Concentricity tolerances in deep hole drilling are achieved when the hole follows the desired axis of the part, eliminating drift from the point of entrance to the exit. In a round part with on-center drilling, this is easily illustrated; some applications may include deep holes which are off-center, or in non-round parts, but still have tight concentricity requirements.
Low concentricity in some applications can result in parts with weak sidewalls, mismatched holes, or even scrapped parts. In other cases, manufacturers may decline production of these components, because of perceived impossibility or unproductiveness. With the addition of a counter-rotating process on deep hole drilling equipment, critical concentricity tolerances can become both achievable and economical.
- Typically used for non-symmetrical components, or round parts with off-center holes
- Cutting speed is determined by tool spindle speed
- Drill drift can be significant when compared to rotating workpiece, or counter-rotating process
- Typically used for round parts with a deep, on-center hole
- Cutting speed is determined by part, balanced to allow high rotating speeds
- Drill drift is reduced compared to rotating tool only
Rotating Tool and Counter-Rotating Workpiece
- Ideal process for round parts with a deep, on-center hole
- Cutting speed is determined by a combination of tool and workpiece rotation
- Provides optimal hole straightness and concentricity
Drilling a deep hole is commonly achieved by rotating the cutting surface of a tool against the metal of a workpiece with two opposing spindles in a horizontal setup. Typically, this consists of a stationary workpiece and rotating tool, but can also be configured with a rotating workpiece and stationary tool, or a third option, with a counter-rotating tool and workpiece.
A tool-rotate configuration is the least accurate when it comes to concentricity. In this setup, gravity is believed to act on the base and shank of the tool, not the drilling tip, and along with the rotation of the tool. Because of the relative position of the tool and gravity to the workpiece, this configuration produces the poorest results. This tool-rotate process is common for shallower holes on a machining center, but as holes become deeper, and tolerances become tighter, this no longer works as a solution.
Workpiece-rotate-only setups produce holes that are approximately twice as concentric as tool-rotate. A rotating workpiece changes the relative force of gravity compared to the workpiece position, negating some of the effects on the finished hole. A rotating workpiece can be done on a lathe with limited capability, but is ideally performed on a dedicated deep hole drilling machine.
Counter rotating tool and workpiece improve significantly upon both of these, 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.
Counter-rotation is easily achievable with the right equipment and setup, whether it is for smaller gundrilled holes, or larger, longer BTA drilled components.