Herein disclosed are a method of forming a rectangular groove having accurately rectangular cross-section at an apex of an angular workpiece, and a combination tool therefor. The method comprises the steps of broaching the apex portion of the angular workpiece to form a preparatory dove-tailed groove having its bottom width substantially equal to that of the finally obtainable rectangular groove of a desired width and its top width slightly smaller than said desired width, and burnishing said preparatory dove-tailed groove to finally obtain the rectangular groove having its bottom and top of said desired width. The method may further comprise the step of initially broaching the apex portion to form a preparatory rectangular groove having its width slightly smaller than the top width of the dove-tailed groove.