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CGEM tight RA axis

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#1 orlyandico

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Posted 17 March 2011 - 07:51 AM

So I finally got my CGEM.. the DEC axis is nice and free-wheeling (almost like a bicycle wheel) when the clutch is disengaged.

However the RA axis is sticky. With no scope, one counterweight and the clutch loose, I try to "pendulum" it and it only makes 1-1/2 oscillation. And makes a grunting sound too.

I tried loosening the RA retaining ring (around the polar scope hole) but no difference.

Help!

#2 nemo129

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Posted 17 March 2011 - 08:50 AM

Have you tried using the mount yet to see if this impacts performance? I realize the sticky RA axis may make getting good balance more difficult. Before you try any remedies, I suggest you try it out first. If it does not perform well, you have your warranty, although I notice you are in Singapore, so I am not sure if Celestron would pay shipping back to the factory (you could always ask) and I am not sure they could improve the issue as some folks report a return to the factory does not correct sticky axes issues. You did not say if you were visual or an imager, for visual this is not the same criticality as it would be if you intend to do imaging with the mount.

When you say grunting sound, is that constant, or only on certain spots as you rotate the RA axis. Also double check that your RA clutch is completely disengaged...it never hurts to check. I know it sounds silly...but :)

#3 orlyandico

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Posted 17 March 2011 - 11:40 AM

Only on certain spots (the grunting sound).

The mount is used, I decided to buy used in an attempt to weed out the dead-on-arrival ones. The prior owner only used it for visual. It actually slews alright and tracks OK (haven't managed a good polar alignment).

But it had been back to Celestron for a busted motor controller. I think the binding RA axis overloaded the motor.

#4 nemo129

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Posted 17 March 2011 - 07:11 PM

That's a tough one, as it is a known issue with some CGEMS. If you lived in the US, I would advise you to contact Deep Space Products and have Ed Thomas give the mount a hypertune. I think the shipping from Singapore would be a bit on the high side though! A self hypertune should only be undertaken by a very competent mechanic with experience in mounts, otherwise you could ruin your mount. If you are just a visual observer it may not be that bad, but for long exposure AP you could be very frustrated.

#5 orlyandico

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Posted 17 March 2011 - 07:54 PM

Yup that's pretty much what Ed has recommended. I can't ship it back to him ($500 each way!) I should've asked the seller to ship it to Ed in the first place. Oh well..

I think I'll have a look inside -- disassembling the RA axis doesn't look too bad -- and if that doesn't work get the DIY Hypertune kit from Ed.

#6 bluedandelion

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Posted 18 March 2011 - 12:15 AM

Orlando,

If you are systematic, follow instructions and take it slow, you should be able to work through the hypertune steps. I started on my own Atlas rework after my DEC axis started to bind. It too was used and it didn't make sense to take on additional expenditures. I am glad I did it. I assume Ed provides instructions along with the kit. He has always been very generous with his advice on CN even to folks who haven't bought his products - so you could pause and ask for help if you run into problems. Once again, if this is your first mount be deliberate and follow instructions.

Good luck.

Ajay

#7 orlyandico

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Posted 18 March 2011 - 04:19 AM

Being the impatient sort that I am, I went home from work early and took the RA axis apart.

There wasn't any China grease inside, nothing seemed out of place. So I sanded at the interior of the RA housing (with some pretty rough sandpaper..) then polished it with a buffing wheel on a Dremel clone (Bosch RTX). Looks like I've added one oscillation to the pendulum-swinging. Or maybe it's just my imagination..

#8 nemo129

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Posted 18 March 2011 - 06:52 AM

Did the rubbing or "grunting" sound go away? One thing I noticed that gave me a hint as to where to sand were marks on the big brass ring gear. I had "tracks" about an inch up from the toothed part of the gear and concentrated on a high spot I found inside the axis housing. Mine was the DEC axis. It now rotates very freely and smoothly. How smoothly do the large bearings that insert into the big brass ring gear turn? I ended up cleaning those out completetly with a citrus based degreaser and relubing them with a teflon based grease called Superlube. I found that helped me as well. It is probably more important to me as I live in an environment where it can get very cold and lower quality lubes can turn to wax. I imagine that in Singapore you do not have that issue. Good luck with your tuning.

#9 orlyandico

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Posted 19 March 2011 - 01:48 AM

Hi Nemo

what did you use to sand the inside of the axis housing?

Seems the axis housing is made of steel and my polishing didn't seem to do anything. Am thinking of using a sanding wheel in a dremel.. but am concerned that might remove too much material. (it works on knives though ;-)

#10 j.w.white

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Posted 19 March 2011 - 05:28 AM

When I did my HyperTune, I used a series of sandpapers, starting at 360, then 600, then 1000 grit. Of course you need to be VERY careful, especially with the more course paper, that you don't remove too much. It's better to go real slow on this (since you can't put metal back). I also used a very soft backing to ensure that finger pressure didn't produce grooves.

After you get it all put back together, but before you reinstall the motors, run the gears with your finger to help you set the worm meshing. What I discovered with setting mine is that if you mesh the worm wheel/gear perfectly (i.e. no backlash) it becomes so stiff that the motor has real difficulty in moving the axis and maintaining tracking. It seems, at least with my CGEM, that some gap is necessary to keep the motors from straining. I also discovered that at least my CGEM is more beholden to good balance than I would have suspected. By moving the gears with your finger, you get a very good appreciation for the amount of torque required to move the axes for a given balance and meshing.

#11 nemo129

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Posted 19 March 2011 - 10:23 AM

Hi Orlando,
Like John I used a series of fine sandpapers. Ed Thomas gave me two very fine grades in the DIY kit, a 600 and a 320. I started very lightly with some 200 that I had then moved to the 320 and finally the 600 followed up with some metal polish ( I was a bit compulsive about it!). Then I washed out the axis housing with water then some alcohol and wiped it clean with a soft rag (old t-shirt) to make sure all of the grit I generated was gone. I would try to avoid any power tool like a dremel because as you said it could remove too much or damage the housing. Slow and easy is the way to go here. Just keep testing in between. Like I said I was also using a very small amount of Superlube on the ring gear and housing ( never try to dry fit or test...that can lead to disaster!). You can also polish (do not sand) the ring gear as well. It gets very shiny and smooth quickly!

#12 nemo129

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Posted 19 March 2011 - 10:35 AM

if you mesh the worm wheel/gear perfectly (i.e. no backlash) it becomes so stiff that the motor has real difficulty in moving the axis and maintaining tracking.


John,
I had this same issue, but only in my DEC axis. I was able to get a very good mesh in RA, so there is so little play and you cannot really feel it when the clutches are on and you try to move it a bit and the RA motor moves very quitely and smoothly without straining. In DEC it has been a bear to get it to mesh well enough and not bind the motors so there is no appreciable backlash. I keep meaning to email Ed about this, as I have gone over his video several times and remeasured all of the spacings. Maybe I am being a bit over particular about it, but I do not like the amount of backlash I still feel. The mount seems to work fine, it just is not as well meshed in DEC as it was from the factory. Of course the unlocked axes now spin like well oiled tops! I have no more stricture and it guides well.

#13 j.w.white

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Posted 19 March 2011 - 10:55 AM

Hi Kirk - yeah, I'm not too happy about not being able to get a perfect mesh between the worm and ring gear, but even being very careful about setting the mesh caused the motor to sound like it had crickets inside (lots of chirping!). It wasn't until I pulled the motors and rotated the gear with my finger that I figured out why the motor was straining so much! I think I've got it about as good as it'll get though. I must be very near success, since the clouds rolled in almost as soon as I had finished!

#14 mich_al

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Posted 20 March 2011 - 09:08 AM

There is a good thread going on this subject on the Yahoo CelestronCGEM forum. http://tech.groups.y...yguid=167505766

#15 EFT

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Posted 20 March 2011 - 12:28 PM

[John,
I had this same issue, but only in my DEC axis. I was able to get a very good mesh in RA, so there is so little play and you cannot really feel it when the clutches are on and you try to move it a bit and the RA motor moves very quitely and smoothly without straining. In DEC it has been a bear to get it to mesh well enough and not bind the motors so there is no appreciable backlash. I keep meaning to email Ed about this, as I have gone over his video several times and remeasured all of the spacings. Maybe I am being a bit over particular about it, but I do not like the amount of backlash I still feel. The mount seems to work fine, it just is not as well meshed in DEC as it was from the factory. Of course the unlocked axes now spin like well oiled tops! I have no more stricture and it guides well.


Kirk,

I won't say too much here since I just get beat on anytime I post here, but one possibility is that the ring gear was machined off-center. The measurements taken for the spacers assume that the center of the gear mesh is in the center of the machined teeth. It is entirely possible that the gear was machined with the center not in the center of the teeth. This could cause the worm and ring gear to have greater backlash than they originally had. I don't know that I have encountered this before but I can tell you that, now that I am installing high precision gears in these mounts, the centering of the machining can certainly make a difference. That's really the only reason I can come up with for there to be more apparent backlash now than before. This is of course assuming that the backlash you observe is in fact coming from the worm and ring gear only and not the spur gears or the motor gearbox. You should contact me offline or on the Yahoo group if you want to discuss this more since the likelihood of this post being deleted is quite high.

#16 nemo129

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Posted 20 March 2011 - 02:49 PM

Thanks Ed. :bow: I will email you.

#17 nemo129

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Posted 20 March 2011 - 07:33 PM

John,
After emailing with Ed a bit, it finally dawned on me with Ed's prodding (as I can be a bit dense) that I was over-tightening my DEC clutch which was causing the ring gear to warp. Fortunately it does not seem to be permanent condition, as loosening its death grip on the ring gear a bit has allowed me to make the backlash adjustment that gives me no wiggle and feels like what I had on the out of the box. Why was this happening now? Simple, I have new longer clutch levers than the stock ones. Apparently I installed it so I can torque the heck out of the DEC one making the ring gear flex out of round. A simple test of loosening the DEC clutch a bit while rotating the DEC axis showed an immediate improvement in the noise coming from the axis. A few adjustments to the backlash screws and it is running great. I readjusted the new clutch lever so it would be difficult to over tighten it. Ed sure knows his stuff. :jump:






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