The modern gundrill is an engineering high tech marvel, a well-designed piece of equipment that does one thing exceptionally well: producing round, straight holes with enhanced cylindricity even at its deepest points. And it does all this while simultaneously providing a fine I.D. finish and excellent tool life.
Like all tools, gundrills wear out. While a talented operator can still drill a hole with a worn gundrill, it will more often result in a loss of hole tolerance and finish at best. As gundrills wear, they require more thrust and torque while producing more run-out and experiencing greater drift . A dull cutting edge will produce irregular chips, which in turn cause spikes in coolant pressure – sure signs that failure is imminent.
Unlike some tools, gundrills are excellent candidates for resharpening. When performed correctly, the same gundrill can be resharpened to perform as well as a new drill as many as 8 to 10 times. The only visible difference will be seen in the length of solid carbide on the gundrill’s tip.
Even coated drills can be sharpened. Naturally, this will reveal the raw carbide on the face, but this does not impact performance. The coating will remain on the wear pads and continue to improve the gundrill’s size control and ability to leave behind a finished surface . Tool life will be impacted, but the only other option is having it fully resharpened and re-coated by the OEM, which will likely be less cost effective.
Straightness control in deep hole gundrilling of is challenging. The drills are found to degrade rapidly on the cutting edges, bearing pads and side margins, largely due to the extreme heat resistivity of the high temperature superalloys. Severe adhesive wear developed on the rake and flank faces deteriorates cutting efficiencies of the cutting edges while diffusive wear on the bearing pads and side margins deteriorates self-piloting efficacies of the drill. Coupled with highly irregular wear rates on the inner and outer cutting edges, the drills are forced against the hole at high rotational speeds – leading to escalations in frictional contact, heating and thermal damage on the bearing pads and side margins. As a consequence, the drills are deflected from the designated drilling course and resulted in straightness deviation on the part of hole drilled. Through the accumulation of partial straightness deviation over the course of high aspect ratio drilling, the final hole produced is deflected in a constant trend as governed by the rate and behaviour of tool degradation.