Prior to 1980's all sewing machines were mechanical sewing machines. We classify them as such because they form stitches using gears and levers. The difference between a straight stitch machine and a zig zag machine is a special cam gear mounted on the upper shaft to create a variety of back and forth movements of the needle bar controlled by ridges on the edge of the cam. Some machines had electric motors and some of these had special electronics to modulate the AC flow. They still used gears and levers to form stitches.