cylinder is closed with a welded in plate carrying a centered drive shaft connected through a conventional ball bearing and gear to a simple drive motor and the fixed frame supporting the whole assembly the cylinder is rotated at about 2 8 rpm usually preferably at about 3 rpm when running with yarn speeds of around 300 ypm the opposite open end of the cylinder is loosely closed against heat loss by a fixed circular collar which overlaps the interior of the cylinder about 2 inches and is welded to an end plate holding a plurality of fixed calrod heaters in place beneath the shell of the cylinder the cylinder preferably turns in the same direction as the moving yarns i e clockwise with left to right yarn travel as viewed in fig2 although the direction of rotation is not critical if the apparatus components are properly placed around the cylinder and provided that yarn speed is not so great as to cause excessive drag and subsequent yarn breaks fig3 more clearly depicts the grooves 9 circling the cylinder in close spaced circumferential lines although their width and depth may advantageously be increased for sizing larger yarn it has been found that for yarns such as 50 50 polyester rayon or 50 50 polyester acrylic in 15 to 30 singles grooves cut 10 per inch 10 15 mils wide and 50 mils deep are both effective and practical although no absolute limits are set for the depth of the grooves they are herein referred to as 34 deep 34 in the sense that they should fully enclose the yarns being sized and should have sufficient volume to provide an ample but not over large excess of size a depth of three to five times the diameter of the.