The drills (drilling cutters) used for strain gauge centre hole drilling are usually of the inverted cone dental ‘bur’ type.  These are readily available in wide ranges of diameters and lengths.

The drills shown below are all of the inverted cone type.  This profile is most suited to hole centre hole drilling because the conical side of the drill provides clearance for the egress of drilling debris to reduce the risk of rubbing on the cut side surface of the hole which can lead to the introduction of spurious stresses.  The end surface should be flat, or slightly concave.  Tungsten carbide drills are suitable for drilling most materials including aluminium, titanium, steel and nickel alloys.  Harder materials (eg hardened bearing components or ceramics) require coated burs (diamond or cubic boron nitride).

It is a requirement of centre hole drilling that the corner of the drill (between the end and side profile) is ‘sharp’.  The sharp corner matches the geometry of the finite element models used to generate Integral Method coefficients.

Drills

The presence of significantly large radii at the corner produces smearing of data, where measured strains contain contributions from the more gradual relief of stresses within the volumes of material containing the corner geometry rather than being confined to material between the clean section changes relating to the sharp corner.

Prior to use, each drill is mounted in a tilting/rotating fixture under a microscope camera.  The flutes and cutting edges are inspected for damage, chipping and profile; the drill diameter is compared to a graticule scale to ensure conformity to the required size range.

Inverted cone drill - side view

Following the drilling process, the drill is cleaned and returned to the microscope to inspect for any excessive or abnormal patterns of wear.

Next section: Drilling Machine