METAL CUTTING: Drills

METAL CUTTING: Drills

1. DRILLING

   • process of producing round holes in a solid material or enlarging existing holes
     • using a multi-tooth cutting tool
       • drill bits
   • Most common
   • workpiece is typically stationary
   • chips are produced within the workpiece and move in opposite direction of the drilling feed
   • Application of coolant fluids are important
     • temperature increases rapidly
   • Drill bits are guided by the margins that rubs against the hole surface
     • low surface quality
   • rack angle and cutting speed of the cutting edges vary from the centre to the edge of the drill
   • leaves a burr on the bottom surface
     • may need deburring operations

2. BASIC OPERATIONS

   • CENTER DRILLING
     • drills a starting hole to accurately establish its location for subsequent drilling
   • COUNTERSINKING
     • the step in teh hole is coneshaped for flat head screws and bolts
   • SPOTFACING
     • Provides a flat surface on the workpiece in a localised area
   • COUNTERBORING
     • Provides a stepped hole, in which a larger diameter follows a smaller diameter partially into the hole
       • used to seat bolt heads into a hole so the heads do not protrude above the surface

3. DRILLING MACHINES

   • RADIAL
     • used for large workpieces
     • spindle mounts on radial arm allow drilling operation anywhere along the arm length
     • spindle feed can be manual or automated
     • table may be swiveled to enable drilling on an angle
   • NUMERICAL CONTROL
     • movements are controlled automatically by computer
     • Multiple operations can be done sequentially
     • Turret may hold multiple tools and rotate to position the required tool
   • MULTIPLE SPINDLE
     • Single drive motot
     • Multiple spindles cut at the same time
     • feed rates are teh same for all drills
   • DRILL PRESS
     • basic drilling machine
     • used when location and orientation of the hole must be controlled accurately
     • composed of a base that supports a column which in turn supports a table

4. PARAMETERS

   • Revolution speed
     • Rotational speed of the drill
     • N - rpm
   • Cutting Speed
     • The speed is not constant along the cutting edge. It is zero at the cetre of the twist drill and has a maximum value at the drill corner
     • V - mm/min
   • Diameter
     • Size of the drill lip outer diameter
     • D - mm
   • Feed Rate
     • Linear speed of the drill along side of the drilling axes
     • fr - mm/min
   • Feed
     • Distance that drill penetrate to the part per revolution
     • f - mm/rev
   • Material Removal Rate
     • Volume of material removed from workpiece per unit of time
     • MMR - mm^3/min
   • V = PI()*D*N
   • MRR = (PI()*D^2/4)*f*N

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