My 300PDS Dob Mount Build

[Uplooker]

Well I have been a bit quiet on this recently so a bit of an update. I have managed to do all the sawing, routing and sanding to get me to the point where I have all the components in a state ready for assembling.

I have however encountered a slight problem. This being that my alt bearings are not parallel when attached to the SW tube rings. Not only does this cause issue with general use of the mount but it may very well alter the dimensions of my mount.

When I attach the alt bearings to the SW tube rings (which are mounted to the OTA), if I measure the distance between the outside edges of the alt bearings on one side of the OTA I get a 422mm. When I then measure on the other side of the OTA I get 430mm. I suppose it is similar to toe-in or toe-out on the front wheel alignment of your car.

Effectively I need to have the tube rings open a bit more at the thumb screw side, which should result in the alt bearings at the tube ring hinge end turning in slightly.

I have cut some strips of flocking material to pad out the tube rings so that they are “open” a bit more at the thumb screw side. There is some adjustment, depending on how tight I do up the thumbscrew, but I think I am going to have to cut a new base for the rocker sides to attach to as I was working to a width of 422mm.

I could pack one side of the alt bearings out with washers, but as above this would alter the width of my rocker base.

The other option is to file off some of the flat on the tube rings. This would be very onerous as I would have to keep removing the tube rings from the OTA and unbolting the alt bearings from the tube rings and then reassembling every time I have filed a bit more off. I also not keen on this option as it taking metal off, which is an awful lot easier than putting metal back on if I do not get it right.

The part of the alt bearings that would actually be seated on the rocker boards / PTFE between elevations of 0 deg and 90 deg is pretty darn close to 422mm. It is just that I need to fit retainers to the sides of the rockers to prevent any lateral movement. I suspect that these will bind and thereby ruin any smooth movement I may attain.

You are probably struggling to understand my garbled explanation; I will try and get a picture up later to illustrate the problem better

I would appreciate any input the more experienced Dob builders may have.

Ian

[Uplooker]

Here are a couple of pics of the various parts I have made for my dob mount

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[Peter Drew]

The parts look very nice Ian, as you have found, it's the sum of the parts that matter. A pic of the assembly will be helpful. 

[Uplooker]

Here is a drawing that hopefully better explains my problem, they do say a picture speaks a thousand words.

Ian

[mapstar]

Hello Ian,

Have you tried the set up for parallel without the OTA inside the rings?

Looking at the diagram you posted if the 430mm section is pulled in it is going to throw the bottom out from 422mm so not much use to your base width measurements.

Are the alt bearings bigger diameter than the OTA?

And what thickness material is it?

Filing is an option to get parallel and to the size you need.

Or parallel them up to say 426mm and put a further piece of wood where the teflon will mount to widen the runners for the side guides (easiest option in my opinion) and could be part of the side guides as it only runs on the teflon not the half moon of the box so doesn't necessarily need to be the same width unless the tube rings are going to foul the inside like this?

Only way to get it to 422mm is to rout out the bit where the bearings bolt onto the rings to decrease the width, but you need them parallel so if the bearings are a bigger diameter than the OTA I would make a split square board (top to bottom same as the tube rings and say 386mm wide if 18mm ply is used) with an OTA sized hole in it to go between both bearings and then you can bolt them on parallel.

A few suggestions others may come up with something more appealing?

Hope this helps and you get my explanations too

Damian

[Uplooker]

Hi Damian,

Thanks for your input.

To answer a couple of your questions:-

1. I am using 18mm birch ply
2. Alt bearings are 1.3 times the primary mirror diameter

I am reluctant to remove material either from the tube rings or the alt bearings as once you have taken it off, its off, very difficult to put back on again. I am able to get the alt bearings parallel when the rings are not attached to the scope, I can't remember the distance once achieved. It is then the problem of having to pack the rings out to allow suitable purchase on the OTA.

I think I am going to shim the alt bearings out with washers. i need stainless steel one, but have not had the opportunit to purchase them as yet, hopefully this afternoon. I have not assembled the rocker sides and base board as yet, no point if the distance between the rocker sides if incorrect. Once I have got the alt bearings parallel by shimming them and am sure of the distance between the outside edges I will cut a new base board for them to be attached to.

I will see how I get on this evening.

Thanks again.

Ian

[Uplooker]

Update.

Well I bought some stainless steel washers this afternoon with the hope that I could fit 1 or 2 between the alt bearings and tube rings on one side so as to push out the alt bearing on that one side and thereby in on the opposing side. Well the washers were only 1.8mm thick and with only using 1 off washer on each side it over corrected the situation by a mile. I played about with packing the rings themselves out with strips of flocking material - still to no avail. I was stumped. I then remembered that I had some small sheets of plastic shim left over from another project. I only had 0.6mm and 0.3mm thicknesses. Well it turns out I only needed 0.9mm either side of the tube rings, sandwiched between the alt bearings.

I now have parallel alt bearings.

At least i can now get on and assemble the dob base. I need to hurry and get this project finished as I still have my Denver seat to make before the observing season starts proper and given the time it has taken me to get this far I need all the time I can get.

Ian

[mapstar]

Well done Ian for getting your problem sorted.

If you need to get a move on then I've no chance as I've so much to finish!

[steppenwolf]

Found this thread far too late to help but delighted that you have resolved the issue. Your cut out parts look great, I love the spokes in the altitude wheels. Good luck with the rest of the build, having just completed my own Dob base, I can only tell you that it was a very enjoyable project to take on.

[Uplooker]

Ha ha Steve, I was just reading your dob mount build thread again. Thanks. I have not made a quick job of this. I am enjoying it but I keep encountering problems I had not envisaged.

Ian

[Moonshane]

Found this thread far too late to help but delighted that you have resolved the issue. Your cut out parts look great, I love the spokes in the altitude wheels. Good luck with the rest of the build, having just completed my own Dob base, I can only tell you that it was a very enjoyable project to take on.

I could have written this post myself it is just what I was about to post!

Glad you are sorted without surgery. It is funny how when faced with something like this we often go through the classic stages:

Oh no, disaster

I am going to have to seriously ruin all my work and start again

I will leave it a bit and think as it has really brassed me off

hang on, what if I..........

Sorted - I am a genius - why did I ever doubt myself

[Uplooker]

Well I have managed to get one of my rocker sides glued and screwed in place this evening, thankfully as I did not have to alter my rocker base board dimensions :-)

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[Uplooker]

I could have written this post myself it is just what I was about to post!

Glad you are sorted without surgery. It is funny how when faced with something like this we often go through the classic stages:

Oh no, disaster

I am going to have to seriously ruin all my work and start again

I will leave it a bit and think as it has really brassed me off

hang on, what if I..........

Sorted - I am a genius - why did I ever doubt myself

You captured it perfectly Shane, apart from the last part "I am a genius - why did I ever doubt myself"

Ian