Function, Requirements, and Limitations
Deep-hole drilling is considered one of the most challenging disciplines in manufacturing technology and is indispensable in the production of ball screws. Whether for ball recirculation, lubricant supply, or liquid cooling, several core functions of a ball screw require holes with a diameter-to-length ratio that far exceeds what conventional drilling technologies can achieve.
Definition and Classification
Deep-hole drilling refers to any hole with a diameter-to-length ratio (D/L) of 10:1 or greater; in other words, the drilling depth is at least ten times the diameter of the hole. At this ratio and beyond, conventional twist drills fail in several respects: chip removal is no longer guaranteed, tool guidance along the length of the hole deteriorates, and heat dissipation from the contact zone can no longer be controlled with standard coolants. For deep-hole drilling, specialized methods are therefore used, in particular single- and double-flute drills, in which coolant is fed under pressure through the tool shank and the chips are removed via an internal chip groove or an external chip removal tube.
The key process parameters—cutting speed, feed rate, coolant pressure and flow rate, as well as tool guidance—must be individually adjusted to the material, bore diameter, and D/L ratio. Tool rigidity and elasticity play a decisive role in maintaining the required bore profile: A tool that is too rigid follows geometric irregularities in the workpiece and generates stresses, while one that is too elastic deviates from the target profile and causes profile errors.
Applications for Ball Screws
Ball Recirculation (D/L 10:1 up to 25:1)
The rolling elements of a ball screw travel along the load-bearing raceway and must then be returned to the start of the nut via a return channel. In the case of internal deflections, this return is achieved via deep-hole bores in the nut body, which guide the ball channel axially through the nut body.
The geometric requirements for this bore are stringent: The bore profile must fall within defined form and position tolerances, as deviations lead to uneven ball entry, increased noise levels, and wear in the entry area. Typical D/L ratios range from 10:1 to 25:1—a range that can be managed with the appropriate tooling and a standardized process, but which requires consistent measurement procedures to monitor the bore profile.
Ball-Return Bore: The KGT 100×25 as an Example
Lubricant supply (D/L ratio up to 40:1)
To ensure targeted lubrication of the raceway, lubricant channels are incorporated directly into the nut body via deep-hole drillings. These channels deliver the lubricant to specific locations along the thread, thereby ensuring an even supply to the rolling contact zone.
The D/L ratios here can reach up to 40:1. Compared to a ball-return bore, the requirements for the bore profile are less critical, since no defined ball transition needs to be created. Nevertheless, the positional accuracy and angular tolerance of the bore are crucial for reliably controlling lubricant distribution.
Deep-hole drilling of a KGT spindle for a cooling system
Cooling Systems: Nut and Spindle (D/L up to 400:1)
Applications subject to high thermal loads require active liquid cooling to minimize thermal expansion and the resulting positioning errors.
In cooled nut systems, the coolant is routed through bore channels in the nut body. The required D/L ratios here can reach up to 50:1, placing them at the upper limit of what can be achieved internally using specialized equipment.
Cooled spindle systems constitute a distinct category in manufacturing technology. In these systems, the coolant must be routed through an axial internal bore along the entire length of the spindle. The resulting D/L ratios can reach values of up to 400:1—a range that can only be achieved by manufacturing partners specializing in deep-hole drilling with appropriate machine designs (including counter-rotating expansion of the tool and workpiece, and active vibration damping).
In addition, deep-hole bores in spindles are also used for media passage, such as for compressed air, electrical power, or data transfer. In the case of telescoping lead screws, bores with D/L ratios ranging from 5:1 to 15:1 are also used.
Structural effect: Loss of stiffness
Each hole reduces the effective cross-sectional area of the component and thus its stiffness. For the nut body, this effect is generally negligible: With standard nut geometries, the remaining wall thicknesses are sufficient to ensure the required stiffness.
This does not apply without restriction to deep-hole drilling in the spindle. The axial stiffness of a spindle is largely determined by its cross-section, which depends on the bore diameter and the shaft diameter. For long strokes, this loss of stiffness, combined with the control behavior, can limit the positioning accuracy that can be achieved.
The decision for or against a deep-hole bore in the spindle is therefore a design trade-off: between the thermal benefits of cooling and the mechanical disadvantage of reduced spindle rigidity. This trade-off must be made on a case-by-case basis and based on the actual load conditions.
Vertical Integration and Process Control
At Kammerer, deep-hole drilling for ball recirculation as well as for lubricant and coolant supply is performed exclusively in-house. Tooling concepts, manufacturing, and measurement processes have been systematically developed and transformed into reproducible standard processes. Bore quality is defined by form and position tolerances and verified through metrological measurements.
For D/L ratios of up to 400:1, achieved in cooled spindle applications, Kammerer has been working in close technical collaboration with specialized partner companies for about 15 years. This arrangement makes it possible to implement even the most technically demanding manufacturing requirements in series production.
