Sheet Roll bending machine

Found a few surplus bits of 20mm round linear rail and thought I'd have a go at building a sheet roll-bending machine. It's for light ally.
Had a look, and there seems to be several roll placement and gearing systems. Hope this next bit isn't too confusing to read, it wasn't easy to write.
Given one powered roller:
On one system there are two rollers vertically above each other, which are tightened together until the sheet fits snugly between them. These are geared together with two gears on one end of each shaft. There is a third unpowered roller whose axis is is movable and which does the bending. The sheet curls round the top geared roller.
On another system there are two rollers horizontally next to each other with a larger fixed space in between. These are geared together with a 3-gear chain, so they each have a gear on the end and there is a third gear which is driven by one roller and drives the other. There is a third unpowered roller whose axis comes down between the powered rollers. The sheet curls round the third unpowered roller.
The two geared rollers turn in the same direction in this second system, unlike the first system where they turn in opposite directions.
A third type has no gearing.
Questions, are there any more systems?
Which system is best?
Is there any system which doesn't leave unbent edges?
How does the gearing in the first two-gear system work, if the separation is variable? Do they use special gears?
I though a couple of standard 10-tooth mod 2 gears might work, pressure circle is 20mm same as the rollers, outer diameter is ~23mm, and if they were separated by a mm or so more than nominal to allow for the thickness of the ally sheet they would still mesh. If I used a smaller mod the possible separation range would be less. Opinions?
Peter Fairbrother
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It is thought that the ones with the two rollers pinching and driving the work and a third (and fourth, on the other side of the driving ones for big sets) do a better job of avoiding flats at the end of the workpiece.
There is a further tweak on the vertically geared setup, produced by G.H.Thomas, that is to use four gears to link the two rollers together, with two idlers in series mounted on links to keep the gears' spacing constant. The advantage of this is that as the pinch roller is moved closer and farther from its other half, the gear meshing is kept constant instead of varying to the point where it's hard to mesh the gears. In ASCII art (fixed width font) form it's like so:-
O-O | O-O
Picture is here:-
Regards Mark Rand

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