A method and apparatus for preparing a sheet workpiece (122) comprises a pair of rollers (142, 144) defining a nip (146) therebetween through which said sheet workpiece (122) passes in a single pass, and means (56, 58) for driving said opposed rollers (142, 144) in opposite directions and at sufficiently different lineal surface velocities for said sheet (122) to be both milled and stretched simultaneously in said nip (146), as said sheet workpiece passes through said apparatus in a predetermined direction of travel. Modifying means (143, 188, 190, 192, 211, 213, 227, 235, 237, 239) to modify the sheet workpiece upstream or at the said nip (146) so that, on leaving the said nip (146) the sheet has an increased degree of uniformity. The modification comprises heating means (143, 188, 190, 192) for heating an outer surface portion, especially an edge portion of said sheet workpiece (122) which is not disposed in contact with a first (142), upstream one of said pair of opposed rollers to a temperature level which is greater than the temperature level of an inner surface portion. This ensures that the temperature levels of said inner and outer surface portions of said sheet workpiece (122) are substantially the same whereby the density of said sheet workpiece (122) across its thickness dimension is substantially uniform. Alternatively, the modification comprises providing the workpiece (122) with a configuration in which its edges are thinner so that after passage through the nip (146) it has an increased degree of flatness. IMAGE
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