A rectangular insert of cutting material having top and bottom surfaces intersected by longitudinal edge surfaces to form four main longitudinal cutting edges. The top and bottom surfaces have ribs normal to the edge surfaces at each corner and sinusoidally spaced ribs normal to the edge surfaces starting at graduated distances from each corner rib so that when the inserts are stacked edgewise in different index positions, the corner ribs register with each other and the other ribs are out of phase with each other. The ribs of the cutting edges all extend edgewise of the bit or in the width direction so that the width of the insert is available to take the cutting thrust. When four of the inserts are stacked edgewise, each in a different one of the four index positions, the corners of the bits are all in alignment, thereby providing the finish of four identical inserts and the serrated edges provided by the sinusoidal ribs are in staggered relation so that each rib takes a fine chip which reduces the chattering.
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