There comes a moment in every technicians life when your own confidence and your ego can take a bit of a knock back. This was mine !

Now I like to think that its our failures that teach us the most and help to keep self confidence balanced at a point safely short of arrogance. The recent
rebuild of a Skywatcher EQ6 tested my patience and abilities to the limit and I felt it worth writing up the experience.

I have not written this up with any sense of smugness but rather that it gives people considering a mount rebuild some clues as to the possible issues and
how I overcame them in the hope it might save some of you time. It also taught me a lot about the engineering of these mounts which wasn’t discovered in the
original rebuild guide or the super-tune guide for reasons which will become apparent when you read through this case study.

It also shows how difficult a mount rebuild can be even when you have a lot of experience and a lot of tools to work with. It might also scare some people
who are convinced their mount is OK. If that’s you then move along because this part of my EQ6 guide might well give you sleepless nights.

Rather than rewrite the original EQ6 strip down guide with a lot of extra information, much of which wont be used in the average rebuild I have written
this as an addendum to be read with the main guide. Even if your rebuild goes smooth some of the information in this section will be worth a read.

This EQ6 presented with a problem with its backlash and tracking. The mount would track reliably but when the user overrode tracking using a small
amount of slewing to correct for tracking errors the mount failed to maintain tracking afterwards and in some cases demonstrated unacceptable amount of backlash. Repeated manual slewing would cause the mount to eventually regain some reliability of tracking.

The mount was tested by me prior to rebuild and appeared to work quite well. The motors were relatively quiet with little gear noise and there was no
observable play in either axis.

The backlash problem appeared to me to be poor set-up on the motor/worm meshing. The mount was quite old with a history of at least one previous owner and as the mount showed some signs of having been opened up I suggested a full strip down and rebuild. Its impossible to estimate the age of this mount but I would imagine its at least 4 years old (ie manufactured around 2007).
I felt it likely its overall assembly may be a bit poor (some of the early EQ6s did appear to suffer all manner of ill often associated by poor quality control at the
factory). If nothing else I felt sure the mount would be filled with the black goop that passes for lubricant in some of the Skywatcher mounts.

What followed was the strip-down from hell with a mount I christened the ‘demon’ mount for the huge amount of problems it was suffering internally. These included seized and broken bearings and every single internal shim washer being in a state of decay/destruction which was unexplainable.

This isn’t a complete strip down guide – just the edited highlights to show you how bad a mount can get while showing no outward signs of distress. I suspect given time the problems would have gotten worse.

DEC RemovalThe DEC axis is usually the easier of the two to strip down.

On this mount this proved not to be the case. After removal of the DEC collar and loosening of the hex bolts around the worm carrier......

Tap Out DEC Shaft...normally a tap at the end of the DEC shaft would be sufficient to free it from the mount.

On this one I was faced with what I came to call the bearing from hell.....

Seized Taper ShaftThe DEC shaft taper bearing was seized to the shaft and no amount of careful tapping would extract it either directly or with a drift tool. Across the course of about 5 evenings I repeatedly lubed it with WD-40 to no avail. The bearing material is too hard to cut with a hacksaw or drill and with the bearing recessed and surrounded by light allow and paintwork heating the bearing up seemed inappropriate. My bearing extraction tool lacked the strength to remove this bearing.

Club Hammer AttackAfter trying to freeze the shaft itself by packing it with ice while heating the bearing gently achieved no results I decided on a radical solution. A metal club hammer ! The metal/metal impact proved sufficient to eventually drive the shaft out from the bearing. This had to be done EXTREMELY carefully and during this process I probably came close to a heart attack at times.

The bearing is shown above after extraction - only perhaps 50% of the damage was done during the extraction process showing what bad shape the bearing was in before I started work.

The CauseHere was the cause of the problems. It doesn't show well in this photograph but there were shards of metal raised out from the DEC shaft where the bearing would seat. These together with no lubrication had caused the bearing to seize solid to the shaft.

The shaft itself also suffered a small flaring at its tip from the metal to metal impact during removal. This prevented the DEC collar being refitted and had to be filed gently back to shape.

After CleaningOnce the seized bearing was removed and the DEC shaft could be slipped out the main mount block was cleaned up. In this picture the motors and electronics have been removed so that the entire casing can be cleaned out. The casing showed all kinds of sludge and particles inside.

The top face where the worm carrier sits was cleaned and polished free of any paint to give the worm carrier a perfectly flat surface.

Taper Bearing JammedAt one stage with the DEC axis - with the taper bearing appearing to be completely jammed and non removable I attempted to remove the crown from the top of the DEC shaft. If this were possible then the DEC shaft could be extracted from below the mount and the bearing either heated off or ground off. The top of the DEC shaft is retained by a circular nut shown above.....

Recessed Screws..and around the crown there are three silver cross head screws. Underneath these are some small Allen headed grub screws that are deeply recessed.

Circle Nut DamageThe circular nut on this mount showed a lot of damage. Possibly a previous owner had attempted rebuild and also been frustrated by the lower taper bearing.

I found that the top of the DEC appeared to be fitted so tightly that it would be impossible to move without damaging the paintwork and opted for the approach of using a hammer to drive out the shaft.

DEC Shim Washers in a State

FrangibleHere is the DEC top shim washer during removal. Like the lower shim washers this one had shattered. The material had become frangible and the slightest movement caused them to start to break up.

You can see the entire inside of the assembly is coated with black grease.

Synta GoopThe DEC worm carrier is shown here. As in the previous pictures you can see the famous Synta goop is very much in evidence. WARNING - this stuff gets everywhere and its like tar. Its extremely hard to clean and this mount required about 5 wash downs in very hot soapy water to get it shifted. It took about a day just to clean the components up ready for rebuild.

Molten TarA close up of the Synta goop which gives you an idea of its consistency. It really is like molten tar.

Its very hard to remove and gets into everything. As ever I found hot water and domestic washing up liquid worked best but it gets expensive in washing up liquid.

Damaged SealMore problems. The worm end roller bearings were in a very bad state on this mount. This one had suffered damage to its seal on the face against the worm roller. All of the worm roller end bearings were replaced during rebuild with new bearings.

All Cleaned UpThis is the DEC worm carrier after cleaning and rebuild. All flakey paint was removed from the inside of the carrier and the edges where it contacts the main assembly were smoothed by placing the carrier on a sheet of very fine emery cloth on a glass table and gently moving the carrier in a circular motion to remove any burrs, paint flecks or imperfections from its mating faces.
New worm roller end bearings were fitted as per the supertune guide.

Replacement TaperThe picture above shows the replacement taper bearing with other components of the DEC shaft cleaned up and ready for reassembly. Apart from the taper bearing all of the shaft bearings were OK and in good condition although very light on lubrication.

Filed DEC ShaftThe DEC shaft end was filed to shape to remove a slight lip which has been created by its removal.

The small lip of sub millimeter size would prevent the DEC collar going on later during assembly.

IndentationsHeres the bearing face on the DEC shaft after careful work to remove the nasty metal burrs which were causing the bearing to seize. The indentations are all that remains. These wont cause the mount problems as they are slightly above the bearing area.

Restored DEC ShaftAnd finally the fully restored DEC shaft. The end piece has been repainted with a good quality metal/enamel paint. Its not critical but I like everything to be perfect when I rebuild these mounts.

Clean DEC CrownThe DEC crown with all of the grease and broken washer parts removed awaiting reassembly.

DEC Worm Assembly ReassembledFinally the DEC worm assembly is reassembled with new worm end roller bearings and new Delrin shim washers.
The shims presented a special problem on this mount and are covered in more detail below. The gears are coated in a good quality lithium grease.

RA Axis - More GoopThe RA axis of this EQ6 was in better shape and in the main stripped down exactly as per my EQ6 strip down guide. However as with the DEC axis the Synta goop was very much in evidence along with broken shim washers.....

Shim Fragment...here is a fragment of one of the shims that was rattling around the gears. Like the DEC shaft the RA shaft had its share of nasty stuff - mostly mashed up pieces of the washers which had been ground down into a kind of paste.

Random GlueThe nasty stuff on the RA gear included this. The picture shows something that resembled glue stuck to the base of the RA Worm gear. It came away quite easily and had the consistency of dried plastic gel beading.

Bad Shim Washer StateJust like the DEC axis the RA axis had every single shim washer in a bad state of repair. This one just disintegrated into fragments at the lightest touch.

Loose PaintHeres the same washer as the previous photograph seen from the other side.
Loose paint was much in evidence and this was cleaned away along with the remains of the Synta grease.

Lower Shims SplitHere are the two RA lower shim washers after removal. One was split neatly. The other was frangible and just broke up. The missing fragments were ground up in the gears and inside the worm carrier.

Cleaned and ReplacedJust like the DEC axis the RA was fully cleaned up and degreased and the worm roller end bearings were replaced with brand new bearings.

Disintegrated Shim Washers EverywhereThe disintegrated shim washers had found their way in all areas of the mount. This small fragment was found wedged underneath one of the motor gears. Because this mount was in such poor shape internally it was stripped completely and cleaned with repeated washing out.

Bearings, after CleaningMost of the mount bearings after cleaning. The small worm roller end bearings are to the front with their black plastic seals. The other bearings were completely cleaned and washed out to remove any contamination before being greased with white lithium and their seals being replaced.

Shim WashersUnlike previous mounts I have rebuilt the fact that this one needed a wholesale replacement of all of its washers created some problems.
The shim washers control different aspects of the mounts fittings. The lower shim shown in the diagram above controls the height of the worm gear while the upper shim adjusts the height of the axis above the main mount block.

Shim PlacementThe simplified diagram above shows the lower shim in red which controls the height of the worm gear to get correct engagement with the worm roller. Getting the correct height is explained in the supertune guide HERE.
The upper shim shown in green controls the distance between the axis casing and the main worm carrier. This is easier to see with the DEC axis but the principle is identical for the RA axis. The axis casing and worm carrier are shown as the dotted line.

Mount BindingIf the upper shim is too small you will find the mount may bind against the worm carrier along the line shown by the pencil in the above picture. This is where the axis joins the main mount casing and (on the DEC axis) is just below the DEC scale. On the RA axis it is below the RA scale.
If the upper shim is too large it will create an unsightly gap between the scales and the mount. I corrected this by trial and error fittings of different sized shims until I was happy.

8 - Epilogue

After rebuild the mount was thoroughly cleaned and any small chips in its paintwork were touched up with good quality enamel paint. When finally ready the mount looked as good as new only showing a slight yellowing of its paintwork. After suitable tuning the EQ6 in this part of the guide responded well. The mount was not quite as quiet as it had been running with the Synta grease (although still far better than most) but it was quite definitely better behaved. Both axis were smooth and free flowing. The DEC axis was never quite as free as I would have expected and hoped but I believe given time it will settle down in actual use. The bearings being filled with new grease may have contributed to this aspect. The mount was rebuilt with zero play in either axis and with a flawless backlash. When the mount comes to a halt a press on the slewing keys in either direction will set the mount moving immediately. Its tracking performance was tested against a stationary target and timed and was consistent and true. The pictures to the left show the mount in a complete state of disassembly and the mount after rebuilding and cleaning.