The ‘wavy’ part has to do with flex in the rollers. Which has to do
with how thick (diameter) they are, in relation to their width
Production mills that do this industrially look downright weird, by
manual rolling mill standards. The a roll that does 6" wide sheet
may well be nearly 6" in diameter as well. Just massively thick in
relation to their width. That way they don’t flex (much) under the
pressure of rolling sheet. Which keeps things flat.
The answer on a manual mill is to go with the narrowest rolls, with
the thickest diameters you can find, that’ll fit the work you want
to do. Then take small bites as you get closer to your target
(assuming all other factors (like quality of build) are the same.)
You can also take multiple passes at the final thickness setting.
(or a smidge above. Try it while measuring with a micrometer, and
you’ll be surprised how much more it comes down on the second pass
on the 'same’setting. That was all from flex in the rollers, and
spring in the frame.) Flip the sheet right-for-left between passes
to even out any unevenness in the roll settings. Annealing between
final pass 1, and final pass 2 will help as well. I’ve not seen a
hand mill that will give perfectly flat sheet of any great width.
Much beyond 2-3 inches wide is where things start to wave, even on
the good mills. Want to be really surprised? Roll some wide sheet,
then measure the thickness of the edges that were closest to the
sides of the mill with a micrometer, and compare that with the
thickness at the center of the sheet. Do it on final pass 1, before
you even it out with the second pass. Should give you some sense of
how much your rolls are flexing. Then measure it again after final
2. You should see a surprising amount of change, especially in the
middle. Do a third pass, and it should come in even closer.
Whether the third pass makes any difference depends on how stiff
your mill is.
I just picked up a used Durston D4, but one of the older ones with
the frame made out of steel bars. It has rollers 158mm wide
(6+inches) but they’re only 60mm thick, which means they can flex a
fair bit if you’re trying to roll a full 6" sheet. (It’s the biggest,
nicest mill Durston makes, and a very good mill, but I still had to
stop and think about it.) The consideration was that my current mill
is an older ‘unknown’ brand, probably American, probably circa 1930.
It only has 3" rolls, but they’re nearly 2" (50mm) thick. So they’re
very thick, in relation to their span. (Half the span of the Durston,
and nearly the same diameter, so roughly 3.5x the resistance to
flex.) Gives great sheet, but only about 2.5" wide. Given as I do a
lot of gold foil, where ultimate compression matters I had to think
hard about whether or not to either keep it, or get the Durston at
all. I finally decided to get the Durston, and sell off the little
one, (already gone, I think.) Frequently the right choice isn’t about
name, or size, or ‘quality’, it’s about understanding why machines
work the way they do, and which one is closest to ideal for the job
you want it to do.
PS. > As far as the PEPE mills, I don’t recall that I’ve ever used
one. I’ve seen them at shows, and they look OK as $500 units. The
’special hardening’ thing is just a deep case hardening, but RC60 at
the surface, and.120" deep is a pretty serious case. I made a set of
rollers for one of my mills (when I was young and dumb) and I only
had mine cased.050" deep, at ?RC55? or so. So that’s an indication
that they’re not fooling around. (I wasn’t fooling around either,
and they’ve gone more than twice as deep as I though was enough, if
that gives you any clue.) I have heard of issues with the gears on
the old ones, but maybe they’ve gotten that sorted? I’d certainly
look at that before I’d start looking at the Indian or Chinese ones.
They also do have a manufacturing facility here in the states.
(Oklahoma City) So it’s not all offshored. I don’t know what all
they make there, but they do make at least some things in the US
now. Surprisingly. I also know, from talking to them at shows, that
they really are trying to make the best tools they can, within the
limits of the pricepoint they’re aiming at. Take that for what it’s