On 12/14/2010 10:29 AM, Bob La Londe wrote:
"Bob La Londe" wrote in message
...
I've got this urge to own a tapping head, so I've started doing a
little reading on tapping speeds. One formula I have read is:

For inch taps
RPM = SFM / (.26 * Tap OD)

http://www.newyorktwistdrill.com/tap...l_data_03.html

A middle of the road SFM for aluminum ( I know it can vary
dramatically depending on hardness, alloy, etc ) is about 600.

When I plug that in for a 1/4" tap it comes up with 9230.769 That
seems insanely fast to me. Are you guys who do this really tapping
holes in a fraction of a second?

Seems like for small jobs the clamping and positioning requirements to
do that would take longer than tapping a couple holes by hand.


Oh, heck. They also list aluminum on their chart at 90-100. Now I am
really confused.

At 95 that comes out as 6.175. That makes more sense, but then it
becomes a problem to turn that slow and still apply full power with most
machines guys like me can afford.

And why do they list aluminum at 90-100 and many of the milling sites
list aluminum from 500 to as much as 1200?

I had thought up until this moment SFM was SFM.




It all depends on the tap I would say.. If its a standard HSS tap with
no coatings, we normally run them at 100 RPM with oil in steels, and
coolant in plastic or aluminium. If we get into some type of production,
then we look at better taps so we can get faster feedrates.

We tend not to push a tap too hard, because it is normally a final
operation, and its a bitch to drill them out if they break.