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Old 04-08-2006, 07:58 PM
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For those who are considerig DIY dobson

Hi,
this solution of Az drive with friction rollers may be interesting for someone...
I designed it this way because I wanted the specific reduction ratio (it is visible on the pic that rollers provide for reduction 1:3) and some degree of "self-aligning" at the same time, to allow for larger tolerances in positioning of the rollers and axles. This solution was sort of forced upon me, because I was using various parts from waste.. and it turned out pretty well.
Why two rollers? I tried with just one (forced between the driver and driven disk by spring), but that solution turned out bad, because when the telescope was driven in one particular direction, the roller was pushed out and the friction was decreased to the point of not moving at all in that direction.
The most important thing here is that both rollers are identical, and the tension between the rollers is sufficient, which is provided with two springs (another one is below the rollers and not visible on this image).
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Last edited by bojan; 04-08-2006 at 08:01 PM. Reason: typos
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Old 10-08-2006, 01:58 PM
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Looks good. Do you have more pics of the mount. I love the way you guys can make bartelized type dobson mounts out of stuff you have laying around I would love to be able to make one
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Old 13-08-2006, 11:10 AM
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Here they are.. of course, everything is still being modified from time to time...
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Click for full-size image (Hor_drive_2.jpg)
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Old 13-08-2006, 01:54 PM
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Nice work ther Bojan, did you make the gears yourself?

Regards
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Old 13-08-2006, 05:58 PM
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No, I was lucky to find them on my friend's machine shop waste (he likes to collect those things just like me). You never know when you may need something like that ;-)
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Old 13-08-2006, 06:06 PM
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The first picture you posted kind of remind me of my childhood train set. It had similar shiny wheels. And the motion is similar to that of a train wheel on a track, very ingenious method.
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Old 14-08-2006, 04:27 PM
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Now thats a roboscope very well done
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Old 14-08-2006, 06:54 PM
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Thank you all for your kind comments :-)...
This is another example of roller coupling that I am using on my small MTO1000 macsutov cassegrain... same principle, but very simple to implement and auto adjustable to the certain degree (so you do not have to drill those holes for axles very precisely), and better than gears, because there is no backlash and no periodic errors.
Bojan
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Old 14-08-2006, 08:57 PM
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The picture is a bit dark on my laptop screen. But i think i got it. your using the drive shaft to turn to rollers that are runing in a milled track. Very nice.

I came accross this other similar idea that seems pretty cool as well using friction to drive a worm type flywheel. They used a Threaded rod to move a square pipe along the rod that turned the flywheel. Saw it posted on CN forums will dig it out. To stop the square tube turning the placed a second roller on the other side of it similar to your drive shaft sitting in the middle of two rollers.
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Old 14-08-2006, 09:06 PM
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Yes, and springs are holding them in place. It is similar to those "shiny wheels" from the first picture but here there is no additional reduction (apart from ratio of shaft and milled track diameters). Also, you can not break anything (like gears) because the drive will slip if forced too hard... very useful feature in dark :-)
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Old 14-08-2006, 09:13 PM
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better and new picture or rollers

Here is better picture :-)
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Old 01-05-2007, 07:28 PM
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thank you all
Lex
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Old 02-05-2007, 09:57 AM
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dont the two bearings work against each other ifthey are bothin contact with the milled part ofthe drive
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Old 02-05-2007, 06:40 PM
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Encoders coupled to drive wheels

This is how I did it.....
No slippage observed.
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Old 02-05-2007, 06:43 PM
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Quote:
Originally Posted by Lex View Post
dont the two bearings work against each other ifthey are bothin contact with the milled part ofthe drive
Nope... they work together. Of course, they are not supposed to touch each other. Both of them are only in contact with driving axle and the milled part.
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Old 17-03-2010, 09:08 PM
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Hi,
Had a couple of questions on your alt-az drive

are you using any rubber bushes or coating on the metal rollers to aid friction thereby reducing the need for added tension to the rollers and avoid slippage? what would happen after a period of wear on the metal rollers?
when you say bartelized dobsonian, I presume its adopting methods described by mel bartel, but just wanted to understand what it actually does. does it auto track using a secondary guide star, or is it for motorized slew on alt and azimuth axes or is it for normal tracking using fixed rate motion
if for tracking, how do you get around field rotation thats inherent with alt-az mounts?
do you use prime photography or field projection if you're using it for astrophotography, and if prime, did you have to move the mirror?
I was thinking of prime with a reflector, but by making the focusser detachable, and using an alternate support for the camera thats low profile, that may get away with the need to move the mirror forward to achieve focus.
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Old 17-03-2010, 10:21 PM
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There is no rubber, just metal-to-metal (brass, steel, al.. with additional pressure in a form of springs holding wheels together... and there is no slippage. Or if there is, it doesn't matter enough to be noticed as a problem).
When it wears off sufficiently (big aluminium wheel, brass and steel will be OK), I will be long gone from this world :-) If not, I will machine the new one.
Tracking is as per Mel Bartels' : software running on 100MHz pentium, DOS, controlling both AZ and EL steppers appropriately.
It does slewing, Goto and tracking at sideral or lunar or solar rate.
It also understands LX200 protocol for autoguiding (but I do not use it, not yet).
I do not have field de-rotator yet (I am using this setup only for visual and occasionally for planetary photography), but Mel's software supports FR. Perhaps one day...
My focuser is easily detachable (Carl Zeiss style) and I am working now on focuser for camera at prime focus.
Mirror is mounted such that I have focus ~95mm outside the tube (my own design), and it will be OK with new camera focuser. I think even if field flattener id fitted, the focus is out enough.

For Moon and planetary photos (I tried couple of times, I saw that I can do it and that's it) I used projection, with camera (kyocera M400R or any other with 50mm filter thread) mounted on adapter (inside which was 9mm eyepiece)

Last edited by bojan; 18-03-2010 at 09:21 AM.
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  #18  
Old 11-10-2018, 05:16 PM
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After some time spent in storage, I am doing some modifications to my Bartelized Dobson (10"Newt), in encoder area.

Vertical axis is first, I added 3D printed timing pulley and a bit of up-gearing, to achieve 6600 ticks per 360 (Ek's interface).
The idea is to avoid friction coupling -it was OK, but because the driving plate was also friction-coupled, it was tricky to adjust frictions such that encoder reading is reliable
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Last edited by bojan; 11-10-2018 at 06:42 PM.
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Old 20-10-2018, 01:54 PM
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Today I printed elevation sector for encoder.
It will be fixed on the inner side of the round plate, to replace the existing friction coupled roller transmission.
With this sector (400 teeth for full circle) and 10-teeth pinion, I will have resolution of 6400 tics/rev, which is 3.38 '/arcmin (error due to plastic parts use will probably be larger).
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Last edited by bojan; 20-10-2018 at 08:07 PM.
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  #20  
Old 29-10-2018, 04:58 PM
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...complete with period errors arising from such a small pinion.

Would have been much better if the teeth had been helical, or use a toothed belt between the two.
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