What I mean is that putting feeler gauges or pieces of paper in between rollers to check their alignment is just plain silly.
Actually, all the methods described so far, including mine with paper
are workarounds of the only correct way is to use toolmaker gap gage,
or feeler gage if you will. It is quite expensive in good quality and
have no other use for goldsmith, so other methods were developed.
Leonid Surpin
I have another theory about this.
When same-sized gears are correctly meshed, any rotation of the
driving gear will be faithfully reproduced in the driven gear. Gears
are correctly meshed when their Pitch Circle Diameters (PCDs) are
tangential; the gear teeth then roll against each other rather than
slide.
In small rolling mills the PCDs of the gears are tangential at only
one point of separation of the rolls. If you adjust them closer or
further apart then the gears will not be in perfect mesh and the two
rollers will not rotate in perfect unison. For each tooth, the driven
roller will speed up and slow down relative to the driving roller,
and it’s this mismatch that causes the wavy effect.
I’m not suggesting that this is the entire cause of waviness, but
I’m pretty sure it makes a contribution…
Regards, Gary Wooding
What I mean is that putting feeler gauges or pieces of paper in between rollers to check their alignment is just plain silly.
Often, yes. It’s useful, though, for some things, including
detecting the degree to which the rolls are compressed slightly in
the middle rather than flat. (see below…)
Your average under ten years old mill will be just fine with a visual.
Of course. Who said I was talking about new mills? My own favorite
flat hand rolling mill was made somewhere around 1920, I think. We’ve
got another, virtually the same, where I work, as well as an old as
the hills power mill there too. None of these three have actual
springs holding up the upper roll. Instead, they have a strap
attached to the bearing, that holds the roll amost up in contact with
the bearing, but only barely. You can’t adjust the gap on these just
visually. You have to put a slight load on them.
I grant you that even then, you’re right that you don’t need feeler
guages. If you roll a piece of sheet, and it curves to the right or
left, adjust the top gears accordingly to fix. Don’t even have to
look at the rolls. It might take a couple tries to home in on the
closest adjustment, and with old mills where there’s some wear on the
rolls, you might have to take almost perfect instead of absolutely
perfect.
bottom line is that there are a number of ways to adjust a mill.
Seat of the pants by seeing what’s not going straight, or visually,
or with feeler guages, or who knows what all else.
By the way, one other point that nobody’s made yet in this thread is
that sometimes, you may intentionally want the mill to be
misadjusted. If one side of the rolls is tighter than the other, the
metal rolled through it will curve away from the tight side. Bending
a piece of sheet metal to the side, in the plane of the sheet, is
difficult. You can do it with hammers, forging one edge and side to
stretch it, but not the other, so it curves. If it’s a narrow strip
of thicker guage metal, you can sometimes figure out how to get it to
just bend without buckling anyway, but for wider or thinner sheet,
the easiest way is with the rolling mill adjusted tighter on one
side. How much tighter you do experimentally with a piece of scrap.
Oh, and if you’re starting off with a properly adjusted mill, use a
marker to mark the proper position of the gears so that when you’re
done, it’s easy to put them back where they were.
Why would you do it this way? Well, consider how you’d make a sheet
metal blank that will bend around into a cone shape, rather than a
cylindar, such as you might wish to do for, say, a neckpiece made of
wider sheet metal. You can, of course, saw out the arc shaped blank
from wider sheet, but for something larger like this, that needs a
lot more metal. Doing it with the mill, if you get it right, you can
make that preform or blank with no waste…
Peter
Peter:
Your point about deliberately canting the rolls on a mill is
outstanding. I’ve leveled out more mills than I can count, and I use
little strips of brass to do it, so I’ve seen lots of little flat
curved strips. Yet it never crossed my mind to use that ability.
Outstanding! (in my own defense, I rarely make things that use that
form of a blank, but still.)
Learn something new every day…
Brian.
people,
wavy metal from rolling could well mean your platens are wonky -
re-align them and check alignment often. also making many
sequential, minor adjustments and reducing torque to decrease gauge
could keep the thick sheet from screwing up (a technical term) the
platens’ alignment. rolling mill diet: better to nibble away with
small bites than to choke on a huge chunk.
ive
to avoid embarrassment: think, people, think!
In small rolling mills the PCDs of the gears are tangential at only one point of separation of the rolls. If you adjust them closer or further apart then the gears will not be in perfect mesh and the two rollers will not rotate in perfect unison. For each tooth, the driven roller will speed up and slow down relative to the driving roller, and it's this mismatch that causes the wavy effect.
Yes, there is a truth to that. All mills have a gap at which metal
will be warped. This gap is usually way out of useful range. On my
mill it is around 2.5mm. After, metal straightens out. If mill
continuously warp the metal, there is definitely something wrong -
either with the mill or technique.
Leonid Surpin
The jeweller’s rolling mill is a wonderful tool with limitations.
It’s a crude step much like a hammer and anvil only faster and more
uniform in accuracy. It can be utilised for very good accuracy but
this will take more time, care and understanding. A wide faced
hammer or a mallet and a flat anvil will get more flatness after
rolling in the mill, but at the expense of surface finish. A file
will get even more flattness also at the expense of surface finish.
Hand lapping will get ultimate flatness together with the perfect
polish.
The factors mentioned in previous replies all play their part and
here’s another…
The gears that transfer the force from the bottom cylinder to the
top cylinder may be the cause of wavey metal. Older and/or cheaper
rolling mills have a single row of gear teeth attached to each
cylinder. The teeth are fully meshed when the cylinders are touching
each other, but as the gap between the cylinders is widened the
teeth are meshing differently and at the widest opening the gear
teeth are meshing only on their tips. During the meshing one pair of
teeth let go while the next pair engage. The teeth cannot mesh
perfectly unless their centres are locked at a fixed distance. A
rolling mill must make do with a variable distance and as a
consequence the top cylinder will not move exactly the same as the
bottom cylinder except at the smallest gap. If the two cylinders are
out of sync by a tiny amount then the metal will be wavey.
Edward, I also practise using the power stroke in different places
to spread out the wear. Another thing to be aware of is rolling
Forwards & Backwards at less than a full turn of the cylinder.
Rolling forwards squeezes some of the oil out of the bearings.
Rolling backwards immediately means you are relying on the residual
oil layer. Going forwards and backwards within a quarter turn of the
cylinder will soon deplete the oil film and wear out the bearings in
your favourite F&B place. I like to give the handle a full 2 turns
(I have a 4:1 reduction gearbox) every now and then when doing the
F&B.
Regards, Alastair
I don’t know from rolling mills much but it seems to me that if your
bearings are sloppy not only will you get up and down slop but also
slop in regards to the two rollers’ axis being parallel when viewed
from the top. I would guess it gets worse as the rollers get smaller
and handcranking gets more uneven. That might be why industrial
mills are huge and very expensive.