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Re: [TowerTalk] pipe tap drill size

To: "'Tower and HF antenna construction topics.'" <>
Subject: Re: [TowerTalk] pipe tap drill size
From: "Ed Swiderski, KU4BP" <>
Reply-to: "Tower and HF antenna construction topics." <>
Date: Fri, 12 Mar 2010 09:20:26 -0500
List-post: <">>
If you are doing a 1/4' pipe thread (NPT), then your size is 1/4-18 NPT. Tap
drill is 7/16(.4375).

If you're doing a straight hole, then your thread size is either 1/4-20 or
1/4-28. Best way to check is to measure the hole with calipers to see what
the tap drill size is.

Tap drill for 1/4-20...#7(.201) and 1/4-28...#3(.213)


-----Original Message-----
[] On Behalf Of Charlie Gallo
Sent: Friday, March 12, 2010 8:14 AM
To: Tower and HF antenna construction topics.
Subject: Re: [TowerTalk] pipe tap drill size

On 3/12/2010 jimlux wrote:

> And sometimes, it's stamped on the shank of the tap.

OK - The answer given is accurate enough, as it is (given the fact that you
had to ask the question)

That said, the answer isn't as clear cut as #7 drill for 1/4-20

First - there are basically 2 types of tap for our purpose here - "cutting"
taps (which are the taps you will see about 99% of the time - be they 2
flute, 4 flute, hand, spiral point, whatever) and Forming taps (which don't
remove metal, but push the metal from the bottoms of the thread to form the
crests - they only work on ductile materials, and the threads look weird,
like they have a groove at the TOP of the thread)

Anyway, sticking with "cutting" taps, a #7 drill will give you a 70% thread,
so it's actually a fairly good answer, but there are cases where you want a
HIGHER percentage of thread (you never go to 100) or lower percentage
(easier to tap, somewhat weaker threads)

IF you don't have a #7 drill around, you COULD, in theory go as LARGE as a
#4 drill bit, and only get 57% of thread (I wouldn't ever GO that low except
for threads that don't really hold anything), and go as small as a #9 drill
bit(which will give you 77% of thread).  The danger there is breaking taps,
and if you have a slightly large screw/bolt it won't go in real easy)

IN that range is a 13/64 inch drill bit, which will give you a slightly
looser thread (66%) than the #7, BUT might be a LOT easier for you to come
73 de KG2V

For the Children - RKBA!

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