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ATTENTION DOB OWNERS


derekm
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If you want to improve your azimuth movement, a lazy Susan bearing is the usual choice.

Here is a good source at sensible prices:-

Lazy Susan Bearing - Power Tools, Cabinet Fittings, Ironmongery & Woodworking Machinery

A 12" bearing, £5.36 inc VAT but plus carriage. Zinc plated and made in USA, not China!!

I got my last one from Axminster a few years back after a tip on here, but now they are over £14 for the same bearing!!!

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the bearing should match the diameter of the feet spacing - i.e. the bearing position should be directly above all the feet.

In which case you will need a 420mm bearing for the 250PX

I've just fitted a 400mm Ebonystar ring to the base of my 250 Flextube and the outer edge just falls short of the feet.

The Revelation dobs took a different approach with their lazy susan. They had Teflon pads as normal over the feet with a smaller Lazy Susan around the central bolt.

Edited by russ
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the Revelation method would also work and actually reduce stiction on the bearings if e.g. the underside of the rocker was smooth. it's a balancing act though to ensure it does not whizz round like, well, a lazy susan. :)

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I should add that the important features of a smooth dob bearing are that it is 1) smooth and 'buttery' to start moving with no 'letting go' 2) stops when you stop moving and does not move when e.g. your eye is pressed against the eyepiece 3) fully supports the weight of the scope and are rock solid and as wide apart as possible (azimuth bearings anyway).

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the bearing should match the diameter of the feet spacing - i.e. the bearing position should be directly above all the feet.

Sorry, but no.

I had a 12" bearing on an 8" Skywatcher and it was nowhere near the diameter of the original pads. It worked perfectly.

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The Revelation dobs took a different approach with their lazy susan. They had Teflon pads as normal over the feet with a smaller Lazy Susan around the central bolt.

My Revelation 12" has no teflon pads - only a lazy Susan. Sounds like any scope with both would be old parts from whenever they changed from teflon to bearings.

Edited by derekm
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Thanks Derek really helpful and I will soon be giving it a go :)

Does anyone know of a DIY guide for this mod?

Thanks

Don't know of a detailed guide, but:-

Cut a circular piece of 8mm ply (or a bit of laminate flooring) 179mm diameter.

Drill a central hole to suit your central bolt.

This will be the guide for the top part of your mount. Screw it to the underside of top section of your mount, concentric with the central bolthole.

Screw the bearing to the groundboard of the mount, with the inner (rotating) ring uppermost, making sure it too is concentric with the central bolthole. Carefully assemble the top section of mount onto the bearing using the 179mm circle as a guide.

Add the nut to the central bolt and Bob's your father's brother, almost!.

You will now find that rotation is too free, so make up some washers about 80mm dia. from empty milk cartons/jugs and add them between the 179mm wood circle and the groundboard.

You can now adjust the friction with the central nut. It helps if you can anchor the bolthead under the groundboard to prevent it turning. Stamped steel "cycle" spanners are ideal, if they coincide with your bolt & nut sizes. You will have to add/remove plastic washers until you get the right amount of friction.

Before assembling the bearing to the groundboard, get some grease into the balls of the bearings to prevent rust. I am assuming that they are not stainless.

If you wish, you can mount the bearing to the upper section of the mount, with the 179mm guide on the groundboard.

If anyone is wondering why 179mm, it's because the hole in the bearing is 180mm and I've just spent an hour sanding down two 180mm discs to fit!!!

Edited by derekm
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Sorry, but no.

I had a 12" bearing on an 8" Skywatcher and it was nowhere near the diameter of the original pads. It worked perfectly.

it depends whether or not you want maximum stability. if not then a smaller bearing would be fine.

if you get a bearing that is smaller than the feet then the force is applied between the feet, in other words into the board and not the ground. it may well work OK but there will be more 'springiness' in the mount than if spaced to match the feet. if you space to match the feet you could (literally) use a piece of cardboard as your groundboard. :)

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Here's one mounted on a 250PX I used to have. You just need to add some friction pads to stop it turning too easily. I used chair glides with washers cut from a milk bottle to get the spacing right.

John

this would also provide a stable platform with the smaller LS bearing as the bulk of the weight would presumably rest on the pads which are above the feet (more or less).

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Originally Posted by derekm viewpost.gif Sorry, but no.

I had a 12" bearing on an 8" Skywatcher and it was nowhere near the diameter of the original pads. It worked perfectly.

it depends whether or not you want maximum stability. if not then a smaller bearing would be fine.

if you get a bearing that is smaller than the feet then the force is applied between the feet, in other words into the board and not the ground. it may well work OK but there will be more 'springiness' in the mount than if spaced to match the feet. if you space to match the feet you could (literally) use a piece of cardboard as your groundboard. :)

As you wish.

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My Revelation 12" has no teflon pads - only a lazy Susan. Sounds like any scope with both would be old parts from whenever they changed from teflon to bearings.

Sounds like the previous owner of my 12 added some pads, which was good move. Still needed tweaking though.

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Teflon pads may well be OK in addition to a bearing however the plastic washers not only work fine but are also infinitely adjustable for friction.

Stiction is no problem, as the plastic washers are of small diameter relative to the large turning moment and overcoming the stiction is not noticeable.

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Teflon pads may well be OK in addition to a bearing however the plastic washers not only work fine but are also infinitely adjustable for friction.

Stiction is no problem, as the plastic washers are of small diameter relative to the large turning moment and overcoming the stiction is not noticeable.

What plastic washers would that be? And out of interest what age is your Revelation 12?

Edited by russ
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What plastic washers would that be? And out of interest what age is your Revelation 12?

Plastic washers cut from milk cartons (see post 17).

My Rev. was bought Feb. 2010. It has a large roller bearing lazy Susan, but nowhere near as large as the pitch circle of the feet.

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