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 don't know if you can find any commodity-grade stuff that is
much better. You can get certified aircraft-grade fasteners
from an approved source, but they will cost a lot more. Still
have to watch out for counterfeit stuff there, too.

I buy stuff like this in 100-piece boxes. For electronic-style
stuff (ie binding head phillips machine screws), I buy it from
Digi-Key. Lately I have had to go to stainless for ROHS
reasons, the zinc-plated stuff is non-compliant for some reason.

For other stuff like socket head cap screws and button head
(Allen) screws I get them from MSC. I have had some with no
threads, way off-size allen sockets, etc. But, I have had no
problem with screws breaking off - from either source.

6-32 is the weakest standard screw thread, due to the ratio of
minor diameter to major dia., so if that is the size you are
using, it's an added complication. You might also want to check
the quality of your taps, the way you use the taps, the size of
the pilot hole, etc. The screws should go in effortlessly. If
there is significant binding, then the thread is off in some
manner, or the hole isn't clean. I use spiral-flute taps (not
spiral POINT) as much as possible, they can be run in in one
continuous motion, producing a chip similar to a long, curly
drill chip. This makes it much easier to clean the hole after
tapping. I rip a little piece of a paper towel off and twist it
into a thin cone and poke it into the hole and twirl it to get
the chips out.
(Professional shops use compressed air, but my family would
object to this late at night.)

Jon