Re: [ATM] holding a tube in place in an inexpensive dobnewt

["RodShea" <RodShea@comcast.net>]

A quilting or embroidery hoop sometimes fits or can be made to fit around 
the tube at the uphill end of the cradle.  Alternatively, a very large hose 
clamp meant for HVAC ducts with small blocks between it and the sonotube. 
I've done one of each, and both allowed the tube to rotate but not slide 
lengthways.  I think I glued sandpaper to the inside of the ring and/or 
blocks to help it not slip.  This would be pretty much your solution #1, but 
either were pretty quick to do.

I tried a strip of aluminum lengthways in the cradle with screws with knobs 
to tighten the strip against the tube, but, at least the way I did it, the 
tube still slid, and I could not rotate it.  Little wedges between the 
cradle and the tube did work, but I could not rotate the tube.

Rod

----- Original Message ----- 
From: "Guy Brandenburg" <gfbrandenburg@yahoo.com>
To: <atm@atmlist.net>
Sent: Monday, November 05, 2007 6:36 PM
Subject: [ATM] holding a tube in place in an inexpensive dobnewt


>I designed, and sawed out, and had kids assemble, five dob-newts (anywhere 
>from 4,4 to 6 inch diameter, and f/ ratios from f/5 to f/10) that either I 
>or another local ATMer had ground and/or figured. They work OK except that 
>the tubes have an excessive tendency to slide around in the cradle. I had 
>tried routing out and then sanding/filing out a circular disk to fit just 
>above the cradle, to prevent this, but it took an excessive amount of time 
>to do this, especially since I discovered that some of the Home-Depot-style 
>non-Sonotubes were actually not cylindrical in cross-section, but slightly 
>conical.
>
> I am now considering two possible remedies:
>
> (1) a split ring made out of either plywood or metal strapping material or 
> a combination of the two, that sits just above the cradle, and can be 
> clamped as tight as one wants, so that the tube can be rotated if there 
> are really little'uns trying to view somthing or if there are tall 
> teenagers / adults. This will require a lot of routing and fiddly stuff 
> where the gap in the circle is located. For five telescopes.
>
> (2) An entirely different sort of clamp that fits into one of the corners 
> of the cradle and uses some sort of a large-handled screw to wedge it onto 
> the tube. The cradles have trunnion bearings on their sides made out of 
> PVC pipe, and they follow the length of the tube for about 2 feet or so. 
> The screws for the clamps would be held in place by threaded inserts, and 
> would press against a piece of wood that is parallel to the tube and is as 
> long as the cradle. Not sure how to prevent the screw from eventually 
> passing through the wood clamping piece. Disadvantage of this method is 
> that the tube could not be easily turned without unclamping it.
>
> I also found that a Richard Berry type of trunnion tends to move too 
> easily. I think I have a plan for fixing that.
>
> Any thoughts and suggestions will be read with interest, though not 
> necessarily followed.
>
> Guy
>
>
>
> Guy Brandenburg, Washington, DC
> My home page on astronomy, mathematics, education:
> http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
> or else
> http://tinyurl.com/r6fh2
>
> =============================
> "Does anyone seriously believe that the reason U.S. companies are shipping 
> jobs to Mexico and Asia is that they believe those countries' schools are 
> better?"
> --  asks Alfie Kohn
> _______________________________________________
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> 


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