Re: ATM aluminium cage design

["Dave Pitou" <dpitou@ix.netcom.com>]

The problem with a parallel tube design is that the stiffness of the
structure is just 8X the cantilevered stiffness of an individual tube. If
you hold a tube straight out with a 2 pound weight on the end you'll see how
flexible it is with this kind of loading. The strength of the truss design
is that the members are partially loaded in compression because of the
triangular placement. This is like placing the 2 LB. weight on the top of
the tube if it's pointing straight up. With the design you are considering I
expect you'll loose all the machining accuracy in tube flexure. Why can't
you machine the rings so the tubes go through them at an angle? This will
get you the best of both worlds!

Dave


-----Original Message-----
From: tofkas@uswest.net <tofkas@uswest.net>
To: atm@shore.net <atm@shore.net>
Date: Sunday, August 01, 1999 5:29 PM
Subject: ATM aluminium cage design


>
>I'm designing a 12.5" newtonian and am considering an open
>tube design.  Instead of a truss design, I'm looking at 8
>1/2" aluminum tubes in an octagonal pattern running through
>a series of 1/8" thick aluminum rings.  This will connect
>the mirror bucket (made out of laminated hardboard) to the
>focuser cage.  The mounting will be "dobsonian" sort of ala
>Mel Bartel but I want the pivot point further up the tube so
>that the change in eyepiece height isn't so drastic and
>remains within a reasonable viewing range.
>
>I have not seen many of these designs around..Is there a
>good reason?  Is there anything I should watch out for?  I'd
>really appreciate some advise here.  One reason that I am
>considering this design is because I can get the aluminum
>cut extremely accurately and fairly cheaply with a water-jet
>cutter.  The aluminum tubes would be welded to the rings
>where they pass through.
>
>I would definitely appreciate some input on this.....thanks.
>
>Stephen Houle       tofkas@uswest.net
>