. The 20th century toolsmith and steelworker; a complete, practical, and scientific book. Fig. 85. Illustrating side and end views of a machineor cross drill. Fig. 86. Showing the shape of a machine drill bit before cuttingbevel to form cutting edge. are also used in stone quarries, but when making amachine drill for soft rock such as limestone, the bitshould be made thinner and should also have a longerbevel to form the cutting edge, than a drill which isto drill hard rock. When hardening a machine drill, heat the wholebit evenly but not exceeding % of an inch back fromthe cutting edge, then
. The 20th century toolsmith and steelworker; a complete, practical, and scientific book. Fig. 85. Illustrating side and end views of a machineor cross drill. Fig. 86. Showing the shape of a machine drill bit before cuttingbevel to form cutting edge. are also used in stone quarries, but when making amachine drill for soft rock such as limestone, the bitshould be made thinner and should also have a longerbevel to form the cutting edge, than a drill which isto drill hard rock. When hardening a machine drill, heat the wholebit evenly but not exceeding % of an inch back fromthe cutting edge, then plunge into the hardening., bathand cool off entirely. Drills to cut hard rock require TOOLSMITH AND STEELWORKER 151 no temper drawn, but a machine drill to drill softrock should be drawn to a dark blue. The Breaking of Drills when Drilling and the Cause. The main cause for drills breaking is due to overand uneven heating and also by having too long aheat when hardening, but a great many times the drillwill break and although the broken fracture shows avery solid, fine and close grain
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