Bob, N7IP,
A number of years ago I built a large multi H frame system, using
large aluminum square booms, for a 28' long boom ,16 yagi 2M array.
I welded all of the four smaller H frames , made of square tubing (2"
x 3") together and the and the mounting plates on the large 30' 6"
square cross boom and 28' vertical risers and their mounting plates
. Also the perpendicular guy support tubes on the large cross and
vertical tubing pieces.
I was a beginning welder and only had just recently taken a couple of
welding classes at the local Jr. College. My recently purchased
Miller 175 amp, 220 VAC, MIG welding system, also included a recently
released option of a spool gun for use with soft aluminium welding
wire . The spool gun installation instructions didn't include any
set up temperature and speed information regarding welding aluminum
material . I wrongfully assumed that it would be less than for
steel. Later released instructions suggested much hotter temps
for aluminium than for steel. So my first few welds had to be ground
off and redone.
TIG type of welding (by an accomplished welder), I think is a much
better method for welding aluminum and most beautiful commercial
aluminum welds that you see are TIGed. However for most HB Amateur
use, the MIG technique seems to be quite satisfactory as far as
strength is concerned, as long as adequate weld penetration is
achieved. Fortunately my finish welds would not be too visible, at
30' up in the air. Pure argon is used for the shielding gas.
I did develope on idea/aid for use in extending and joining sections
of the square tube to make the long boom and the long vertical
risers. I was very much concerned on how to accomplish perfect
horizontal and vertical alignment of the two pieces in this
procedure. What I finally came up with, was to use a pencil type
lazer pointer that I mounted on a plate, which I laid on one end of a
section and placed a target block of wood on the other end of that
section. I then transferred the target onto the far end of the 2nd,
roughly aligned, extension piece of square tubing, I then fine tuned
the two pieces of tubing so that the lazer spot was dead on the
target for the full length of the to be extended tube, hence perfect
alignment in both H and V planes.
I then butt welded the two sections together and followed up with two
1/4" thick x 2' long large pieces of aluminum angle along the
diametric opposite sides of the joint, that were then also welded on
to the two pieces.
I've found that discretely using welding techniques can not only make
an assembly lighter but may accomplish it much quicker, than using
nuts and bolts and maybe with backing plates or angles. This is
especially true when assembling large structures for tower mounting,
where minimum weight and permanent alignment can be of high
priority. Getting rid of heavy mounting plates and multiple
corrosion resistant U bolts can be significant, and even cost effective.
Chuck, W7CS
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