I am thinking of doing worm lapping. I see some people had great success using 1-micron diamond paste from mcmaster.
What is the latest wisdom on this subject?

Hi all, great forum. I’m not sure if what I’m experiencing is normal, hence the post. I have a new G11G/Gemini 2. PA to start is decent but not perfect, probably within a dozen arc minutes of where it should be.

Mount level, CWD down, Quick start. Mount type, location, time and offset all correct. Finder and OTA aligned with each other. Start the model, slew to (in this case) Spica, and Spica isn’t anywhere to be found in my Meade 7x50 finder. After some major centering gymnastics, I do the usual model-align-back routine and go to Arcturus. Same issue, rinse, repeat. Try something in the east - same problem. I’ve been using a 55 Plossl just to make things easier, then fine centering with a 9mm. The same routine is necessary on subsequent gotos; it’s a night-long struggle. ?I have had some success synching, but it’s still not nailing the gotos.

I MUST be missing something. Any ideas?

Thanks!

You say I need replace the just acquired OPW bearings?

I'm hard pressed to see the larger periodic error being ?associated with the worm. You might be looking in the wrong place. May I suggest you change the the thrust beings. I would recommend buying them from McMaster, not Losmandy.

Alan,
I will check it, but I think it is optical illusion.

What I mean is that it is out of round, due to the set screw mounting, or some other reason.? You should check it with an indicator like John suggested.

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Hi Andre -

If your mount originally came with stainless worms, from what I understand it should be a marked improvement.

In my opinion, you ideally want to reduce PE on the RA axis since that will continually move during tracking/guiding.? Backlash does not mater as much since you are constantly moving in the same direction.? Wind handling may come into play, but that can largely be mitigated with eastward weight bias, or the string trick. ? ?

One the DEC side, PE does not matter much, however you want to reduce backlash since the DEC will receive small corrections in both directions.? This is why if it were my mount, I would put the OPW on the DEC side.

The OPW upgrade is separate from the spring loaded worm upgrade.?? The SLW upgrade requires the OPW, but it is a modification? that is only done in house by Losmandy.? That said, many people have done various DIY interpretations of the spring loaded worm, with or without the factory OPW.? If youre comfortable drilling and tapping your mount, adding a spring should be a relatively simple modification.? Alternately, you might be able to get creative with some rubber bands or some other method of applying pressure to the worm block.?? The hole you mention in the OPW is indeed for the spring mechanism, but requires drilling and tapping the worm mounting plate.

I am currently using the Wemos R32 and CNCv3 onstep controller.? I have TMC2130 drivers in SPI mode.? Previously I was using LV8729s and those worked well too.

I am also using bellows style couplings.? 16mm diameter, 5mm x 6.35mm.? I was concerned the helical couplings might have been winding up (motor movement without worm movement), so I moved to bellows just to be safe.? The are less flexible, but as long as your alignment is good, it should be OK IMO.? Here's the link if you're curious:?

Good luck,
-Tony

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Hi Andre -?

Did your original mount come with the HP brass worms?? Or were they stainless?? ?Assuming they were HP brass already, the new OPW might not offer much less PE than your original ones, however it should be a whole lot easier to set the backlash.? And if you ever decided to go DIY spring loaded worms, it should be a relatively easy mod for you.

Also, I would think you want the High Precision worm to go on the RA side to reduce periodic-error.? ?Personally, I would put the OPW on the DEC side with the expectation that I would eventually do a spring loaded mod to reduce backlash, or at the very least, make routine backlash adjustment easier.

I also have a ten+ year old G11 that's been converted to onstep.? ?To give you an idea of how my RMS guide error progressed:

~ .85 - 1.0 arc-sec RMS:? ?Direct drive 400 step motors and stock two piece worm blocks
~ .6 - .75 arc-sec RMS:? ?Belt Drive with 16:60 reduction and stock 2 piece worm blocks
~ .5 - .65 arc-sec RMS:? ?Belt Drive with 16:60 reduction, and DIY spring loaded worms, new HP worm (old was brass, but high PE) and bellows style couplings

The above assumes nights of good seeing and little wind.? ?I am still using a 60mm guidescope, and? I've had 30 lbs of 8" newt and equipment guiding under .5 arc-sec RMS under perfect conditions.

I should also note that certain stepper drivers perform better than others.? The TMC2130 seems to contribute the least error.? ?Also, mechanical reduction is more accurate than microstepping for two reasons:? Coarser microsteps are more accurate and have higher holding torque, and mechanical reduction also reduces load on the motors, resulting in less angular error.? If you can move towards mechanical reduction (planetary gearbox or belt drive) that should give you additional benefit IMO.? But you have to make sure that the reduction benefits outweigh the added complexity, friction, and NVH (noise, vibration, harmonics)

Good luck, -Tony

Alan,
I will check it, but I think it is optical illusion.