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Initial considerations

1.The hole

The three most basic parameters for the hole are:

  • Hole diameter
  • Hole depth
  • Hole quality

The hole type and the required precision affect tool choice. Drilling can be affected by irregular or angled entry/exit surfaces and by cross holes.

Hole types


  1. Holes with clearance for bolts
  2. Holes with a screw thread
  3. Countersunk holes
  4. Holes that have a good fit
  5. Holes for tubes (heat exchangers)
  6. Holes that form channels
  7. Holes to remove weight for balancing
  8. Deep/coolant hole


2.The component


When the hole is analyzed, have a look at the workpiece material, component shape and quantity.

Workpiece material
  • Does the material have good chip breaking qualities? Long or short chipping material?
  • Machinability?
  • Material hardness?
  • Alloy elements?
Component shape
  • Is the component rotation symmetric around the hole, i.e. can the hole be machined with a non-rotating drill?
  • Is the component stable or are there thin sections that can cause vibrations?
  • Can the component be fixed in place? What stability problems need to be considered?
  • Is a tool extension needed to reach the surface where the hole shall be drilled? Need to use long tool overhangs?

The batch size affects which drill to use.

  • Large batch size - Use optimized drill, TM/Special drill
  • Small batch size - Use drills optimized for versatility


3. The machine​

It is important to know how to perform a secure and productive drilling operation in the machine. The machine influences the choice of:

  • Type of operation
  • Which type of tool holders and/or collets to use.

Always consider the following:

  • Machine stability, in general and especially the spindle
  • Is the spindle speed (rpm) enough for small diameters?
  • Coolant supply, is coolant volume enough for large diameter drills?
  • Is coolant pressure enough for small diameter drills?
  • Clamping of the workpiece, is the stability sufficient?
  • Horizontal or vertical spindle? A horizontal spindle enables better chip evacuation
  • Power and torque, is the power enough for large diameters? If not, can a trepanning drill be used or can helical interpolation with a milling toll be used instead?
  • Is there limited place in the tool magazine? Then a step and chamfer drill can be a suitable solution
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