piclist 2009\02\05\212841a >
Thread: ::Drill & Tap Calculator
BY : Jeff Latta email (remove spam text)

> One more thing: There is a number, used in the calculation of the
> drill size, which is found in every machinists handbook I looked in:
> 0.64952
>
> Googling that number provides a surprising number of results. That
> number seems to be quite popular, but nothing tells me why. It must
> be some sort of magic number in the universe like pi or something but
>  I don't know enough to figure out why. Does anyone know what 0.64952
>  means?

James,

The number 0.64952 comes out of the 1:2:sqrt(3)relationship for a 30
degree triangle and the definition of a 60 degree screw thread.  The
distance from the major diameter to the pitch diameter is
called the addendum and is specified as 3H/8 where H is the height of a
perfect thread (before truncation of the crest and root).
Two times the addendum works out to 0.64952P where P = thread pitch.

The formula and thread profile are shown on Wikipedia here:

My copy of Machinery's Handbook gives two formulae for tap
drill size:

#1 American Unified Threads,  Hole Size = D - 1.08253 P X
#2 American National Threads, Hole Size = D - 1.29904 P X
X = fraction of full thread.

The second one is what you have on your web site and agrees with tap
drill formulae that I have from various tool suppliers.  In this formula
the thread height is twice the addendum which means the dedendum is
equal to the addendum.  However, the Unified Thread Standard was adopted
in 1949 to replace the American National Thread.  In the UTS the
dedendum is H/4 which results in a thread height of 0.54127P and formula
#1.  See the Dmin formula on the Wikipedia page.  Using Formula #2
will result in an undersize hole which will require greater torque to
drive and also result in a reduced tap life.

Another point on tapped holes, supposedly there is very little increase
in thread strength for threads above 60% of full thread.

Jeff

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