A Direct Comparison of Process Principles, Tools, and Surface Quality
The manufacture of ball screws places high demands on process stability and dimensional accuracy. Long, slender components react to the thermal effects of heat treatment by warping, which results in changes to their profile and deviations in pitch. In machining, this typically requires a multi-stage process: machining in the soft state, followed by heat treatment, and finally finishing after hardening. Both process stages—grinding and whirling—are well-established in practice and meet the specified requirements; however, they differ in process control, tooling concepts, and the achievable surface properties.
Process Principle: Thread Whirling as a Special Type of Milling
Thread whirling, also known as thread peeling, is a special type of milling. An internally toothed whirling head encloses the workpiece and rotates eccentrically relative to the center of the workpiece. In the process, the cutting edges create the thread in a single pass through a continuous cutting-in and cutting-out action. The chip thickness remains in the range of ten-thousandths of a millimeter. Unlike in grinding, the tool does not move away from the workpiece.
A key feature of the process is that the thread is cut or formed directly to the desired thread depth in a continuous process.
In addition, whirl grinding can be used for both soft and hard machining. For hardened ball screws, hard skiving allows roughing and finishing to be combined into a single operation. This reduces the number of logistical steps and eliminates clamping errors between stages.
Tool Design for Hard Machining
The suitability of the whirl machining process for hard machining depends largely on the tool design. CBN (cubic boron nitride) cutting inserts are used for machining hardened steels. CBN exhibits crystallographic and mechanical properties similar to those of synthetic diamond: high temperature resistance, good chemical reactivity with iron-based materials, and no wear-inducing reactions between iron, chromium, vanadium, molybdenum, and carbon. Reactions with atmospheric oxygen do not occur until temperatures exceed 1000°C.
Another process-related aspect concerns cooling. Conventional cooling lubricants cannot be used due to the high temperatures at the cutting edge and the resulting vapor pressure; this would cause uneven stresses on the cutting inserts. The process therefore operates dry, which also offers an environmental benefit.
Swivel Blade
Close-up of the grinding process
Grinding Processes: An Established Standard in Spindle Manufacturing
The grinding of ball screws has been tried and tested in industry for decades and is considered the gold standard for achieving high surface finishes. In thread grinding, the profile is machined by a rotating grinding wheel with a negative profile; this is typically performed as a separate manufacturing step following heat treatment. The process enables very tight tolerances and reproducibly smooth flanks. In combination with a subsequent polishing process, surface roughness values are achieved that rank among the best when compared to other methods. This requires careful coordination of the grinding wheel bond, dressing parameters, and coolant delivery, as thermal effects at the contact zone can influence the properties of the workpiece’s edge zones. The multi-stage process sequence—pre-machining, hardening, grinding—involves a corresponding logistical and time investment, but offers the advantage of a long track record and broad machine availability.
Metrological Comparison: Swirling and Grinding
To verify the achievable surface qualities through metrological testing, 40 × 10 mm ball screws were manufactured in three variants and evaluated by an independent testing institute: swirled, fine-ground using conventional manufacturing methods, and ground and then polished. For each screw, 30 measurements were taken at nine positions, and the mean values were calculated.
|
Procedure |
Rmax (mean value) |
Ra (mean value) |
Measurement Accuracy |
|
Whirled |
0,214–0,589 μm |
0,160–0,371 μm |
0,023 / 0,018 |
|
Sanded |
0,460–0,813 μm |
0,333–0,430 μm |
0,076 / 0,036 |
|
Sanded & polished |
0,150–0,220 μm |
0,093–0,127 μm |
0,022 / 0,015 |
The grinding process—especially when followed by a polishing step—produces very good surface finishes and has been established in spindle manufacturing for decades. Without any additional machining steps, the swirled spindles achieve surface roughness values that are more favorable than those obtained by grinding alone. Compared to the combined grinding and polishing method, these values are of a comparable magnitude.
Additional polishing of the swirled spindles yields only minor further improvements, since the surface quality is already high after the swirling process. This additional step is generally not necessary.
Accuracy and Measuring Spindle
When the process is carefully controlled, thread whirling enables pitch accuracies of IT1 class. Linearity is a characteristic of the machine and depends primarily on the tool, the material, and the process parameters.
A linear encoder (ERA 180:2003, Heidenhain GmbH) with 9,000 divisions per revolution and a measuring length of 4,040 mm was used for the metrological evaluation. The accuracy is ±5 μm. The C-axis (feed axis) is used to automatically generate the pitch; the pitch is electronically controlled via an upstream low-backlash planetary gearset and an incremental linear encoder.
Analysis and Conclusion
Grinding and whirling are both established methods for the final machining of ball screws. The grinding process has a long history of industrial use and, especially when combined with polishing, delivers consistently high surface quality.
Thread whirling, on the other hand, offers the possibility of combining roughing and finishing in a single pass, achieving surface finishes that meet the requirements for the vast majority of applications without the need for additional machining operations. Combined with the long service life of CBN tools and the dry machining process, this results in economic advantages that vary in significance depending on the part geometry, batch size, and quality requirements.
The choice of process ultimately depends on the application; the specific requirements regarding geometry, surface finish, and production volume are the deciding factors.
