Synchronous Polygon Turning for Implant Manufacturing

A complex machining process is proposed to generate the shank of a dynamic hip lag screw multi-faceted with both planar and cylindrical surface types. By installing a power driven tool on each of two sides of a workpiece, a synchronous polygon turning process is setup to complete the prescribed cutting in a single stroke. The tool on one side is to cut the planar faces using the conventional polygon turning process. Under the constraints of the above polygon turning, the tool on the other side demands special assignments for polygon turning of cylindrical faces. Assigned with a non-integer cutter-disk/spindle speed ratio and a number of blades mounted on a cutter disk, enormous number of line segments can be generated to approximate circular cuttings. The use of non-integer speed ratio enables the generation of enormous number of sides with relatively low speed ratio and few number of blades. This research also simulates the motion paths of blade tips and calculates the error between the approximate curve and true circular arc under the arbitrarily assigned parameters of blade numbers and speed ratios. For implementation, bars of titanium alloy Ti-6Al-4V and stainless steel 316L were used to generate orthopedic implants, dynamic hip lag screws, on a Swiss lathe. This synchronous polygon turning has shown the efficacy and efficiency on the cutting of screw shanks.