On Sep 27, 8:26*am, Ignoramus21149 ignoramus21...@NOSPAM.
21149.invalid wrote:
On 2010-09-27, Richard J Kinch wrote:





Ignoramus24898 writes:

I would like to drill and tap them with 5/16" drill and 3/8" tap, say
spaced at 1" interval. That makes for about 200 holes to be drilled
and tapped on my CNC mill.

How frequently do you break taps? *If you're not an utterly reliable
tapper, you'll never be able to complete this job. Consider that if your
workpiece is ruined by breaking a tap, then you must succeed at all 200 in
a row. *For a 90 percent chance of success, then, each individual hole must
be tapped with a 99.95 percent chance of success.

If you break a tap, say, once every hundred holes, then you have an 87
percent chance of failure on this piece (1.0 - 0.99**200 = 0.87).

Now you know why these cost $$$$:

http://www.edmundoptics.com/onlineca...t.cfm?productI...

Richard, I was hoping that I would tap on my CNC mill, so whatever
process I do, would be repeatable and not as random as manual tapping.

i- Hide quoted text -

- Show quoted text -

For aluminum, you may want to use a Thread Forming Tap, instead of a
Thread Cutting Tap...Since you will be using coolant/cutting fluid,
the going will be much easier, and the thread will be stronger. A
thread forming tap has the additional advantage of no chips.

_kevin