John Doe wrote:
I'm doing an important project. Trying to thread a machine screw
into some aluminum. Halfway through, the machine screw twists apart
just below the head. The heads breaking off is not uncommon here. I
don't put all that much pressure on them, so either I'm Superman or
the machine screws I buy from Lowe's are made out of junk steel.

I guess the upside is that at least the screwheads don't strip first
/

Is stainless steel necessary, or is there an assortment of zinc
plated (or whatever similar type) machine screws/nuts made out of
decent steel someplace on the Internet or at a USA chainstore? I
need sizes like 6, 8, & 10.

I assume that you're tapping the holes first, right? If not, that's your
problem.

Screws should thread into tapped holes very easily. If your tap is
extremely worn, that might be the problem. Or you could be using the
wrong size of tap. Or you might not be cleaning out the swarf from the
hole after tapping.

But I doubt you could break even the cheapest screws by simply threading
them into correctly tapped and cleaned holes.

Best wishes,

Chris