Charlie sent this off list to me, great advice.
Grant KZ1W
-------- Forwarded Message --------
Subject: Re: [TowerTalk] Mast slippage
Date: Wed, 29 Jul 2020 11:43:48 -0400
From: Charles Gallo <charlie@thegallos.com>
To: Grant Saviers <grants2@pacbell.net>
Geez,
How big a hole are you planning on drilling in that poor mast that you
need a rotabroach?
Let's face we are probably talking somewhere in the 3/8-1/2 inch size
range (I could not find a manual to see the size of the hole in the clamp)
4130 is tough, but it isn't THAT bad (I drill a lot of 4140 pre hard)
Get a good 135 deg split point cobalt drill. something like
https://www.travers.com/nachi-cobalt-jobbers-length-twist-drill/p/01-021-332/
don't want to spend that much, try
https://www.travers.com/ttc-surface-treated-jobbers-length-twist-drill/p/01-001-232/
or
https://www.travers.com/triumph-hss-jobbers-length-twist-drill/p/01-331-032/
4130 is .30% carbon, which puts it on the border between mild and
alloy steel You probably want to keep it < 70sfm - which, for a 1/2"
bit means 535rpm. Let's be conservative, and keep it down to 400rpm or so
Use a coolant (cutting oil, not lubricating oil), and just go drill it.
Use a simple jig to start things because pipe is round.
If you aren't getting a good chip, because you can't apply enough
pressure, SLOW DOWN
You can also pre-drill with a smaller bit (why I recommended a split
point, less need to pre-drill, and a 135 deg has shorter cutting edges
than a 118, so less pressure needed )
Charlie
On 2020-07-29 10:41, Grant Saviers wrote:
Ron,
I shouldn't generalize, as it is possible to mag drill on a tower as
you are doing. But rigging my 60# mag drill on a tower with a custom
fixture and using Rota-Broach drills is not in most ham skills and
toolboxes. Maybe the fact my mags get heavier every year is another
factor ;) .
Grant KZ1W
--
73 de KG2V
Charles Gallo
http://www.thegallos.com
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