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Did I do something wrong or does this type of mount really suck?

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#1 Jbhillman

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Posted 18 January 2020 - 06:36 PM

I have a DIY 15” truss tube scope.  I built it using information gathered online.  Everything about it works like I expect it to except for one small detail.  Up/down and rotational movement are fine.  Smooth and even. Except if I only want to move the image a little when I am using a high magnification eyepiece.  Then, it sucks.  I can not shift the image only from the edge of the fov to the center.  It will not move that tiny amount smoothly.  What is wrong, and is there a better way than Teflon pads on black star Formica?



#2 Auburn80

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Posted 18 January 2020 - 06:44 PM

Sounds like a bit too much stiction. Try rubbing the formica with some bar soap. No need to use a lot.
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#3 WadeH237

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Posted 18 January 2020 - 07:02 PM

Yup.  It sounds like classic stiction.

 

My suggestion is to post the question over here in the ATM forum.  You will probably get lots of great suggestions.



#4 Joe1950

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Posted 18 January 2020 - 07:18 PM

Do you have a center PTFE or plastic washer that takes some of the load off the circumference Pads?  The weight of the scope pressing on the center of the baseboard is sometimes too much to allow smooth movements. There should be a washer, plastic or PTFE on the center bolt between the top board and the bottom that is thick enough to take up most of the weight so it doesn't all ride on the outer pads. IOW, just a little less than the thickness of the pads themselves.

 

You can also use a series of washers made from empty milk carton containers. They can be cut to a large (2-3") diameter with a hole that allows the center bolt easily through. Make several and build up the center until the weight distribution allows the baseboards to move effortlessly (with the scope in position).

 

Makes a big difference. And the soap helps a lot too. But too much weight bearing sown on the center of the top baseboard is a usual culprit for stiction.


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#5 Pauls72

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Posted 18 January 2020 - 09:24 PM

On the Formica try using some car wax to make it slicker. Or you can use a little silicon spray or graphite as dry lubricant.

Is your load spread evenly across the teflon pads?


Edited by Pauls72, 18 January 2020 - 09:26 PM.

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#6 Berny

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Posted 18 January 2020 - 11:25 PM

On the Formica try using some car wax to make it slicker. Or you can use a little silicon spray or graphite as dry lubricant.

Is your load spread evenly across the teflon pads?

Or Molybdenum Disulfide in powder form, not grease. Probably better than graphite.



#7 csrlice12

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Posted 19 January 2020 - 01:00 AM

You could install a lazy susan bearing in place of the teflon pads and formica.



#8 alphatripleplus

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Posted 19 January 2020 - 07:38 AM

Moving this from Mounts  to ATM, Optics and DIY for a potentially better fit.



#9 macdonjh

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Posted 19 January 2020 - 07:43 AM

You could install a lazy susan bearing in place of the teflon pads and formica.

I did this once on the azimuth bearing of a small Dobsonian.  Then I had the opposite problem: not enough sticktion.  The scope wouldn't stay where I aimed it.  To fix that problem I added a bolt that went through the rocker box and pressed against the ground board.  On the end of the bolt I put a washer that had a felt pad on it.  That prevented the bolt from gouging my ground board and made the bolt slide smoothly against the ground board.  Now I had an easy method to adjust the amount of friction in the azimuth axis and the scope moved smoothly.

 

I was told, after I'd done all this, I could have had the same results by making the PTFE pads a bit smaller.  Less surface area rubbing against the Formica means less stiction/ friction.  Seems the Dobsonian builders had already solved that problem.

 

One advantage of the lazy Susan bearing, though, is I never had to worry about how clean my ground board was.  Movement was always smooth and easy.



#10 airbleeder

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Posted 19 January 2020 - 09:47 AM

   Try bar soap before you start spending money and rebuilding. Wet some soap and rub on the formica and teflon. After that, you can even bathe with it. It's cheap, usually on-hand and it works.



#11 davidmcgo

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Posted 19 January 2020 - 10:01 AM

Make sure you don’t have the azimuth bolt locked down too tight, the ground board should be able to drop a little away from the rocker when you lift it.

 

Dave


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#12 Pinbout

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Posted 19 January 2020 - 10:16 AM

What is wrong, and is there a better way than Teflon pads on black star Formica?

 

show us your mount

 

black star formica isn't a thing.... so what laminate are you using.

 

pad placement can help. also pad size can help...



#13 Jbhillman

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Posted 19 January 2020 - 10:52 AM

show us your mount

 

black star formica isn't a thing.... so what laminate are you using.

 

pad placement can help. also pad size can help...

Good question. I bought it 15 years ago.  It was the Formica recommended on the ATM list I belong to



#14 Dale Eason

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Posted 19 January 2020 - 01:30 PM

You probably mean Ebony Star Formica instead of black star formica.  If it has small  bumps and is not perfectly flat  that is the key.  If you have that and true Teflon you have the best or ideal parts.

 

If so then you have a different issue.  It cold be that you have too much friction from the center bolt or that your structure has too much spring in it.  That is it is not stiff enough.  My diagnosis is because if you have the right bearing surfaces as you describe then you should have close to the correct amount of stiction and friction for really nice movements.  

 

Dale


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#15 Jbhillman

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Posted 19 January 2020 - 03:37 PM

You probably mean Ebony Star Formica instead of black star formica.  If it has small  bumps and is not perfectly flat  that is the key.  If you have that and true Teflon you have the best or ideal parts.

 

If so then you have a different issue.  It cold be that you have too much friction from the center bolt or that your structure has too much spring in it.  That is it is not stiff enough.  My diagnosis is because if you have the right bearing surfaces as you describe then you should have close to the correct amount of stiction and friction for really nice movements.  

 

Dale

It is the ebony star Formica.  Found the cause I think.  My upper cage has a tiny bit of flex.  Probably developed due to being moved around on rough ground.  I am going to either rebuild the upper cage or convert to a tube since it is going to be installed in a permanent observatory.



#16 Dale Eason

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Posted 19 January 2020 - 04:22 PM

There are ways to stiffen structures after the fact.  Sometimes you can string wires attached in a x pattern between the truss poles to stiffen it all up once they are under tension. 


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#17 Bob4BVM

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Posted 19 January 2020 - 10:24 PM

There are ways to stiffen structures after the fact.  Sometimes you can string wires attached in a x pattern between the truss poles to stiffen it all up once they are under tension. 

Dale's is an easy solution to added stiffness , for a permanent mounted scope its a great idea.

 

Regardless of what else you do, top it off with the bar soap treatment of bearing surfaces. The only people who poo-poo that are the ones who have never tried it.



#18 Jbhillman

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Posted 23 January 2020 - 04:55 PM

On the Formica try using some car wax to make it slicker. Or you can use a little silicon spray or graphite as dry lubricant.

Is your load spread evenly across the teflon pads?

The load is spread evenly.  I sprayed the Formica with silicone spray.  That made it better.  While I had the rocker box off I also countersunk a bearing in to separate the rocker box from the bolt.  Discovered that there was a lot of resistance there.  I also drilled the hole out oversize so the only thing in the rocker box that the bolt touches is the bearing.  Now it works the way I always thought it should.  Final test will be after I get it assembled and out in the field to use.




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