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Would this work? (air gap in ballast core)

Original poster: "J. Aaron Holmes" <jaholmes@xxxxxxxxxxxxxxxxxx>

I recently mailed the list about making a ballast from
one of two large "C" cores, extracted with much
physical effort from a junked 15kVA pole pig.  I have
the cores free and all cleaned up now, but making an
air gap is going to be difficult.  The layers of steel
are strapped together and the gaps in each layer are
staggered such that making them all align would
required re-bending each layer (visualize a roll of
steel tape and you'll have a good idea what these
cores look like).  I'd like to avoid that.  In fact,
if I can, I'd like to wind the ballast without opening
the core.  Fortunately the window appears large enough
for me to get away with it.

My question:  What if I were to wind on the "C" core
without opening it, then drill holes in it
(essentially perforating it)?  Would this work?
Basically, I'd put some small 60Hz voltage across it,
drop it on the drill press, and drill down through the
laminations again and again (forming a nice line)
until the current through the thing indicated that I'd
reached the desired inductance.  Seems like this would
work....any thoughts?  I understand this'll make it
pretty hard to adjust the thing down the road, but I'm
not going to worry about that.  I've got two of 'em
anyway :)