A drilling machine is used for this operation, and a typical counterboring tool is shown in Fig. 14.26. The operation involves enlarging existing holes, and the depth of the enlarged hole is controlled by a stop on the drilling machine. 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. A typical use for a counterbored hole is to provide a recess for the head of a screw, as shown in Fig. 14.27 or a flat surface for an exposed nut or bolt, as in Fig. 14.28. The flat surface in Fig. 14.28 could also be obtained by spotfacing.
Figure 14.29 shows methods of dimensioning counterbores. Note that, in every case, it is necessary to specify the size of counterbore required. 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.
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