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Ummmm, how to design the center bolt for a dobsonian?

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#1 opticsguy

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Posted 20 April 2017 - 10:29 PM

OK, seems like this is an easy build and a little embarrassed to ask, a bolt, a few washers and a nut or two to hold the ground pad and ground board for rotation on a dobsonian.

Done this many times, however.................................. One time I saw a much nicer assembly and now can not remember how it was done.

A center bolt with appropriate nuts and washers etc that do not tighten or loosen as the scope is rotated.

Thank you for helping me out with my poor memory.

 



#2 Pinbout

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Posted 20 April 2017 - 10:33 PM

it doesn't have to be tighten to begin with

 

the rocker box could just sit on a pin

 

but I use a 3/8-16 bolt thru a 1/2" bronze bushing in the rocker box and into a t-nut on the underside of the ground board.


Edited by Pinbout, 20 April 2017 - 10:34 PM.

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#3 Cames

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Posted 21 April 2017 - 12:08 AM

[snip]

A center bolt with appropriate nuts and washers etc that do not tighten or loosen as the scope is rotated.

[snip]

You should consider using a 'nylon-insert lock nut' in conjunction with flat fender washers. Nylon-insert nut

 

The washers prevent chafing of the base components when you lift them in transport.

 

The nylon insert prevents the nut from tightening or loosening as you rotate the telescope while stargazing. With a normal nut, one has to cinch down the nut pretty tightly in conjunction with a lock washer in order to prevent its undesirable rotation. That arrangement won't work very well on a design where the restrained parts are supposed to rotate like the base of a telescope. 

 

On the other hand, a nylon-insert nut will hold just about anywhere on the thread of the azimuth bolt due to the friction that results from deforming the nylon insert as it is screwed on to the threads.   There are quite a few solutions to your problem but I think the nylon-insert nut is the simplest.

--------

C



#4 xrayvizhen

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Posted 21 April 2017 - 07:17 AM

This is what I did. I would only add that if you ever think you will have an inclination in the future to add digital setting circles you should substitute the hardware store bought 3/8 bolt with one from Astrosystems which has a machined out 1/4" dia. recess and a setscrew for an encoder.Azimuth Pivot.jpg


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#5 Megiddo

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Posted 21 April 2017 - 07:40 AM

I was thinking the same as I saw my turning circle pointer would would contact my belt. (I haven't built it yet). So I was thinking of using the center bolt to rotate on using some old bicycle gears. But how to keep the center bolt from turning.

So I'll be studying this thread.

#6 calypsob

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Posted 21 April 2017 - 08:52 AM

Would it make sense to epoxy the bushings to the 3/8 bolt and feed them through needle bearings?  Im no pro when it comes to Dob's but just curious to know.  I see there are some cheap ones http://www.ebay.com/...c0AAOSwWxNYrqo1

 

and some heavy duty looking options http://www.ebay.com/...q_qq4MzNasM4hoQ



#7 Pinbout

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Posted 21 April 2017 - 01:42 PM

if you want to stop the bolt from spinning you really need a plate with a welded nut that has a setscrew.

 

non rotating nut.jpg



#8 MitchAlsup

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Posted 21 April 2017 - 05:06 PM

While all of the above will work::

 

I simply countersunk a hole to fit a 1/4-20 bolt in the Y-frame ground board (3/5 thick plywood) slightly small, and pounded in the hex portion of the bolt prior to urethane coating.

There is a teflon washer between the ground board and the rocker box, and there is a nylon washer on the top side of the rocker box.

But instead of using the bolt as a bolt, I carefully drilled a hole for a cotter pin (the ones with the bump in the middle) cut off all of the threads, and the result is only 1/8 of an inch of rise off the top of the bottom of the inside of the rocker box (that is thinner than the thinnest 1/4-20 bolt.)

 

The thing comes on and off with needle nose pliers, screwdriver, and a <butter> knife if you have none of the above.

 

Structurally, there is almost no force on this particular component--at least compared to what a 1/4-20 grade 5 bolt will take. My y-frame ground board is just over 1 pound, and while sideways forces can be fair, proper sizing of the drilled hole prevents the bolt from movements. One could epoxy the bolt if one is paranoid, I suppose. Anyways, mine has survived 19-odd years (so far).



#9 Oberon

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Posted 21 April 2017 - 05:23 PM

You don't need a bolt. Any form of pin will do. Here I've used a short piece of 50mm plastic pipe.

gallery_217007_4913_64486.jpg

 

gallery_217007_4913_41336.jpg



#10 Pinbout

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Posted 21 April 2017 - 06:09 PM

In EU a lot of people make ctr'less ground boards 



#11 roscoe

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Posted 21 April 2017 - 08:17 PM

Mine's just a 3/8 hex bolt with a no-thread section under the head, stuck through a washer atop the rocker baseboard, with a t-nut pushed into the ground board from the top, and a nylok nut and a washer on the bottom.  The three teflon pads ( I didn't even use real teflon, I just used 'magic sliders' ) around the base hold it up above the rim of the t-nut, so it doesn't rub.  Maybe someday. I'll make some milk-jug washers for between the groundboard and the rocker base, but it works without any....



#12 Jon Isaacs

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Posted 21 April 2017 - 09:03 PM

My thinking:

 

A bolt is nice because the ground board is attached to the rocker box so if you are using wheel barrow handles to move the scope or just carrying the rocker box, it all stays together.

 

As a bearing, the diameter (radius actually) is very small so the torque required is inconsequential.  Just about anything works.

 

As far as shimming the center to reduce the torque required to turn the scope, this is tricky and if the wood is not perfectly flat, then the force is not constant. Furthermore, adjusting it requires disassembling and reshimming it and the assembling it to test it..

 

What I did was design a center bolt assembly that had a heavy spring that pushed between the ground board and the rocker box.  The force could be adjusted with two nuts on that were on either side of the top nut, I could adjust it with the scope in place. It actually worked quite well but once I discovered using bar soap to lube Dob bearings, it was no longer needed.

Dob Center Bearing Design CN.jpg
 
These days, if the bearings on one of my Dobs are not smooth and free of striction, i just clean them and lubricate them by rubbing moistened bar soap on the laminate.. it's amazing.. that's how I am able to track stuff like the Saturn nebula and the Ghost of Jupiter at over 1000x.
 
Jon

 


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#13 roscoe

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Posted 21 April 2017 - 09:58 PM

parrifin wax - cheap candle wax, not beeswax, works great as a lubricant, too.  I'm sure there is some on my pivot bolt....

 

I regularly wax my (woodworking) machine tables with wax-in-a-can, the brand I use is called 'Butcher Wax' - it is rubbed on, wait 15 minutes for it to dry, buff it out.  the difference before and after sliding a board across my table saw or whatever is amazing!!  I use it on all manner of stuff I want to be either shiny (like every scope I own...) or slide well.  That would also work well if either axis of a scope seemed stiffer than desirable.

 

Also, my center bolt, mentioned above, is not tight against the rocker box bottom - There is at least 1/16" of extra space between the washer and underside of the head of the bolt....solves any warpage or irregularity issues....



#14 photiost

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Posted 21 April 2017 - 10:29 PM

I also like to use a 'nylon-insert lock nut' - you adjust the tension once and you're done



#15 Kunama

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Posted 21 April 2017 - 10:33 PM

I used a 1/2" bolt with a brass bushing which screws down through the rocker box and ground board into a nut welded onto a plate with a locking nut underneath.

I then drilled the bolt for a 1/4" encoder shaft with a grub screw through the side of the bolt head to lock the encoder into place.

Then of course the bolt etc needed to be painted to match colour scheme of the scope.....

 

My bearing surface on the bottom of the rocker box is a 3mm thick aluminium plate that was powder coated in Aztec Gold.

Attached Thumbnails

  • MS2_2230.jpg
  • MS2_2267 copy.jpg

Edited by Kunama, 21 April 2017 - 10:34 PM.


#16 member010719

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Posted 21 May 2017 - 11:21 AM

Any of the methods shown above will work.
I'm not criticizing , I don't want polemics , just sharing the  solution made available by Stellafane site and which I believe is the best.

 

Simple , elegant and effective - the center  bolt as suggested in the ''Build a Dobsonian'' of ''stellafane.org''.

It is very easy to retrofit.

 

https://stellafane.o...unt/ground.html

 

It resembles the solution posted by xrayvizhen -see post #4.

But in the drawing  the T nut is in the wrong place.

The T nut should be put on the upper face of the ground board as suggested by Stellafane.

The  nut should stay as per drawing.

No need for  any bushing into the rocker base.

Actually one need a radial play between the rocker base and the bolt  which should be large enough to allow the rocker to sit only on teflon pads - that is the bearing.

Not the bushing.

The bushing and the teflon pads are working one against the other , with bushing and teflon pads you have double determination.

This is not working well in real world.

At a moment the bearing consist of teflon , few degrees further it consist by bushing.

 

 

The Stellafane center bolt could be retrofitted.

I done it already to  my 125mm Dobsonian  telescope 

I made many observations with the 125mm Dobsonian after retrofitting this type of center bolt and is working great.

 

Ground board.T nut.DSCN5199

 

And is under implementation on the 8'' F/6 Dobsonian.

 

Rocker placa De Sol perne teflon Toleascop
 
Ziridava


#17 Jon Isaacs

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Posted 21 May 2017 - 05:20 PM

The bushing and the teflon pads are working one against the other , with bushing and teflon pads you have double determination.

 

Ideally, they work together.

 

The advantage of the spring loaded center bearing is that it maintains a constant load on the center bearing (and therefore the Teflon pads) even though the base is warped.  

 

Jon


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#18 pjmulka

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Posted 21 May 2017 - 10:10 PM

I'm glad I saw this thread. I have some bee's wax left from a make your own lip balm project my daughter wanted to do. I was planning on using it on my ground board ebony star replacement. I'm curious why won't the bee's wax work.
Also can I literally use a candle on the laminate?

parrifin wax - cheap candle wax, not beeswax, works great as a lubricant, too. I'm sure there is some on my pivot bolt....

I regularly wax my (woodworking) machine tables with wax-in-a-can, the brand I use is called 'Butcher Wax' - it is rubbed on, wait 15 minutes for it to dry, buff it out. the difference before and after sliding a board across my table saw or whatever is amazing!! I use it on all manner of stuff I want to be either shiny (like every scope I own...) or slide well. That would also work well if either axis of a scope seemed stiffer than desirable.

Also, my center bolt, mentioned above, is not tight against the rocker box bottom - There is at least 1/16" of extra space between the washer and underside of the head of the bolt....solves any warpage or irregularity issues....



#19 drneilmb

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Posted 22 May 2017 - 08:09 AM

I'm glad I saw this thread. I have some bee's wax left from a make your own lip balm project my daughter wanted to do. I was planning on using it on my ground board ebony star replacement. I'm curious why won't the bee's wax work.
Also can I literally use a candle on the laminate?


You can most definitely rub a candle on the laminate. The wax is a bit hard when you start, but it will still rub off on the texture of the laminate with the right pressure.

When we were skateboarding hoodlums we used to rob our mother's dinner candles to lubricate the curbs in our neighborhood to make them more slippery for certain tricks.
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#20 member010719

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Posted 23 May 2017 - 06:09 AM

 

The bushing and the teflon pads are working one against the other , with bushing and teflon pads you have double determination.

 

Ideally, they work together.

 

The advantage of the spring loaded center bearing is that it maintains a constant load on the center bearing (and therefore the Teflon pads) even though the base is warped.  

 

Jon

 

Thank you Jon for showing that.

Is missing from my post /opinion the fact that I'm referring to small telescopes -up to 8 inch - with which I have experience.

For small telescopes the ground board could be considered a rigid body.

 

I understand large telescopes need another aproach.

If one wants to keep a large telescope to be portable , the mounting can  be considered a rigid body  only at  first aproximation.

My hat off to all telescope makers who succeeded to build lightweight  large telescopes but which mantain colimation.bow.gif

 

Ziridava


Edited by ziridava, 23 May 2017 - 06:10 AM.


#21 Jon Isaacs

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Posted 23 May 2017 - 07:33 AM

 

 

The bushing and the teflon pads are working one against the other , with bushing and teflon pads you have double determination.

 

Ideally, they work together.

 

The advantage of the spring loaded center bearing is that it maintains a constant load on the center bearing (and therefore the Teflon pads) even though the base is warped.  

 

Jon

 

Thank you Jon for showing that.

Is missing from my post /opinion the fact that I'm referring to small telescopes -up to 8 inch - with which I have experience.

For small telescopes the ground board could be considered a rigid body.

 

I understand large telescopes need another aproach.

If one wants to keep a large telescope to be portable , the mounting can  be considered a rigid body  only at  first aproximation.

My hat off to all telescope makers who succeeded to build lightweight  large telescopes but which mantain colimation.bow.gif

 

Ziridava

 

 

It's not a question of the ground board being rigid as much as it being an issue that everything is made of wood so the bottom of the mirror box cannot be considered flat and parallel.  If one is trying to shim the center bearing to reduce the load and therefore the friction on the outer bearings,  any warpage is problematic.   The other problem is that adjusting it requires trial and error and each trial requires removing the scope from the mount and then removing the ground board. 

 

With the spring design,  the spring evens the load even though  everything is not perfectly flat and it is very easy to adjust,  the two top bolts,  adjust the load,  no need to disassemble it. 

 

As far as a lubricant.  I think I have tried them all..  Last year, out in the desert with a wind blown scope that needed help, my wife suggest bar soap. I gave it a try.. It was amazing.. Smooth, no stiction.  

 

The other night i was looking at NGC-6153, a small planetary in Scopius.  It was low on the horizon but the seeing was pretty good,  I was using 806x in the 22 inch.

 

 No nudging,  just a constant pressure allowed me to keep the target centered and steady.  

 

A number of factors made that possible,  the motion was nearly all in the azimuth direction,  the structure is stiff,  I know the scope and have literally thousands of hours experience with Dobs. 

 

But without essential perfect motions,  that would not happen. And the soap as a lube is important part of it.                   

 

Jon 


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#22 Achernar

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Posted 23 May 2017 - 05:01 PM

 

[snip]

A center bolt with appropriate nuts and washers etc that do not tighten or loosen as the scope is rotated.

[snip]

You should consider using a 'nylon-insert lock nut' in conjunction with flat fender washers. Nylon-insert nut

 

The washers prevent chafing of the base components when you lift them in transport.

 

The nylon insert prevents the nut from tightening or loosening as you rotate the telescope while stargazing. With a normal nut, one has to cinch down the nut pretty tightly in conjunction with a lock washer in order to prevent its undesirable rotation. That arrangement won't work very well on a design where the restrained parts are supposed to rotate like the base of a telescope. 

 

On the other hand, a nylon-insert nut will hold just about anywhere on the thread of the azimuth bolt due to the friction that results from deforming the nylon insert as it is screwed on to the threads.   There are quite a few solutions to your problem but I think the nylon-insert nut is the simplest.

--------

C

 

A better way is to get an all-metal locknut, and weld or braze it to a thin, square metal plate you attach to the ground board with screws. Don't try that with a nylon insert locknut, the insert will melt. If you are going to use digital setting circles, this will eliminate any tendency of the bolt to rotate as you turn the base. If the bolt rotates as you move the telescope, the DSC's will be way off. You don't have to weld it all the way around, three tacks that penetrate into the nut and the plate 120 degrees apart will do the trick. I've also put another plate on the rocker bottom with a close fitting hole and no locknut held in place with screws, which acts as a center bearing and prevents wobbling.

 

Taras


Edited by Achernar, 23 May 2017 - 05:04 PM.

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#23 member010719

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Posted 24 May 2017 - 08:27 AM

I tought it easy to see on the Stellafane drawings but maybe is better to underline this : the T nut in the upper face of ground board and the nut below the ground board are blocking the center bolt which cannot rotate.

I tested the solution and I can confirm that rotation of the bolt is not allowed , the axial play it stay as was set by me a month ago.

I understand the refinements requested by DSC's and described by Taras.

 

Ziridava 



#24 drneilmb

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Posted 24 May 2017 - 10:10 AM

I tought it easy to see on the Stellafane drawings but maybe is better to underline this : the T nut in the upper face of ground board and the nut below the ground board are blocking the center bolt which cannot rotate.

I tested the solution and I can confirm that rotation of the bolt is not allowed , the axial play it stay as was set by me a month ago.

I understand the refinements requested by DSC's and described by Taras.

 

Ziridava 

I agree, the Stellafane design is straightforward AFTER you realize that the bolt is only being used as a post, so the T-nut on the TOP of the group board and the nut on the bottom of the ground board are holding the bolt rigidly while the rocker box rotates around the post. The head of the bolt and the washer on the top of the rocker box are just there to keep the rocker box and ground board attached together.

 

It's a clever way to implement a simple post like Oberon showed above. I'm planning to use that design for my 8" f/6 truss dob.



#25 xrayvizhen

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Posted 28 May 2017 - 02:58 PM

I was going to start a separate thread about a problem I discovered with pivot design I illustrated in Post #4 but figured this is as good a place as any to bring this up and solicit a solution.

 

The problem I've discovered with this design is the pivot bolt doesn't rotate with the rocker box...the rocker box rotates smoothly around the pivot bolt. So what's the problem, you ask? Well I'm adding encoders and a DSC system and just realized that the pivot bolt needs to rotate with the rocker or else how else will the 1/4" encoder shaft locked into the the milled out recess in the head of the bolt turn?  I copied this pivot design from the Astrosystems website figuring that if I were to add an encoder all I would need to do is replace the hardware store bolt with one with the milled out head for the encoder shaft, but it's not working.

 

I tried tightening the lock nut underneath the ground board and that did get the pivot bolt to start rotating but as it turned the whole rocker assembly started to tighten down unacceptably. I'm kind of stumped at the moment and hope the mechanical-engineers in the group can provide a solution.

 




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