TM 9-243 USING THE PORTABLE ELECTRIC DRILL – CONTINUED drilling thin sheet stock, back up the stock with a piece of wood to prevent the stock from being bent out of shape. When   the   location   of   the   desired   hole   is   selected,   it should   be   center   punched.      This   will   aid   the   drill   in starting  to  cut  and  will  prevent  it  from  wandering.    Turn the drill chuck a few times by hand before turning on the power   to   insure   that   the   drill   bit   is   properly   installed. Keep the drill cutting at all times while in contact with the metal.  A steady and uniform pressure should be applied at  all  times  to  insure  continuous  cutting.    The  drill  will become excessively hot if permitted to turn on the metal without   cutting   When   the   drill   point   is   about   ready   to break through the metal, ease up on the pressure.  This point  in  noted  by  the  difference  in  pressure  and  cutting feel.    Don't  permit  the  drill  to  project  through  the  hole. When the hole is complete, remove the drill immediately by  pulling  it  back  as  it  continues  to  turn  in  a  clockwise direction. A  drilled  hole  will  often  have  rough  edges  or  burrs  on both  surfaces.    To  remove  these,  select  a  drill  bit  twice the size of the hole and hand rotate the point against the burrs.    When  possible  insert  the  drill  bit  in  a  file  handle when performing this job.  Be careful not to ,burr the hole too  much.    The  hole  should  be  a  true  cylinder  and  not countersunk. When the hole to be drilled is large, a pilot or guide hole should be drilled first.  This small hole will help guide the larger   drill   and   will   also   prevent   the   larger   drill   from wandering across the metal. REMOVING RIVETS To remove a defective rivet or disassemble a unit joined by rivets, use the following procedure: 1. Select  a  twist  drill  equal  in  size  or  smaller  than  the rivet shank. 2. Drill  into  the  exact   center   of   the   head   to   a   depth equal to the head thickness. 3. Insert  a  pin  punch  in  the  hole  and  pry  off  the  rivet head. 4. Drive   the   rivet   shank   out   of   the   metal   with   a   pin punch. DRILLING SAFETY PRACTICES A   misused   drill   can   result   in   personal   injury.      When performing    a    drilling    operation,    the    following    safety precautions should be adhered to: 1. Be  sure  that  the  drill  selected  is  of  the  proper  size, free of rust, and that the flutes are clean. 2. Keep the drill bit tight in the chuck. 3. Be   sure   that   the   drill   motor   switch   is   off   when inserting the line cord into the receptacle. 4. Never use a bent drill. 5. Be  certain  that  the  cutting  edges  and  point  are  not dulled. 6. Be sure that the metal stock is properly secured. 7.     Always wear safety glasses. 8. Turn off the drill motor before laying it down COUNTERSINKS Countersinks  are  used  to  bevel  the  end  of  drilled  holes to fit countersink type screws and bolts.  The countersink stocked   at   the   sites   is   equipped   with   a   micrometer adjustment  (adjustable  to  .001  inch)  and  a  stop  pin  that prevents  the  countersink  from  cutting  deeper  than  the desired depth.  The micrometer adjustment of this tool is calibrated . The countersink is designed to hold a variety of sizes of cutting  heads,  both  in  820  and  1000  point  angles.    The reason for the two different joint angles is that, at a site, screw  and  bolts  are  available  with  two  different  types  of countersunk  heads.    The  countersunk  cutting  heads  are also  designated  according  to  the  size  of  the  hole  being countersunk,  which  is,  in  effect,  the  minor  diameter  of the countersink cuter pilot.  The purpose of the pilot is to insure  that  the  countersunk  hole  is  concentric  with  the screw hole. Change 3  52-5.1