How do you adjust (remove) excessive backlash (dec) in a Vixen Atlux?

[g__day (Matthew)]

I've just realised my Vixen Atlux has ALOT of slop in the dec gear - put a slight force on the OTA and it can shift +/- 5 arc minutes - even with the clutch on really tight! RA is perfect but what the heck with the DEC?

Is this indicative of the worm just isn't close enough to the main DEC gear - or is there some other adjustment that is needed?

Can anyone tell me how to diagnose adjust the likely source of this error please?

Many thanks!

Matthew

[g__day (Matthew)]

I think I've found the answer to this after a bit of inspection - but I'll leave it to a bright Saturday morning to try and adjust this.

First confirm the degree of the error - I lightly pushed the end of the OTA with it powered down and their is a 2-3mm movement at the end of the OTA in the dec axis - RA is beautiful. Secondly DEC adjustments take time on slow speed - RA are instanteous. Third on PHD I have watched it calibrating where it clears DEC backlash - this sometimes takes 70 * 500ms pulses!

So my conclusion is this was poorly adjusted on arrival.

The good news is it seems a quality finish that is all metal and once dissassembled - ready to adjust. The process would appear to be:

1. Remove the DEC casing to expose the motors and gears
2. Remove the motor by unscrewing 3 metal bolts
3. Using an Allen key slightly loosen 4 metal blots that attach the worm to the case, be careful loosening the 4 bolts that are adjacent to the brass 60 tooth wheel - ensuring you don't slip and damage this gear
4. There are two bolts that control the distance of the DEC worm to the main DEC gear - assuming these are parallel already - tighten each of these very carefully and slowly - turning the same amount on each to bring the worm slightly closer to the gear - after each adjustment (maybe only 2-3 mm movement at the end of the Allen key) stop and try and move the OTA to see if there is still excessive slop between the worm and the main DEC gear.
5. Once you think its right - tighten the 4 bolts holding the worm firm and do a final tension test - slow small steps are crucial in steps 4 and 5.
6. Re-attach the DEC motor and check their is some play but very little between the two 60 teeth gears (the key thing is to avoid sticking).
7. Tighten and lock the 4 bolts holding the DEC motor - do a final tension test
8. Replace the DEC housing - pray - switch it on and cross fingers - run it through a complete revolution of the worm in both directions at a slow - medium speed first - then a high speed test lastly

Tricky but do-able - does that seem right to folk here?

Matt

[gbeal]

Seems pretty good as far as I can tell Matt. A little "slop" isn't really the end of the world, especially in Dec, as once it is up and running, the drift should all be in one direction anyway. Programs like Maxim actually allow you to input a dec backlash correction when auto-guiding. Fix it though, but don't get it too tight.

[Dennis]

This sounds similar to the Vixen GPDX worm adjustment.

There are x2 M6 socket head cap screws (pull-in) that clamp the worm housing to the mount and a small grub screw (push-out) in between them. By very slightly tightening the x2 M6 socket head cap screws the worm mesh increases and then by tightening the small grub screw the action pushes the worm housing slightly away from the assembly to ensure the fit is not too tight.

There is also a large hex nut at the end of the worm shaft which can be adjusted to remove any excess end float in the worm shaft.

As you rightly say, small iterative adjustments are the key. I adjust mine so that I can still grab the short worm shaft and turn it manually with my fingers. If I am not able to turn the shaft using finger strength, I judge the mesh to be too tight and the MT2 motors will labour on the GPDX.

Cheers

Dennis

[g__day (Matthew)]

Thank god someone replied!

Basically I looked at this multiple times - adjusting the DEC clamp as tightly as I thought sensible and moving the end of the OTA back and forth slowly maybe 3 - 4 mm - trying to check whether the large brass DEC gear attached to the shaft was actually moving whilst the worm wasn't! The movement is very small - but I think it's there - so I perceive I'm in for a very careful dismantle - adjust and re-construct exercise!

[Dennis]

One of the ATM’rs at the ATMQ (ATM Qld) told me of an old engineer’s trick. To ensure the correct mesh between the bronze gear on my MT2 motor and the same size bronze gear on the end of the worm shaft, place a single sheet of cigarette paper between the gears and tighten the grub screws onto the D flats on their respective shafts and then wind the paper through to get rid of it.

It seems that cigarette paper is the correct thickness!

I have never smoked and I don’t know of anyone who rolls their own so I haven’t tried this yet.

Cheers

Dennis

[g__day (Matthew)]

Well adjustments all done - seems silky smooth - will listen to gear noises and check it needs no final tweaking - a tad of backlash left I'd say - but minimal and as all say - better a little than too tight!

Secondly I used lan cable to wire a 6 metre Serial to min sub 8 port DIN for the RS-232C PC serial port to Skysensor 2000-PC hand controller - and it all worked - very fiddly job given the size of soldering iron versus the size of the connecters - very fiddly. But it cost $11 + labour only - and checked it all with a multi-metre and its and exact duplicate of the $50 2 metre part from Astro Optical - plus its not going through a 5 metre extension and a 25 pin -> 9 pin convertor - so hopefully I'll have alot better signal.

Lastly I'm awaiting version 2.10 EPROMs from the USA to upgrade my version 2.05 hand controller - then I hope that's the last major surgey I have to do for a while!

Please now I want a beer and clear nights! Thanks to folks for their useful suggestions.

Matt