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I've had about a half dozen or so nights out imaging with my OnStep converted G11.? My guiding has been pretty good, averaging around .65 to .75 with the old two piece worm blocks and HP brass worms.? After seeing a few posts here and on cloudy nights about DIY spring loaded worm mods, I decided to give it a try myself.?

The raw materials were some 3/4 inch aluminum 90? angle stock, 1/8 thick, some 3D printed attachment points,? a 3D printed drill jig, and some off the shelf hardware.?? Here is the aluminum angle stock marked up for drilling.

I ordered fresh worm blocks so if this mod doesn't quite work out, I can always go back to stock.? I already had extra bearings.? The new blocks were drilled front and top, but only tapped on the front side.? I tapped the top holes to match the front, 10-24 threads.? I also had to file down the corner to get the worm blocks to sit flat in the angle stock.? There is a tiny radius in the extruded aluminum you need to accommodate.

Drilling into a nice mount can be a bit unnerving.? I 3D printed up a jig to make sure I drilled straight, and got the depth right.

A 6-32 tap cuts the threads.?? The rear drill hole does go into the worm block mounting hole, but the bolt does not protrude into it.? No big deal.

One side done, one more to go.

The 3D printed attachment point is in place.

The finished assembly on the DEC side.? The empty outboard hole next to the spring is for a push set screw, if I find it's needed.

And here's the RA side.

Hello little worm, I see you!? You can also see my DIY RA extension I needed to clear the motor brackets.? I'm using 1-1/4 aluminum spacers, and a 3D printed trim ring.?? You can see I did something similar with the DEC as well.

So far, so good.? The pivot point is the bottom worm block bolt closest to the motor.? It's located in place with an m4 x 10mm OD plastic washer.? It is very lightly snugged in place.?? The worm block bolt away from the motor is the sliding side.? It's left loose, only tight enough to keep the worm from walking upward.?

When testing the worm in the OPWB, if I snugged the stock worm blocks into the aluminum angle stock, the worm was difficult to turn.? It could be a burr or defect on both pieces of angle stock, but more likely, the worm blocks / bearings are just off by a fraction of a degree.? I found that torquing the top bolts tightly first, then lightly snugging the front bolts allows the worm to spin freely.

I can just barely get to the motor couplings without taking everything apart, which is nice.? I'm using flexible couplings attached to the belt drive brackets, courtesy of Rockmover on Thingiverse.? I have 16:60 belt reduction, and I'm running the 0.9? stepper motors at 1/16 microsteps, which gives me 0.15 arc-sec or resolution.? Control is handled by OnStep.? Personally, my gut feeling is that attaching the motors directly to the OPWB is unnecessary for the microscopic amount of runout the worm wheels have.? The flexible couplers are more than adequate for taking up any angular error.? I could probably even get away with a rigid coupler and rely on the shaft to flex a tiny bit.? And that's to say nothing of all the plastic. :)

There's clouds in the foreseeable forecast, but I'm hoping there's a break in the weather soon so I can get some testing done.

Any thoughts, comments or criticism are both welcome and appreciated.

-Tony

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Hi Michael -

Please, no apologies necessary.? I'm one to find the humor in a good autocorrect fail, but not quite sure I've spotted the gaffe.? Regardless, no worries at all, I never gave it a second thought (or a first for that matter).

Thanks for the comments and suggestions, and I'll be sure to roll it into a PDF once I finalize it and get some baseline results.? It may still be a dud.? Anyway, since the clouds rolled in, its getting painted flat black right now in the garage, and have proper stainless button head screws coming tomorrow.? I'm sure I'll continue to tweak and make revisions before I'm ready to leave it be.

Thanks again,
-Tony

Hi Tony that looks great.? The basic idea is not that different from my humble construct below (for DEC)



Namely, both blocks are screwed fixed against the aluminum cover (and are aligned to prevent the 75 Hz vibration), both bottoms screws are loose and the one nearest the motor serves as the pivot.? I added 2 Belleville washers tightened lightly at the bottom one near the motor to keep the blocks flat against the plate and prevent wobble.? Note that the flex couplings are not quite symmetric around the axis because there is always some play and the setting screws pull them off center.? That in addition to the not perfectly centered motor axis yields significant periodic forces that try to wobble the block assembly.

That's the balancing part.? On top of that there is the additional friction because of the continuous spring pressure as compared to a fixed worm position with some play.? That will cause stronger up and down vibrations and may also push the worm away from the RA axis if the pressure is not right.? I've noticed that the Losmandy SLW have a setting screw, is that because of this perhaps?? So it's not really full time spring loaded, only at high points.? Altogether a lot of forces that need to be managed.

Here's the RA one,



The idea is that the pivot screw in its hole is pushed against the side away from the motor and stays there.? For RA, since the worm pulls itself away from the motor during tracking, this will hopefully be true.? But nevertheless we have two screws floating of which one is only loosely fixed in position.? Same with yours if I understand it right.? My spring system is more of the Flintstone kind than your professional looking tunable spring.? I regulate the force by the position of the screw around which it sits.? Further outside, more momentum.

So far it has worked very well for slewing.? This used to be my main problem (the ring gear is not round enough to work with a tight fixed setting and binds up), and it solved that.? As far as RMS is concerned, when I get less than 1" total RMS I'm happy (it happened once).? I have not had the chance to do much testing because I bought a lot of electronic gear that I am learning how to use so the focus has been elsewhere.

So far I'm still hopeful that this system will work.? If not, all I have to do is turn the bottom screws tight and I'm back to the old fixed system so it can't hurt to try.? Or just use the spring loaded configuration for slewing and the fixed on for tracking.?

Good luck and keep us posted on how it works!

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Hello Henk -

Thanks for your comments.? I'm of course familiar with your SLW modifications, and were part of the inspiration in trying this for myself.? Your use of the sideways spring to apply both the worm pressure as well as locating the worm block was especially clever.

Thank you for your suggestions on mitigating the periodic forces from the flex couplers.? I was looking into the belville washers for both the bottom bolts, as well as potentially to keep the worm from walking (bearing preload).? I may first try a regular compression spring and longer bolt under the sliding bolt.? I was also thinking about using some delrin or low friction plastic under the pivot, both for the small bit of compression, and the anti friction properties.

I do agree with you that the biggest room for improvement is how the worm bock is located by? the loose pivot bolt.? It's less than ideal.? The spring is a novel approach.? A set screw with a rounded nose may also be an option to push the worm block assembly to one side.? It's been something I've been pondering since the beginning, but haven't settled on a solution yet, at least not before seeing how the current m4 plastic washer works.? But its certainly my primary suspect from any large backlash.

I'm lucky that my static worms were pretty good.? I had somewhere around 8 to 10 arc sec peak to peak of native PE in the worm.? Not sure if swapping them would have improved things or not.? But guiding wise, it was relatively easy to get under 1 arc-sec RMS when? the conditions supported it.? In trying to dial the backlash out with the static worms, I also had some occasional binding, though only when running the worms quite tight.? Still, the backlash would vary depending on where you were on the worm ring.? I'm also hoping the SLW will help the mount handle just a tiny bit more wind without having to resort to excessive intentional weight imbalance.

Thanks again for your comments, and I'd love to hear your thoughts on my results or any other unexpected difficulties I may encounter.? Thanks again.
-Tony

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Made a couple of minor changes...

Painted the worm block assembly black.?
Changed to button head socket screws.?
Added very thin washers so the button head screws don't get stuck in the paint.?
Added nylon washers under the pivot bottom bolt.
Added a longer bolt and spring under the bottom sliding bolt to apply constant pressure.

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Got some initial guide results with the spring loaded worms last night.? My initial reaction is that it may have helped, but certainly didn't hurt anything.

Conditions last night were mediocre at best, so my results should be taken with a grain of salt.?? We had LOTS of passing clouds, and a bit of variable breeze.? I was guiding on M13, near zenith, historically a difficult spot for me because the intentional weight imbalance on the DEC (camera heavy) doesn't work well here.? I was shooting 30 and 60 second subs, dithering every other frame. ?? Seeing was good but not great, but transparency and clouds were below average.? This is using a 60mm guide scope with a 240mm focal length.? I seemed to have the best results from a 1 second exposure with multi star enabled.? I have an OAG that I will eventually move to, but since I'm used to using the guide scope, I wanted to keep things apples to apples.

First, my guiding assistant results (guiding disabled) were lackluster at best.? My peak-to-peak RA numbers seem to be artificially inflated due to some RA tracking drift.? My drift rate is somewhere around 2.4 arc-sec/min, so the RA error peak to peak climbs the longer it runs.? Smoothing it visually, the worm appears to have roughly 12-15 arc-sec native PE.? This is right in line with the results I had with the static mounted worms, so no big surprise here.? I think I should swap the RA and DEC worms to see if it improves at all.? I might also apply some tracking rate adjustment.? I suspect the RA drift is due to a sub optimal RTC in the cheap Arduino board that runs the whole show.? Tracking rate adjustment may help if the drift is consistent from night to night.

Next is the declination backlash.? I really expected this to tighten up, but it did not seem to.? Measured backlash seemed to be between 650? to 900 ms (ran twice), which was higher than the static worms.? I suspect this is from some wiggle room in the pivot bolt.? Perhaps if I can tighten this up, things will improve here.?

Now for the actual meat and potatoes guiding.? Here, it seemed to shine.? I saw .61 and .58 arc-sec RMS overall average.?? While sitting there watching, there were a few times where my number (current screen average) dropped to .48!? And most promising, the breeze did not seem to move the mount nearly as much as usual.? Typically I would set up a wind break to calm it down, but didn't feel like setting it up at 11pm.? These numbers are the best I've seen on any mount without using a wind break.

I think my next order of business is figuring out how to remove any play in the pivot bolt side.? Then maybe swap worms between the DEC and RA to see which has less native PE.? But overall, I think the spring loaded worm mod shows some promise.? I can't wait to test it on a night with better conditions.

Thanks,
-Tony

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Thanks Michael -

Those were peak to peak numbers, though the RMA number was still above 5 if I recall.? I agree that something is not quite right and requires some fiddling.

While the unguided error seemed a bit larger than I expected, the guiding performance was very good in my opinion.? Ultimately that's what I care about, since I do not shoot unguided subs.? Unguided error is purely a tool for troubleshooting any mechanical issues.? I'm very happy that the native PE appears to be relatively smooth and regular.? And no signs of the dreaded 76 second PE, or any other obvious harmonics.? Predictive PEC was able to tune out the worm error and any other PE with ease.

The geartrain is not stock Losmandy.? I am using stepper motors, belt drive reduction, additional bearings, shafts, and helical couplings.? As you can see, there are a lot of unaccounted variables here.? But starting with what I do have control over, the worm bearings are indeed preloaded while snugging them into the makeshift OPWB.? I do not have any noticeable side to side movement of the worm between bearings, and there is no perceptible notchiness or catching of the bearings as the worm spins. ? With a 1140mm focal length refractor, any side to side backlash from the worm moving between bearings is quite easy to feel.? However the method I'm using to locate the pivot bolt seems to be an obvious weak link.? I could still have other issues that may be taken out with a belville washer like you suggest, but I think I need to be certain the pivot location is sound first.

I have spent some time doing my best to center the shafts and to get the least amount of runout from the flexible helical coupling.? Like the oldham coupler, the set screws do pull it off center a touch.? I will readdress this once again after I improve the pivot bolt issue.? As a temporary troubleshooting method, I may simply shim the gap between the makeshift OPWB and the motor bracket to keep the worm assembly pressed towards the outside.? Ultimately I'd like something a bit more elegant, but that should be enough to tell me if I'm on the right track.

Thanks again for your thoughts,
-Tony

Tony, I have an older G11 that I have been trying to do a DIY spring loaded worm. I am not able to upgrade to Losmandy newer spring load worms so I have
been playing around with a DIY spring loaded worm. I am impressed with your approach. I am using a modified gear system base on Rockmover and use the onstep?
system.??
Could you make available your 3d printed jig for drilling the holes in the base plate and the 3d printed spring attachment design.

Thanks,

Brent

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Thanks Brent -

I'll gladly make them available.? I haven't created a thingiverse account yet, but I'll try to attach them here for now.?? If that doesn't work, I can email them to you directly. ?

I drilled the plates out with what I believe was a 7/64 drill bit, and tapped for a 6-32 x 1" socket head screw.?? I printed the jig at 100% infill, and very slowly drilled out the plastic to keep it from melting.? Make sure you're satisfied with the holes being straight.??? You might want to try some light oil in the jig/guide to keep it from melting when you actually drill into the mount.? Mine melted a bit, but the holes were still pretty straight.? To set the depth, I simply chucked the drill bit so there would be about 3/8 depth before bottoming out against the jig.? Make sure you remove your worms and blocks of course, and tape over the worm wheels to avoid any errant chips or shavings getting in there.

For the anchor / attachment point, I think those were pretty much good to go right off the printer.? I printed in PETG for heat resistance.? The spring hole was just barely snug enough for the 8-32 x 1-3/4 screw to cut its own threads.? I used springs I had on hand from my 3d-printer bed, but the rough dimensions are about an inch in length, and maybe 1/4 inch ID.

Please double check everything just in case my memory is a bit off.?? And of course, feel free to ask any questions.

Thanks,
-Tony

Thanks so much and I will let you know my progress and results.
I also am anxious? to follow your results and enhancements.

Thanks for sharing,

Brent

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I did some more testing last night.? I swapped the RA and DEC worms and almost instantly regretted my decision.? Almost 38 arc-sec peak to peak PE on this worm.? Seems super high for a high precision brass worm.? Looking back today, the worm spins freely.? The coupling is straight, no rubbing, and almost no runout.? I will note that last night was considerably cooler than it was when I last set the bearing preload, and compared to today.? Perhaps the aluminum OPW contracted more than the brass worm, causing increased bearing preload to the point the bearings did not spin smoothly?? I guess its one possibility, and one where the belleville washer mod might help.? Another possibility is the helical coupling loading up and unloading as the worm mesh friction changes.? I have bellows style couplings on order, hopefully that will remove that possibility for error.? Alternately, perhaps I could engage the set / push screw to limit worm mesh, resulting in less drag and presumably less windup.

Anyway, I have swapped worms back to the original locations.? That RA worm gave me roughly 15 arc-sec peak to peak PE, still not great, but much better than 38!?? I also got rid of the free play in the (motor side) pivot bolt.? Measuring the worm block protrusion, I saw .373 inches, and the corresponding hole was .375.?? I had some plastic strapping tape, and cut a small sliver to sandwich on one side while inserting the worm blocks.? The OPW still move freely, but the side to side play has been eliminated.? No need for anything more fancy or complicated, this seems to do the trick.

I'm going to wait and see how the worms perform once the bellows couplings finally arrive from overseas.? If they still show 15+ arc-sec of PE, I think I'm going to have to order at least one new HP worm for the RA, and put the better of the two stock worms in the DEC.

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Hi Brent -

Glad to hear the mod went well, would love to see some pics!

I'm no PHD2 expert by any means, but looking at your graph, I'd say something is loose, big time!? It was guiding well enough, but then started oscillating more and more wildly.? Some primary suspects I would investigate include:

1. Play in the left block pivot point.? I shimmed mine with plastic strapping tape so that the block protrusion was a snug fit, but would still easily turn.? Don't snug the bolt, it needs to be free.
2. Lateral movement of the worm between the two bearings.? Did you compress them slightly while tightening them to the one piece worm block? Just enough for some preload, but not so much the bearings feel notchy.? Make sure the holes in your OPWB allow for enough adjustment.? Michael Herman suggests belleville washers in? there to allow for temperature changes.? I may eventually go that route, but for now, Ill reseat it if temperature swings affect it.
3. The worm block pivoting away.? Either the pivot point is too tight, the sliding bolt is too tight, or simply insufficient spring tension to keep the worm against the wheel.? As the dec worm works back and forth, the threads want to climb, pushing the worm block assembly away from the worm wheel, increasing backlash.? If the pivot point is too tight, it may hold the wormblock out of place and keep it from returning.? Make sure everything moves freely, and you shouldnt need too much spring tension.? I also greased under my worm blocks, and the aluminum conical fittings for the bottom bolts.? I want everything to move as freely as possible, but without any play.

If you can safely rule out the first two suggestions, then number 3 seems like a strong possibility.? I think it's always going to be a bit of a balancing act to get the spring tension correct, and to get the bottom bolt tension right.? To give you some idea, I have my left bolt loose enough that I can easily spin the aluminum conical fitting the bolt sits in.? On the right side, I switched to a long bolt and another spring.? This has just enough tension to keep the worm block flat against the worm mounting plate.? You could just leave the stock bolt loose, but I thought some light but constant pressure would be good here.? With no worm spring tension, the whole assembly moves quite easily.? As for how much worm spring tension to use, I just gradually increase it until I start to feel an increase in effort required to turn the stepper motor by hand.? Once I feel a little added drag, I figure the worm is being pushed into place.? I dont want too much because thats? added friction, wear, and load on the motor.? And if youre using helical couplings, it could cause some wind up.? For that reason, I switched to a rigid coupling on the DEC axis.

With TMC5160 drivers, and I assume big 1.5-2.0 amp stepper motors, you should have plenty of available torque, as long as the worm block moves freely, and your spring tension is not crazy tight.? I moved from direct drive to belt drive (1:3.75 reduction) before I moved to the spring loaded worms, but the belt reduction did seem to help while I was running the two piece worm blocks.? It greatly reduces the load on the motors, and I felt the increased accuracy there overcomes the added complexity and additional bearings, shafts, etc.? Still, its probably not a total game changer, and I think there will always be healthy competition between direct drive and belt reduction, as they both have pros and cons.

I tend to constantly tinker with my PHD2 settings from night to night.? Lately I have been running the Predictive PEC algo, but for simplicity's sake, lets assume the default "hysteresis" and "resist switch" algos.? On nights with good seeing, my settings are typically in the following ranges:

RA Agr:? 60-75
Hys: 7-10
MinMo: 12
DEC Agr: 80-90
MinMo: 15

I'm running 2.6.9 dev 5, and using multi-star.? My exposure is typically 1 second, but Ill increase to 2 when necessary.? I typically adjust RA to try to get the RA Osc in the 20-30 range.?? I'm guiding at .5x sidereal rate, and I start with 200ms backlash compensation on the DEC.? Ive seen this grow to 400ms, or drop to 60ms as needed.? My OnStep is version 4.24d, and I do not use any backlash compensation in OnStep directly.? I'm running a WeMos ESP32 and CNC3 shield with LV8729 drivers and 400 step 2.0A motors, and currently using 1/16 microsteps.?? I'll be switching the drivers out for TMC2130s with SPI to allow for changing microstepping during gotos.? I just bench tested the new brains yesterday, and just have to swap them out now.?

But honestly, I'm pretty confident your wild DEC swings are mechanical in nature, and not due to any hardware or software differences.

Good luck, and please share some pics.? I'd love to see your spin on things.

Thanks,
-Tony

Thanks soo much for your detailed reply.? You were spot on with the number 3 option. I found loose lateral movement in the dec. I have
the bellville washers but they were not compressed enough to make them effective.?
I had better results last night but still have some work to do. I very much appreciate your response and insights.? I will send pic's when I iron out some more kinks.

So I had a reduction based on and modification of rockmovers 3d printed design in thingverse (3 to 1). But had some issues. SO I went back to direct couple to test your DIY spring loaded design.
I think you have also a modified version. Can you share the STL file so I can compare what I did to see if I can make improvements?

Thanks so much,

Brent

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I manually modified rockmover's brackets after printing.? Basically, I just had to drill a big hole to move the RA motor to the outboard location.?? I attempted to tweak rockmover's brackets, but couldn't convert them due to the half millions facets being too complicated to reverse engineer back to an editable .STL, at least with the software I tried.? I was going to design my own brackets, but never got the push to really get started.? The rockmover ones seem to be working well enough for now, so Ive been putting my efforts towards dialing in the spring loaded worms and tweaking things there instead.

Glad to hear you made some progress.? Please feel free to share any adjustments or modifications that you found to be helpful along the way.? I'm sure there's plenty of tweaks I haven't considered, and the more information we can get, the better. And it'll help the next guy, too.

Thanks and good luck with it,
-Tony

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I made a few more minor tweaks to the mount over the last week.

1. I changed up the OnStep brains a bit.? Changed the stepper drivers from LV8729 to TMC2130s instead.? These allow for changing microstepping on the fly, which results in both faster slew speeds (4+ degrees / sec @ 1/8 microstepping) and finer resolution/accuracy when tracking (.15 arc-sec / step).? They also interpolate the 1/32 microstepping while tracking? and guiding down to 1/256 microstepping, resulting in dead silent tracking, with no detectable pulses or vibration.? It brings the stepper motor a bit closer to servo motor smoothness.? The TMC2130 drivers are also supposedly more accurate between full steps, and have less angular error under load.

2. I swapped out the old helical and rigid couplings to a new bellows coupling on both axes.?? The helical coupler was probably suffering from wind-up.? The rigid coupler eliminates wind-up, but offers no flexibility.? The bellows coupling should be the best of both worlds.? They did require quite a bit of modification to fit.

3. Added wavy washers under the pivot side (motor side) bottom bolt.?

In daytime testing, with the pivot bolt relatively loose, I could feel the whole OPWB assembly rocking as the worm attempted to climb the worm wheel on high points.? I wound up having to run the wavy washer almost fully compressed in order to eliminate any rocking.? The worm block assembly still pivoted, but requires considerably more spring tension to keep it meshed.? Knowing just how much PE my worms have, I'm inclined to eventually replace them with new ones, at least on the RA axis.? But for now, Ill work with what I've got.

So, the results??? Good, maybe even Very Good.?? Factoring out gusts of wind, the mount effortlessly held .50 to .55 the whole evening, despite the continuous 6-7mph breeze.? We had gusts to around 10 which would push RMS error up to .7 or so, but it would quickly settle back down.?? This was guiding 30 lbs worth of 8-inch newt, guidescope and DSLR.? I did not use a windbreak, and I was using the regular old hysteresis algo, not PPEC.? During quite a few calm periods, I saw my error drop to .44 arc-sec on screen.? My backlash compensation seemed to settle in the 300-400 ms range. ? I'm pretty darn happy with numbers like those, especially considering the use of PPEC and a windbreak can only improve them? But the mount is doing exactly what I had hoped, its holding steady in the breeze that would have buffeted my old mount around.

Thanks,
-Tony