as quenching is completed the filament curtain is directed through a narrowing lower end 30 of the quenching chamber into the impingement point slot opening 32 where the quench air attains a velocity that can be anywhere in the approximate range of 3 000 to 21 000 meters per minute the drawing slot extends across the full machine width and is preferably formed by two identical air knives 34 having an angle in the range from about 15 u00b0 degrees to 80 u00b0 with a preferred angle of 45 u00b0 spanning the width of the machine in the preferred embodiment the bottom surfaces of the knives 34 are co planar and substantially horizontal however it is to be understood that these surfaces can be angled to converge toward one another for certain applications the convergence if provided would typically be such as to provide a protruding or convex bottom surface for the chamber although in some instances a recessed or concave bottom surface may be provided the movable air knives can be retracted from one another under the chamber assembly using a manually actuable and lockable slide arrangement or a hydraulically or otherwise actuable arrangement fig1 and 2 depict knives forming a 90 u00b0 entry angle between them the blades may either slide laterally or instead pivot about a point near the upper corner of the blade whereby the tip of each blade would swing down in an arc until the upper blade surfaces came.