[TowerTalk] Shorty Forty Hose Clamp thread

Grant Saviers grants2 at pacbell.net
Sun Aug 9 13:15:19 EDT 2015


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 at 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 at contesting.com>" <towertalk at contesting.com>
>>> Subject:    Re: [TowerTalk] Shorty Forty Hose Clamp thread
>>> From:    Robert Harmon <k6uj at 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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>>
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