![]() A counterbore is a recessed cylindrical hole with a flat bottom. It is not sufficient to state 'COUNTERBORE FOR M10 RD HD SCREW', since obviously the head of the screw will fit into any counterbore which is larger than the head.Ī counterbore callout is typically a shorthand way to describe the size, shape, and requirements of a counterbore. Note that, in every case, it is necessary to specify the size of counterbore required. 14.28 could also be obtained by spotfacing.įigure 14.29 shows methods of dimensioning counterbores. 14.27 or a flat surface for an exposed nut or bolt, as in Fig. A typical use for a counterbored hole is to provide a recess for the head of a screw, as shown in Fig. The location of the counterbored hole is assisted by a pilot at the tip of the tool which is a clearance fit in the previously drilled hole. The operation involves enlarging existing holes, and the depth of the enlarged hole is controlled by a stop on the drilling machine. ![]() ![]() ![]() A drilling machine is used for this operation, and a typical counterboring tool is shown in Fig.
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