Figure 4-53.  Drilling large holes. Figure 4-52.  Drilled sheet metal. 1.  Measure and lay out the drill locations carefully and  mark with crossed lines. 2.  Use a sharp prick punch or spring-loaded center punch  and magnifying glass to further mark the holes. 3.  Seat a properly ground center punch (120°–135°) in  the prick punch mark and, holding the center punch  perpendicular to the surface, strike a firm square blow  with a hammer. 4.  Mark each hole with a small drill bit ( 1 ⁄ 16-inch  recommended) to check and adjust the location prior  to pilot drilling. 5.  For holes  3 ⁄ 16-inch and larger, pilot drilling is  recommended. Select a drill bit equal to the width  of the chisel edge of the final drill bit size. Avoid  using a pilot drill bit that is too large because it would  cause the corners and cutting lips of the final drill bit  to be dulled, burned, or chipped. It also contributes  to chattering and drill motor stalling. Pilot drill at  each mark. 6.  Place the drill point at the center of the crossed lines,  perpendicular to the surface, and, with light pressure,  start drilling slowly. Stop drilling after a few turns and  check to see if the drill bit is starting on the mark. It  should be; if not, it is necessary to walk the hole a little  by pointing the drill in the direction it should go, and  rotating it carefully and intermittently until properly  lined up. 7.  Enlarge each pilot drilled hole to final size. Hole Drilling Techniques Drilling Large Holes 1.  Pilot drill using a drill bushing. Bushings are sized for  1 ⁄ 8,  3 ⁄ 16, or  1 ⁄ 4 drill bits. 2.  Step drill bits are used to step the hole to approximately  1 ⁄ 64-inch smaller than the final hole size. The aligning  step diameter matches the pilot drill bit size. 3.  Finish ream to size using a step reamer. The aligning  step diameter matches the core drill bit size. Reamers  should be available for both clearance and interference  fit hole sizes. Precise location of drilled holes is sometimes required.  When locating holes to close tolerances, accurately located  punch marks need to be made. If a punch mark is too small,  the chisel edge of the drill bit may bridge it and “walk off”  the exact location before starting. If the punch mark is too  heavy, it may deform the metal and/or result in a local strain  hardening where the drill bit is to start cutting. The best size  for a punch mark is about the width of the chisel edge of the  drill bit to be used. This holds the drill point in place while  starting. The procedure that ensures accurate holes follows:  [Figure 4-52] NOTE: The chisel edge is the least efficient operating  surface element of the twist drill bit because it does  not cut, but actually squeezes or extrudes the work  material.  The following technique can be used to drill larger holes.  Special tooling has been developed to drill large holes to  precise tolerances.  [Figure 4-53] Figure 4-51.  Bushing holder. 4-20