I'll note along with the others that a 1/2" bit needs to have 4x the
pressure as well as 1/4th the speed of a 1/4" bit. The little 1/4" bit will
dig in very nicely with the pressure but the 1/2" bit will really need to
have a lot more pressure in order to dig in properly.
Also going too small in the steps of hole sizes does tend to dull the outer
part of the cutting edges relative to the center. Drill a hole that is a
little bit larger than the center part of the bit and then use the larger
bit to do the hole.
Finally, I'll note that with a 1/2" bit, the angle iron gets to be a bit
thin so you have to start using thin metal techniques to drill such a hole
otherwise the bit will grab the angle iron and spin it.
A half inch unibit is probably going to work best in this application.
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Why isn't there an Ozone Hole at the NORTH Pole?