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Equipment Discussions >> ATM, Optics and DIY Forum

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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
Vendor - Zerochromat
*****

Reged: 06/17/09

Loc: UK
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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Crayfordjon
Vendor - Zerochromat
*****

Reged: 06/17/09

Loc: UK
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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Crayfordjon
Vendor - Zerochromat
*****

Reged: 06/17/09

Loc: UK
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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Crayfordjon
Vendor - Zerochromat
*****

Reged: 06/17/09

Loc: UK
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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PrestonE
Carpal Tunnel
*****

Reged: 04/29/05

Loc: San Miguel de Allende,Mexico
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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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]
      #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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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]
      #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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Crayfordjon
Vendor - Zerochromat
*****

Reged: 06/17/09

Loc: UK
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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Crayfordjon
Vendor - Zerochromat
*****

Reged: 06/17/09

Loc: UK
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
Vendor (Clement Focuser)


Reged: 02/02/11

Loc: Running Springs, California
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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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]
      #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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Crayfordjon
Vendor - Zerochromat
*****

Reged: 06/17/09

Loc: UK
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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John Carruthers
Skiprat
*****

Reged: 02/02/07

Loc: Kent, UK
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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Crayfordjon
Vendor - Zerochromat
*****

Reged: 06/17/09

Loc: UK
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
professor emeritus
*****

Reged: 02/04/06

Loc: Madisonville Louisiana
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
Vendor (Clement Focuser)


Reged: 02/02/11

Loc: Running Springs, California
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
scholastic sledgehammer


Reged: 12/22/06

Loc: Plymouth, Minnesota, USA
Re: A new design, the R&P Crayford. [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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Crayfordjon
Vendor - Zerochromat
*****

Reged: 06/17/09

Loc: UK
Re: A new design, the R&P Crayford. [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
Vendor (Clement Focuser)


Reged: 02/02/11

Loc: Running Springs, California
Re: A new design, the R&P Crayford. [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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