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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 964
Loc: Waco, TX
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I like the idea of helical focusers and I wanted one for my Coulter 8" (which has the infamous friction focuser), but I am cheap, and the cheapest helical I could find one was $50. So I am making my own helical Crayford. Here is how it currently looks. As you can see, it's still missing a hole for the draw tube! That's because I had experimentally mounted the posts, expecting it wouldn't work, and was pleasantly surprised that it worked. I expected I might need to put rubber rings on the bearings to compensate for poor tolerances, but it didn't seem to be necessary.
Unfortunately, the result of my unexpected success is that the posts are glued only with Aleene's craft glue, and then have two screws. I may unscrew, sand, JB Weld, and re-screw, or maybe I'll just add JB Weld gussets to strengthen them. I am also thinking of running picture wire between them or putting webbing around them to keep them from bowing outward or attaching some kind of particle board ring to them.
Cost was about $6. The bearings are roller skate bearings ($3 for 8 on ebay) with 3/8" ID, JB Welded to a 1.25" long pieces of 1/4" square rod. The square rod is then JB Welded on top of the head of a #12 screw, for a T-shape with the bearings on the arms of the T, and the stem going into the post. I used a lot of JB Weld on all sides, so that I am relying on the internal strength of JB Weld rather than its lower adhesive strength. I used masking tape to mask off the moving parts of the bearings. I was scared that the JB Weld would penetrate behind the edges of masking tape, the way paint sometimes does, but it didn't, even though there was quite a bit of glue on the masking tape. But no--masking tape keeps JB Weld really well at bay. (I did have a spot of masking tape that I couldn't remove afterwards, maybe a 1mm x 2mm spot. So I just buried that under more JB Weld.) It's kind of weird to have wheels on a square axle, but with ball bearings it doesn't matter, and the square rod was easier to attach to the screw.
The screws then go into hardwood posts, except that one of the posts has a Teflon pusher. The pusher was made by JB Welding a square of metal to the top part of a bondable piece of Teflon, with the bottom part JB Welded to the post.
Overall view:
Closer view of bearings:
With draw tube:
Edited by arpruss (09/13/09 05:05 PM)
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timo4352
super member
Reged: 04/16/07
Posts: 194
Loc: Northeast Ohio
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Very Cool 
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Orion 3.6CA Reflector
and 2 homebrew scopes --
8" Hubble Bubble ballscope
8" F/8 CHief - nearing completion 
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 964
Loc: Waco, TX
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I just realized (based on an email I got) that some beginners -- like me until recently -- might not know how a helical Crayford works. The idea is that wheels are tilted away from horizontal, so as you rotate the tube, it goes up and down. It's a helical focuser with much less (none if it's done well enough) play than a standard helical focuser, without the need for any threads, damping paste, etc.
It's kind of surprising that the friction of steel on aluminum is sufficient, but it is. The one eyepiece I tested this with is my 13mm Hyperion, and the friction is sufficient to keep it from falling in.
The speed of up/down movemet is adjustable by changing the angle of the rollers, but changing that will move the focuser axis.
In case I didn't say it, the drawtube is from the original friction focuser on the scope. It's aluminum, ID 1.25 and OD 1.50, with a nicely textured end. The only problem is that it's shiny inside!
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Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm GSO SV, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer
Edited by arpruss (09/13/09 01:31 PM)
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 964
Loc: Waco, TX
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I ripped the posts off the original design, drilled a hole for the drawtube, sanded the particle board, and slathered on JB Weld. I JB Welded a hardwood fence joining the three posts, to keep them from bowing out. Now it's all drying.
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Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm GSO SV, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer
Edited by arpruss (09/13/09 07:06 PM)
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 964
Loc: Waco, TX
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Still needs to be painted, and the JB Weld is still curing:
Edited by arpruss (09/13/09 07:05 PM)
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 591
Loc: UK
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I am intrigued by the helical CF, I must make one. Congrats to the genius who thought up this very interesting design. I like the way people are making focusers out of wood, it works doesn't it! it shows that a crayford can be made out of anything.
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 964
Loc: Waco, TX
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I am convinced that the simple three wooden post and thumbscrew design can handle a traditional Crayford, too. One of the three posts would get two bearing pairs, one above the other (they can be mounted on T-constructions just as I mounted them here, though smaller bearings--mine have 22mm OD--would be needed for a lower profile focuser). The shaft with knobs goes through the other two posts. The shaft holes in the posts are elongated parallel with the focuser board (one could do that by wriggling the drill or a small file or a small router). Thumbscrews (either nylon, or metal + teflon) in each of the posts (or just in one?) press the shaft against the drawtube. As a result, one would have a cheap and simple Crayford. If I had thought of this before doing a helical, I might have done it instead.
The Crayford really is the Dobsonian of focusers, as they say. Simple, elegant, infinitely variable, easy to execute by hand out of the cheapest materials or elegantly machinable, smooth-running, not requiring difficult to achieve tolerances (I did all my drilling with a somewhat wobbly drill press and cutting with a hand saw).
By the way, any thoughts on how to mount my helical CF to a 10" OD sonotube?
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Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm GSO SV, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer
Edited by arpruss (09/14/09 10:31 AM)
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Bill Kocken
sage
Reged: 08/07/04
Posts: 202
Loc: Coon Rapids, MN
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Quote:
By the way, any thoughts on how to mount my helical CF to a 10" OD sonotube?
Glue or screw 2 small strips of wood down the length of your board on the bottom side. The strips should be big enough so that you can set the focuser on the sonotube without the middle bottoming out. Maybe about 1/4" by 1/4" You can bevel the bottom of the strips so that they sit flat on the sonotube. To attach, use small bolts drilled through the focuser board, the small stips and through the sonotube.
Bill
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16"F4.5 Homebuilt Truss Dob
2 - 8"F6 Homebuilt dobs
Member, Minnesota Astronomical Society
Edited by Bill Kocken (09/14/09 02:21 PM)
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 964
Loc: Waco, TX
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And then shim as needed?
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Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm GSO SV, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer
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Darenwh
Pooh-Bah
Reged: 05/11/06
Posts: 1470
Loc: Covington, GA
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The strips on the side that the flat board attaches to should be attached seperate from the focuser board. The focuser board should have two screws going through it at each corner. One screw just goes through the board and pushes against the strip, the other goes through the board and into the strip. Depending on how far through the board you put the four screws that contact the strips will determine how the board sits in referrence to the tube. With this you will have fine control of the setup of the focuser board. You will need to find a way to put a threaded piece into the board though for each of these four screws and the screws should be fine threaded.
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Daren
Covington, GA
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 591
Loc: UK
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This is the way to do it, parallell strips on the base of the focuser can be adjusted to cope with the slope of the tube, you can move them in until you get three points of contact, with the tube on the centre line, and at the edges
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 964
Loc: Waco, TX
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Thanks for all the advice. I actually ended up cutting it down to a triangle, to save weight (plus it looks better). I may do three point support, align with that, and then glue in wooden strips / weatherstripping. Before I do that, I have to find a good way of removing the old focuser, which is epoxied in place. Worst comes to worst, I'll have to cut out a hole around it.
After cutting and painting, the helical CF is now 7.5 oz (compare to the KineOptics HC-1 which is 2.5oz; on the other hand, the HC-1 costs $100, while this costs well under $10). Of that, 2 oz is bearings, and 1 oz is the wooden fence that keeps the posts from bowing out. The posts could be replaced by aluminum to make them lighter, and smaller bearings would be better. A 2" version would be easy--just move the posts further out--but would be heavier and I don't have any 2" EPs.
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Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm GSO SV, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 591
Loc: UK
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Why dont you make a triangular fascia plate to tie the three posts together, it could be secured by screws into the ends of the pillars. You can see the general idea from a metal CF I made last year. the fascia could be cut from 3mm ply. It would look very neat and match the base plate.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 591
Loc: UK
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here it is
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474747
member
Reged: 04/19/08
Posts: 11
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Very,very nice.
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 591
Loc: UK
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Thanks but dont be put off, I am very interested in giving advice on building CF,s as I have been doing it since 1969, about that time I made one from wood to prove that it could be done, and it could, I built a wood one last year, and got a lot of joy doing so, you guys making CF's from scraps of timber and pipe fittings, and using ingenuity is what ATM is all about. Back to the fascia plate idea, you can see from my metal one what I mean about joining up the three pillars of your design, I am sure it would look nice.
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 964
Loc: Waco, TX
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That is a really nice focuser!
Next time I make a focuser (if there is a next time--I think I may have enough scopes!) I might use a plate like that. Currently, my hardwood fence (about 8mm x 12mm in cross-section) is attached with a lot of JB Weld and the whole assembly is rock solid. Also, my posts aren't all the same height (I don't try hard for uniformity when the design doesn't require uniformity, and am not very good at sawing straight without a table saw), and they slope down away from the central hole, so putting a plate on would have required a fair amount of JB Weld anyway to bridge the differences.
There is one slight problem at this point in my focuser. One of the four bearings doesn't always make contact with the draw tube. The resulting amount of play is miniscule, however--while screwing the draw tube in and out, I don't feel any play. I am not going to fix it. But there is a lesson here. While the original Crayford may not have much in the way tolerances, the helical Crayford when made in my design does have some tolerances, though not very difficult ones (else I couldn't have made it).
My way of getting the post placement as close to right as I could was this. I attached the posts with craft glue, and before the glue had set, I moved the posts so all four bearings would have contact with the draw tube. I was able to make fine adjustments when the glue was almost set. Once the glue set enough, I drilled for screws. Then ripped the posts off, re-glued with JB Weld, and used the screw holes for getting the placement right. On reflection, it would have been better to use JB Weld in the first place, let it get fairly thick over two hours, then fine-tune the post positions, then let it all set, and only then drill for screws.
I still haven't mounted the focuser to the scope--been busy with other things, like generating a fun theorem in tense logic.
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Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm GSO SV, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer
Edited by arpruss (09/17/09 12:34 PM)
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 964
Loc: Waco, TX
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OK, the project is basically complete (a few touches of paint are needed, and there are a few places I want to go over with JB Weld). Thank you everybody for encouragement and advice!
By arpruss
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Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm GSO SV, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2316
Loc: salem, OR
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Wow, you have achieved a vintage look - for a vintage approach that never existed! Cool!
Best,
Mark
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Crayfordjon
Inventor
Reged: 06/17/09
Posts: 591
Loc: UK
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Very nice clean design, it looks really good, you have made a smart focuser there.
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