Roger,
I think we agree that rivets need a hole "to be snug, clean, and burr
free". Snug in my shop is in the sequence of tolerances of fits -
(examples for a 3/16 or #10 diameter oversize of the hole, and how it is
made, with pro grade tooling)
interference (dowel pins not to be removed), minus 0.0005 to 0.0015
clearance (undersize drill and reamer)
line (+/- zero tolerance, pins can be pressed out, might need loctite,
undersize drill, same size or slightly under reamer)
snug/tight (hand fit but tight) +0.002/0.003 (oversize reamer, pilot
point drill or step or core drill secondary operation)
close (easy fit) +0.006 (larger drill, stub length split point or pilot
point drill)
free (allowance for position errors) +0.012 (even larger drill)
Snug to me also means a round (low TIR) hole with square entry/exit.
With a top grade 2 flute chisel point drill bit in a hand drill the
holes I've done/seen in 0.058 wall tubing will be somewhat triangular,
and the inside burr significant. The drill flutes don't engage the
material before the drill breaks through. Fish tail or DeWalt pilot
point (formerly Black&Decker Bullet) drills are far superior for thin
material drilling but are hard to find in other than 64th sizes. These
drills produce pretty close to snug/tight fits in thin materials.
I don't build aircraft parts but do have a collection of step and double
margin drills used for aircraft rivet hole drilling. These are usually
not in the average ham's tool box. Blind hole inside deburring tools
are made, but fortunately for most antennas parts access to the interior
burr is not too hard and a half round file cleans them up.
OTOH, enough 4/5/6(?) rivets you suggest may hold up in close/free or
worse fit holes. I don't know how the antennas you rebuilt were drilled
for rivets, the F12 EF180C (85' tip to tip) I rebuilt had missing/loose
rivets and worn holes (2 or 3 rivets per joint).
Grant KZ1W
On 8/9/2015 0:59 AM, Roger (K8RI) on TT wrote:
Having built aircraft, or parts of them with lots of "blind rivets", I
feel safe is using standard "blind rivets". The hole needs to be
snug, clean, and burr free. I would prefer 4 rivets per joint in sets
of 3 at 120 degrees with the second set of 2 off set from the first
set by 60 degrees. I'd use good quality, blind rivets, not
necessarily aircraft "Cherry rivets" But the Cherry rivets for
homebuilts are not all that expensive.
Just make sure the tubing is clean, inside and out. I've never had
them come loose.
I purchased a used Force 12 C19XR and WARC7. After they had been up
years, the joints were still good. The rivets were drilled out to
transport the antennas. Putting them back together was a quick
cleaning with fine scotchgard pads, a thin coat of Al noalox and pop
rivet back together.
73
Roger (K8RI)
On 8/9/2015 12:57 AM, Grant Saviers wrote:
I'll offer a contrarian view - I don't use hose clamps or rivets. One
exception is a pair of hose clamps when the tip of the element is
intended to be adjusted e.g. cw to ssb. Usually a 1/2" to 3/8" tube
joint.
Riveted antennas I have acquired and rebuilt had missing and loose
rivets. I drilled the remaining ones out and used 2 bolts/machine
screws per joint. Structurally, pop rivets squeeze a joint together,
they don't fill the holes with rivet material. Bridge rivets (hot
forged) and Boeing rivets do fill the hole and prevent motion
(airplane rivets are very precise fasteners, essentially each hole is
reamed to very tight tolerances and then the rivet is cold forged
closed). A pop rivet allows the tubes to move which eventually
loosens the rivet or shears it off and it falls out. I don't think a
pop rivet can achieve the force needed for a "slip critical" joint -
https://en.wikipedia.org/wiki/Slip-critical_joint
Hose clamps are for hose, not aluminum and the thread strength is
poor (why there is this thread!). There is no secondary resistance
to loosening - no lockwasher, nylock, etc.
I subscribe to the Dave Leeson ("Physical Design of Yagi Antennas")
bolt the elements together philosophy, particularly two cross bolts
since the bolts tighten against each other as the tubing goes a bit
oval from the force. Nylocks or K-L nuts help keep them from
loosening but cross bolted, they do get tight enough to stay tight. I
have yet to see one loosen when properly tightened and then
re-tightened when the connection stress relaxes over a few days.
Cross bolts also restrain the elements in two planes, just as a pair
of orthogonal set screws are best for stuff attached to shafts. For
my 2- 3" center sections I use 5/16-18 and for small 1/2 to 3/8
diameter joints 6-32 works fine, stepping down as needed as element
diameters decrease - 1/4-20. 10-32. 8-32.
I use hex head bolts and then socket head (allen) for smaller sizes
which are preferable to phillips, although one pro builder who built
some of my antennas managed to find hex head #10 and #8 machine
screws, but those are pretty rare. With Penetrox on the threads of
SS fasteners and on the overlapped tube sections,
disassembly/reassembly with threaded fasteners is simple. Just did
that for seven large yagis that had been up 7 years. Not a single
fastener was loose or missing.
Grant KZ1W
On 8/8/2015 20:19 PM, john@kk9a.com wrote:
Hose clamps come in different widths, materials and styles. I am not
sure
what MFJ/ Cushcraft uses or what failed for K6UJ, but on my homebrew
antennas I use worm drive hose clamps with a 9/16" wide band and a 300
series stainless steel screw. For 1 1/4 or larger tubing I use a
bolt style
hose clamp.
John KK9A
To: "<towertalk@contesting.com>" <towertalk@contesting.com>
Subject: Re: [TowerTalk] Shorty Forty Hose Clamp thread
From: Robert Harmon <k6uj@pacbell.net>
Date: Fri, 7 Aug 2015 22:02:00 -0700
Doug,
I no longer use hose clamps after having them strip as you had
happen or the
connection
loosening after flexing back and forth in the wind (I am also in the
Pacific
NW)
Now I only use rivets. I have had a number of Force 12 antennas and no
problem
at all with the connections. Their riveted conns sold me. The HF
beams I
have
fabbed have riveted elements
and no problems. I wipe on Penetrox when assembling and later when
taking
apart
the tubing is like new. Plus to change element lengths it is super
easy to
drill
out the center of the rivets, they pop right out. Now I can sleep easy
while
the winds blow, hihi.
73,
Bob
K6UJ
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