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Re: [TowerTalk] Drill bits

To: "Charlie Gallo" <>, "Tower and HF antenna construction topics." <>
Subject: Re: [TowerTalk] Drill bits
From: "Jim Miller KG0KP" <JimMiller@STL-OnLine.Net>
Reply-to: "Tower and HF antenna construction topics." <>
Date: Wed, 24 Mar 2010 09:40:50 -0500
List-post: <">>
Finally, somebody who KNOWS about cutting oil.  It's absolutely amazing to 
me that so many people think you can use motor oil for drilling or cuting 
threads and how few of the want-to-be hardware stores using kids for 
employees haven't heard of cutting oil.  I am not a machinist but Dad showed 
me when I was a young kid how using motor oil when cutting threads tears the 
threads and using cutting oil would cut perfect threads every time.  Back 
then I didn't know there was light and dark cutting oil and just used what 
Dad had and now when I neded to buy it, I found out.

Now, MY question, I bought the "light" cutting oil as I am cutting threads 
by hand and not with a machine and suspect the "dark" cutting oil is for the 
machings, TRUE ????

Thanks es 73, de Jim KG0KP

----- Original Message ----- 
From: "Charlie Gallo" <>
To: "Tower and HF antenna construction topics." <>
Sent: Wednesday, March 24, 2010 12:03 AM
Subject: Re: [TowerTalk] Drill bits

> 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
> "But when a long train of abuses and usurpations,
> 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."
> My Website:
> My Blog:
> Hi, my name is Annie Key. Please don't hit me!
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