Counterbores or spotfacers are often used to provide a flat bearing surface for nut or bolt hesds on rough castings or a raised boss (Figure H-109). This operation is called spot facing. Because these rough surfaces may not be at right angles to the pirot hole, great strain is put on the pirot and counterbore and can cause breakage of either one. To avoid breaking the tool, be very careful when starting the cut, especially when hand feeding. Prevent hogging into the work by tightening the spindle clamps slightly to remove possible backlash. Back