Enjoyed your post. I would like to just add to one of your points.
The cutter has more trouble expanding the surrounding sheet than
in the actual cutting.
This is due to the amount of clearance, or allowance between the
punch and the die.
The tool and die maker when designing a blanking tool, will
determine whether it is the outside/skeleton part, or the slug as an
example that will be saved, and therefore design accordingly to stress
the scrap rather than the required saved part.
If you have a square sheet, and you require a doughnut as an
example, this as you know, is a two stage process.
The first stage would be to cut the main doughnut diameter out of
the sheet, and this will be the saved part.
The outside skeleton will be the scrap. In this instance, the gap or
cutting allowance between the tools will be tight, and the stress will
be taken up by the scrap/skeleton, and in essence, cause the edge of
the cut diameter hole in the skeleton, to bulge up, leaving you with
a nice and clean disk/part.
This happens because the metal wants to flow outwards. In fine
blanking, they will use stinger plates to prevent this from
happening. Very important in a progressive / multi stage die.
Now the opposite comes into play when you want to get a nice hole by
now removing the slug for the center hole of the doughnut. Plastic
deformation is now transferred to the slug which of course becomes
the scrap. We do not want to see the edge of the doughnut hole bulge
like we did on the square sheet, because now the doughnut of course
is the good part. Therefore, the way to control this, is to
essentially have more cutting clearance between the punch and the
die. This causes the slug to dome downwards because the punch is
smaller, and the point of pressure engagement is away from the
cutting or supporting edge of the die. Quite obvious if you think
about it. Instead of pushing metal outwards like in the first Op, this
wants to essentially, push the whole material through the hole but
of course the shearing edge does the job before that happens. Granted
with the thicknesses used in jewellery, this is quite minor, but with
thicker material in industrial applications, it’s a big deal.
Now this of course is using a press with everything aligned on
centre. Manual stuff with a hammer etc where deflection now enters the
mix, is a whole different animal. Theory is the same, but in
practice, another story
Just thought it may be of interest.