John Doe wrote:

snip?

And that is something stronger. Besides a stronger machine screw (at
least stronger than junk steel), maybe something different will work
too. I might go with stainless steel. The fitting must be extremely
tight. A correctly tapped hole will not work. I will look into
thread forming fasteners, but I've heard that a machine screw into
aluminum can do that.

So you are attempting to drive a machine screw into an untapped hole?
Please don't see this question as being confrontational; I'm just trying
to figure out exactly what's going on.

There could be a number of reasons why you need a close fitting thread.
If it's for location, try to rethink the design. Use dowel pins or a
milled slot for location, and screws in clearance holes to hold the
joint together. But if you must have precise, close fitting threads you
can buy special taps for the purpose. Screw threads are assigned various
classes of fit, which range from a close fit to a loose fit. You will
need to buy special taps for a close fit. They're used in instrument
manufacture. They'll cost more, but they are available.

If you want a close fit to prevent the screws from coming undone due to
vibration, use spring washers or loctite instead.

Either way, driving machine screws into unthreaded holes will break
them. Even if they're really good screws.

Best wishes,

Chris