Telescope Reviews: CGEM Declination axis shift

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telescopemullet
professor emeritus

Reged: 11/16/09

CGEM Declination axis shift
#5389588 - 08/27/12 11:31 AM

Last night I noticed a slight shift or rocking in my dec axis. It is locked down tight but I am able to rock the axis back and forth a slight, very slight, amount. I noticed this when looking at the moon last night. As I was focusing, the scope would the slight distance with the pressure from my fingers. The moon moved 1/2-way across the eyepiece before the slop was gone.

Any idea on how to fix this. I check all the allen heads and they were tight.

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RTLR 12
Post Laureate

Reged: 12/04/08

Loc: Capistrano Beach, Ca

Re: CGEM Declination axis shift new

new

[Re: telescopemullet]
#5389591 - 08/27/12 11:34 AM

Worm gear to ring gear adjustment or possibly worm shaft end play adjustment.

Stan

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telescopemullet
professor emeritus

Reged: 11/16/09

Re: CGEM Declination axis shift new

new

[Re: RTLR 12]
#5389738 - 08/27/12 01:04 PM

How would one go about doing either of those two adjustments?

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RTLR 12
Post Laureate

Reged: 12/04/08

Loc: Capistrano Beach, Ca

Re: CGEM Declination axis shift new

new

[Re: telescopemullet]
#5389798 - 08/27/12 01:41 PM

This site shows the EQ6, but the CGEM is very similar.

http://timsastroblog.blogspot.com/2007/04/little-tune-up-for-eq6.html

Stan

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RTLR 12
Post Laureate

Reged: 12/04/08

Loc: Capistrano Beach, Ca

Re: CGEM Declination axis shift new

new

[Re: RTLR 12]
#5389802 - 08/27/12 01:44 PM

Here's a better site.

http://timsastroblog.blogspot.com/2007/04/little-tune-up-for-eq6.html

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rmollise
Postmaster

Reged: 07/06/07

Re: CGEM Declination axis shift new

new

[Re: telescopemullet]
#5390010 - 08/27/12 03:59 PM

Quote:

Last night I noticed a slight shift or rocking in my dec axis. It is locked down tight but I am able to rock the axis back and forth a slight, very slight, amount. I noticed this when looking at the moon last night. As I was focusing, the scope would the slight distance with the pressure from my fingers. The moon moved 1/2-way across the eyepiece before the slop was gone.

Any idea on how to fix this. I check all the allen heads and they were tight.

A slight amount isn't going to hurt anything. Leave it alone is my advice.

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Wembley2000
super member

Reged: 01/07/12

Loc: NW Ohio

Re: CGEM Declination axis shift new

new

[Re: rmollise]
#5390215 - 08/27/12 06:29 PM

Make sure the 3 hex bolts that hold the dovetail on are tight, the ones on mine we loose.

Wem

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WarmWeatherGuy
professor emeritus

Reged: 08/27/11

Loc: Orlando, FL 28° N, 81° W

Re: CGEM Declination axis shift new

new

[Re: telescopemullet]
#5390800 - 08/28/12 01:43 AM

One reason I did a hypertune on my CGEM was so I would know how to adjust the slop out. Deep Space Products http://www.deepspaceproducts.com/ has a kit that comes with some DVDs with instructions showing in great detail how to take the mount apart and put it back together again. When you're done you know how to adjust the slop with two simple Allen wrenches.

I found that the gear in my declination axis is not perfectly round. If I adjust the slop out on one side and rotate it 180° I then have a small amount of slop. If I adjust the slop out on that side then when I rotate it 180° it will make noise telling me it is too tight.

Measure your slop and then rotate the scope 90° and measure again. Keep going all the way around and you may notice the amount of slop varies with position. Note that you have to move the scope with the motor. If you just release the clutch and move the scope by hand you haven't moved the main gear relative to the worm gear.

After doing the hypertune I had to make the final adjustment which gives the amount of slop you end up with. I had no scope or counterweights attached. The thing moved around and then suddenly stopped. My heart sank. I thought that it had binded the gears. I was able to move in the direction I came from but it wouldn't move past that one point. It didn't make any sound like something was wrong, it just stopped at this one place. Soon I realized that it was the firmware doing that as part of the protection from allowing your camera to smash into the tripod. All I had to do was turn off the mount and turn it back on. Then I could continue moving in that direction.

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Carl Lydon
journeyman

Reged: 01/12/13

Loc: Stamford, Ct

Re: CGEM Declination axis shift new

new

[Re: WarmWeatherGuy]
#5716237 - 03/06/13 11:46 AM

I have the same problem! When I first got my CGEM DX everything seemed rock solid, but later it seemed that the alan bolts for the dovetail lock were loose, and now dec has a wobble that no amounts of bolt tightening will fix. The wobble varies from very slight to quite significant at high magnifications when doing planetary imaging. Thanks for posting the above instructions but I'm a bit scared to take this expensive piece of equipment apart.

Edited by Carl Lydon (03/06/13 11:47 AM)

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RTLR 12
Post Laureate

Reged: 12/04/08

Loc: Capistrano Beach, Ca

Re: CGEM Declination axis shift new

new

[Re: Carl Lydon]
#5716264 - 03/06/13 12:04 PM

Carl, You don't have to take it apart to do the worm gear adjustment. That is where you want to start.

Stan

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WarmWeatherGuy
professor emeritus

Reged: 08/27/11

Loc: Orlando, FL 28° N, 81° W

Re: CGEM Declination axis shift new

new

[Re: Carl Lydon]
#5716283 - 03/06/13 12:19 PM

It is not a matter of tightening the allen bolts. In fact you should be VERY CAREFUL not to over-tighten them, they strip very easily.

What you have to do is loosen some allen bolts and then make an adjustment with other allen bolts. Then tighten the allen bolts. You have to spend some time doing this because when you re-tighten things change and you have to start over.

The point I was making is that by doing the hypertune myself I knew what was inside and could comfortably make the adjustments.

I was originally worried that the hypertune job would be too big for me. When you order the kit you get beautiful photos / diagrams showing all the internal workings. You also get two DVDs where Ed Thomas shows you step by step how to do the entire hypertune (except for the cleaning of the grease part). You will need to buy some tools and that can take some time trying to find them. It also adds to the expense. Also, it takes much longer to perform than advertised, in part because you have to go buy tools you didn't know you needed until you open things up.

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orlyandico
Carpal Tunnel

Reged: 08/10/09

Loc: Singapore

Re: CGEM Declination axis shift new

new

[Re: WarmWeatherGuy]
#5716364 - 03/06/13 01:13 PM

Ed has pointed this out in other posts, but the M6 hex bolts that hold the worm blocks have a tendency to strip their holes very easily.

Sometimes this means you cannot torque down the worm blocks enough and this causes slop.

The solution is to replace all the M6 hex head bolts that hold the worm blocks with longer ones; I used 25mm ones and they still clear the internal parts. Since these bolts protrude slightly into the mount housing, they engage all the threads and are far more secure.

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RTLR 12
Post Laureate

Reged: 12/04/08

Loc: Capistrano Beach, Ca

Re: CGEM Declination axis shift new

new

[Re: orlyandico]
#5716848 - 03/06/13 05:02 PM

I installed Heli-Coil inserts in all of the worm gear housing bolts on both axis, for both the motor mounting bolts, and Keensert repair sleeves in the ALT adjusting screw threads. This was a much more proper fix to the problem than the use of longer bolts that would only strip out the already damaged and weakened threads at a later time. I found the casting of the CGEM main body to be very brittle. I found the use of a specifically for aluminium cutting fluid was required to do the job properly. I no longer have to handle the threads with kid gloves or worry about further thread damage.

Stan

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WarmWeatherGuy
professor emeritus

Reged: 08/27/11

Loc: Orlando, FL 28° N, 81° W

Re: CGEM Declination axis shift new

new

[Re: RTLR 12]
#5716904 - 03/06/13 05:27 PM

Quote:

I installed Heli-Coil inserts in all of the worm gear housing bolts on both axis, for both the motor mounting bolts, and Keensert repair sleeves in the ALT adjusting screw threads. This was a much more proper fix to the problem than the use of longer bolts that would only strip out the already damaged and weakened threads at a later time. I found the casting of the CGEM main body to be very brittle. I found the use of a specifically for aluminium cutting fluid was required to do the job properly. I no longer have to handle the threads with kid gloves or worry about further thread damage.

Stan

Beautiful! I had heard about Heli-Coil inserts but not Keenserts. I found these YouTube videos that show how to install them if anyone is interested.

Thread Repair Using A HeliCoil Insert
http://www.youtube.com/watch?v=Z-uxtuE1xKM

Thread Repair Using a Keensert Insert
http://www.youtube.com/watch?v=XyCTh23Z6o0

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Lane
Post Laureate

Reged: 11/19/07

Loc: Frisco, Texas

Re: CGEM Declination axis shift new

new

[Re: RTLR 12]
#5716967 - 03/06/13 06:02 PM

Quote:

I installed Heli-Coil inserts in all of the worm gear housing bolts on both axis, for both the motor mounting bolts, and Keensert repair sleeves in the ALT adjusting screw threads. This was a much more proper fix to the problem than the use of longer bolts that would only strip out the already damaged and weakened threads at a later time. I found the casting of the CGEM main body to be very brittle. I found the use of a specifically for aluminium cutting fluid was required to do the job properly. I no longer have to handle the threads with kid gloves or worry about further thread damage.

Stan

How do those inserts work? If you had an M5 aluminum hole and stripped out the threads what would be the procedure to fix it?

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RTLR 12
Post Laureate

Reged: 12/04/08

Loc: Capistrano Beach, Ca

Re: CGEM Declination axis shift new

new

[Re: Lane]
#5717017 - 03/06/13 06:32 PM

You need to purchase the Heli-Coil kit for the specific size thread you need to repair. These kits are expensive and can run from $35 to $75 or more depending on the thread size. The kit comes with the repair inserts, tap, and insert tool/tools. A proper repair takes some practice and careful attention to the work being done. In the case of the 5mm you suggested, you first drill the damaged thread with the proper drill size suggested by the Heli-Coil kit. Next step is to tap the hole with the special Heli-Coil tap supplied with the kit. Then the Heli-Coil repair insert is installed using the installation tool. I use some Loctite to insure the insert does not shift after installation and to make the insert permanent.

I already had the size kits that I need to do the repair with in my tool box. Otherwise this repair would be very expensive to preform. You can take your mount to a machine shop and have the work done professionally, but this too can be very expensive.

Stan

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skywolf856
sage

Reged: 01/25/08

Loc: SE Michigan

Re: CGEM Declination axis shift new

new

[Re: RTLR 12]
#5718775 - 03/07/13 04:35 PM

The Keenserts basically are a larger standard threaded hole
for the given screw size you want to replace.
You would have to make sure you have enough material to tap a larger hole.

Link:
http://www.alcoa.com/fastening_systems/aerospace/en/product_category.asp?cat_...

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Lane
Post Laureate

Reged: 11/19/07

Loc: Frisco, Texas

Re: CGEM Declination axis shift new

new

[Re: skywolf856]
#5718824 - 03/07/13 04:59 PM

So for a 5mm existing hole that is damaged, if you wanted the new hole to also be 5mm, then you have drill out to 5.5mm? 6mm? 6.5mm? 7mm? Just trying to figure out if there is enough room to do this. I guess I could go to a smaller bolt.

What I need to fix is a part on my lawnmower's aluminum deck.

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EFT
Vendor - Deep Space Products

Reged: 05/07/07

Loc: Phoenix, AZ

Re: CGEM Declination axis shift [Re: Lane]
#5718983 - 03/07/13 06:12 PM

Quote:

So for a 5mm existing hole that is damaged, if you wanted the new hole to also be 5mm, then you have drill out to 5.5mm? 6mm? 6.5mm? 7mm? Just trying to figure out if there is enough room to do this. I guess I could go to a smaller bolt.

What I need to fix is a part on my lawnmower's aluminum deck.

The biggest problem with using inserts for the mount is not the area available for a larger hole, but the thin metal of the aluminum housing to start with. Inserts are generally designed for either blind holes or holes in metal that is 1/2" think or more. The relatively thin (<1/4") thick cast aluminum housings are not a ideal place to use inserts. In general it would be easier and probably better to do a better job of drilling and tapping a larger hole that to try to use an insert.

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RTLR 12
Post Laureate

Reged: 12/04/08

Loc: Capistrano Beach, Ca

Re: CGEM Declination axis shift [Re: EFT]
#5719175 - 03/07/13 07:41 PM

The use of inserts, regardless of type, is not something to be done by just anyone. I have many years experience installing treads repair inserts in all kinds of situations. The tread repair of the worm gear housing bolts and motor mounting bolts are of the most elementary type of installations however they should not be done by someone that does not feel comfortable doing them. I would recommend that if you are not experienced with this type of work, then you should have it done by a professional.

Stan

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