ed Holland]
      #6285586 - 01/01/14 01:49 PM

I wish I could remember the thread from a couple years ago, where some CN member showed a drawing of exactly what you describe, Ed. Only using threaded rod to serve as the rack/worm.

-Tim.


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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: tim53]
      #6285696 - 01/01/14 02:51 PM

Quote:

I wish I could remember the thread from a couple years ago, where some CN member showed a drawing of exactly what you describe, Ed. Only using threaded rod to serve as the rack/worm.

-Tim.




Tim,

Could it have been similar to this: http://clementfocuser.com/images/Flexible_Rod_Drive_Print.pdf

Don


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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: tim53]
      #6286235 - 01/01/14 07:01 PM

Quote:

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.




As an alternative to the half-nut idea (good idea BTW): In my junk box today and came across an old Velmex Unislide with rapid advance feature. http://www.velmex.com/manual_rapid_advance.html As shown below on my junk box Unislide the rapid advance feature consists of a internal threaded rod that fits onto the leadscrew. A clamp on the outside of the rod allow for rapid changing position and also increases the travel from 1" to 4". This allows for one to keep the minimal backlash feature and have rapid positioning also. In fact in the new version of the Unislide with rapid advance feature a micrometer head is used. http://www.velmex.com/manual_micrometer_head_stage.html

Don



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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]
      #6286387 - 01/01/14 08:34 PM

Quote:

Nice idea about half nuts,you can disengage and push the slide up or down for rapid transit, re engage and fine up the focus.The arrangement can work without poking your eye out on the focusing knob. Dovetail slides are great of course but you have to be an expert machinist to make one to slide evenly, also the pro slides have adjustment slips built in, and the whole shebang is smeared with goop to make it move smoothly. The R&P CF moves unusually freely for an R&P focuser, there is no goop just mechnical freedom as everything runs on ball races.




An alternative to the more difficult to machine dovetail is a box way slide. I found an old Bausch & Lomb microscope focusing mechanism today in my junk box that uses box ways and a helical rack and pinion as shown in the photo below. The slide is made from what looks stainless steel with a rectangular cross-section. The gibs are made from a self-lubricating polymer plastic so no goop is needed for lubrication. Backlash in the box ways is taken up by set screw (grub screw). The helical pinion is held against the rack by what appears to be brass spring strips tensioned by a separate set of set screws. Box ways may not be as elegant as dovetail ways but are possibly easier to fabricate for the amateur with limited machine tools.

Don



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Crayfordjon
Vendor - Zerochromat
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Reged: 06/17/09

Loc: UK
Re: A new design, the R&P Crayford. new [Re: don clement]
      #6286816 - 01/02/14 01:59 AM

These mechanisms are all sophisticated engineering, difficult to make without machine shop expertise and equipment. The focuser I describe uses kinematic principles which are not found in any other focuser type other than a Crayford. The Crayford is easy to build, that is why it was made by amateurs for 30 years before the commercial guys finally cottoned on to it. The rack and pinion Crayford can still be made by the amateur with the minimum of tools,the focuser on this thread it is still a Crayford but uses a rack and pinion instead of a smooth pinion. We are drifting away from the issue here.

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Ed Holland
Carpal Tunnel


Reged: 06/16/10

Loc: San Jose, CA and Oxford, UK
Re: A new design, the R&P Crayford. new [Re: Crayfordjon]
      #6286904 - 01/02/14 04:30 AM

Good point John

Although I will comment that the standard of workmanship evident in your projects certainly demonstrates considerable expertise.

Ed


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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: Ed Holland]
      #6287325 - 01/02/14 11:12 AM

John,

I believe the average amateur can make a leadscrew much easier than cutting a rack & pinion which requires the use much more sophisticated machine tools. IMO it's good to explore different ideas even if sophisticated or different from one's own. I have been looking into all aspects of focuser positioning for some time now and am open to ideas not limited to just my own. As you said there are many ways to skin a cat.

Why post to this list if one does not want feedback?

Don

Insanity: doing the same thing over and over again and expecting different results. ~Albert Einstein


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Crayfordjon
Vendor - Zerochromat
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Reged: 06/17/09

Loc: UK
Re: A new design, the R&P Crayford. new [Re: don clement]
      #6287357 - 01/02/14 11:28 AM

Yeah I see your point, OK then. The average amateur does not even have to make a lead screw, you buy ready made screwed rod in metric sizes here in one metre lengths. Elsewhere no doubt other thread sizes are made too, you can get this stuff in any DIY store. Electromail will sell screwed rod all sizes in stainless steel or brass as well as Zinc plated mild steel. I used this rod for the worms in the post I did some months ago on worm drive reduction gears and how easy it is to cut a wormwheel on the lathe.

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Geo31
Carpal Tunnel
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Reged: 01/28/13

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Re: A new design, the R&P Crayford. new [Re: Crayfordjon]
      #6287432 - 01/02/14 11:59 AM

Sorry to belabor the point, but I remember the S&T article that started the widespread popularity of the Crayford. The Crayford was characterized by two essential things:

1. Focusing by friction between the focusing shaft (or a friction material on the shaft) and the barrel (but no R&P).

2. Bearings that the barrel slid along.

From the Wikki Crayford page:

"The Crayford is similar in appearance to a Rack and pinion focuser, but has no teeth on either the rack or the pinion. Instead, a round axle is pressed (for example by a spring-loaded or thumbscrew-tightened piece of PTFE plastic) against a flat on the side of the focuser drawtube, relying only on friction to move the drawtube as the axle is turned. This also presses the drawtube against a set of four ball bearings against which it moves smoothly with minimal friction."

I'm not seeing how this could to considered a Crayford.


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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: Geo31]
      #6287494 - 01/02/14 12:29 PM

Is it your intention to make an R&P with the tensioning of a Crayford???????? If that is the case, Corect me if I am wrong but I believe it is considered a design flaw to preload an R&P this way. Tooth mesh is not supposed to bottom out and by squeezing it you make it wear incorrectly and it will eventually seize.

Edited by sonny.barile (01/02/14 12:33 PM)


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m. allan noah
scholastic sledgehammer


Reged: 08/14/09

Loc: Virginia, USA
Re: A new design, the R&P Crayford. new [Re: sonny.barile]
      #6288249 - 01/02/14 06:46 PM

Quote:

Is it your intention to make an R&P with the tensioning of a Crayford???????? If that is the case, Corect me if I am wrong but I believe it is considered a design flaw to preload an R&P this way. Tooth mesh is not supposed to bottom out and by squeezing it you make it wear incorrectly and it will eventually seize.




He said he preloaded the tube separately from the rack...

allan


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Ed Jones
Carpal Tunnel
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Reged: 04/06/04

Loc: Sin-sin-atti
Re: A new design, the R&P Crayford. new [Re: m. allan noah]
      #6288525 - 01/02/14 09:22 PM

If the intent is to prevent slippage with heavy loads couldn't you just sand blast the shaft and tube areas that are in contact to increase friction or possibly knurl both in one direction to act like fine helical gears?

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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: Ed Jones]
      #6288561 - 01/02/14 09:38 PM

Quote:

If the intent is to prevent slippage with heavy loads couldn't you just sand blast the shaft and tube areas that are in contact to increase friction or possibly knurl both in one direction to act like fine helical gears?




The helical R&P is an involute gear form. http://en.wikipedia.org/wiki/Involute_gear Can't expect just random sand blasting to replicate the smooth rolling motion of an involute form.

Don


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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: don clement]
      #6288637 - 01/02/14 10:13 PM

Oh! Thanks, Now I get it. Please forgive my ignorance of the design as I am trying to work out a simple but precision acting focuser for a scope I am building and I do not have any experience in this area.

Are these tubes all the same material? Would I need to worry about galling if I did something similar?


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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]
      #6288842 - 01/03/14 12:54 AM

Jon,

I built a 2” Crayford hybrid for an 80mm finder scope that uses a leadscrew for focus positioning . The leadscrew is actually the drawtube that is threaded on the OD. On the outside of the threaded drawtube I machined three equally spaced axial flats the full length of drawtube in which five bearings support the drawtube in the same way as a Crayford. A captured nut threads onto the drawtube and when rotated positions the drawtube for focus. Shown below is the 2" Crayford hybrid. Note the Capitan's wheel focusing knob. This hybrid design evolved from a non-rotating helical focuser I built and shares many of the same parts. The main difference between the non-rotating helical is that the drawtube is supported by five bearings riding in the three axial machined flats and not by the threaded drawtube. The five bearings also prevent the drawtube from rotating when the focusing.

Don



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Crayfordjon
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Re: A new design, the R&P Crayford. new [Re: sonny.barile]
      #6288963 - 01/03/14 03:33 AM

Sonny. The pinion is not pressed against the rack, pressure on the focusing tube is affected by a separate roller which is preloaded, and is independant of the rack and pinion. The pinion engages the rack at any running clearance desired. Yes you are right, jamming the pinion into the rack is very bad news and as a design engineer I would not do this.

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Re: A new design, the R&P Crayford. new [Re: Crayfordjon]
      #6288966 - 01/03/14 03:36 AM

Don that is a clever design, neat!

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Re: A new design, the R&P Crayford. new [Re: Crayfordjon]
      #6288971 - 01/03/14 03:51 AM

Ed. sand blasting the tube is an option, but it would introduce gritty focusing which would prevent very fine adjustment for imaging, Knurling would be worse, I have considered these options but rejected them, I actually built a large focuser for a thirty inch scope using high powers, but the three inch focusing tube had been CNC machined and had a very fine threaded surface due to the rapid transit of the tool, roughing out in the old days when we actually handled machine tools ourselves, the action of the smooth pinion produced a burring vibration, and I turfed the tube back to be remachined smooth. In my original specifications for the design of the Crayford focuser, I stipulated the use of centerless ground stainless steel tube for the focusing barrel, this does not gall, the alternative is to use smooth aluminium tube which had been anodised.

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Re: A new design, the R&P Crayford. new [Re: Crayfordjon]
      #6289191 - 01/03/14 08:43 AM Attachment (19 downloads)

To clarify the principle I have hand sketched a diagram showing how the R&P Crayford works. The upper pressure rollers straddle the rack which is fastened to a flat track mounted on top of the focusing tube. The assembly coloured red is adjustable and is secured to the base plate via a flat spring hinge. By tightening the pressure screw the tube is loaded against the bottom rollers, the system so far is similar to a classical CF in that the focusing tube is mounted kinematically in the Vee rollers, allowing the tube one degree of freedom, IE in and out. Instead of the smooth pinion a rack and pinion is used to focus. The R&P is mounted directly to the fixed bridge and acts independently to the pressure rollers, so that the pinion does not at any time bind down onto the rack. The roller bracket ( red), does not really move but applies pressure only, so that the clearance in the R&P is virtually unaffected. As has been stated before, the R&P Crayford is a Hybrid designed to accept really heavy payloads without slipping, yet retains the smoothness and freedom from stiffness that ordinary R&P's do not have.

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Re: A new design, the R&P Crayford. new [Re: Crayfordjon]
      #6289215 - 01/03/14 09:09 AM

Wonderfully simple design John to fix the problem with heavier loads on the normal Crawford .

Best Regards,

Preston


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Re: A new design, the R&P Crayford. new [Re: don clement]
      #6289320 - 01/03/14 10:30 AM

In my Crayford-hybrid design there is no metal-to-metal contact in either the threaded "leadscrew" or the five bearing contacts because Turcite self-lubricating material is used at the typical metal-to-metal interfaces, thus the issue of the roller bearing small area, high pressure, metal-to-metal contact is eliminated. So no galling. Also soft aluminum (without the need for hardened inserts) can be used for the bearing surfaces and leadscrew threads with out the need for "goop" grease or oil type lubrication. The use of the leadscrew also eliminates the slipping of the friction roller found in a typical Crayford. Unlike the friction roller with limited reduction ratio, the use of a leadscrew also allows for a wide range of reduction ratios by simply choosing the thread pitch from fine pitch to multi-start coarse pitch. Addition gear reduction would not be required as there is with the friction roller. There still is stiction, however for visual applications like the 80mm finder that the Crayford-hybrid was designed for works extremely well.

Don


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Re: A new design, the R&P Crayford. new [Re: don clement]
      #6289940 - 01/03/14 03:38 PM

Jon Thanks for sharing your sketch. So if I am seeing this properly, the tube is sliding as it is constrained between bearings with one bearing point being spring loaded. So the linear motion (driven by the rack) is the only degree of freedom. ????? So the maximum movement that can be achieved is the length from the back bearing to the edge of the focuser tube?



PS I am also a mechanical designer by trade........

Edited by sonny.barile (01/03/14 03:39 PM)


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Re: A new design, the R&P Crayford. new [Re: sonny.barile]
      #6291055 - 01/04/14 03:39 AM

The tube rolls along four ball races set at 45 degrees in vee configuration to emulate a Vee block as used in engineering workshops. This configuration is a kinematic support in that it constrains the tube to just one degree of freedom, that is, it can only move back and forth axially. This is what is desired in a focuser as no other movement will occur, in this way the problems in an R&P focuser, such as side wobble sticking, erratic movement etc are eliminated. In the R&P Crayford, these parameters are retained, so all that is required is to preload the tube against the rollers using another set at the top. The movement of the tube axially depends on how long you want the tube to be before it runs off the rollers.

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Re: A new design, the R&P Crayford. new [Re: Crayfordjon]
      #6291057 - 01/04/14 03:41 AM

Don, why dont you open another thread on your focuser design, I am sure it will generate a lot of interest, with diagrams too.

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don clement
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Re: A new design, the R&P Crayford. new [Re: Crayfordjon]
      #6291326 - 01/04/14 09:23 AM

Quote:

Don, why dont you open another thread on your focuser design, I am sure it will generate a lot of interest, with diagrams too.




Jon,

Don't think so. This cat has been skinned enough. You continue on with the Crayford taxidermy. I am going on to something completely different Nudge, nudge. Wink wink. Say no more.

Don


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Re: A new design, the R&P Crayford. new [Re: don clement]
      #6293315 - 01/05/14 07:57 AM

Jon,

Both your R&P and my leadscrew do essentially the same thing: that of overcoming the limitations of slipping and limited reduction with the friction roller in the Crayford design. However the R&P is offset from the center axis which violates the Abbe rule (Ernst Abbe not NCIS Abby) but the centrally located leadscrew is in alignment with Abbe to minimize error.

Don


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Re: A new design, the R&P Crayford. new [Re: don clement]
      #6293906 - 01/05/14 01:18 PM

The Crayfed is fine for ordinary eyepiece work, there is no problem there as eyepieces are not heavy, but the classical CF will not handle heavy cameras unless you really load up the pinion. I have successfully built several heavy duty CF's that will take heavy loads that use this method and published these on this forum. Without avail I am afraid. The best solution is to use an R&P or leadscrew as you suggest, as this is a positive solution to the slipping problem, without heavy loading. It is time the commercial boys took notice of what is suggested here on the ATM forum and got on the ball on this one, but again without avail!.

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Re: A new design, the R&P Crayford. new [Re: Crayfordjon]
      #6297385 - 01/07/14 03:01 AM

Good morning John, just turned on the tv and saw you doing a piece for BBC1 :-)

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Re: A new design, the R&P Crayford. new [Re: John Carruthers]
      #6297401 - 01/07/14 03:17 AM

I saw it in the Midlands news, a full version will appear at 18.30 hrs tonight, you saw it on News SE no doubt.

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charles genovese
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Re: A new design, the R&P Crayford. new [Re: Crayfordjon]
      #6297506 - 01/07/14 07:29 AM

Orion already has one

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don clement
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Re: A new design, the R&P Crayford. new [Re: charles genovese]
      #6297517 - 01/07/14 07:44 AM

Quote:

Orion already has one




Why no patent? Prior knowledge?

Don


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Dick Jacobson
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Re: A new design, the R&P Crayford. new [Re: don clement]
      #6297832 - 01/07/14 11:13 AM

Quote:

Quote:

Orion already has one




Why no patent? Prior knowledge?

Don



Maybe there isn't enough money in the amateur astronomy market to make patenting worthwhile.


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Re: A new design, the R&P Crayford. new [Re: Dick Jacobson]
      #6297867 - 01/07/14 11:29 AM

I did think of it independently but do not fool myself that it is original, I posted it for interest only. I also invented the reverse focuser about a year before JM brought it out, and rejected the prototype as inadequate. It is interesting that Jim's version was exactly the same as mine, covergent evolution I guess!. Jim's focuser works very well as you have to have accurate machining to get the platen exactly parallel with the tube.

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don clement
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Re: A new design, the R&P Crayford. new [Re: Crayfordjon]
      #6297967 - 01/07/14 12:29 PM

Quote:

I did think of it independently but do not fool myself that it is original, I posted it for interest only.




Jon,

That could be true as John Strong had proposed something very similar in his 1938 publication “Procedures in Experimental Physics” in Chap. XIV ‘Construction and Design “. Fig. 20 shown below is from that chapter which discusses kinematic design: “ The different ways in which the principle of kinematic design may be used for positioning the various elements of instrument are illustrated by Figs. 11 to 23.”

Don



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Re: A new design, the R&P Crayford. new [Re: don clement]
      #6299309 - 01/08/14 03:26 AM

I have seen this patent before. many years after I invented the CF, I had not seen it before then. so I did not plagerise the idea. This design uses a flat bed veeblock, and so is not a Crayford focuser, it is subject to friction and errors of linear movement. My design uses four ball races from the onset of the original concept, so that is how the CF differs widely from Strong's ideas. Note that it did not take off like the Crayford did, back in 1969.

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don clement
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Re: A new design, the R&P Crayford. new [Re: Crayfordjon]
      #6299490 - 01/08/14 08:08 AM

Jon,
No one is proposing that you plagiarized the idea at all as independent ideas are thought up all the time.
True you did popularize the Crayford design. John Stong did not patent the idea AFAIK. However there is no real difference between John Stong’s use of a Vee configuration in his kinematic focuser and your idea of configuring roller bearings to emulate Vee block in your kinematic Crayford. In your own words: “The tube rolls along four ball races set at 45 degrees in vee configuration to emulate a Vee block as used in engineering workshops.” Flexbar actually make a Vee block using ball bearings: http://www.newmantools.com/flexbar/flex160.htm

Don

IMO idealized kinematic principles are only realized on paper in or in low load laboratory conditions. In the real world with real world loads (e.g. heavy camera) the assumptions used when applying idealized kinematic principles simply break down. In the real world materials are not rigid but are in fact elastic, surfaces are not really smooth but are rough, and the very act of reducing physical contacts to "points" cause immense pressures and stress concentrations in real world materials with real world loads.


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steveastrouk
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Re: A new design, the R&P Crayford. new [Re: don clement]
      #6299593 - 01/08/14 09:18 AM

Quote:


IMO idealized kinematic principles are only realized on paper in or in low load laboratory conditions. In the real world with real world loads (e.g. heavy camera) the assumptions used when applying idealized kinematic principles simply break down.




Absolutely right, and a Crayford is overconstrained anyway, surely ? It should only HAVE three contacts on the sliding cylinder, not four.

Thanks for the heads-up on the Strong book Don - what a goldmine.


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Re: A new design, the R&P Crayford. new [Re: steveastrouk]
      #6299699 - 01/08/14 10:18 AM

Have you tried to support a cylinder on three supports?? Try it and see how far you get, then report back.

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George Kiger
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Re: A new design, the R&P Crayford. new [Re: Crayfordjon]
      #6299793 - 01/08/14 11:01 AM

Three points of contact is the same as for a table or stool - three legs are sufficient. For the Crayford, just replace two of the rollers in a line with one roller at a point between the two replaced rollers. Not, in my opinion, as stable as the Crayford, but certainly possible.

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starcanoe
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Re: A new design, the R&P Crayford. new [Re: George Kiger]
      #6299807 - 01/08/14 11:10 AM


Put two rollers on one side on a bar with a pivot half way between them.

More stable and ALL rollers can still maintain contact.


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Re: A new design, the R&P Crayford. new [Re: George Kiger]
      #6299902 - 01/08/14 12:09 PM

Quote:

Three points of contact is the same as for a table or stool - three legs are sufficient.



Yes, for a relatively flat surface 3 points are fine - but you can slide a table
or stool in any direction.

For a cylinder, where you want to constrain it to linear movement
parallel to the cylinder axis, 4 points are necessary.


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don clement
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Re: A new design, the R&P Crayford. new [Re: EJN]
      #6299918 - 01/08/14 12:22 PM

Is a ball bearing race kinematic or simply over constrained?

Don


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George Kiger
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Re: A new design, the R&P Crayford. new [Re: EJN]
      #6299922 - 01/08/14 12:24 PM

Quote:

Quote:

Three points of contact is the same as for a table or stool - three legs are sufficient.



Yes, for a relatively flat surface 3 points are fine - but you can slide a table
or stool in any direction.

For a cylinder, where you want to constrain it to linear movement
parallel to the cylinder axis, 4 points are necessary.




Sorry, but no. See reply by starcanoe above where the explanation is clearer than the one I gave.


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Re: A new design, the R&P Crayford. new [Re: George Kiger]
      #6300031 - 01/08/14 01:20 PM Attachment (7 downloads)

Quote:

See reply by starcanoe above where the explanation is clearer than the one I gave.



Without a diagram or illustration, the explanation is clear as mud.

Here is another example: you can attach an OTA rigidly to a mount with a cradle made
using 2 V-blocks and a strap. The tube will naturally form 4 contact points even if not
perfectly round. I don't see how only 3 contact points could be stable.


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Crayfordjon
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Re: A new design, the R&P Crayford. new [Re: EJN]
      #6300068 - 01/08/14 01:31 PM

I agree the statement is based on a profound well of ignorance. If you support a cylinder on three points two will be in a Vee form and the third must touch the cylinder further back and on the centreline, it will work fine as a tripodial support, but what is there to stop the cylinder sliding off to one side.???

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don clement
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Re: A new design, the R&P Crayford. new [Re: EJN]
      #6300071 - 01/08/14 01:32 PM

If the cylinder had three flats equally spaced milled axially the length of the cylinder as in my leadscrew-Crayford hybrid, would only three contact pads be needed instead of four and be stable? Note that the pads not points "bend" the kinematic rules, but hey this is the real world. If using roller bearings like in the Crayford there is no point contact but a line contact from the roller anyhow.

Don


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steveastrouk
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Re: A new design, the R&P Crayford. new [Re: Crayfordjon]
      #6300116 - 01/08/14 01:59 PM Attachment (11 downloads)

Quote:

I agree the statement is based on a profound well of ignorance. If you support a cylinder on three points two will be in a Vee form and the third must touch the cylinder further back and on the centreline, it will work fine as a tripodial support, but what is there to stop the cylinder sliding off to one side.???






Jon,
Turn that 90 degrees, or look at the attached photo. In perfect stability.


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Re: A new design, the R&P Crayford. new [Re: steveastrouk]
      #6300130 - 01/08/14 02:10 PM

But there is nothing constraining the tube from rotating left-right
in the setup in the photo. I just tried it, as shown, and the slightest
bump causes it to rotate and fall off my fingers, now matter how widely
spread the points are.

Now if you machined flats at the contact points it would be more stable, but
that ruins the basic simplicity. 4 points are stable using just a simple
unmodified cylinder. 4 points constrain it from any rotation orthogonal
to the axis of the cylinder, it's just basic geometry.


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steveastrouk
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Re: A new design, the R&P Crayford. new [Re: EJN]
      #6300159 - 01/08/14 02:25 PM

Jon claims its not stable, it is. The rotational constraint would be applied by the drive roller on top. The Crayford will also twist on its axis with out the pinch roller.

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Re: A new design, the R&P Crayford. new [Re: steveastrouk]
      #6300189 - 01/08/14 02:39 PM

Quote:

Jon claims its not stable, it is. The rotational constraint would be applied by the drive roller on top. The Crayford will also twist on its axis with out the pinch roller.




I have 3 Crayford focusers. With the pinch roller disengaged, the only way it can
twist is if it loses contact with 2 support points, which even the slightest
pressure from the pinch roller prevents. In the configuration you show, it will
twist even when in contact with all 3 support points. Even with a drive roller.

In geometry, 3 points (a triangle) can only uniquely determine a plane. The drive roller
will not work as a 4th support point in the configuration shown. In the picture I posted
of the tube mounted in V-blocks, it would not be stable in your configuration
even with the strap tightened.

The drive roller in a standard Crayford does not act as a 5th support point, but
as a constraint to maintain contact with the 4 support points.

If you think I'm wrong, then build one with 3 points & post the results here.


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don clement
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Re: A new design, the R&P Crayford. new [Re: EJN]
      #6300244 - 01/08/14 03:11 PM

In the Crayford that uses the outer rim of roller bearings there are no contact "points", rather there is a line of contact by the rim of each roller bearing. Van Slyke patented a modified Crayford that uses a whiffle tree so that two bearings with two lines of contact for each bearing of the four so-called "points" of contact in the original Crayford.

Don


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Re: A new design, the R&P Crayford. new [Re: don clement]
      #6300256 - 01/08/14 03:19 PM

The rim of a roller bearing can only contact a curved surface at a single
point which is the intersection of the tangent vectors of the 2 surfaces, unless
the axis of the 2 curved surfaces are parallel, in which case the tangent vectors
are also parallel. In the Crayford using roller bearings (all 3 I own do), the
axis of the bearing is perpendicular to the axis of the focuser tube. If they
were parallel, you could spin the focuser tube really nicely, but it wouldn't
move in & out very well, if at all.

If flats are made on the focuser tube for the bearing surfaces as well
as the pinch roller, then you would have a line contact surface.



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steveastrouk
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Re: A new design, the R&P Crayford. new [Re: EJN]
      #6300257 - 01/08/14 03:19 PM

My photo was illustrating the point that three points is stable. The implementation would have to be on rollers anyway.

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Re: A new design, the R&P Crayford. new [Re: steveastrouk]
      #6300268 - 01/08/14 03:26 PM

Quote:

My photo was illustrating the point that three points is stable. The implementation would have to be on rollers anyway.




The configuration you show is, in physics, called metastable, like a pencil
balanced on its tip. The slightest perturbation causes it to become unstable.


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don clement
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Re: A new design, the R&P Crayford. new [Re: EJN]
      #6300335 - 01/08/14 04:00 PM

Quote:

The rim of a roller bearing can only contact a curved surface at a single
point which is the intersection of the tangent vectors of the 2 surfaces, unless
the axis of the 2 curved surfaces are parallel, in which case the tangent vectors
are also parallel. In the Crayford using roller bearings (all 3 I own do), the
axis of the bearing is perpendicular to the axis of the focuser tube. If they
were parallel, you could spin the focuser tube really nicely, but it wouldn't
move in & out very well, if at all.

If flats are made on the focuser tube for the bearing surfaces as well
as the pinch roller, then you would have a line contact surface.






The assumption is that materials are rigid and non elastic. In the real world there is no such thing. Particularly the Crayford with hardened rims bearing on a soft aluminum tube. Kinematic contact "points" exist only on paper or in someone's mind. BTW what are the kinematics of the caged roller bearing?

Don


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Re: A new design, the R&P Crayford. new [Re: don clement]
      #6300353 - 01/08/14 04:08 PM

I'm talking general principles. In the case of a Crayford, unless you apply
hundreds of pounds of force to the pinch roller, the contact area is for all
practical purposes a point. This is getting like a debate of how many angels can
dance on the head of a pin. Were talking about a focuser for amateur telescopes,
not a piece of heavy industrial machinery.

You seem to like to argue just for the sake of argument. Not a good strategy
for a vendor.



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George Kiger
member


Reged: 05/19/06

Re: A new design, the R&P Crayford. new [Re: steveastrouk]
      #6300383 - 01/08/14 04:31 PM

Quote:

Quote:

I agree the statement is based on a profound well of ignorance. If you support a cylinder on three points two will be in a Vee form and the third must touch the cylinder further back and on the centreline, it will work fine as a tripodial support, but what is there to stop the cylinder sliding off to one side.???






Jon,
Turn that 90 degrees, or look at the attached photo. In perfect stability.




I do not understand why some cannot see that three points is sufficient - maybe if you had your finger supports 120-deg apart in your photo then it would be more obvious that the only way for the tube to 'fall' would be to raise it off of the supports (which would also cause havoc with the Crayford sans pincher).


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steveastrouk
super member


Reged: 08/01/13

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Re: A new design, the R&P Crayford. new [Re: EJN]
      #6300402 - 01/08/14 04:42 PM

Sorry, I believe you are wrong. The system, as I had it balanced on my fingers was completely stable,since the COG was within the support triangle

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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: EJN]
      #6300471 - 01/08/14 05:21 PM

Quote:


You seem to like to argue just for the sake of argument. Not a good strategy
for a vendor.



EJN,

Sounds like a threat. I call it as I see it and in the real world deformation of materials, galling and Brinelling http://en.wikipedia.org/wiki/Brinelling is a real world issue particularly with the hard rollers on soft (aluminum) tube. BTW have you actually built a focuser yourself?

Don


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EJN
Carpal Tunnel
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Re: A new design, the R&P Crayford. new [Re: don clement]
      #6300488 - 01/08/14 05:29 PM

Quote:

Sounds like a threat.



Are you serious?



Double facepalm

To clarify: Every person on this forum is a potential customer.
It pays not to antagonize them.

Another vendor (call him X) started arguing with someone in another thread,
finally the other person said: note to self - never buy a ***** from vendor X.


I'm done with this thread.


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starcanoe
super member


Reged: 11/07/13

Re: A new design, the R&P Crayford. new [Re: EJN]
      #6300541 - 01/08/14 06:04 PM


While it is true that there are no such things as points or lines in the real world when it comes to things in contact with each other, for all practical purposes they still are for this discussion when it comes to the kinematics.


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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: starcanoe]
      #6301225 - 01/09/14 01:10 AM

OT.

I think the customers for Don's focusers wouldn't be dissuaded by his forum manner. The reality is if you're putting out $1000 for a focuser, you know your requirements very, very well, and argumentative posts on a forum don't really matter.

Reminds me of Roy from Shipping Wars.

And yeah, I think Don is overly argumentative (plus the recurring theme of evil cheap Far East labor) but it's a free forum. And he's right. Every darn Far East crayford I've owned has brinelling at certain points on the aluminum flat from the over-tightening of the pinion in an attempt to arrest slippage or to lift a heavy camera.

Hence any mechanism to arrest slippage without loading the focuser tube (like this R&P crayford) is very welcome.

I know a very well-made crayford like a Feathertouch will have very minimal slippage, but on mine I still have to set the brake to prevent slipping and that does tighten up the focuser pinion (although not as much as a Far East model, and it doesn't feel rubbery, show focus shift, or stiction). A solution that does not cause any tightening up would be very interesting...


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EJN
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Re: A new design, the R&P Crayford. new [Re: orlyandico]
      #6301251 - 01/09/14 01:49 AM

OK, 2 comments then I am really done.

Quote:

Every darn Far East crayford I've owned has brinelling at certain points on the aluminum flat from the over-tightening of the pinion in an attempt to arrest slippage or to lift a heavy camera.



I tore down and reworked a GSO and 2 Synta Crayfords (all single speed), now they work silky smooth,
but I use them strictly for visual, have no idea how they would handle a camera.



Quote:

And yeah, I think Don is overly argumentative (plus the recurring theme of evil cheap Far East labor) but it's a free forum.



Other than being annoying, its a potential TOS violation:

Quote:


Vendors:

<snip>

Vendors are expected to act professionally and are held to a high standard when posting.





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Crayfordjon
Vendor - Zerochromat
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Reged: 06/17/09

Loc: UK
Re: A new design, the R&P Crayford. new [Re: EJN]
      #6301256 - 01/09/14 01:59 AM

Yes we are arguing infitesimals here are we not. The solution to a soft tube is to use a hard one, I advocated ground stainless steel for the tube, why manufacturers will not do this is a mystery, centerless ground stainless tubes are easily available and still cheap. The rubbish churned out from the far east ia a disgrace, and is undermining the reputation of the CF.

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orlyandico
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Re: A new design, the R&P Crayford. new [Re: EJN]
      #6301274 - 01/09/14 02:38 AM

Quote:

I tore down and reworked a GSO and 2 Synta Crayfords (all single speed), now they work silky smooth,
but I use them strictly for visual, have no idea how they would handle a camera.




I also tore down my Synta 2" single-speed from an Orion 100ED, added some teflon slides to replace the cheap plastic.

Also tore down, lubricated, and cleaned two William-Optics 2" and a Zhumell 2" (all two-speed Long Perng crayfords). All have brinelling from over-tight pinions. And tightening the brake deflects the tube (gravity also does this) which makes the CCD non-perpendicular to the OTA axis. Which in turn causes non-round stars at two opposite edges of the chip. When you've spent big bucks on a mount so that your stars are round, having those round stars ruined by a sub-standard focuser is extremely annoying.

I also have a 2.7" Long Perng rack and pinion, which is much better than the 2" LP crayfords, because the focuser doesn't sag with a long/heavy imaging load (probably due to the more than 3" OD of the barrel). But it does image shift when reversing and when tightening the brake, and feels "rubbery" when reversing direction which makes precise focusing hard. Still, I would choose the 2.7" R&P over the LP crayfords any day of the week. At least it delivers round stars.

After finally biting the bullet and getting a Feathertouch, I now see what it's all about. No image shift when reversing direction or tightening the brake. No backlash that I can detect. Simply no comparison to the Synta and Long Perng offerings. I am led to wonder how much better the Feathertouch R&P is.. but it's way more expensive.

The only flaw of the Feathertouch 2" crayford is that if the brake is loose so that focusing is truly buttery smooth, the focuser will slip down to its lowest position when I let go of the fine focus wheel. This is where a solution like the one posted by the OP would help - the rack prevents slippage. My Long Perngs would slip even with the focuser knob fixed, unless the pinion were tightened significantly using the grub screws - hence the brinelling.

I have never seen the FT R&P, but based on their description, the drawtube rides on stainless steel and teflon, not bearings. Hence I think this R&P crayford is a fantastic idea!

Note that almost everything I've said is irrelevant for visual. And unless your guiding/tracking is perfect, mount imperfections will mask the focuser issues.


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Crayfordjon
Vendor - Zerochromat
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Reged: 06/17/09

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Re: A new design, the R&P Crayford. new [Re: orlyandico]
      #6301833 - 01/09/14 11:26 AM

I hand built a 3 inch focuser for an imaging friend and it works perfectly with heavy equipment on it, it has a micro stepper motor and there is no issue there either. Fact is that a lot of Crayfords rely upon an interference fit for tightness of the tube on the rollers, also the rollers are far too small. I use 16mm dia ball races, large I know but the smooth action is worth the "lack of styling" which is more important than good design in many cases. I also use variable pressure on the pinion using a knurled screw, forcing a flexihinge pinion bracket down onto the tube, You have obviously fallen victim to bad design, in your focusers.

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Chuck Hards
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Re: A new design, the R&P Crayford. new [Re: don clement]
      #6301882 - 01/09/14 11:48 AM

Quote:

I call it as I see it and in the real world deformation of materials, galling and Brinelling http://en.wikipedia.org/wiki/Brinelling is a real world issue particularly with the hard rollers on soft (aluminum) tube.

Don




Don, you are 100% correct, I've noticed formerly smooth commercial Crayfords get progressively worse over time, all use steel rollers on aluminum tubes. And once I bought a Baader Steeltrack focuser, I realized that I didn't really know what a smooth focuser was until that purchase.


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ausastronomerModerator
Carpal Tunnel
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Reged: 06/30/03

Loc: Kiama NSW (Australia)
Re: A new design, the R&P Crayford. new [Re: Chuck Hards]
      #6303318 - 01/10/14 12:27 AM

Keep it civil please people; and play nice otherwise this thread will be locked. Vendors should note that the bar is set higher in this regard.

Cheers


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Crayfordjon
Vendor - Zerochromat
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Reged: 06/17/09

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Re: A new design, the R&P Crayford. new [Re: ausastronomer]
      #6303389 - 01/10/14 02:02 AM

Enough has been said by myself on this thread, after all, all I did was to put forward a new design to combat slipping in the traditional CF, I did not intend this to degenerate into bad critiscism of the CF concept. You allways get these people intruding whenever a new idea is launched on ATM, all too ready to tear down and grind the ideas into the mud.

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orlyandico
Postmaster
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Re: A new design, the R&P Crayford. new [Re: Crayfordjon]
      #6303497 - 01/10/14 04:37 AM

I personally think that it's a concept that's crying out to be productized.

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steveastrouk
super member


Reged: 08/01/13

Loc: Rochdale, UK
Re: A new design, the R&P Crayford. new [Re: Crayfordjon]
      #6303644 - 01/10/14 07:46 AM

Quote:

I did not intend this to degenerate into bad critiscism of the CF concept.




Did it ? As I saw it, it was a criticism of the materials of construction on some implementations of the Crayford, and some comments on kinematics.


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Chuck Hards
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Re: A new design, the R&P Crayford. new [Re: steveastrouk]
      #6303658 - 01/10/14 08:02 AM

^ Exactly. My comments concerned commercial execution of the design, including materials choice. The design itself is quite elegant and a masterful demonstration of constrained kinematics.

The current trend of manufacturers to pair steel tracks with steel bearings demonstrates that the Crayford is still evolving in the marketplace, and improving. A hybrid is one more step in the right direction, in my opinion.


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charles genovese
professor emeritus
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Reged: 02/04/06

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Re: A new design, the R&P Crayford. new [Re: Chuck Hards]
      #6303665 - 01/10/14 08:12 AM

I still don't see how the idea is different from this:

http://www.telescope.com/Accessories/Telescope-Focusers/Orion-2-Low-Profile-D...


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George Kiger
member


Reged: 05/19/06

Re: A new design, the R&P Crayford. new [Re: Crayfordjon]
      #6303786 - 01/10/14 09:40 AM

Quote:

Enough has been said by myself on this thread, after all, all I did was to put forward a new design to combat slipping in the traditional CF, I did not intend this to degenerate into bad critiscism of the CF concept. You allways get these people intruding whenever a new idea is launched on ATM, all too ready to tear down and grind the ideas into the mud.




Backlash is a typical issue with RP focusers - how would you say this hybrid addresses that issue?


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Crayfordjon
Vendor - Zerochromat
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Reged: 06/17/09

Loc: UK
Re: A new design, the R&P Crayford. new [Re: George Kiger]
      #6303941 - 01/10/14 10:51 AM

It does not, you will always get backlash in an R&P as you must have clearance in the meshing of the pinion to the rack. Total meshing will work of course but you get binding to replace backlash. However if the heavy camera etc is pointing up at the sky, the weight of the equipment will take up the back lash, I have applied a variable brake to the pinion shaft and it can be given roll out resistance, I have lifted 15kg up with the brake on and it does not roll back when pointing vertically. I will publish full specs on this R&P Crayford soon so that other people can make them.

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orlyandico
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Re: A new design, the R&P Crayford. new [Re: George Kiger]
      #6305304 - 01/11/14 12:34 AM

I'd say it does not. But since the tube is constrained by bearings the rack and pinion won't deflect the tube when reversing. Making the backlash only a slight inconvenience.

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Crayfordjon
Vendor - Zerochromat
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Reged: 06/17/09

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Re: A new design, the R&P Crayford. new [Re: orlyandico]
      #6305472 - 01/11/14 04:07 AM

The tube on its ball race supports is free from side wobble and sticking, but if the rack is a bad one it can introduce "jump" into the motion, that is, while focusing, there can be a sudden resistance then the tube will jump forwards within the limits of the backlash clearance. I cannot make a rack and pinion as I do not have machining facilities that will cut a rack, or a pinion. I recycle these items from ex equipment, in the case of the demonstration R&P CF, the items were removed from a piece of precision optical equipment that was surplus. You can get a rack and pinion from a focuser of course, and cannibalise it, there are plenty of these around.

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steveastrouk
super member


Reged: 08/01/13

Loc: Rochdale, UK
Re: A new design, the R&P Crayford. new [Re: Crayfordjon]
      #6305516 - 01/11/14 06:01 AM

Didn't you already mention helical rack Jon ?

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Crayfordjon
Vendor - Zerochromat
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Reged: 06/17/09

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Re: A new design, the R&P Crayford. new [Re: steveastrouk]
      #6306001 - 01/11/14 11:30 AM

The rack I am using is a helical rack, but there is still backlash, I have left some slack in the mesh.

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Crayfordjon
Vendor - Zerochromat
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Reged: 06/17/09

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Re: A new design, the R&P Crayford. new [Re: Crayfordjon]
      #6318308 - 01/17/14 10:48 AM Attachment (10 downloads)

Some more pics of finished sub assemblies, these shew the tube, track rack assy, the pressure rollers impinging on the tube track to load it against the support rollers underneath the tube, and the almost complets focuser shewing the variable brake to control stiffness of rotaton of the pinion.

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Crayfordjon
Vendor - Zerochromat
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Re: A new design, the R&P Crayford. new [Re: Crayfordjon]
      #6318325 - 01/17/14 10:52 AM Attachment (7 downloads)

The loading rollers on the semi complete focuser

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Crayfordjon
Vendor - Zerochromat
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Re: A new design, the R&P Crayford. new [Re: Crayfordjon]
      #6318332 - 01/17/14 10:56 AM Attachment (12 downloads)

The partially finished focuser.

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