Kum is the only manufacturer worldwide to equip their sharpeners with a curved blade. Due to the curved knife, the knife edges are under constant tension and are not lifed by the pressure during sharpening. This prevents the pencil tip from springing out or tearing. All sharpeners are equipped with ultra hard (65 hrc) and razor sharp high carbon steel blades. These blades are harder than ball bearings or any high quality knife. This is needed because pencil leads are made of a mixture of graphite and various types of clay which are extremely hard. Each blade is razor-sharp. A felt sharpening wheel (similar to leather) is used as the last step in the automatic sharpening process. Note: Stainless steel cannot be used because it cannot be sharpened that much.