but all the transformers do it

(Disclaimer: I know I'm electronically impaired.)Why isn't it the
case that all transformers are not isolation transformers? The wire
in the primary and secondary coils is insulated -- and so isolated --
right? Don't the sparks only cross the divide via induction?(Talking
plain vanilla single phase VAC here.)

The answer must be something to with the grounding or treatment
of "netral" whatever that means. My AC understanding is limited to
the notion of a sine function.

In any event, I'd like to avoid laying out big $ for big isolation
transformers. I've got to knock down the service voltage (110 or 220)
to 70VAC to feed my servo amps--can an ordinary transformer be
converted to an isolation transformer by ordinary methods?

Alternative 2. Big 110 to 110 isolation units don't seem to be that
hard to find surplus, and neither do ordinary transformers (110 or
220) --> 70. So presumably one could cascade the two to make a 70V
secondary isolation transformer--but which one should go first? Does
it matter?

The little Bport CNC retrofit project is well on the way to a rack
mount cabinet. I didn't think this would happen.

Thanks in advance.