Center drills are used with the lathe to drill a hole in the end of stock for mounting between centers in the lathe. They are designed to drill a small diameter pilot hole followed by a 60° countersink that provides a bearing surface for the 60° point of the lathe center. Center drills are also used extensively with the milling machine to accurately start holes. The hole locations can be dialed in using the milling machine’s calibrated handwheels, then the hole is started using the center drill. The large diameter shank on the center drill holds the drill point right on center. Once the hole is started, the center drill is replaced with a drill of the desired diameter and the hole is finished. It is imperative that a cutting lubricant is used, and the center drill is backed off several times to prevent the smaller tip from twisting off.
This center drill set includes a #1, #2 and #3 double-ended high-speed steel drill. The #1 drill (P/N 30240) has a 1/8" shank, the #2 drill (P/N 30260) has a 3/16" shank, and the #3 drill (P/N 30270) has a 1/4" shank. The tip diameter of the #1 is .047", the #2 is .078" and the #3 is .109".