On 2/6/09, James Newton <jamesnewton@massmind.org> wrote:
> 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?
>
> I believe it is common because it is 3*sqrt(3)/8 which comes from the fact
> that the height of the thread is sin(60')*pitch = sqrt(3)/2*pitch
> http://en.wikipedia.org/wiki/Unified_Thread_Standard#Basic_profile But that
> doesn't explain the 2* in the formula I'm using, or why that number is found
> so many other places.
>
> 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.

Well, things are much easy in practice with metric sizes (I'm sure in
inch too, despite your accurate computer). For small drill sizes (up
to 8mm diameter, which are often used by people doing electronics and
rarely programming...) the magic number is 0.8. Need a 3mm tap? Drill
with 2.4mm or 2.5mm, whatever you have near your fingers. Things are
changing a little bit for larger drills than 8mm.

Hurray, again two different worlds, with two different systems,
populated with the same unhappy people.

Vasile
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