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'[TECH]::Drill & Tap Calculator'
2009\01\31@224732 by

The following is from James Newton:

I wrote up a page that rather nicely gets you the drill to use for a tap
hole, given the tap size, TPI, and depth of thread you want. It's better
than the tables one can find in engineering handbooks and online, because
you can feed it ANY size, thread count and depth and it will give you not
only the ideal size, but also all the standard drill sizes that are close to
that, and how close, and how deep the threads will be if you use that drill.
It also is quicker, because all the common bolts are listed in a pull down
at the top so you can instantly load all the required values and get the
answers you need without scanning through a table or typing in everything
for yourself. You can also specify the tap size in fractional, number gauge,
or UTS. On the other hand, you must be online to use it... :o)

http://techref.massmind.org/techref/taps.htm#drillcalc

Now, I think I did everything right, and I've checked against a few common
sizes and it seems to give reasonable results, but I would love to have a
few people who really know what they are doing with a tap and drill tell me
if it looks right to them or not.

It doesn't support metric sizes. If someone from that side of the pond wants
to adapt it, I'll include your changes on the web page and credit you with
saving the world from imperialism. ;o)

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?

Last question: Did anyone else know there was such a thing as a drap? I
totally want one for #4-40, but \$27 each is a bit dear.

James

_______________________

RMc says: One may find that in about 1950 the 0.64952 officially changed to
0.61343

'[TECH]::Drill & Tap Calculator'
2009\02\05@212841 by
> 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:
http://en.wikipedia.org/wiki/Unified_Thread_Standard

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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