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CGEM modified counterweight shaft

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#1 coper4

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Posted 23 June 2009 - 06:34 PM

Those of you that have the new CGEM mount more than likely have noticed that the CW shaft that comes with the mount only extends 9" beyond the hub and that it is rather thin. If you have the 11" Celestron OTA as I do and you put everything on the tube you will find that the two 17 pound weights are not enough to balance it and that it flexes a bit. I purchased a 24" long, 1" OD hot rolled steel bar and a friend used his lathe to make a new shaft that is 19" long. I bored out the weights to 1 1/16" and dropped from having to use 43 1/2 pounds to 28 pounds to balance it all. Here is a link to a pic of the modified shaft. Hope this helps some of you.
http://lowcountrysta...fied CGEM 1.jpg

#2 Jeff55

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Posted 23 June 2009 - 09:55 PM

Take a look at this offering that will be available at the end of the month. www.Criterionmachine.com

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#3 coper4

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Posted 24 June 2009 - 09:16 AM

I saw that shaft. The problem with it is that it is the same diameter as the original shat and has the tendency to bow when the weights are put further down. The diameter of that shaft is only .787 inches. Did you get a price for the new shaft when it is available? I hope it works for you if you decide to get it.

#4 Jeff55

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Posted 24 June 2009 - 04:48 PM

I've forwarded your response to Criterion Machine I'll let you know what they say (and I'm curious as well). The price for the EQ-6 version is $95 + shipping so I assume the CGEM version will be about the same. With the screw on casting on the DEC axis that the CW shaft screws into the real length of the new CGEM shaft is actually 18 inches. I do appreciate your comments.

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#5 coper4

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Posted 24 June 2009 - 09:42 PM

Jeff, is that 18" beyond the hub or total length? Please do let me know what they have to say about the flex in the shaft. Thanks for the info.

#6 Jeff55

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Posted 25 June 2009 - 08:15 AM

Jim;
It is 18 inches beyond the hub...I specifically asked that question as I felt 20 inches was more than I needed and thought I might request a custom size...but...18 inches is only about couple of inches longer then my initial thoughts of a 15 inch shaft. BTW I'm still waiting for a response to your comments...I'll let you know when I hear more.

#7 coper4

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Posted 26 June 2009 - 01:20 PM

Update on this new shaft I have; I have noticed that due to the longer length of the shaft and the fact that the weights can be put further down, it seems to have eliminated almost all vibration when you try to focus as well. Very nice side benefit.

#8 Deven Matlick

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Posted 30 June 2009 - 12:24 PM

About our new shaft for the CGEM Mount it is 20" overall length with 2" of that being threaded leaving a little less than 18" protruding from the nut. The new shaft is made from 303SS and is 20mm dia.. The toe saver is the same as the one we use on the Atlas shaft.

As for the flexure question I and the beta tester have found it to be insignifigant, adding no addititional dampining time or vibration in normal sensible use.

As for weight, I have used the Atlas mount with the 18mm dia. 24" length prototype of the atlas shaft with my 12" f/8 newtonian and 44lbs of counterweights on it. Although it did bow slightly that had no effect on dampening time, ofcourse this is an extreme example.

The weakest point of any shaft that can be made for the CGEM is the M16x2 thread since it is the smallest diameter on the shaft and the only part of the equation that can't be changed. We did not use a thread relief (groove) on the new shafts in order to increase ridgidity of this critical end.

The 20" shafts for the CGEM Mounts are now available and are selling for $100.00 + S&H

#9 Bob D

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Posted 01 July 2009 - 09:07 AM

The weakest point of any shaft that can be made for the CGEM is the M16x2 thread since it is the smallest diameter on the shaft and the only part of the equation that can't be changed. We did not use a thread relief (groove) on the new shafts in order to increase ridgidity of this critical end.


Deven, How do the CGEM and Atlas mounts compare in this regard?

Also, after installing your repacement shafts on both the CGEM and the Atlas, which mount then gives the better overall performance on supporting counterweights?

#10 Deven Matlick

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Posted 01 July 2009 - 11:38 AM


The weakest point of any shaft that can be made for the CGEM is the M16x2 thread since it is the smallest diameter on the shaft and the only part of the equation that can't be changed. We did not use a thread relief (groove) on the new shafts in order to increase ridgidity of this critical end.


Deven, How do the CGEM and Atlas mounts compare in this regard?

Also, after installing your repacement shafts on both the CGEM and the Atlas, which mount then gives the better overall performance on supporting counterweights?


It is basically a tie. The pros and cons of each for the most part level out.

#11 Jeff55

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Posted 01 July 2009 - 12:32 PM

Just placed my order with Deven at Criterion for the new CGEM counterweight shaft...I'll report back to the group with my initial impressions. I expect to be able to lighten my current load of 45 pounds of counterweights with this longer shaft. I also like the idea of a higher quality SS material as part of the CW construction.

#12 coper4

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Posted 04 July 2009 - 04:33 AM

The CW shaft that my friend made for me for my CGEM mount cost less than half of what Criterion is asking for theirs and it has a 1" OD, not 20MM, or .787". This affords no flexing of the shaft at all and DOES make a difference in dampening. The original shaft that is on the CGEM is a 20MM shaft and only extends out 9" past the hub and it flexes with the amount of weight needed to balance the 11" celestron SCT. Extending that shaft by twice and leaving it at the same diameter only increases the amount of flex and thus less dampening. I am sure that Criterion is a very reputeble company but I believe their claim is flawed.

#13 Ed J

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Posted 06 July 2009 - 07:33 PM

I just do what is needed, see http://www.flickr.co...ker/3654124857/
Just now found 7 1/2# was right for 102mm scope.

#14 HaleBopper

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Posted 07 July 2009 - 08:54 PM

Hi coper4,

I am considering buying a CGEM with an 11" OTA, but have been reading some reports of bugs some people have had to contend with.

I had heard that the shaft was short, but this is the first time I've heard that someone could not get it balanced. Are you adding extras to the OTA that make it too heavy like a guidescope for example?

I'm really wondering if it's worth buying the 11" SCT on a CGEM if I have to buy separate pieces of equipment to use on a mount that should have had these potential problems worked out well ahead of time...

Thanks.

#15 AntarcticDave

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Posted 08 July 2009 - 11:59 AM

I have a 28 lb refractor on mine, no issues with balancing the scope, or with operation of the drive.

#16 coper4

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Posted 08 July 2009 - 08:53 PM

HaleBopper,I have the CGEM 1100. Added to that is the 50MM finderscope supplied with the scope and I also added a William Optics 2" dual speed focuser and a Quartz dielectric diagonal. Not enough to throw the balance off by much at all and it does not add enough weight to it to make a difference. With the Celestron 11" OTA mounted on the CGEM, and the length of the stock CW shaft, the 34 pounds of weights did not balance it because the shaft was too short. I had to add 7.5 pounds of extra weight to the shaft in order to balance the OTA. By extending the shaft and extra 9" I was able to go from 41.5 pounds of weight to 28 pounds and, with the longer shaft and the weights further down it actually improved the dampening when focusing quite a bit. Two of us have done this and both of us have noticed a great improvement. Hope this helps.

#17 HaleBopper

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Posted 08 July 2009 - 09:19 PM

Thanks. Maybe I'll just get a 12" dob instead :grin:

The CGEM should work well enough with my C8 and AP gear. The total weight would be about 20 pounds. I'll maybe buy the mount separate.

#18 Mike Harvey

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Posted 09 July 2009 - 02:12 AM

I received my 20" shaft from Criterion today. Looks great...and I can readily tell that the counterbalance weight needed will be well LESS THAN HALF of what I was forced to use with the CGEM-provided shaft.
Hopefully, weather will permit some actual trials tomorrow night.

Mike Harvey






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