piclist 2009\01\31\224732a >
BY : apptech email (remove spam text)

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