Counter-rotation gundrilling delivers required concentricity for deep hole applications in precision medical devices.

For deep holes in components to be accurate – such as surgical tooling – they need to uphold tight concentricity tolerances. In gundrilling, this is best achieved with counter-rotation. For a manufacturer, this is vital. For a surgeon and patient, it can make all the difference.

Manufacturers achieve concentricity tolerances when the hole follows the desired axis of the part, eliminating drift from the point of entry to the exit. In a round part with on-center drilling, this is easily illustrated; some applications may include off-center deep holes or holes in non-round parts that still have tight concentricity requirements.

Low concentricity in some applications can weaken sidewalls, lead to mismatched holes, or force producers to scrap parts. Adding a counter-rotating process on deep-hole drilling equipment allows manufacturers to economically achieve critical tolerances.

With the right equipment and setup, counter-rotation is possible for smaller gun-drilled holes or larger, longer, Boring Trepanning Association-drilled (BTA) components.

Deep-hole drilling machines, designed to manage accurate counter-rotation in gundrilling and BTA processes, must include suitable components, machined and assembled to maintain superior alignment.

From the machine base to rotating bearing groups and spindles, to tool and workpiece support, the machine must maintain accuracy while moving and hold concentricity tolerances throughout the hole’s depth. For deep-hole machine builders, alignment considerations begin with the base. Counter-rotation may be possible on machines retrofitted with a second rotating group, but they will often need an alignment improvement process which creates additional challenges.

A general starting point for counter-rotation is to allow one-third of the total speed to come from the workpiece and two-thirds of the speed to come from the tool, which can be adjusted for the specific application.

The ability to hold improved concentricity tolerances with counter-rotation, which consistently produces a more concentric drilled hole and typically allows higher surface cutting speeds, offers clear benefits in accuracy and efficiency. Manufacturers can increase capability, improve hole tolerances, and optimize productivity while cutting costs and gaining a competitive manufacturing advantage.

Drilling test performed on a 4140HT workpiece 30" length, 3/4" OD, 1/4" drilled hole, 120:1 depth to diameter (D:d):

Drilling holes with a 40:1 or less D:d can be done with limited drift using standard drilling practices. Higher than 40:1, counter-rotation delivers benefits with minimal drift.

A tool-rotate configuration is the least accurate when it comes to concentricity. Because of the relative position of the tool and gravity to the workpiece, this configuration produces the poorest results. The tool-rotate process is common for shallower holes on a machining center, but as holes become deeper and tolerances tighter, this is no longer a solution.

Both improve significantly as the forces are never static – changing relative gravity and orientation will provide drilling conditions without a single constant net direction that the tool will follow. In this setup, the tool is restricted from drifting, and will produce a much more concentric finished hole.