DIY One Piece Wormblock for Losmandy GM8/G11


smith_barry
 

Hi Bob, I dont really know as it was second hand to me.  I have got the mount out from the store and remeasured.  The original was 8mm thick and my replacement (in clear anodized) is 12mm.  I also made up and fitted an extension block to help the axis hitting the motor.  At the time I did this I had been looking at variations in worm and axis rotation hence the rather neat canon encoder attached to the worm shaft (81,000 ppr) and the Heidenhain ERN180 encoder and IBV interpolation box giving if I recall right around 6 million ppr.  These were all obtained very cheap on ebay.  Unfortunately my plans to use the mount on a pier in my garden have been put on hold due to new light pollution issues where I live.   As you can see the system has dc servos on at the moment, my real interests with OnStep are to use it to finally finish a large harmonic drive mount thats been on the go for the past 20 years and to make a smaller mount I can set up in the field.
Barry

  


Robert Benward
 

Barry,
What vintage is your G11?  The thickness of my main plate is around 0.330.  The mount is probably 25 years old.

Bob


smith_barry
 

One thing I found out when doing a conversion on my G11 (Pittman DC servo and Ovision worm/block and Scitech II controller) was that you can replace the 1/4" plate the worm block mounts to, with a thicker version.  I went up to a 10mm thick plate using water jet cut Al plate and once you machine the kerf square it produces a more solid mounting plate for the motor bracket.  Not sure if you can do this on the G8.   I have the cad file if it's of use to anyone.
Barry


Alexander Varakin
 

Hi Peter,
I did not see that video while I was designing my drive, I found the video only recently.
Alex


Peter Boreland
 

Alexander,

I curious, why did you not implement the screw hole arrangement in the video you posted that allows for rotational belt tensioning?  https://www.youtube.com/watch?v=2YMXr6ewAjs&t=1133s

Peter


Alexander Varakin
 

Peter/Robert,
I created a more detailed write-up with additional images and CAD drawings of the components and posted it to the Losmandy group.
Attaching it here too.


Peter Boreland
 

Alexander,

I cross linked your reverse OPW design to this thread in the Losmandy forum. We have very active discussion going on. Love for you to join us. We also out how to post high res images into these forums. I say we but is was Bob!

Peter


Robert Benward
 

Alexander,
Very nice work!  Can you post this on the Losmandy group?  Some of us there are having the same discussions and issues.

I like the way you mounted the motor directly to the worm assembly.  This way the worm can float and move without the belt tension becoming an issue or changing as it rides the worm wheel.

Regards,
Bob


Peter Boreland
 

"there is a lot of slop in the gear, and in some places, it is very tight"

It may be that your ring gear is not excentric, but rather the axis shaft is not a tight fit to the needle bearings, so that there is tilt, which gets lock-in as it were, causing tightness and loose within on rotation. In the tight spot loosen off the lock and re tightening might fix the problem.

I went with 48T. I do not think on my G11 there is enough room for anything bigger. I do like the screw arrangement on the side bracket for belt tightening, but I may use a horizontal plate simply because I want to mount a small 1" x 1" x 1.75" box next to the stepper motor.  Perhaps I could extend down the side bracket an 1" to side mount the box below the stepper motor. Only problem is I do not have a CNBC setup to do those nice recessed slots.  

Peter 


Alexander Varakin
 

Peter, 
The pulleys are 16 and 60. A little odd ratio, but it works fine. I was not able to find pulleys with 15 teeth available for immediate delivery, so had to use 16.
The large pulley has the locking screws very close to the edge, so it is holding the worm just fine.


Alexander Varakin
 

Thank you, Peter, for your kind words. 
The spring is for spring-loaded worm. As you said, it is more trouble than it's worth. 
Eventually, I removed the springs and tightened up the screws.
The reason why I was trying spring-loaded worm is that the ring gears on my GM8 are not exactly circular, so in some areas, there is a lot of slop in the gear, and in some places, it is very tight.
I read somewhere that it is pretty common for GM8. I also have G11 and it does not have this problem.

Re: how I tension the belt:
You should be able to see the slots for motor mounting screws, so I have some wiggle room for moving the motor in and out. Otherwise, I don't have anything else. Counterintuitively, it is not a good idea to tighten the belt too much. 
See this video about belt tensioning of AP mounts:
https://www.youtube.com/watch?v=2YMXr6ewAjs&t=1133s
BTW, I like the arrangement of the holes on the motor bracket of the AP mount, you just have to rotate the motor to change the belt tensioning.


Peter Boreland
 

Also, how do you tension the belt?


Peter Boreland
 

This is a beautifully machined implementation. Very impressed! What purpose does the spring in the picture serve. As to spring loading Dec, I would not do it. I did and found no benefit and it just increased backlash. Better to set up with feeler guages and lock it down. What gear ratio/pulley sizes are you using? What size belt? I'm surprised the worm stuck out far enough to attach the large pulley?

Peter.


Alexander Varakin
 

Hi All,
Wanted to share my recent build of One Piece Wormblock for Losmandy GM8/G11.
It is based on the reverse worm approach, where the worm is flipped, which allows a much simpler attachment of timing gear.
I drilled and tapped holes in the original worm blocks and then screwed them to the housing using #8-18 3/8" long screws. I normally use metric screws, but in this case, I wanted to have coarse threads, because there was not much material to thread into. 
I built housing out of two pieces of 5mm thick AL plate, screwed together with 3mm screws.
All the parts were cut using CNC, but it is also possible to build the housing out of 1" AL angle with 0.25" thickness.
So far it is working pretty well on my GM8. It should also work on G11. 
I tried setting up spring-loaded worm and it worked ok with RA, but it did not work well with DEC, where I ran into long period oscillations during guiding. I need to work more on this aspect.