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Homage to Homemade Crayford Focusers

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#26 pjmulka

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Posted 19 December 2016 - 05:04 PM

All wood with LOTS of clear coat.

20160724_151019_crop_741x767.jpg

 


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#27 pjmulka

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Posted 19 December 2016 - 05:06 PM

20160724_151005_crop_350x415.jpg


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#28 ZeroID

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Posted 20 December 2016 - 07:39 PM

Hello,
 
Hopping that helicoidal Crayford do qualify as home made Crayford, here are 2 other entries. Contrary to most of the other posts here, they are for 1.25" focusers.
 
The focus of the construction were low profile, weight, cost and simplicity.
Althrough they ar enot able to carry heavy eyepeice, they do work very well, and at around $8 a pop, they are cheap. They are also very easy and fast to build (less than 1/2 hour a pop). At less than 1" height (actually, 2cm + whatever travel you want), they are also low profile (the pictures there are using a "test tube" which is 5cm total, longer than the 3.5 cm that I use once things are calibrated).
 
the 5° angle on the bearings give a 10 mm course on a full circle, which gives quite a lot of precision.
 
The first one is at the top of a single pole scope.
 
The 2nd one is on a small 114, FL 500 scope.

  
The hardest part of course is to get the aluminum tube correclty made (ie, concentric enough).
Cyrille


Could we get an explanation how a 'helicoidal' focuser works? Just seems so simple looking. I ask because I've been working on a collapsible portable scope I can leave in the car (6F5) and the focuser has always been the bug bear. I actually built a 1.25" Crayford but my workshop skills let me down a bit and it doesn't work as well as I'd hoped.
Thanks in anticipation ..

#29 Pierre Lemay

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Posted 20 December 2016 - 07:58 PM

Could we get an explanation how a 'helicoidal' focuser works? Just seems so simple looking. I ask because I've been working on a collapsible portable scope I can leave in the car (6F5) and the focuser has always been the bug bear. I actually built a 1.25" Crayford but my workshop skills let me down a bit and it doesn't work as well as I'd hoped.

Thanks in anticipation .

Sure. The drawtube of the crayford-helical focuser has a smooth surface which rests on four small ball bearings, just like an ordinary crayford. However, unlike the crayford, notice how the axis of rotation of the ball bearings are collinear with the optical axis.

 

Here's a picture of one of my home made helical-crayford focusers for 2 inch eyepieces:

 

DSC01942 - copie.jpg

 

Actually, the bearings look collinear but their axis of rotation are actually inclined a few degrees. You can kind of see it on the image above.  Here is a closeup of the slot I milled in the ring that holds the bearings. You can see a 4 degree inclination:

 

DSC01944.JPG

 

The angle of the bearings forces the smooth round drawtube to follow the angled bearing points of contact. So when you rotate the drawtube it goes up or down because it is following the angle of the ball bearings. the amount of angle determines the "speed" of focusing. It's the same as if it was following an imaginary thread, which is how real helical focusers work. The advantage is that there is no slop and no grease and the focuser is extremely lightweight.

 

A fifth contact point pushes on the drawtube through a nylon or teflon screw.

 

Hope that clears things up.


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#30 gregj888

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Posted 20 December 2016 - 10:39 PM

On my 6" Trischiefspiegler and the reason for the tilted EP holder

 

DSCN0794-web.jpg


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#31 brebisson

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Posted 21 December 2016 - 02:25 AM

Hello ZeroID,

 

Pierre explained the principle of the helicoidal Crayford quite nicely.

The gist of it is: 2 sets of bearings, with a small angle (5° in my case) compared with the axis of the draw tube, one opposite pressure point to push the drawtube against the bearings and you are done.

You can make the focusser separately and attach it to the scope. I personally make them as part of the scope to simplify the whole thing and make it low profile.

They are tricks to making such focussers:

1) the outside and inside of the draw tube must be coaxial, else, when you rotate the drawtube, you move (in a small circle) the axis of the EP and mass up the colimation. Depending on the scope (fast/slow...) it might not be a big problem, or it might.

2) you need to drill the 2 angled bearing holes correctly.

3) if you use the same system as me,remember that the 'bottom' bearing and part of the draw tube will be IN the OTA, so the OTA must be large enough to, as much as possible, remove these from the light cone.

 

In the pictures that I posted earlier, I was using a 34mm outside diameter and 32mm inside, which is perfect for 1.25" focussers.

WARNING, a tube that you buy from a store is most likely NOT a good cylinder and will need to be trued on a lathe!!!

 

Here are some fabrication pictures for a 2"

This first picture shows the focusser plate with the focusser layout, and the 'prop plate' that I used to drill my 5° (actually, I think that this focusser is 10°).

I first drew my drawtube hole (55mm if my memory serves me well) and then the construction circle for the bearing rods at 'draw tube radius + bearing radius - 1mm' (to have 1.5mm of bearing overhang). 2 lines at 120° intersect the construction circle to tell me where to drill.

Part of the 'secret' is the use of a stepped drill bit (visible on the left). This is a 'Kreg Jig' drill bit. The reason for the use of such bit is that it will, at the same time (and hence concentrically) drill a pilot hole AND a shoulder for a nut to rest on to ensure that my bearing rod stays at my given angle.

The prop plate is made to allow me to drill a consistent angle without having to rely on my craptastic drill press.

DSC_6156.JPG

 

This second picture shows the drilling process. You will need to drill 2 holes, from both sides (to make the 2 shoulders). Be careful to respect the angles! This is where the construction lines come along. Keep them parallel to the bottom of the 'prop board' to place the angle correctly.

DSC_6157.JPG

 

Here is a picture of a hole with the shoulder. You can see the taper on the shoulder, deeper on the top of the picture and shallower at the bottom.

In my case, I then thread the holes so that the bearing rod holds well in place.

DSC_6158.JPG

 

Hogging the big hole. For smaller focusser, I user forestner bits...

DSC_6159.JPG

 

And here are 2 pictures of the mostly completed focusser plate. It is attached at the top of a single pole (similar to Ross's designs http://stardazed.com/MoonsilverI.html ), presented here with a 2" to 1.25" adapter.

The second picture shows nicely the bearing stack (although here is is the wrong way around, the screws "start" should be on the light cone side to lower the profile). it also lets guess at the angle and shows wear marks on the tube that helps understand how the system works.

What is missing from this "Work in Progress" is the plastic Nylon "Push Screw" which is threaded at the bottom of the plate (parallel to the last remaining construction line), to push the draw tube against the bearings.

DSC_6160.JPG DSC_6161.JPG

 

The advantage of these focussers is the low cost (4 bearings, 2 bolts, 6 nuts and not nylon screw), low weight and simplicity in building.

The disadvantages include the sometimes problematic coaxiality of the system and the fact that it is not suitable for heavy EP.

 

Cyrille


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#32 Charl

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Posted 21 December 2016 - 07:05 AM

Glad to see so many non-metal variations. Here is my refractor and Newtonian versions machined out of scrap mdf on my home made cnc router, soaked in epoxy resin and hand painted with a brush.


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#33 brebisson

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Posted 09 January 2017 - 03:12 AM

Hello,

 

Sorry for reviving an old topic, but I have a question.

 

Why are 4 bearings used in a Crayford and not 3 (in a triangle pattern)? It would seem that 3 bearings would make a sure 'plan' while 4 might not...

 

Cyrille


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#34 Rusted

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Posted 09 January 2017 - 09:24 AM

Hello,

 

Sorry for reviving an old topic, but I have a question.

 

Why are 4 bearings used in a Crayford and not 3 (in a triangle pattern)? It would seem that 3 bearings would make a sure 'plan' while 4 might not...

 

Cyrille

Alignment of the moving tube relies on two, widely spaced pairs of bearings. Three would not offer the same 'staying on the rails' effect.

Any free play between the tube and four [possibly uneven] bearings should be overcome by the pressure from the drive shaft.


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#35 Bob4BVM

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Posted 09 January 2017 - 06:37 PM

 

Hello,

 

Sorry for reviving an old topic, but I have a question.

 

Why are 4 bearings used in a Crayford and not 3 (in a triangle pattern)? It would seem that 3 bearings would make a sure 'plan' while 4 might not...

 

Cyrille

Alignment of the moving tube relies on two, widely spaced pairs of bearings. Three would not offer the same 'staying on the rails' effect.

Any free play between the tube and four [possibly uneven] bearings should be overcome by the pressure from the drive shaft.

 

That is correct

To put it another way, the pressure of the drive shaft maintains straight, parallel tracking of the drawtube on the bearings, which in turn function essentially as a low-friction V-block

 

CS

Bob


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#36 dave brock

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Posted 09 January 2017 - 07:34 PM

Just to add.

The tube will align itself to 4 bearings even if they are poorly aligned onto their mounting. The tube just won't be aligned to the mounting.

 

Dave



#37 Bob4BVM

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Posted 09 January 2017 - 09:39 PM

And actually it IS a triangle.

The shaft is the 3rd bearing point, the tip of two triangles, longitudinal & radial.


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#38 Rusted

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Posted 10 January 2017 - 02:34 AM

And actually it IS a triangle.

The shaft is the 3rd bearing point, the tip of two triangles, longitudinal & radial.

Or, a truncated, square-based pyramid with constrained, linear translation?  :waytogo:



#39 jtsenghas

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Posted 10 January 2017 - 11:10 AM

Sorry to be joining this conversation so late, but for those planning on heavy eyepiece loads on helical focusers, something like this idea of mine,  might fit the bill.  

 

This design does require excellent roundness of the drawtube, and could have a soft spring-loaded roller at one or two places in each of the two planes instead of the Vlier screws.  The advantage of this design, though, is that for alt- azimuth scopes (including dobs), gravity will keep the drawtube against the rollers. This concept could be used for linear motion focusers too. It's just very important to have tight tolerances on the tube for taper and roundness- -cylindricity. 



#40 Jon Isaacs

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Posted 10 January 2017 - 05:22 PM

Hello,

 

Sorry for reviving an old topic, but I have a question.

 

Why are 4 bearings used in a Crayford and not 3 (in a triangle pattern)? It would seem that 3 bearings would make a sure 'plan' while 4 might not...

 

Cyrille

Cyrille:

 

Others have explained it clearly, let me try to confuse the situation..  :)

 

Three points define a plane but they do not constrain a tube.  To constrain a tube so it is only free to move in the axial direction takes 5 points.  

 

That said, there are some many wonder, creative works of beauty..  WOW.. it's fun just to imagine them all.. 

 

Anyone built a two speed?

 

Jon


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#41 astroelliottuk

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Posted 10 January 2017 - 07:39 PM

Hi

Here is a picture of a 3 inch crayford focusser built for me by john wall ,the crayfords inventor.

Form function and art.AAA1.jpg


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#42 daviddecristoforo

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Posted 10 January 2017 - 10:28 PM

Hi

Here is a picture of a 3 inch crayford focusser built for me by john wall ,the crayfords inventor.

Form function and art.attachicon.gifAAA1.jpg

That's gorgeous!!!



#43 Bob4BVM

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Posted 10 January 2017 - 10:51 PM

 

And actually it IS a triangle.

The shaft is the 3rd bearing point, the tip of two triangles, longitudinal & radial.

Or, a truncated, square-based pyramid with constrained, linear translation?  :waytogo:

 

Yeah, THAT"S what I meant to say !  :)  :)


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#44 Oberon

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Posted 11 January 2017 - 06:03 AM

Hello,

 

Sorry for reviving an old topic, but I have a question.

 

Why are 4 bearings used in a Crayford and not 3 (in a triangle pattern)? It would seem that 3 bearings would make a sure 'plan' while 4 might not...

 

Cyrille

 

The same question applies to the altitude bearings on a Dob, which virtually always have 4 points of support rather than 3. So why is it stable? Why doesn't it rock?

 

Think about it...  :cool:



#45 jtsenghas

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Posted 11 January 2017 - 06:13 AM

The same question applies to the altitude bearings on a Dob, which virtually always have 4 points of support rather than 3. So why is it stable? Why doesn't it rock?

Think about it... :cool:

On both setups the forces are essentially radial due to contact against a cylinder, so each pair of contact points determine the location of the center of a circle. Both are the same distance from that center and the radius is a fixed value. Those two centers at different sections are the two points that determine a line- the axis itself.

Edited by jtsenghas, 11 January 2017 - 06:20 AM.

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#46 flseadog

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Posted 09 August 2018 - 07:06 PM

Here is a motorized 3 1/4".  I have machine tools though.

Nice job. Do you have plans for that you would share?



#47 Chucke

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Posted 10 August 2018 - 02:04 AM

Flseadog,

 

The focuser was inspired by a VanSlyke Toad Loader. His website is still up and many of the basic dimensions and his design philosophies can be found by digging through it. I was looking for a large heavy focuser and it seemed to be a good sturdy design to modify for my needs.  If he had still been in business I might have bought one from him instead of making it. It probably would have been expensive.  It took me about a week to make over a Xmas holiday. The hard part was figuring out the setups to keep the parts square and parallel.  It would be really easy with a multi-axis machining center.  Not so much with a manual mill and lathe like I have.

 

My focuser is a little unusual since the focuser bore is 3.234" to fit my coma corrector. 

 

Other important dimensions:  The focuser tube has a wall thickness of approx. 3/8" with an OD of 4".  The flat is 1" wide and stops about 0.1" from each end of the tube.  The bearings are shielded  SS with 7/8" OD with a 3/8" bore.  The focuser body is 4.8" OD 4.05" ID.  The Crayford shaft is 1/4" dia precision ground SS rod.  I tried regular SS rod but it was lumpy so I switched to precision ground.  It is nice and smooth.  The 2 tension screws are 3/8-24 x 1/2" socket head set screws with the cones machined off flat.  It's fun tapping their mounting holes obliquely on the convex surface of the focuser body. If you try to make one, be sure to drill and tap the holes for the pressure screws before you cut the slot for the focusing rod.  (Been there- made that mistake). The pressure pads are Turcite with a small nub that fits in a hole drilled into the flats on the ends of the tension screws.  I tried Teflon but it is too squishy. Turcite works well for the purpose.  Make a few extras.  They are easy to lose. Even though you can but large amounts of pressure on the rod because of the big bearings and the Turcite, I have a long heavy imaging train and still had problems with the focusing tube deflecting under load.  I solved it by putting in a bunch of nylon tip set screws at various strategic locations around the focuser body.   They are not applying pressure but touch the focusing tube so it can't deflect.  I think they are 10-32 SS.  I'm not sure off the top of my head because I didn't add them to the drawing.  One of my bad habits is not always updating the CAD drawings to reflect on the fly changes. The Robofocus motor is very strong and is geared down with SS anti-backlash gears so it wont stall.  The Turcite pads will dent from the pressure so when you first put the focuser into service you will have to adjust the tension periodically until they stabilize.

 

To mount it to the 16" I made a 5.8" square radius block to fit the tube.  The 22" uses a flat mounting plate.

 

The aluminum was from a SoCal scrap yard and the fasteners, etc, were from McMaster-Carr.

 

I do have drawings but I'm not sure anyone but me would understand them.  Also, they were done in an old and uncommon CAD program, DesignCAD 2000.  I've been using it since 2000 and the drawings are in a version from that era.  Nice thing about the old version is it does not use the windows registry so I can run it from any folder I feel like putting it in.



#48 brebisson

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Posted 27 August 2018 - 12:17 PM

Hello,

 

Here is a nearly 100% wood Crayford.

Main body is Cherry, tube and knob are Jatoba with a brass pression screw.

 

DSC_6925.JPG

 

Cyrille


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#49 Bob4BVM

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Posted 27 August 2018 - 12:30 PM

Hello,

 

Here is a nearly 100% wood Crayford.

Main body is Cherry, tube and knob are Jatoba with a brass pression screw.

 

attachicon.gif DSC_6925.JPG

 

Cyrille

Nice try  Cyrille, but you're not gonna get away with it !

We need to see the rest of that work of art, the whole scope.

C'mon, cough up !

:)

Bob



#50 brebisson

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Posted 27 August 2018 - 02:33 PM

Hello,

 

"Nice try  Cyrille, but you're not gonna get away with it !

We need to see the rest of that work of art, the whole scope.

C'mon, cough up !"

 

:) see https://www.cloudyni...45-galadrielle/

 

Cyrille




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