Page 109 - CITS - Fitter Trade Pactical - Volume - 2
P. 109

FITTER - CITS




           Set the spindle speed of the drilling machine RPM. Use the  formula

           Substitute the recommended speed of the countersink. (V = 1/3rd of the cutting speed for drilling)
           Countersink hole to a depth equal to the head length of  the   screw head. (Fig 3)
           Check the countersink hole with a suitable countersink  head screw for proper seating. (Fig 4)

              Fig 3                                       Fig 4














            Counter boring


           Objectives : At the end of this exercise you shall be able to
           •  state counter boring.


           Selection  of counter bore  sizes
           •  B.I.S. recommends different sizes of counter bores based  on the sizes of the clearance holes.

           •  Select the counter bore according to the screw size.
           •  Fix the job in the machine vice, square to the axis of the  machine spindle. Use parallel blocks. (Fig 1)
           Set the location of the drilled hole position using the correct diameter drills.
           Align the spindle axis with the drilled hole. For accurate work, drill and counter bore in one setting.

           Mount and fix the counter bore  tool on the drilling machine spindle. (Fig 2)

             Fig 1                                        Fig 2













           Set the spindle speed of the drilling machine to the nearest calculated RPM. Use the formula
                            x  d  x  n
                      V =
                           1000
           (Consider the value of `V` as 1/3rd of the cutting speed  for drilling)
           Counter bore the hole to a depth slightly more than the thickness of the screw head (Figs 3 & 4)
           Use the depth stop arrangement for controlling  the depth of the counter bore hole.

           Check the depth of the counter bored hole. (Use the correct  screw for checking the depth and seating).






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                                            CITS : CG & M - Fitter - Exercise 31
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