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Mirror side support - captive ball bearing - LB16"

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#1 bilgebay

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Posted 30 July 2012 - 06:51 AM

I will tell you this story backwards :)

Here is what I did after I got rid of the silicons Meade used to fix the mirror in the cell:

Posted Image

I have placed 2x 3/8" torlon balls into the cavities I prepared on the mirror cell. The cavities are at 45 degrees from vertical at the bottom of the the mirror cell and in line with COG of the mirror.

Here is a short clip showing how the torlon balls are freely rotating.

Short movie

I did the same test when the mirror was at 45 and 60 degrees and the system worked without a problem. However, since I was alone and two handed only, couldn't film those trials.

If the experts on this forum approve this solution and you want to know more, I will continue.

Thanks for your time.

Sedat

#2 rockethead26

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Posted 30 July 2012 - 09:48 AM

I'm certainly no expert, but wouldn't the ball bearings create two stress points by not evenly supporting the edge of the mirror like a sling or cable would?

#3 Pinbout

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Posted 30 July 2012 - 10:00 AM

that is great. love the movie.

#4 Howie Glatter

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Posted 30 July 2012 - 10:27 AM

Hi Sedat,

Looks good! I think the cavities are a little bit wider than the balls because you can see a tiny bit of side motion of the ball when you reverse the mirror rotation, but I don't think that's a problem. Does the ball bear against a countersink in the aluminum cell casting?

#5 bilgebay

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Posted 30 July 2012 - 02:52 PM

Hi Howie,

I am glad you liked the idea. You are absolutely right, there is some lateral play caused by the imperfections of my hand held drilling. I will do this again ( by rotating the aluminium mirror cell by 120 degrees) but this time in my machine shop and with great precision. This was just a test run and in fact will function as is if I leave it alone.

You can see where the ball sits in the photo below:

Posted Image

Hi res photo is here

Initially, I planned to use plastic screws but they didn't look strong enough to me, especially with the angle I drilled the holes. There simply was too much side load on these 4mm plastic screws.

Posted Image

Why did I drill the holes at such a stupid angle ? To leave enough material at the edge of the mirror cell while keeping the tip of the screw on the COG of the mirror. It wasn't a good idea though.

#6 bilgebay

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Posted 30 July 2012 - 02:55 PM

Hi Danny, thank you.

#7 bilgebay

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Posted 30 July 2012 - 03:07 PM

Hi Jim,

I reckon the sling cable is the best solution. However, it would be difficult for me to implement that solution to the existing mirror cell. A while ago, there was a thread by Project Galileo where 2-point edge support was discussed.

Once I marked the COG on the metal edge I set and drilled a hole in the side so I could tap new threads. A tap and some lubricating oil had me in business soon. Be careful. It is an aluminum cell and is very soft and easy to work with. I opted for a 10/24 screw size so I had many threads in the thinner metal cell side.

Posted Image Posted Image Posted Image Posted Image Posted Image

In the close up picture you can see the hole I drilled is 2.5 mm below the COG mark above it. I chose to back up the support to allow for more metal at the lip of the hole and cell. I felt I would be drilling and tapping a hole too close to the edge using the true COG.

The Edge Support Calculator showed that by changing the support position relative to the COG 1/20th of the mirror thickness (2.5 mm in my case) it would change the RMS Surface Error to 3.3 nm. Still within the 5 nm goal and undetectable with the eye. I opted to move the screw in trade for more strength.

The new 10/24 nylon screws fit perfectly. I will round and smooth the tips of the screws to lower their friction on the mirror's edge too. Finally, I will use left over UHMW tape pads stuck to the side of the mirror where it contacts the nylon screws to lower friction even more.

Posted Image

Ta da! Mirror cell improved and done for now!


You can read about M42's evolution here.

I will be following Doc's footsteps for a while :) Thank you for this beautiful thread once again Doc.

#8 bilgebay

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Posted 30 July 2012 - 03:30 PM

Howie, I forgot to give you the crucial dimension, here it is:

Posted Image

You should be somewhere between 11.00 and 11.40 mm to keep the mirror at a distance from the cork material around the periphery of the cell.

And for those who are wondering how I was able to rotate the mirror so easily, here is the magical chemical I have used for that: McLube Sailkote.

We are using this stuff a lot on our sailboats to help everything move much easier so I said why not here :) I have sprayed the back of the mirror with this and it made a huge difference. Of course, I have removed the original support pads and the mirror is on bare floating supports. I have also used Sailkote at the fulcrum of the floating supports both on the mirror cell and the support side. They are really floating now. Needless to say, the seats where the 2x 3/8" Torlon balls are seated were also sprayed with Sailkote. It takes most of the friction out of the equation.

#9 rockethead26

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Posted 30 July 2012 - 04:28 PM

Hi Jim,

I reckon the sling cable is the best solution. However, it would be difficult for me to implement that solution to the existing mirror cell. A while ago, there was a thread by Project Galileo where 2-point edge support was discussed.

Once I marked the COG on the metal edge I set and drilled a hole in the side so I could tap new threads. A tap and some lubricating oil had me in business soon. Be careful. It is an aluminum cell and is very soft and easy to work with. I opted for a 10/24 screw size so I had many threads in the thinner metal cell side.

Posted Image Posted Image Posted Image Posted Image Posted Image

In the close up picture you can see the hole I drilled is 2.5 mm below the COG mark above it. I chose to back up the support to allow for more metal at the lip of the hole and cell. I felt I would be drilling and tapping a hole too close to the edge using the true COG.

The Edge Support Calculator showed that by changing the support position relative to the COG 1/20th of the mirror thickness (2.5 mm in my case) it would change the RMS Surface Error to 3.3 nm. Still within the 5 nm goal and undetectable with the eye. I opted to move the screw in trade for more strength.

The new 10/24 nylon screws fit perfectly. I will round and smooth the tips of the screws to lower their friction on the mirror's edge too. Finally, I will use left over UHMW tape pads stuck to the side of the mirror where it contacts the nylon screws to lower friction even more.

Posted Image

Ta da! Mirror cell improved and done for now!


You can read about M42's evolution here.

I will be following Doc's footsteps for a while :) Thank you for this beautiful thread once again Doc.


Thanks for clarifying Sedat. Looks like the stress change to the mirror's figure is insignificant. Looks like a good solution for you.

#10 GeneT

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Posted 30 July 2012 - 04:52 PM

Very interesting. Please post how it works in the field.

#11 bilgebay

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Posted 31 July 2012 - 01:31 AM

You are welcome Jim.

#12 bilgebay

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Posted 31 July 2012 - 01:33 AM

Hi Gene,

I bet it will be much better than the original but I will certainly report my findings.

#13 bilgebay

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Posted 31 July 2012 - 01:39 AM

Another implementation can be ball point support screws. Just like the ballpoint pens...

Posted Image

This was not applicable to LB16 mirror cell but could be used by others and ATM'ers. In this case you will not be limited to 2 contact points. You could use 4 points and more with careful craftsmanship.

#14 The bear

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Posted 31 July 2012 - 10:24 AM

do they make structures like the ball on a stalk like you show in your diagram i have some ideas i could use such a thing thanks
doc
:grin:

#15 bilgebay

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Posted 01 August 2012 - 01:17 AM

Any machine shop should be able to do it for you.

#16 Pinbout

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Posted 01 August 2012 - 06:03 AM

do they make structures like the ball on a stalk like you show in your diagram i have some ideas i could use such a thing thanks



mcmasters sells them, but the ball is spring loaded.

http://www.mcmaster....lungers/=inp0r1

#17 piaras

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Posted 05 August 2012 - 09:18 AM

Some of the "plunger ball screws" have a tension screw on the opposite end. One can remove the spring and replace it with either a pin, like a dowel pin, or even a longer screw to lock the ball in place.

Also one can weld ears on the mirror support ring to allow the screws to be in the correct place and orientation.

#18 Mark Peterman

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Posted 11 December 2012 - 05:21 PM

do they make structures like the ball on a stalk like you show in your diagram i have some ideas i could use such a thing thanks




mcmasters sells them, but the ball is spring loaded.

http://www.mcmaster....lungers/=inp0r1


They also have these set screws with a ball bearing in the end. (non-spring loaded)

McMaster

#19 Project Galileo

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Posted 11 December 2012 - 10:32 PM

I love this idea. The captive ball bearing precision adjustment set screw may be just the thing for me too. I can only see them as decreasing friction and freeing up the mirror to float better.

#20 d.sireci51

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Posted 11 December 2012 - 11:18 PM

I like the spring loaded ball bearings. If you cant set the ball bearings exact, the spring would compensate. Also it would hold the mirror snug while moving the tube from horz to vert.

D

#21 mtb54703

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Posted 12 December 2012 - 04:16 PM

I like the spring loaded ball bearings. If you cant set the ball bearings exact, the spring would compensate. Also it would hold the mirror snug while moving the tube from horz to vert.

D


I don't think you want any sort of edge support with a spring in it - that would only allow the mirror to compress the spring as you put more load on the edge support when dropping the scope in elevation.

Also - I'd be a little concerned about a small set screw will a metal ball tip - you are putting metal against glass.

#22 Starman1

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Posted 12 December 2012 - 06:46 PM

I will tell you this story backwards :)

Here is what I did after I got rid of the silicons Meade used to fix the mirror in the cell:

Posted Image

I have placed 2x 3/8" torlon balls into the cavities I prepared on the mirror cell. The cavities are at 45 degrees from vertical at the bottom of the the mirror cell and in line with COG of the mirror.

Here is a short clip showing how the torlon balls are freely rotating.

Short movie

I did the same test when the mirror was at 45 and 60 degrees and the system worked without a problem. However, since I was alone and two handed only, couldn't film those trials.

If the experts on this forum approve this solution and you want to know more, I will continue.

Thanks for your time.

Sedat

I like this. Good thinking!

#23 bilgebay

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Posted 13 December 2012 - 12:51 AM

Thank you Don.

I'd be a little concerned about a small set screw will a metal ball tip - you are putting metal against glass.



Mike - this sounds scary to me as well. This was one of the reasons I used Torlon balls instead of steel balls.

Clear skies

Sedat






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