Countersinking, Counterboring, and Spotfacing - 4: Spotfacing
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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.

Spotfacing provides a seat or flat surface at the entrance and surrounding area of a hole. This flat surface allows the bottom of a cap screw or bolt to seat squarely with the material even if the clearance hole is not normal to the surrounding material. Spotfacing is commonly done on castings where irregular surfaces are found. Spotfacing may be performed on a drill press with a counterbore of suitable size for the operation. A proper size pilot must be used whenever this is done on the drill press. Spotfacing, when machined on a mill, a center cutting end mill may be the tool of choice. Spotfacing on the lathe is commonly done with a facing tool or form tool.

When drilling on a curved or angular surface, spotfacing is performed before the drilling operation. This process will help the drill start in the proper location and to follow-in straight; in this case, the operator will setup a milling machine with a center cutting end mill (left). The diameter of the end mill should not be smaller that the diameter of the drill. A circular face will then be machined in the proper spot before producing the hole.

Back spot facing is the same process from the inside out as shown in this video
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label:Counter sinking, counter boring, spot facing