5.4 DRILLING AND SPADE DRILLING

Drilling is a machining operation for producing round holes in metallic and nonmetallic materials. A drill is a rotary-end cutting tool with one or more cutting edges or lips and one or more straight or helical grooves or flutes for the passage of chips and cutting fluids and coolants. When the depth of the drilled hole reaches three or four times the drill diameter, a reduction must be made in the drilling feed and speed. A coolant-hole drill can produce drilled depths to eight or more times the diameter of the drill. The gundrill can produce an accurate hole to depths of more than 100 times the diameter of the drill with great precision.

Enlarging a drilled hole for a portion of its depth is called counterboring, while a counterbore for cleaning the surface a small amount around the hole is called spotfacing. Cutting an angular bevel at the perimeter of a drilled hole is termed countersinking. Countersinking tools are available to produce 82 ,90 , and 100 countersinks and other special angles.

Drills are classified by material, length, shape, number, and type of helix or flute, shank, point characteristics, and size series. Most drills are made for right-hand rotation. Right-hand drills, as viewed from their point, with the shank facing away from your view, are rotated in a counterclockwise direction in order to cut. Left-hand drills cut when rotated clockwise in a similar manner.

Drill Types or Styles

• HSS jobber drills