1,fast ,smooth and long lifespan 2,Stable quality for long term 3,free shipping 4,technology for bit using by cnc engraver Diamond Engraving Tool 30 degreeC series: Smelting and Fusing Diamond grinding head, making by military high-tech " Smelting and Fusing Diamond technology" . It is more sharp ,higher efficiency , not deformation and precise engraving .So, it is better for engraving marble, ...