az_100 wrote:

Hi Andrew,
The process is called metal spinning, its done on a mandrel in a lathe
or similar machine. Somehow I do not think its an easy process to get
the hang of in a few tries.

What I would do is make a sleeve with a cut in it (so its diameter can
be adjusted) and clamp that between the bigger and smaller tube. You
may have to experiment to roll the sleeve out of just the right
thickness alu for a good fit.

If I were to build a yagi big enough for the 20m band I would not use
alu tubing at all. Instead go for the fibreglass fishing rod blanks and
run a 2mm copper wire inside for the element. That would be much
lighter still and also stiffer, as the fishing rod is tapered. The
blanks come quite long, the ones I used for the cubical Quad I'd built
some years ago were over 3 m ( 12' I think) long. You could always
extend them at the centre with an alu tube epoxy glued (or sikaflexed)
to the inside of the fibreglass blank.

Have fun

Klaus VK6AKS (not active)

Andrew VK3BFA wrote:
Hi All,
new to this metalworking stuff, so please excuse my stupid newby
questions. I am trying to build an HF Yagi antenna from telescoping
aluminium tubing(to reduce windloading and weight) - trouble is,
Imperial size tubing was a good fit into the next size ID, so
telescopic sections could be made easily. All thats available now is
Metric sizing, and its too sloppy a fit into the next size up. HOW do I
roll/reduce the diameter of the bigger tubing so its a nice
interference fit onto the smaller size down? - whats the machine
called, can I homebrew one (I have basic hand tools, a welder, lots of
"stuffit - lets try this"...misguided enthusiasm -- well equiped
here...) - any tips, references. pointers appreciated.

PS I built the tower its going on, 8.5 metres, teaching myself to weld
as I went. Welding gets better as you go up.......so, the next logical
step is to build the antenna that goes on top of that.........

Andrew VK3BFA.

Interesting idea with the fishing rods.

I think the aluminum spinning for the relatively simple task of necking
down a short section at the end of a tube is a bit less complex than a
lot of form spinning tasks.

What I would suggest trying (still requires a lathe) is to put the piece
of tubing in a suitable collet in the lathe with a few inches sticking
out. Lube and insert into the tubing, a solid rod with an OD to match
the desired ID for the tubing, this could be held in a tailstock drill
chuck. Replace the knurling rolls in a scissors type knurling tool with
plain ball bearings (smooth OD). Running the lathe at a fairly low speed
and starting closest to the collet and drawing towards the tailstock
proceed to spin the AL down until it is a snug fit on the inner mandrel.

I've not tried this myself, but since you appear to be talking about a
small amount of slop in the fit, I think this method would work.

Pete C.