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Telescope Mount: DIY Azimuth Bearing Suggestions?

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Dr_GMJN17/01/2021 18:21:54
827 forum posts

All, I've been making a Dobsonian telescope mount for my long neglected Meade 10" Starfinder. The original equatorial mount was hopeless, because it was designed for a smaller telescope.

Anyway, with reference to some online builds, I drew up some MDF shapes for a tube frame and a base. In theory, it can all be pretty much balanced over the azimuth axis in order to avoid any significant tipping moments. A friend cut it all out on his CNC router, and it all went together OK.

I designed and made some 3D printed quadrants with adjustable clamps to go around the tube to stop it falling through the tube frame. This also allows the tube to be rotated in-situ to get the eyepiece in the right place. The contact faces of the tube frames will eventually be lined with felt to make it all look and feel nice.



The altitude bearings are machined Teflon blocks, screwed into 3D printed holders. The trunnions are CNC routed hardwood blanks, finished in the lathe. They need some textured Formica edging wrapping around them to reduce stiction (as online sources suggest), but so far so good:



The aluminium discs will eventually have a couple of small Teflon rubbing blocks next to them to keep everything aligned laterally.

The issue I've go is the azimuth bearing. The same online sources again suggest using Teflon pads, or furniture glides, running on an old vinyl record; the grooves in the record apparently reduce stiction. However, the base of mine seems way too big for this to ever work smoothly. Here it is with a c. 400mm diameter cake turntable on the base (the white outline is filler due to an error when CNC routing I think - doesn't have any effect of function):



Since I could fit a slew bearing of up to 400mm diameter on that footprint, I can't see a 12" (300mm) diameter record being anything like stable enough.

Even if I stole the ball bearing slew ring from the middle of the cake turntable for the primary support, and used Teflon blocks outboard of that, for stabilisation, I think any slight out-of-balance of the top half would cause enough friction to make rotating the base very jerky, plus I'd need a continuous ring of something with low stiction for the blocks to run on.

So what I'm after is suggestions as to how to make, or adapt, something into a c. 400mm diameter, low friction slew bearing.

I thought of a central hub, with axles radiating from it with small wheels on their ends, but again the issue is finding something very smooth for the wheels to run in.

Thanks.

Brian H18/01/2021 09:07:35
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2032 forum posts
111 photos

I'm no use at all on this one but just thought it would be good to bump it up the list in the hope that someone helpful sees it.

Brian

Bob Mc18/01/2021 09:24:33
155 forum posts
3 photos

Hi Dr GMJN....

This may be helpful, https://stellafane.org/tm/dob/resources/bearnings.html

seems that after some experimentation that kitchen laminate was a good candidate.

I have made a vertical azimuth telescope bearing using the material that my wife has for doing oven cooked chips, I don't know the name of it but its a golden cloth type material, in my application it slides on a metal shaft however you can't glue it so it would have to be held in place at the edges for a Dob.

Bob.

Cabinet Enforcer18/01/2021 09:29:55
98 forum posts
3 photos

eBay search for lazy Susan bearing.

Not that dear and available in 14" size or even bigger.

Chuck Taper18/01/2021 09:37:57
21 forum posts
7 photos

First off that's a real nice job. Put some effort into an "Equatorial Dob" myself... haven't given up yet but....

Have you considered using a "Lazy Susan" type bearing?

Available in many sizes and qualities.

Extreme precision in a Dob. is one of those diminishing return things. (my own experience only)

tr12c_1.jpg

Looking forward to some pictures of the final instrument!!

Frank C.

Dr_GMJN18/01/2021 10:32:42
827 forum posts

Thanks guys that's great. I didn't know that Lazy Susan bearings went up to that diameter.

If I can source one, I might make the base itself from a triangle of dimpled formaica kitchen worktop, use the Lazy Susan bearing in the centre to take the majority of the z-loads, and then make three height-adjustable teflon pads in light contact with the Formica base, purely as stabilisers to keep the bearing races parallel.

That should do it.

Dr_GMJN18/01/2021 10:39:57
827 forum posts

Just to add - I found aluminium/ball bearing type Lazy Susan bearings up to 1m diameter on Amazon just now....

Dr_GMJN31/01/2021 11:44:04
827 forum posts
Posted by Brian H on 18/01/2021 09:07:35:

I'm no use at all on this one but just thought it would be good to bump it up the list in the hope that someone helpful sees it.

Brian

Thanks Brian - it worked!

Dr_GMJN31/01/2021 11:44:50
827 forum posts

All,

I finished the telescope mount, a brief account of what I've ended up with is here:

DIY Dobsonian Telescope Mount | Model Engineer (model-engineer.co.uk)

Thanks again for the advice.

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