RE: [ATM] Dob/quatorial Truss?

["mbyorick" <mbyorick@starband.net>]

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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