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Old 06-02-2019, 02:24 AM
pete_pumpkin (Pete)
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12" sax dob base rebuild, bearing choice

Hi, I picked up a 12" saxon dob the other day but it has been left oustide and the base has warped a little and this is causing it to stick. I'd like to rebuild it by copying the existing design in 18mm mdf. From what I can see there is already a half decent looking bearing in between the two bottom sections. Should I clean this up and keep it or do those lazy susans work better? They are pretty cheap on ebay https://www.ebay.com.au/itm/Lazy-Sus...item3f83629de2

Thanks,
Pete
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Old 06-02-2019, 03:31 AM
pete_pumpkin (Pete)
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I think there may be some disadvantages going with the ebay unit. One being that it would need to be centered really well and screwed/bolted into both panels- so additional access holes probably would be needed. Plus another disadvantage is that it's already 10mm thick and I'd probably need washers to raise one of the panels a bit as both surfaces of the bearing appear to be flush with each other. I might stick to the original one for now, I just wonder why the previous owner slapped so much grease on it (he did say he was a mechanic so that could it explain it lol)
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Old 06-02-2019, 06:01 AM
Malcolm
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A lazy susan bearing is too loose, you really need some sticktion. Clean up the original bearing with degreaser and spray with silicone spray or what you would use on a sliding door runner, a dry lubricant. MDF is a compressed wood, so it will absorb moisture, prime it, oil paint it and seal that with a spray lacquer. You can buy the adhesive edging on a roll, apply with a heat gun. Have built a dob base for a 12 inch GSO, tried the lazy susan and had to fit pieces of carpet to increase the friction. I used plywood and hardwood bracing glued and screwed.
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Old 06-02-2019, 08:00 AM
Rod
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Hi Pete

I suggest you go with Teflon and Formica on the azimuth bearing. Itís a much nicer feel and probably easier to build.

I suggest plywood too. MDF is heavy. If you are on a budget formply is a cheap option.

Rod
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Old 06-02-2019, 12:04 PM
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DavidU (Dave)
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From experience 15mm hoop pine ply is the only way to go.
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Old 06-02-2019, 12:46 PM
pete_pumpkin (Pete)
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Thanks for the replies - I'll look into using some plywood instead. Looks like bunnings has a nice selection.
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Old 06-02-2019, 01:26 PM
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DavidU (Dave)
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Quote:
Originally Posted by pete_pumpkin View Post
Thanks for the replies - I'll look into using some plywood instead. Looks like bunnings has a nice selection.
No they dont, their ply always warps.I build amps with hoop pine ply and it is always stable
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Old 06-02-2019, 02:30 PM
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mental4astro (Alexander)
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Hoop pine ply, hardwood ply or marine grade ply. Any one of these is a good option.

Don't use pine ply as it is too soft and it will flex too much.

If you are going to the trouble of building a new dob mount, it is a good idea then to improve upon what Saxon, Skywatcher and GSO punch out. Their mounts are plagued with issues. Leaving crappy particle board aside, the biggest problem their mounts have is the alt bearing is set too low towards the primary mirror, and the alt bearing is too small. Result is needing some type of friction mechanism in order to control the "balance" in altitude. This is not a balanced situation, is it now!

Redesign the entire mount, and you will end up with a dob that does not require some dicky friction mechanism for the alt bearing, a quality of action that is constant and unchanging, and it won't matter if you swap between a wee 6mm plossl or a 1kg lump of an eyepiece the scope will stay put. This will also eliminate backlash, or flex, in mount as there is no excessive force that needs to be exerted before the friction mechanism yields to allow the scope to move.

Just copy the original mount that came with the scope and you just reproduce the same mechanical problems that all these mass production dobs have instead of eliminating them. Redesign the mount from the start, and you end up with a scope that will be a sheer delight to use, not wrestle with.

Alex.
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Old 06-02-2019, 02:33 PM
Rod
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Hi Pete

I agree with David. The ply at Bunnings is rarely flat. An exception is the formply which is normally very flat but then you have the black film on the surface. I usually go to a specialist plywood shop and buy Russian birch. I find the price is not a lot more. I haven't tried hoop pine for a mount. I did use 1.5 mm sheets of hoop pine for two cylindrical tubes and that worked well.

Rod
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Old 06-02-2019, 06:32 PM
pete_pumpkin (Pete)
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Thanks for the advice fellas. Looks like I'll need to read up a bit more on dob design. I was going to go marine ply but now maybe the formply is the go - I don't mind the black surface and if it's lighter then that seems like a bonus. I need to go in to the store and get a feel for the materials I think. Besides the shape of the arms going up from the base I was going to use the current mounts on the tube for ease of construction - though if you guys reckon the design is flawed then I may investigate other options to get a more balanced setup.
Any recommendations on how to work out the ideal pivot point on the tube?
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