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Latest Project - 18" f3.9 Truss Dobsonian

Moonshane

By Moonshane
in DIY Astronomer

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Moonshane

Moonshane

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Hi Alan, sure will be with my usual made up as I go along, sorry, I mean intuitive tweaks to make it very user friendly and a joy to use as all good dobs should be. ordered the truss poles, some 5mm ali angle (for the truss connections) and 6mm ali plate for the mirror cell triangles. truss poles will fix inside the mirror box.

the cell will be 25mm box section steel.

the alt bearings will be of the half cartwheel style.

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Moonshane

Moonshane

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well, today I have mostly been making.......alt bearings.

after a long gap (sorry), due to a death in the family and the college essay from hell, I have finally restarted work on this project.

As always I made a template for the alt bearings as I like to make them a bit different with my 'cartwheel style' which involves quite a lot of cutting out to reduce weight and to make them look half decent. I follow broadly the Kriege and Berry design as do many others and therefore use large bearings. for this 18" scope mine are just a shade under 25" in diameter which will be approximately the diameter of the ground board. This equality between alt and az bearings means the scope should move in both planes with equal force.

I made the template from 12mm birch ply which may seem extravagant but this can then be used in future projects as it's robust enough to last. then after pencil marking the boards, I cut them out roughly from 15mm birch ply, two per side making bearings 30mm thick. The outer one was routered on each side from the template and then the two boards each side were glued and screwed (screws on the inside) to allow a guided router bit to do its thing. this ensures that the two sides are identical on the bearing surfaces.

I think they look pretty good. as most of the weight is borne on the middle sections I purposefully left these segments small and the outer ones slightly larger as this ensures that there is more meat holding the scope at most normal elevations.

next will be the mirror box although the cell will be done first (I am probably going to get someone to make this for speed).

here's some pics. you can see that there's a slight difference between these bearings and those on my 12" f4.

post-5119-0-37733200-1370286453_thumb.jp

post-5119-0-53896800-1370286474_thumb.jp

post-5119-0-24439000-1370286496_thumb.jp

post-5119-0-46385100-1370286519_thumb.jp

post-5119-0-83951100-1370286539_thumb.jp

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Sfarndell

Sfarndell

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Your carpentry is outstanding!

only 1 comment though: MORE MORE MORE!! It's like waiting for your favourite show to come around again and every week at the right time, there's an unannounced delay! :icon_bounce:

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Sfarndell

Sfarndell

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are you near half-way with your scope sfarndell?..or just started?

I finished the 18" scope, but not the mirror as I got sidetracked with a 16" which I'm now rebuilding (http://stargazerslounge.com/topic/187886-16-dob-remake/). 18" still under my bed waiting for final figuring (about 50% parabolisation so far) hence it's still "under construction" in my signature :embarrassed:.

Rgds

Scott

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Moonshane

Moonshane

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hi all

I managed to complete the mirror box yesterday at last and will upload some pics shortly. it's quite light and strong and will sit quite low in a low rocker box - should be able to do that and the ground board next weekend with a following wind. that will (almost) be the woodwork completed and then the work really starts!

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Sfarndell

Sfarndell

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Nice going Moonshane. Looking forward to seeing your latest pics!

A question re: mirror box rigidity - did the focuser bracket/strut make a significant improvement or were the 4 main columns enough to make the cage quite rigid? Im using a similar design to you on my 16" and it feels flimsy with just the 4 supports.

Rgds

Scott

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swamp thing

swamp thing

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Hi Scott

The upper tube assembly needs to be sturdy with or without the focuser board. It is unwise to rely on the focuser board to give stability alone. Try rebating/morticing the struts into the tube rings. This should strengthen it up nicely.the fixings need to kinds pull the tube rings tightly together this will give it tension and strength. How thick are your rings; and from what ply are they made? If you can use birch or apple ply this is best. The more ply's the wood has the stronger it will be.

Sent from my iPhone using Tapatalk

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Moonshane

Moonshane

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hi Scott

I agree with Steve, it should be quite sturdy with just the struts. the focuser board does add strength though as does the liner and even to some extent the secondary fixing. that said, with the 18" I am making, I am considering lining with 1.5-2mm bendy ply to effect further strength. I may even change the struts slightly, possibly adding some further strips of 12mm ply to each side of the 18mm and then (ask my father in law to) turn the whole thing down to a round profile or simply use a half inch roundover router bit.

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