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Plastic shim between Mount and Tripod

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#1 Donnie D

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Posted 02 May 2011 - 07:54 AM

I had read somewhere that someone used a thin plastic shim between their CGEM Mount and the top of tripod. They did this so that when the two assemblies are relatively tight, the person could still make adjustments to the Azmuth. The way it stands (with me and others) is that if you tighten the two assemblies then realize you need more adjustments, you have to reach under and loosen the bottom hand knob to loosen the mount. I can not remember if this was some link that I stumbled across or I read it here.
If it was here, then I hope that whoever would show me the link so that I can read an account of how that shim works. Getting to be slight annoyance when doing my polar scope alignment to have to fight those knobs.

Also, out of curiosity, what about the newer CGEM DX mount. Is this still a factor? I will admit that I have not taken the time yet to read up on that mount. It seems that the only change is that it is able to handle more weight. I need to read up on that.

Donnie
Thanks in advance for any help.

#2 Eddgie

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Posted 02 May 2011 - 08:30 AM

I don't think I saw the posting, but one of my mounts has a plastic bearing like you mention between the mount head and the tripod plate.

The Meade LXD-750 has a very large plastic bearing that fits between the head and the tripod.

It is just a circular "Gasket" made from a very thick material similar to the plastic used in milk bottles in its composition. The circle is about an inch wide around the outside edge, and the back third is completly filled in.

It is about 1/16th inch thick. It is simply glued to the bottom of the head so that when the head is lifted off, the bearing comes away with the head.

Now the LXD-750 was I think DESIGNED to be used with this bearing.

The LXD-750 has a post on the bottom of the head that is a couple of inches long. The mound will not sit flat on the floor.

The post on the bottom goes into a recess on the top of the tripod plate.

The purpose of this shaft is to keep the assymetrical weight of the mount (I can do the azimuth adjustment with the scope and counterweights in place) from pinching on the north side of the bearing because the shaft prevents this from happening.

The base of the head is also very large, being maybe 1- inches in diameter, so there is a LOT of bearing surface.

In a mount without a posts, you might find that the bearing pinches if you have counterweights on.

But it sounds like it would be cheap to try.

A better choice might be to use the material they use in Dob telescopes to cut your bearings from this material. I would think this would give you the best chance of being able to rotate the base even with pressure on the bottom knob.

#3 Donnie D

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Posted 02 May 2011 - 09:55 AM

Well, now that I think about it, the problem (that I see) is that the counter weight on front of rig is "pulling" the scope downward and forward. If I were able to loosen that hand knob and the whole assembly was "balanced" that would be different. The only way around this is to do a polar align with no weights on it at all. But this would change some dynamics and after weight(s) put on, then have to touch it again.
I guess it all comes down to manufacturing / design / and cost. If in the design was a seriously robust ball-bearing design that the azimuth could be rotated with good pressure, then that would add to cost, etc. Now, what would really be nice is to have a separate motor driven platform that would control azimuth alone.

ps. I guess one thing that I need to think about is getting a plastic shim like you say that will have almost no tolerance for reduction of thickness when pressure is applied so that when the Scope is slewing around, the thickness of the plastic will not "give" under the pressure of the RA moving around. When the RA is moving around, it alone is going to change the dynamics of where the pressure is being applied to the different points of the shim. Then the shim can't have any friction to it.
I wonder what would happen if two shims were used.

Donnie

#4 fetoma

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Posted 02 May 2011 - 10:50 AM

I have heard of this, but it was recommended to use a very thin piece of teflon.

#5 Jwemes

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Posted 02 May 2011 - 10:52 AM

The posts regarding the plastic shims may be on the CGEM Yahoo group. Just about any thin, low-friction material (nylon, teflon, HDPE) will work. Thin stock can be cut with scissors and razor knives. Two shims probably would not work, as the azimuth post will interfere with any kind of rotation around the center line.

They work very well...at least mine does.
Clear, Dark, and Steady

#6 PHampson

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Posted 02 May 2011 - 11:56 AM

I don't have a CGEM but I've used abrasion-resistant Fiberglass Tape coated with PFTE successfully on my mounts. It's plenty slippery enough, holds up well, and is easy to apply, take off, or replace as necessary. Only .005" thick.


Paul

#7 Stew57

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Posted 02 May 2011 - 12:18 PM

I used a plastic milk jug cut to fit and it works great. teflon sheet would probably be best but the "poor man's teflon" work fine and is cheap. Give it a try and you will wonder why celestron doesn't include some type of factory installed plastic spacer.

#8 Eddgie

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Posted 02 May 2011 - 12:25 PM

Yes, that is what I suggested as well. The Meade LXD-750 bearing looks almost exactly like milk bottle plastic.

Some light grease on the face isn't going to hurt anything either, espically if the head is normally left in place on the tripod when not in use. This way, the grease film won't pick up dirt.

I used a thin coat of Superlube on mine. When I bring the mount in, I always put the head back on though, and this keeps dirt off.

Wiping it clean though would be simple enough.

So, plastic milk jug with superlube would work too I suspect.

#9 Eddgie

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Posted 02 May 2011 - 12:30 PM

There is another option of course. I didn't offer it before, but now that the dialog is open, you might be interested.

The real problem here is that you don't have enough leverage on the little knobs that drive the scope in Azimuth.

Have you considered just going with bigger knobs and a little grease?

A good knob may do more for the situation than a plastic bearing.

Just though I would mention it...


http://www.mcmaster....p-knobs/=c4r190

#10 Donnie D

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Posted 02 May 2011 - 12:45 PM

Nice point Eddgie and the others. Also, to see now if I can find (more out of curiosity) the abrasion-resistant Fiberglass Tape coated with PFTE that you mentioned.

Donnie

#11 Donnie D

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Posted 02 May 2011 - 12:46 PM

I actually just found this:
http://www.grainger....ZRF2?Pid=search

#12 Donnie D

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Posted 02 May 2011 - 01:07 PM

Well, I decided to go ahead and order some shim stock Plastic Teflon GP
SKIVED Sheet VIRGIN 0.06" and 0.031"
Very reasonable prices, but had to do the shipping thing - not bad.

got it from
https://www.onlinemetals.com/index.cfm

#13 Jwemes

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Posted 02 May 2011 - 01:23 PM

Out of the box, the threaded holes in my CGEM were so sloppy that I went with the plastic shim immediately, rather than risk a stripped azimuth adjusting knob. At some point, the holes will get retapped with inserts. With the sloppy threads, and the soft Aluminium mount casting, larger diameter knobs were not in the picture.

National Astronomy Day THIS Saturday

#14 Donnie D

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Posted 02 May 2011 - 06:31 PM

I did notice that my azimuth threaded holes were very loose tolerance. I am sure that it time, I can see them getting worse.

Donnie

#15 RTLR 12

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Posted 03 May 2011 - 06:53 PM

I used this UHMW (Ultra High Molecular Weight Polyethylene) tape on my CG-5 and CGEM. Makes it 'slide' a lot easier. I also installed steel threaded inserts on my CGEM azimuth adjustment bolts.

http://www.grainger....aheadSearch.y=8

Stan

#16 GizmoDave

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Posted 03 May 2011 - 07:17 PM

My CGEM had both surfaces painted with wrinkle paint.I sanded the contact area of both parts with wet/dry paper to produce a smooth finish then used a light coat of Super Lube. Azimuth can be adjusted with the mount fully loaded and the center knob loosened 1/16th of a turn.

#17 jpkozic

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Posted 05 May 2011 - 09:19 PM

I beleave I saw a similar thread here in Cloudy Nights but I can't find it any where. If any of you know its whereabouts please let us all know.

Thanks Joe

#18 Donnie D

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Posted 06 May 2011 - 01:22 PM

Well, those teflon shims should be arriving any day now. I will let everyone know. The sanding of those surfaces sounds like a good idea as well.
Thanks
Donnie

#19 Donnie D

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Posted 19 May 2011 - 12:37 PM

I finally had a chance to try out my shims last night. I had ordered .031 teflon shims. I cut out a small shim that fits down inside mount and then cut out a ring type of shim that rides along that upper rim.
It made a huge difference. I can really lock down the mount and still use the AZ knobs to make fine adjustments without the need of turning the knob below to loosen the mount.
This turns out to be a worthwhile mod.

Donnie






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