The work can be done on many home shop lathes - 12x36 and even some
smaller, so check around for who might be willing to help.
A piece of tubing or a mandrel of most anything the size of the mast
minus 0.050" or so makes the setup easier - centering the two halves and
then boring to the desired diameter, recenter and repeat as necessary to
get a square bore and enough teeth and area on the teeth. Three or 4
cleaned up teeth per row on each half with 1/4" contact areas is what I
decided was about right. While I bored for a 3" mast, I think the
casting should work for a 2" bore.
An hour or two of lathe time if you can find a commercial shop willing
to take it on.
It's clear the design intent was to work with all mast diameters, but
that limits the possible tooth engagement. Then adding casting
variability created a marginal clamp system. Mold wear yields more
variability and base machining is difficult to get square on the cast
teeth. It's a classic problem in machining an iron casting - what is
the best reference surface or where should one be machined?
I think many masts slip because bolts are not tightened to spec and not
retightened evenly another couple of times. Also, using stainless steel
bolts, some of which have a tendency to stretch a bit over time. My
DB36 stock mast clamp slipped 3 times before I reinforced the aluminum
"C" clamps on the backside with 1/2" thick steel bar and used grade 8
bolts torqued to spec 3x. The stock "C" collar pair would close to tips
touching and that made further tightening useless.
I am also a big fan of the DX Engineering cast saddle U bolt clamp sets.
The saddle has a large contact area with the mast or tube and my
choice for all home brew antennas. Never had one slip. A great part
for making a multi U bolt mast clamp.
A u-bolt holding a tube against a flat plate may have only 2 contact
points. Using many u-bolts is one approach. Deforming the u-bolts to
increase the "wrap" is another. Or flattening the tube.
Grant KZ1W
On 11/8/2022 17:42, Steve Dyer W1SRD via TowerTalk wrote:
What would a competent machine shop charge to do this? Most of us don't
have a fully outfitted machine shop :-).
73,
Steve
W1SRD
Bolt the two clamp sections to a lathe faceplate and bore them to the
mast diameter. Remove enough material to enlarge the contact area and
insure the bore is true to the bases. Use grade 8 bolts and use a
torque wrench to max spec torque for the size bolt. Repeat this
tightening 3 times.
The clamp bore as cast wasn't straight to the machined base and only a
few "points" would have made contact in the 2800 I have turning a 375#
100ft tip to tip 2L 80m beam. Now it doesn't slip.
Grant KZ1W
On 11/8/2022 10:00, k7lxc--- via TowerTalk wrote:
Howdy, TowerTalkians -
I need more friction between an M2 mast clamp and the mast. How
could I do that? What materials would work? Tnx. Cheers,Steve
K7LXCCell: 206-890-4188
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