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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