Countersinking, Counterboring, and Spotfacing - 3: Counterboring
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WARNING: Machine tools present a safety hazard. Improper operation can result in severe injury. These topics are for non-laboratory study only and are not to be used in conjunction with the operation of any tool or machine described herein. Never use a machine tool without the supervision of a qualified instructor.
Counterboring is the process of enlarging the entrance of a hole with a counterbore, or other similar tool to a specified diameter and depth. Counterbores (below) provide a square shoulder so that a cap screw or bolt head may sit flush or below a surface. Counterbores are available in a variety of sizes and styles. Counterbores may have either a solid or interchangeable pilot to help guide it straight and concentric to the hole. They are held by the shank and are available with straight or Morse taper shanks.

Counterbores have two, three, or four lips or cutting edges known as flutes. Straight flutes are used for brass, spiral flutes are used for most other materials.

Care must be taken on providing solid setups and that proper speeds and feeds are selected for the material being machined. Lubrication will be called for most materials.

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label:Counter sinking, counter boring, spot facing