Jeff et al:
               You raise an interesting question.  Years ago, the
          impact of several drill sizes weighed heavier than it does
          today.  This is because in the past, most drill machines
          had 8 to 10 "pods" for each spindle into which the drills
          were placed.  When that number was exceeded, the drill
          operator had to manually remove and replace drills to the
          larger sizes, and then remember to replace the smaller
          drills before pressing that start button again (or face
          making scrap).  Newer drill machines have rows of pods in
          which to place these drills.  Operators are encouraged to
          touch the drill as little as possible.  There are still
          issues with multiple drill usage:
               
 setup time is increased
               
 drills that still have good life to them are
                 repointed or trashed prematurely if there are only a
                 few hits used per cycle on a particular size that
                 could have been combined with another
               
 run time can increase, with more individual drill
                 paths to complete, and drill changes for the machine
                 to go through
          The number of drill sizes is still a question asked on RFQ's
          (requests for quotations), so this somehow still works its
          way into the price you are charged for a circuit board.
               It is quite common for board houses to use a drill bit
          that is .005" over the nominal finished hole size for plated
          holes.  It is good practice to ascertain a list of
          available drill sizes from your board supplier(s).  Just
          about any size is available, but special sizes usually carry
          a premium from the drill supplier.
               As Columbo would say, "Just one more thing....":
          Drilling is the second-highest expense when it comes to
          fabbing a board (usually about 10% of the board cost).
          Regards,
          Tom Coyle
          Field Services Engineer
          HADCO Corporation

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