Randy Replogle wrote:

snip

I suspect that you don't even know what you don't know.

Drill and tap the "bottom" part correctly for a machine screw of your
choice. Drill a hole in the "top" part slightly larger than the body of
said screw. Place the screw through the top part and turn it into the
threaded hole of the bottom part. The top part will now be held tight to
the bottom part by the head of the screw.

That's another possibility, that he might be tapping the holes in both
parts. If so the starts and ends of the threads may not be aligned, so
that the two parts act like a locknut. If this is the problem, only tap
the hole in the part furthest from the head of the screw. Drill a
clearance hole in the other part.

Best wishes,

Chris