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Drilling

 

Process description

The removal of material by chip processes using rotating tools of various types with two or more cutting edges to produce cylindrical holes in a workpiece.

Materials

All metals (mostly free machining) and some plastics and ceramics.

Drilling

Process variations

  • Variations on the basic drilling machine include: bench, column, radial arm, gang, multiple spindle, turret and CNC controlled turret.
  • Variations on the basic drill types include: twist drill (either three flute, taper shank, bit shank and straight flute), gun drills, spade drill, indexable insert drill, ejector drill, hole saw, trepanning and solid boring drill.
  • Variations on conventional drill point geometry are aimed at reducing cutting forces and selfcentering capability and include: four facet, helical, Racon, Bickford and split point.
  • Wide range of cutting tool materials are available. Titanium nitride coatings are also used to increase tool life.
  • Drilling can also be performed on lathes, milling machines and machining centers.
  • Spot facing, counterboring and countersinking are related drilling processes.

Economic considerations

  • Production rates ranging 10–500/h.
  • Lead times vary from short to moderate. Reduced by automation.
  • Material utilization is very poor. Large quantities of chips generated which can be recycled.
  • Flexibility is high. Little dedicated tooling and generally short setup times.
  • Drill jigs facilitate the reproduction of accurate holes on large production runs.
  • Production volumes are usually low to moderate. Can be used for one-offs.
  • Production costs are significantly reduced with multiple spindle machines when used on large production runs.
  • Tooling costs are low.
  • Equipment costs are low to moderate, depending on degree of automation and simultaneous drilling heads.
  • Direct labor costs are low to moderate. Low operator skill required.
  • Finishing costs are low. Cleaning and deburring required.

Typical applications

  • Any component requiring cylindrical holes, either blind or through
  • Engine blocks
  • Pump components
  • Machine components

Design aspects

  • Complexity limited to cylindrical blind or through hole.
  • Standard sizes used wherever possible.
  • Faces to be drilled usually required to be perpendicular to the drilling direction unless spot faced, and adequate clearance should be provided for.
  • Exit surfaces should be perpendicular to hole.
  • Through holes preferred to blind holes.
  • Allowances should be made for drill point depths in blind holes.
  • Flat-bottomed holes should be avoided.
  • Center drilling usually required before drilling unless special drill point geometry used.
  • Holes with a length to diameter ratio of greater than 70 have been produced, but problems with hole straightness, coolant supply and chip removal may cause drill breakage.
  • Sizes ranging from Ø0.1mm for twist drills to Ø250mm for trepanning.

Quality issues

  • Machinability of the material to be processed is an important issue with regards to: surface roughness, surface integrity, tool life, cutting forces and power requirements. Machinability is expressed in terms of a ‘machinability index’* for the material.
  • Hard spots, oxide layers and poor surfaces can cause drill point to blunt or break.
  • Accurate re-grinding of the drill point geometry is required to maintain correct hole size and balance cutting forces to avoid drill breakage.
  • Rigidity of drilling machine, workpiece and drill holder and concentricity of drill spindle are important in preventing oversize holes, chatter and poor surface finish.
  • Selection of appropriate drill geometry (including relief and rake angles), coolant/lubricant, size of cut/hole, feed rate and cutting speed with respect to material to be machined is important.
  • Drills may require chip breakers for ductile materials to efficiently remove swarf from cutting area.
  • Coolant also helps flush swarf from cutting area in long through holes, and blind holes.
  • Surface detail is fair.
  • Surface roughness values ranging 0.4–12.5 µm Ra are obtainable.
  • A process capability chart showing the achievable dimensional tolerances is provided . Note, the tolerances on this chart are greatly influenced by the machinability index for the material used.

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