Perhaps you are talking about a tower with a welded non-removable top plate.
I opened up the hole of a Rohn 55 top plate buy clamping it upside down on
parallels in a Bridgeport. I used an indicator on a long arm and rotated it
to pick up the center of the three welded mounting tubes. I was amazed at
how far off center existing bearing hole was from the tower center. Tower
components are really not precise! I used a non-Rohn bearing so I drilled
the holes using the digital readout and also bored open the center hole in
my zero position using a boring attachment. I wish that I noticed the
massive amount of zinc inside the mounting tube. I later discovered that it
would not fit on my tower legs and I had additional machining work to do.
John KK9A
To: towertalk@contesting.com
Subject: Re: [TowerTalk] 2 inch Chrome Moly mast needed
From: "Roger (K8RI) on TT" <K8RI-on-TowerTalk@tm.net>
Date: Sat, 9 Apr 2016 19:03:21 -0400
I have both a Plasma torch and a Bridgeport mill.
The plasma torch is hand held and cuts great, but hand held (even using a
guide) is no where near as nice as the Bridegport. A rough set up is all
that is required on the Bridgeport. No need to use a dial indicator to get
it to the nearest thousandth. Just square up the head with the surface to be
cut. Just use an adjustable cutter, or a fly cutter. I don't like fly
cutters. They work well, but I don't like that cutter flying around. The
accessory plates would require no changes from the vertical set-up. In
addition there are no poisonous fumes from the high temp.
73
Roger (K8RI)
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