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A new design, the R&P Crayford.

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#1 Crayfordjon

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Posted 30 December 2013 - 05:26 AM

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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#2 Crayfordjon

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Posted 30 December 2013 - 05:27 AM

And.

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#3 Crayfordjon

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Posted 30 December 2013 - 05:28 AM

Finally a view shewing the rack fully extended.

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#4 Ed D

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Posted 30 December 2013 - 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

#5 Geo31

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Posted 30 December 2013 - 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.

#6 tim53

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Posted 30 December 2013 - 10:13 AM

How is this not just a rack and pinion focuser?

-Tim.

#7 Geo31

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Posted 30 December 2013 - 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.

#8 Crayfordjon

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Posted 30 December 2013 - 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. :D

#9 tim53

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Posted 30 December 2013 - 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.

#10 bremms

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Posted 30 December 2013 - 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.

#11 orlyandico

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Posted 31 December 2013 - 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...

#12 Crayfordjon

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Posted 31 December 2013 - 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.

#13 don clement

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Posted 31 December 2013 - 09:09 AM

Why not use a leadscrew?

Don

#14 Crayfordjon

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Posted 31 December 2013 - 10:45 AM

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

#15 sonny.barile

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Posted 31 December 2013 - 11:10 AM

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

#16 orlyandico

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Posted 31 December 2013 - 01:34 PM

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

#17 don clement

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Posted 31 December 2013 - 03:52 PM

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

#18 bremms

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Posted 31 December 2013 - 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.

#19 Roy McCoy

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Posted 31 December 2013 - 11:31 PM

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.

#20 tim53

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Posted 01 January 2014 - 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.

#21 tim53

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Posted 01 January 2014 - 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:
Posted Image

Racked out:
Posted Image

-Tim.

#22 Crayfordjon

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Posted 01 January 2014 - 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.

#23 don clement

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Posted 01 January 2014 - 06:35 AM

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

Posted Image

#24 don clement

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Posted 01 January 2014 - 07:03 AM

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.c...33/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...ve/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

#25 don clement

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Posted 01 January 2014 - 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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