[TowerTalk] Fwd: Re: Mast slippage

[Grant Saviers]

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 at thegallos.com>
To: Grant Saviers <grants2 at 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