Straddle milling is a CNC milling operation in which two or more parallel vertical surfaces are machined in a single cut using two cutters separated by spacers, washers or shims. For completing straddle milling, two side milling cutters need to be mounted on the same arbor, and set apart to make them straddle the workpiece. Two sides of the blank are cut simultaneously, and the dimensions are exactly controlled. Straddle milling can be used in cutting spline, or squares and hexagons on the end of a cylindrical part. Straddle milling uses sharp milling cutters on either side, it is possible to machine the pieces on opposite sides to parallel in a cut. The indexing head will be utilized to rotate the piece after each cut.
Straddle Milling Diagram
Here is a simple image can help you figure out how does straddle milling work more intuitively.