CRL PD and PDT Series Metal Bond Diamond Drills use a special diamond and brass bond for maximum performance in glass. Metal Bond Drills offer the longest life of any type of diamond drill; 1,000 to 2,000 holes in 1/4" (6.4 mm) glass with proper maintenance and the use of a wet Sharpening Stone to expose fresh diamonds. These Italian made multi-layer drills cut faster and run cooler than any other drill on the market. They have a full 3/8" (10 mm) deep cutting surface filled with diamonds and bronze bond, which allows for extended drill life, but also makes this style of drill more fragile. They should always be used in true running machines on flat glass surfaces. The CRL PD Series features an extra long 1-3/8" x 3/8" (35 x 10 mm) diameter shank for a positive grip on the drill. The CRL PDT Series features Belgian style mounting threads, also known as 1/2" (12.7 mm) gas thread, which are popular among all U.S. and European glass drilling machines. They can be used in portable drill machines such as the CRL AMZ1, but they are especially suited for production equipment.