A pattern of small diamond-shaped points cut into the surface of a clamp die or a clamp insert to increase its grip; the knurled points bite into the tubing material, thereby improving the clamps hold upon the tube. Knurling is functionally the same as serrations and carbide impregnation, but is not generally preferred. The points wear rapidly in comparison and cut the surface of the tube, which can exacerbate fatigue or corrosion of the end product.