upper surfaces of the workpiece have the same radius r and they have the same arc start angle u03b1 and the same arc open angle u03b2 this will be described in more detail in fig7 a vector a 1 a 2 and a vector o 1 o 2 are equal to the vector p 1 p 2 and thus the points a 1 a 2 o 2 o 1 form a parallelogram accordingly a vector o 1 a 1 is equal to a vector o 2 a 2 similarly the points b 1 b 2 o 2 o 1 form a parallelogram and thus a vector o 1 b 1 is equal to a vector o 2 b 2 that is the arcuate locus c 1 of the lower surface of the workpiece is equal to the arcuate locus c 2 of the upper surface of the workpiece both in arc start angle u03b1 and in arc open angle u03b2 in addition the arcuate loci of the upper and lower wire guides 2 and 5 are completely equal to the above described arcuate loci c 1 and c 2 thus the arcuate loci c 1 and c 2 of the workpiece lower.