Skywatcher dob base improvement?

[acey]

I'm interested to hear if anyone has tried improving the azimuth motion of a Skywatcher dob base. After a year I'm finding that my 12-inch Flextube is starting to have a tendency to roll back a little into unwanted positions. Maybe it's due to wear caused by the bearings.

I'm aware of the "milk jug mod" which involves inserting washers, but haven't seen it discussed in connection with roller-bearing mounts like the Skywatcher. Does it help? Are there other mods that people have tried?

Andrew

[russ]

Hi Andrew,

I've not had the 12" Skywatcher, only the 8 and 10. Both of those use teflon pads for azimuth bearings. I found both suffered with stiction. I didn't get the chance to play with the 10 but the 8 i replaced the teflon pads with new ones and played around the placement to improve the feel. Also added the milk bottle washers around the central bolt.

I also wasn't happy with the Altitude bearings either. Ripped off those red rings for starters. Stuck some teflon mouse tape on all the contacting surfaces, moves much better than before.

I never touched the 10 and it showed. The 10 was difficult to track objects with or give gentle nudges. Quite annoying after a while. Where as the 8 is a dream to use now. I spent £4.26 improving the 8, can't understand why Synta couldn't do it in the first place.

Russ

[acey]

Hi Russ - that's interesting. In the 12" there's a cartwheel assembly of roller bearings sandwiched between steel discs, also pads if I remember rightly. Haven't taken it apart to check yet but I'm wondering if those discs might have got dimpled. In any case maybe I should try milk washers and see if it helps.

For altitude I've also heard of people using skate-board bearings instead of the nylon ones - though teflon tape sounds simpler.

[John]

I had those "needle" bearings on my Meade Lightbridge 12". They are very free running but after a while do create little "pits" that they have a habit of settling back into - it sounds like this is what you are experiencing.

Some folks remove the needle bearings and go back to teflon pads (like the ones you buy to put under furniture. Others replace the surface that the bearings ride on with something harder wearing like ebony star laminate.

[russ]

Skate board bearings sound interesting. The Teflon mouse tape is cheap and effective. I also removed the 50mm finder, replaced it with a Rigel MRF. The balance is pretty good, so i only need a very light pressure from the altitude tensioners. If anything, they are only there as a carry handles.

I forgot to mention that my old Meade Starfinder 12.5 had that 'lazy susan' arrangement for the Azimuth. I hated it for three reasons:

moved too easily

never settled in the right place, always moved on a little bit

scope rocked around a bit

I'm guessing it's the preferred method for a large scope like a 12. But i still ripped the thing off, bought a sheet of teflon from Ebay for £1.99 and also did the milk bottle trick. The Meade was like a different scope after that.

Russ

[GazOC]

I've had a lot of bother getting my 14" Dark Star azimuth movement right.

No matter what I've tried theres always too much friction and sticking when the 2 corners (the base is square, not round) overlap. The OTA is very "old school" and I think the weight is just too much for that type of base.

As a last attempt to fix it I've just bought some 50mm luggage trolley wheels to try out. As Russ says I can see them being a little to free and easy with the movement but its got to be better than the sticking and refusal to move every 90 degrees of movement.

[acey]

Took it apart and had a look - the needle rollers had deposited a lot of graphite lubricant on the metal discs, and the centre of the plastic cartwheel that holds the rollers was scratched and pitted. Not sure, though, if that pitting is what needs dealing with. An option would be to use a couple of milk carton washers but I'm not entirely sure where I'd put them - first thought was to have one on each side of the plastic cartwheel, sandwiched inside the large metal discs. But then I reckon I'm effectively lifting the scope free of the rollers. Other option would be the washers outside the metal discs. Either way, there are no other pads or anything, so I'd have all the weight on the washers. Hmmm.

[russ]

You could take off the roller and use 4 teflon pads spaced at the outside with the washers around the central bolt. This seemed to work well. The spacing is trial and error. For example, the closer the pads are to the centre, the easier the scope moves.

A slab of teflon only costs £1.99 on Ebay. So cost is minimal. Worst case scenario, you'll just have to put the roller back on.

Russ

[acey]

Cheers - reckon I'll see how it goes for now and maybe replace with teflon if it worsens. One thing I still haven't completely eliminated is roll-back caused by the dob not being on a perfectly flat surface!

[AWR]

I've just got my 12" SW dob but the first thing I've done is clean and re-grease the needle bearings with some lithium grease. If dirt and old grease on the metal plates is the problem, then this may help (mine feels very, very smooth now). If the the metal plates themselves are pitted then I guess you'll need to replace them or look at another solution.

I'd be interested in hearing ideas on improving the altitude smoothness if anyone has any good tips!

Andrew

[acey]

Re-greasing sounds a good idea. The altitude mod I'm aware of is to unscrew the vinyl rollers on the dob base and replace each of them with a pair of ball races (maybe it's a larger type than the ones used in skateboards etc but it's the same idea), held with a locking nut. I came on another thread discussing going from teflon pads to rollers for azimuth motion:

http://stargazerslounge.com/equipment-discussion/97230-lazy-suzan-bearings.html

Elsewhere I've read that people have noticed an issue with the central bolt in the Skywatcher dob base: there may be some lateral play because the plastic washer is not a perfectly neat fit. And there's the question whether the bolt should be fixed with a plate to one or other of the boards. Additionally I notice that when people describe making a lazy susan mount they screw one of the plates to the dob board, but in the Skywatcher the metal discs sit freely. Also it seems the usual practice is to introduce pads in order to create friction (and stop the base whizzing round). I just did this by tightening down the centre bolt until the base doesn't run on too severely when I let go of it, but maybe that kind of nullifies the whole point of the roller-bearing approach.

All little nit-picking details really as I'm not greatly troubled by the base yet, but it's always nice to improve things if they can be improved. Actually an issue for me right from the start has been vibration, so whatever I do to the base, I'd be looking to reduce rather than lengthen it.