The grooves in a rolling mill have a flattened corner at it’s lowest
point, instead of a sharp corner. This feature helps with the
extreme stress on the corners that making a square wire generates.
However, this flat bottom of each groove also works very well to
"upset" the edge of a strip of metal run through those grooves. The
larger the groove, the wider that flat bottom. This part of the mills
grooves can serve to make a strip less wide, but also to thicken
those outer edges, as well.
If that strip you are running “on edge” through a set of grooves on
your rolling mill is not thick enough, it will tend to collapse on
one side or the other, as you suggested it would. Providing the strip
is thick enough, the strip will collapse onto itself, in a straight
line, upsetting the strip quite evenly on both edges. You will need
to experiment with the strips thickness to see just how thin you can
go before it falls over to one side.
This strip, now with both edges thickened with the mill, if formed
into a ring band and hard soldered together, can be rounded up on a
ring mandril. This forming around the ring mandril with a mallet will
then force this upset strip flat against the mandril, and form the
outside face into a quite regular concave shape.
Yes, hammering can be used to upset the edge of a strip, but the
rolling mill is a fast and quite controllable way to do the same