If by uneven, you mean that the metal curves to one side, then a
feeler gauge or inside micrometer or similar should set you straight
as it were. Place slices of old newspaper cut to fit inside the
rolls, and leave a little space near the edges for measuring, apply
a small amount of pressure and take measurements at both edges. You
can then lift out the center gear and adjust each side of the roll
individually until they are equal.
If the measuring instruments are not available, then you could use
pieces of silver or copper (or any softer metal) cut as evenly as
you can. With the center gear removed, feed the metal strip through
as straight as you can and make adjustments until the metal ejects
straight. This method is only as good you can do every part
perfectly, or nearly so.