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Dobsonian base mechanics

dob
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11 replies to this topic

#1 Eliserpens

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Posted 29 June 2020 - 02:14 PM

Hi - I have recently acquired a custom 12.5", f/6.5 double truss dob (as described in my sig) which, after a few nights viewing, I am very happy with.  However, I think the base could be improved as its rather hard to rotate.  I don't know how standardized these are or if there have been any innovations that I could add. 

 

The design is very simple with a clipped-triangular base platform that has three hockey pucks as feet on each corner.  This is attached to the rocker box by a single central bolt.  There is a large, circular textured teflon sheet on the bottom of the rocker box and opposing ~2.5" diameter teflon circles attached to the base platform (above each hockey puck).  The entire scope weighs about 76 lb, including the base plate. 

 

I find that while the rocker mechanism works fairly well, the rotation of the scope is a bit stiff and can cause you to jerk beyond your target, in particular when you are trying to track an object.  Is there a better way to set this up or maybe a way to tweak it to be a bit more compliant without being too loose.

 

Thanks..

 



#2 whizbang

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Posted 29 June 2020 - 02:30 PM

Do a search on the "lazy Susan" mode.

 

I put a lazy Susan under my DOB and it turned so easy I had to add to adjustable teflon brake pads.


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#3 PETER DREW

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Posted 29 June 2020 - 02:34 PM

You could try reducing the diameter of the teflon pads. Another option would be to re-engineer the rotational bearing arrangement with a "Lazy Susan" bearing, this would require a means of reducing the ease of rotation, the opposite problem but easier to achieve and would be adjustable to taste.


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#4 Couder

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Posted 29 June 2020 - 02:35 PM

Try the easiest thing first - put a coat of wax on the bearing surfaces and see if it helps. And/or, sometimes Teflon will pick up tiny pieces of grit and needs to be cleaned off.


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#5 SteveG

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Posted 29 June 2020 - 02:42 PM

Hi - I have recently acquired a custom 12.5", f/6.5 double truss dob (as described in my sig) which, after a few nights viewing, I am very happy with.  However, I think the base could be improved as its rather hard to rotate.  I don't know how standardized these are or if there have been any innovations that I could add. 

 

The design is very simple with a clipped-triangular base platform that has three hockey pucks as feet on each corner.  This is attached to the rocker box by a single central bolt.  There is a large, circular textured teflon sheet on the bottom of the rocker box and opposing ~2.5" diameter teflon circles attached to the base platform (above each hockey puck).  The entire scope weighs about 76 lb, including the base plate. 

 

I find that while the rocker mechanism works fairly well, the rotation of the scope is a bit stiff and can cause you to jerk beyond your target, in particular when you are trying to track an object.  Is there a better way to set this up or maybe a way to tweak it to be a bit more compliant without being too loose.

 

Thanks..

I have read that some of these can be improved by stacking CD's in the center with the center bolt passing through. Add or remove to taste, but make sure it sits flat and doesn't rock.

 

All of my recent dobs have the lazy Susan bearings, which work very well IMO.


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#6 Jon Isaacs

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Posted 29 June 2020 - 03:30 PM

Elise:

 

The large disk is FRP, sometimes called glass board. The pads should be Teflon. This is the standard bearing used on premium Dobs.

 

I recommend lubricating the bearing with Irovy soap.  Far and away the best ive ever found.  You name. I've tried it.

 

Use a wet rag wipe down the bearing surfaces.

 

Wet the bar of soap and rub it over the surface where the bearing pads ride.   Go around 4 or 5 times.

 

For good luck, rub the bearing pads.

 

It it dry and then assemble.

 

People have been using soap as a lubricant for hundreds of years.

 

Jon


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#7 kfiscus

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Posted 29 June 2020 - 03:53 PM

To the OP, if you measure the thickness of the 3 Teflon pads, I could cut you a center "donut" of Teflon.  It's the fancier version of the stacked CDs method.


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#8 izar187

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Posted 29 June 2020 - 09:59 PM

Stack washers you cut from milk jugs at the center bolt.

This is another well working method.

They are very thin, so fine adjustment is possible... and very affordable.


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#9 Jon Isaacs

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Posted 29 June 2020 - 10:00 PM

To the OP, if you measure the thickness of the 3 Teflon pads, I could cut you a center "donut" of Teflon.  It's the fancier version of the stacked CDs method.

 
This is fancy version of the stacked CD method.  It's a schematic of something I built for my 16 inch. The base is partially supported by a spring. The preload can be adjusted with the scope in place.  That advantage of the spring is that it's not sensitive to warping of boards because the force is essentially constant.
 
Dob center bearing design 2019.jpg
 
Once I started lubing the bearing with soap, this became unnecessary.
 
Jon


#10 kfiscus

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Posted 29 June 2020 - 10:29 PM

Jon, your method is using a strong compression spring to partially lift the rocker box through the red screws, correct?  That is fancy.


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#11 Eliserpens

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Posted 29 June 2020 - 10:32 PM

Wow. I kinda knew you guys would have a ton of solutions and I'm not disappointed!  I think I'll start with the clean and more clean (wash then apply soap) first and see if that fixes it.  If none of them work I'll try a Mcpherson strut...

 

 

Thanks to all, especially the offer to cut me a teflon donut  (doesn't taste good but it slips down the throat very easily :) ) ... 


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#12 Eliserpens

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Posted 03 July 2020 - 07:26 AM

Just a quick followup for future dobbers: Jon's wash behind the ears and soap worked great - indeed, it was so good on the base I did the rocker contacts too.  Something that I may regret because now the scope is prone to move on its own, especially when I am exchanging EPs and throw the balance slightly out.  There really should be a fine-tuning screw :-\

 

What do you guys do for extra weights?  I'm a bit concerned about hanging them on the upper assembly for fear that they could fall through onto the mirror (I already have one weight there and have anchored it with an emergency 'bungee' cord....




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