MW1CFN via rec.radio.amateur.moderated Admin
2018-03-25 06:50:03 UTC
Delrin: a most useful material
Posted: 24 Mar 2018 05:17 AM PDT
If you ever come to build antennas, you might well find yourself wondering
how to physically join two elements together mechanically, whilst keeping
them electrically isolated.
I revisited this problem this week because I wanted to make use of a
successful multiband, end-loaded vertical that can see useful service at my
alternative operating site. The antenna is a twin-fed, homebrew version of
the I-Pro 'Home' antenna.
Having used round timber brush handles for a temporary test run, I found it
almost impossible to remove this from the aluminium tube! The timber had
been outside in the weather for perhaps four years or so, and it was to be
expected it would now be rotten.
The only way I could get the timber out was to apply strong heat from a
butane gas torch. This worked remarkably quickly, the wood just falling
out after a minute or so.
Remarkably, when I cut through the timber, which was a hardwood, I found it
hadn't rotted at all. So if you can find the correct wood, this coupling
could last for a long time.
But I had now embarked on changing things. The alternative material I have
tried is Delrin, a plastic that is meant to be suitable as a replacement
Delrin has interesting properties, not least of which are its mechanical
strength and remarkable heat resistance. This latter property became
apparent as I tried to figure out how to insert a Delrin rod into aluminium
tube that was very slightly (about 0.2mm) smaller than it. Superficially,
Delrin looks and feels like nylon.
If you like mechanical engineering and 'slow TV', this video is moderately
interesting. Having seen a professional engineering friend killed by the
simple act of polishing a metal tube on a lathe, I very strongly recommend
you don't clear waste from a turning lathe as appears in the video.
Without a lathe, all I could so was to try careful, even sanding. This
worked quite well, enough to get the Delrin rod to line up properly inside
the opening of the tube, but no more.
I then tried heating the aluminium tube quite strongly, which eventually
causes the Delrin to start softening. It doesn't melt as such, with only
the slight excess width being sufficiently softened, a bit like thick tar,
to be pushed aside and allow the rod to fit. Even then, it takes quite a
lot of physical pushing to get it to go in.
Perfect friction fit!
I found that after cooling, the Delrin could be removed easily by hand, but
the fit with the aluminium was perfect, with no 'play'. This is very
useful to me, because there is no need to drill holes through the Delrin
and aluminium to secure the fitting; friction is more than enough. It also
allows easy assembly and dismantling without tools.
The end-loaded vertical is quite top-heavy, and puts a reasonable stress on
the Delrin rod. But tests show it is easily capable of taking the load,
although guying is needed in moderate to strong winds.