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Countersinking, Counterboring, and Spotfacing

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COUNTERSINKING, COUNTERBORING, AND SPOTFACING are three machining operations used to enlarge the opening of a hole. In countersinking, a conical, reamerlike tool is used to cut a tapered enlargement at the opening of a hole for receiving the head of a fastener, for receiving a center, or for deburring. The surface cut by the tool is concentric with the hole and at an angle of less than 90° to it. Counterboring and spotfacing do not produce a tapered hole enlargement; instead, these techniques enlarge the hole to a given diameter. The difference between counterboring and spotfacing is that a counterbored surface usually has a shoulder at the bottom of the enlarged hole, while a spotfaced surface is flat and always at right angles with the axis of the hole. In addition, the depth of cut in spotfacing is usually shallower than that in counterboring. Countersinking, counterboring, and spotfacing operations can be carried out on drilling machines and usually follow (or are combined with) drilling operations. Spotfacing, however, sometimes precedes drilling. This is done to provide a contoured workpiece with a flat surface in order to facilitate centering and starting of the drilled hole.
  • From: ASM Handbook Volume 16, Machining (ASM International)
  • Published: 1990
  • Pages: 3
  • Review Type: Peer reviewed