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A new design, the R&P Crayford.

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#26 Ed Holland

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Posted 01 January 2014 - 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

#27 tim53

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Posted 01 January 2014 - 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.

#28 don clement

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Posted 01 January 2014 - 02:51 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.


Tim,

Could it have been similar to this: http://clementfocuse...Drive_Print.pdf

Don

#29 don clement

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Posted 01 January 2014 - 07:01 PM

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.co...id_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.co...head_stage.html

Don

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#30 don clement

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Posted 01 January 2014 - 08:34 PM

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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#31 Crayfordjon

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Posted 02 January 2014 - 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.

#32 Ed Holland

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Posted 02 January 2014 - 04:30 AM

Good point John :)

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

Ed

#33 don clement

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Posted 02 January 2014 - 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

#34 Crayfordjon

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Posted 02 January 2014 - 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.

#35 Geo31

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Posted 02 January 2014 - 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.

#36 sonny.barile

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Posted 02 January 2014 - 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.

#37 m. allan noah

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Posted 02 January 2014 - 06:46 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.


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

allan

#38 Ed Jones

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Posted 02 January 2014 - 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? :question:

#39 don clement

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Posted 02 January 2014 - 09:38 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? :question:


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

Don

#40 sonny.barile

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Posted 02 January 2014 - 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?

#41 don clement

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Posted 03 January 2014 - 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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#42 Crayfordjon

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Posted 03 January 2014 - 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.

#43 Crayfordjon

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Posted 03 January 2014 - 03:36 AM

Don that is a clever design, neat!

#44 Crayfordjon

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Posted 03 January 2014 - 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.

#45 Crayfordjon

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Posted 03 January 2014 - 08:43 AM

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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#46 PrestonE

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Posted 03 January 2014 - 09:09 AM

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

Best Regards,

Preston

#47 don clement

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Posted 03 January 2014 - 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

#48 sonny.barile

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Posted 03 January 2014 - 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........

#49 Crayfordjon

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Posted 04 January 2014 - 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.

#50 Crayfordjon

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Posted 04 January 2014 - 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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