Q&A: Deep-Hole Drilling

Deep-hole drilling is virtually an art unto itself, with a host of complexities and considerations inherent to the process. Greg Forman, technical director of toolmaker Precision Cutting Tools Inc. (PCT), Santa Fe Springs, Calif., answered questions to provide some insight into drilling holes greater than 5 diameters deep.

Greg Forman, technical director of Precision Cutting Tools Inc. Images courtesy PCT.

Cutting Tool Engineering: How can chip control and productivity be addressed when deep-hole drilling?

Greg Forman: Different flute helixes and point combinations can handle a specific material’s chip formation. Deep holes have previously been handled with tubular-body gundrills that have single straight flutes and create small, sliver-like chips. When used with slow programmed feed rates, they control chip evacuation very effectively across many material groups. Solid-carbide and carbide-tipped, straight-flute drills offer a second cutting edge that, compared to gundrilling, can increase productivity. However, chip formation gets trickier, which limits the materials that can be handled effectively compared to using gundrills. Carbide-tipped, spiral-flute drills help in limited applications, but the inherent design of the thin carbide tip does not allow for the more-aggressive point geometries required to break the chips in many materials.

CTE: When should twist drills be applied for deep-hole drilling?

GF: There is an increasing opportunity to introduce solid-carbide twist drills when the customer moves deep-hole drilling from a gundrill machine to a CNC machine that allows more-aggressive speeds and feeds. However, depending on the material, a twist drill’s smaller flute area can be restrictive compared to straight-flute drills and gundrills, potentially lacking the ability to bend and break chips into a controllable size and shape for evacuation. If chip control is not consistent, intermittent chip packing and recutting can severely decrease tool life and increase the risk of catastrophic failure. That said, with many new “designer” carbide grades available for spiral-flute, deep-hole drills, we are not finding any material limitations.