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Equipment Discussions >> ATM, Optics and DIY Forum

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tim53
Postmaster
*****

Reged: 12/17/04

Loc: Highland Park, CA
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
*****

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
Postmaster
*****

Reged: 12/17/04

Loc: Highland Park, CA
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
*****

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
*****

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
*****

Reged: 01/28/13

Loc: Kingwood, TX
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. [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. [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
*****

Reged: 04/06/04

Loc: Sin-sin-atti
Re: A new design, the R&P Crayford. [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. [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. [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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