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Re: [TowerTalk] Drill bits

To: "Tower and HF antenna construction topics." <>
Subject: Re: [TowerTalk] Drill bits
From: Charlie Gallo <>
Reply-to: Charlie Gallo <>, "Tower and HF antenna construction topics." <>
Date: Wed, 24 Mar 2010 01:03:36 -0400
List-post: <">>
On 3/24/2010 Guy Molinari wrote:

> Hi all,

>     I don't know why this is so hard.   I'm trying to drill out
> some hole on some 3/8" steel plates that I'm going to have
> galvanized.   I bought a 13/32" drill bit and it worked great for
> about 10 holes or so.   The problem is that I have about 140 more to
> go.   Any recommendations on a good quality drill bit?   
Hey Guy,
OK, some questions, and a quick tutorial on drill bits

What are you using to drive your drill bit?  What kind of speed, and what kind 
of pressure?  It's going to be REAL hard to get sufficient pressure on a 13/32 
drill bit with a hand drill.  What kind of cutting fluid are you using?

OK - Now let's back up, and go into a bit of theory

Look at the pointed end of a drill bit, end on.  You will notice that there are 
2 cutting edges, and a center area that has a straight line across it joining 
the two edges (flutes).  The BIGGER the drill, the BIGGER and thicker this 
center point.  The big issue is that the point does NOT cut!
You are going to want to drill a "pilot hole" just slightly larger than the 
diameter of the point on your 11/32 drill - this will make life a LOT easier

Now, look at the business end of a drill bit from the side - the angle made by 
the 2 cutting edges on MOST drill bits will be 118 degs.  118 is a nice, 
generally 'comfortable' angle, but it's a compromise.  Wood really wants a 
pointer bit, where steel really wants a point angle of around 135 degs.

There are some other tricks - back to looking end on - if you don't have to 
worry about the bit "walking" when starting the hole (your using a drill press, 
or there is a pilot hole already) there is a way to make what is called a 
"split point" drill, which is where the area in back of the flutes is extended 
to the middle by grinding - this makes the drill cut much easier, but DOES make 
the edge a tad more fragile to shock/impact

As for cutting speed - unfortunately my machinery handbook is downstairs, but 
many (most?) people run their drill bits WAY too fast on large bits - they need 
to turn slowly, and turn out those nice "curly chips" - if the chips are blue, 
your running way too fast

Cutting fluid/oil - for a bit that size, you really need to run with some sort 
of cutting oil, an no, motor oil is NOT cutting oil.  The stuff they sell at HD 
for threading pipe will work nicely for drilling holes that size in steel 
plate.  Yes, it'll smoke and smell, but that's the oil taking heat away from 
the edge of the bit

Of course, the joke is that depending on how thick the plate is, there is a 
real good chance a commercial fab shop would just PUNCH the holes (I've punched 
holes a LOT bigger than that through the flanges of BIG I-beams).  You MIGHT be 
able to find a shop that does moderate duty steel work, and have the plates 
punched for not a heck of a lot of money

73 de KG2V

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pursuing invariably the same Object evinces
 a design to reduce them under absolute Despotism,
it is their right, it is their duty, to throw off such Government,
 and to provide new Guards for their future security."

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