Re: [ATM] Dob/quatorial Truss?

[James P Crombie <jpcrombie@pei.eastlink.ca>]

Mike
 Very nice.  What type of mount is it on, looks a lot like an old one I 
have in the basement for repair for our club, no name on ours.  
Originally came with plain bearings but I am in the process of 
installing ball bearing on the two axises.
  For painting aluminum, you will want to get some aluminum primer 
first, or the paint will eventually flake off.  I think it etchs the 
alum to provide a better grip for the primer and then the regular paint 
will stick to primer.  Should be available at any hardware store. 
 

mbyorick wrote:

>Hey Troops,
>
>I've just finished a truss-tube conversion for a 16" EQ.-mounted (Meade
>'Starblinder') scope, and have posted some (7) photos showing the truss
>and some construction details at:
>
>http://www.atmlist.net/contrib/mbyorick-at-starband-dot-net/
>
>Here's some descriptive info, for any of you who might be interested:
>
>The truss requires no welding or circle-cutting, is dis-assemblable
>(bolted together) and weighs a whopping 14 pounds.  Here's some initial
>description of the thing, which I will supplement if anyone is
>interested in more gory details:
>
>The octagonal truss connecting rings are made from 1/4" x 1" aluminum
>bar, with 1/8" thick gussets on each joint.  The rings are joined to the
>truss poles with some novel(?) connectors I conjured up [see 1st photo].
>The connectors for a 1" O.D. (0.884" I.D.)  alum. tube consist of merely
>a 7/8" flat washer, inserted in one end of the tube, with a bolt (or a
>nut for the inverse case) epoxied to the washer (obviously with the nut
>or head of the bolt on the inside of the washer), and the unit epoxied
>inside the tube.  In the 1st photo, connectors labeled 2 and 3 show a
>'bolt connector'; connector 1 is a 'flush' connector with a nut epoxied
>behind the washer (this is similar to a threaded insert, but
>stronger/cheaper).  
>
>Using two octagonal 'connecting rings' placed in two vices as a jig, you
>can establish the proper compound angle for each connector fairly
>quickly by using a fast setting epoxy (more epoxy discussion below). I
>put a 1/4-20 bolt through a 7/8" washer, put epoxy around the
>bolt/washer & insert the mess into the tube, place Saran wrap over the
>'ring', poke the connector bolt through it, put the truss pole in the
>jig, and apply enough epoxy to fill in the gap where the pole meets the
>ring (at a compound angle).  This method provides a nearly
>indestructible connector, as it reinforces the end of the tube.
>
>I don't think that 'threaded inserts' can be used to connect tubing to
>flat stock at compound angles(?), they don't provide as strong a joint,
>and the prices for those things are absolutely ridiculous, to boot.
>'Split-block' clamps are also more difficult to fabricate, expensive if
>purchased commercially, and heavier than the washer/bolt/epoxy-based
>connectors.  Instead of split-block clamps, the above method can be used
>to attach a pair of truss tubes to an angle bracket, which can then be
>bolted straight-on to a connecting ring.  
>
>The best epoxy that I've found for making the above connectors is
>so-called "Plastic Welder" from Wal-Mart.  This stuff sets up in about 4
>minutes, sticks to nearly anything, and has a 3300 PSI tensile strength,
>which makes it nearly as 'strong' as J-B Weld, at less than half the
>price.  From some crude stress tests I did, I found (not surprisingly)
>that the <adhesive> strength of the epoxy is the limiting factor in
>joint strength, and that the <tensile strength> of a particular type of
>epoxy, which is generally significantly greater than the adhesive
>strength, is irrelevant in applications where a thin layer of epoxy is
>used to glue two together two pieces of rigid material.  J-B Weld, for
>example, has an adhesive strength of only 1200 psi.  
>
>The 1/4" x 1" x ~8" pieces for the octagonal rings are all identical,
>each having a 45 deg., rather than a 22.5 deg., angle at each end (see
>photos '04-FrontEnd' and '02-joint', which show the shape of each piece
>and also that that the end of each piece doesn't coincide with the
>'corner' of the octagon).  I used J-B Weld on one ring, and then found a
>cheaper, and possibly better, metal epoxy, which is the house brand at
>the "Do-It" chain hardware store.  This stuff has a working time of
>about 18-20 minutes, is metal- and KEVLAR- filled, and makes joints that
>are nearly indestructible, perhaps because the Kevlar provides some
>flexibility, the lack of which is responsible for epoxied-joint failure
>in many cases. 
>
>The rear section of the '2-stage cage' has 8 parallel pole members
>spaced equidistantly, which lends itself to easy rotation of the truss
>assembly on the EQ mount (in 8 different positions, of course), as well
>as future attachment to a Dob-type mount by merely adding two supports
>for the altitude bearing (a 'Dob/quatorial'?!). 
>
>I'll add a light baffle when I can find a supplier of ABS sheet
>(one-fourth the price of Kydex) that doesn't require a minimum order of
>$100...  Hey, does anyone know if aluminum tubing can be painted with a
>flat black (epoxy/enamel?) to withstand minor scrapes? 
>
>Cheers,
>Mike Byorick 
>SW New Mexico
>
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>  
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