it is generally constructed by helically winding in opposite directions successive layers of wire over a straight central mandrel wire the flex shaft properties are tailored to the specific application by varying the number of wires in each layer the number of layers the wire diameter and the wire material in this particular application the shaft must be optimized for fatigue life number of revolutions minimum bend radius to allow packaging in the given tool diameter and for conveying thrust another concern is the shaft reliability when applying thrust to the drill bit through the shaft during drilling operations various amounts of thrust are applied to the drill bit to facilitate drilling the amount of thrust applied depends on the sharpness of the bit and the material being drilled sharper bits only require the application of minimum thrust through the flexible shaft this minimum thrust has virtually no affect on the reliability of the flexible shaft duller bits require the application of more thrust that could damage the flexible shaft one solution is to apply the thrust directly to the drill bit instead of through the flexible shaft in this method force applied to a piston located in the tool is transferred by the piston to the drill bit the thrust necessary for drilling is supplied without any effect on the flexible shaft this technique is further described in a u s patent application ser no 08 603 307 docket number 20 2650 filed concurrently with the present application a second solution is to use a sharp bit each time a.