On Oct 31, 12:51 am, Bill Schwab wrote:
Hello all,

I have a design problem requiring a reasonably rigid structure in a
tight space. Forces are not at all large, and structural failure is not
a great threat. However, the parts need to be securely connected to
each other in the sense that we cannot afford to have parts spinning out
of alignment when disturbed. The result is a scientific instrument
package not unlike an indicator holder: it needs to not fall apart every
time it is touched, but does not have to be very strong.

To provide rotational support at the connections[*], I have been
planning for machine screws in pairs. Space is tight, so that has
pushed me toward 4-40, which is a pain to tap. I am thinking of using
one 8-32 screw and making a tight-fitting slot to prevent rotation at
each connection. Any preference or other ideas?

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.
Ordinarily, I try never to get closer than 0.1" from the OD of the
hole to the edge, and usually think more about staying clear of
parallels, so this is new to me.

Thanks,

Bill
I've gotten away with 0.030" from wall to thread OD using a sharp
spiral-point tap but the threads may be visible on the outer surface.
It was a lightly stressed radio enclosure, though, with a cover plate
to take any bending load if it was dropped. If you tap first, then
machine the outside thin you can cut as close as you want to the
threads or even cut into them, for instance for a friction drag plate.

4-40 in aluminum isn't that bad. I redesigned a frequently-opened
microwave radio chassis with 120 0-80 screws to use 2-56. The shop
tapped the panels for about 4 boxes with a thread-forming tap on a
Procunier head without breaking one.

Jim Wilkins