Mad said:
Dunno what was really meant, but since mashine screws often needs to
carry some load/torque, and thus be quite strong/hard, I'd guess mild
steel simply means steel without the carbon component.
What you'd really be after might be the type of soft iron used in i.e.
tranformers. Again, dunno what it's called in expert circles.
You can create soft iron from non-carbon steel by heating it to glow
temperature, IIRC.
You know, poles doesn't have to be round, so another idea might be to
call some shop repairing electric motors or building transformers, and
obtain some of that soft iron in sheet form. cut it in strips, and
laminate those with lacquer to form square shaped poles.
Note that in a laminated constructon, you'll most likely obtain the best
magnetic capabilities when keeping the strips isolated from one another,
else you may have problems with eddie currents.
Your choice of screws is a matter of easy availability plus some
adjustability, I'd guess, no?
If you finesand the finished square poles, you can make them glide up
and down with some applied force for adjustability, if need be, thiugh
once you have worked out the formula, you may not need adjustability.
Welcome to the world of science

(nope, I'm not a scientist, just
electronics educated).
You'll most likely quickly find yourself digging through books of theory
for understanding induction, capacitance, magnetic forces, hysteresis,
saturation in magnetic cores (though the latter is hardly applicable to
pups, it's good understanding it) eddie currents et al...
This is what other puppie designers have gone through, plus lotsa
experiments.
Any idea where to find machine screws
(4-40x1-1/4 round or flat head, with nut) made with this?
Mmnn, mashine tool shops, maybe?