RE: [ATM] sonotube for telescope -- Aluminum no-weld truss?

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

An octagonal tube (truss or solid) is closer to a circle than a
hexagonal one, and wastes much less space, particularly in a truss tube
where the longitudinal truss members have to clear the optical path.
I'll note that you can make all of the bulkhead/truss ring pieces for an
octagonal tube with 45 degree angles at the end of each piece, and avoid
pesky 22.5 degree angles.  The pieces (all identical) fit together as
shown in the attempt at ASCII art below:

       ___________
      /___________\
                \  \
                 \  \
                  \  \
                   \ /
                    /

Gussets overlapping each joint are simply strips of material (also) with
45 degree angles at each end.  The consistent use of 45 degree angles
for all of the bulkhead components means that all pieces can be
generated with a minimum number of cuts (since one cut simultaneously
provides two ends), and minimal waste of material.

I just finished a ~20" dia. all-aluminum truss tube for a 16" scope,
with no welding (or circle cutting).  For the octagonal bulkheads, I
used 8 pieces of 1/4" x 1" aluminum bar (shaped as described above)
glued together with metal epoxy, and gusseted with 1" wide x 1/8"
aluminum strips (~2" long).  I found a house brand metal-filled
'industrial' epoxy (with Kevlar!) at a "Do-It Best" chain hardware
store, which works as well or better than J-B Weld, and costs only half
as much.  After stress-testing several epoxy joints with different types
of epoxy, I found that the adhesive strength of the epoxy is much more
important than the tensile strength in this particular application,
noting that joint failure was caused by 'de-adhesion', rather than epoxy
'breaking', in every case.  For example, J-B Weld states that their
epoxy, which has a tensile strength of 3960 psi, has only 1200 psi
adhesive strength.  Sanding the metal prior to epoxy application
provides optimal adhesion; solvents may help, but not nearly as much as
'roughing up' the surface of the joint.

I fastened the 1" dia. aluminum truss tubes to the bulkheads using
connectors, each made from a 1/4" bolt, placed through a 7/8" washer,
secured to the head of the bolt with a nut, with the assembly epoxied
into one end of an alu. tube at the correct angle (determined by jigging
the bulkheads in two vices).  These truss tube connectors are much
easier to make (and lighter) than split blocks or other clamps I've
seen, and you save big bucks compared to commercial truss connectors.

Btw, Bob, where is that 200" refractor you mentioned?! (I imagine the
Clark brothers are rolling around in their graves waiting to find out!)

Cheers,
Mike Byorick
SW New Mexico


-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf
Of Bob May
Sent: Tuesday, June 15, 2004 11:18 AM
To: atml
Subject: Re: [ATM] sonotube for telescope

Not too sure of what you did for the joints of the octagonal tube but
when I
did the tubes for the 200" refractor, I used cherry that was slightly
less
than 3/8" thick and cut for a hexagonal tube (30 deg. angle) and butt
glued
it together...

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