The hard metal burrs are extremely long-lived and rust free. They have proved to be particularly effective. Working with them is pleasant and low in vibration, thanks to the precise engineering, durable sharp edge, computer guided cutting and high precision burr. For working on nails and hard skin. Recommendation: To obtain the best work results, use hard metal grinders at min. 16,000 to 18,000 rpm. Smaller grinders, e.g. the HM 296 GX 040 medium: min. 25,000 rpm. If possible, use higher speeds of 40,000 rpm.