matt wrote:
On Oct 30, 9:51 pm, Bill Schwab wrote:

Hello all,


In such a situation, how close to the edge of an Al part would you
consider it "safe" to tap for 4-40 and 8-32, respectively? Again, the
concern is not so much ultimate strength as robustness and common sense.


There are already good posts regarding the wall thickness and good
design practice. I would be cautious, though, about using roll-forming
taps if you are by chance tapping cast aluminum (e.g., Mic-6). I
haven't had a
great deal of experience with these taps, but they are designed to
form (displace) the material, and in a more brittle, crystalline
matrix
there will be a tendency toward fracture rather than flow. In a thin-
wall
situation, this is not desirable.
It depends on the alloy. The difference between wrought and cast Al is
the working of the metal. For non-workable alloys what you say is true.
For a cast, but workable alloy, then the tap will compress the metal
initially, converting it into wrought aluminum, at least to some level.
That should be threadable with a thread forming tap. You have to drill
the pilot hole larger to use these taps, and some experimentation with
the specific alloy is needed to find what hole gives reasonable tapping
forces and good thread engagement.

Jon