Machining between two shoulders – a focus on grooving

Machining between two shoulders – a focus on grooving

Grooving, parting off, profile grooving: grooving is one of the most demanding processes in turning. In contrast to conventional longitudinal or face turning, the cutting edge machines the workpiece between two shoulders during grooving and parting off. It is precisely these boundary conditions that fundamentally characterise the process. While the chips can flow relatively freely during longitudinal turning, in grooving the workpiece shoulders confine the cutting edge on the left and right. For more than 50 years, Paul Horn GmbH has been developing solutions for precisely these machining operations and is regarded worldwide as a specialist in this field.

In grooving, the chip essentially leaves the machining zone upwards and against the feed direction. This places greater demands on tool geometry, chip formation, cooling and process stability. Even minor deviations can cause chip wrapping, increased wear, poor surface quality or even tool breakage. Grooving is therefore far from a simple process. Only the coordinated interplay of tool technology, coating, machine stability, clamping technology and process control ensures consistent results. The aim is always to shape the chip in a controlled manner, safely remove it from the groove and, at the same time, achieve long tool life and reproducible machining results.

Importance of cutting edge geometry

The cutting edge geometry has a significant influence on grooving and parting-off. Its main tasks are to guide the chips in a controlled manner and to shape them precisely.
In chip control, the geometry produces short, manageable chip shapes such as helical chips, spiral chips, comma-shaped chips, segmented chips or discontinuous chips. This prevents chip wrapping and reduces process disruptions. Long swarf ribbons, on the other hand, wrap around the workpiece or tool, impede chip flow and degrade surface quality. At the same time, the geometry selectively reshapes and tapers the swarf. This tapering enables the swarf to be safely evacuated from the groove, even in confined machining situations. Without targeted chip breaking, wide swarf is produced that rubs against the sides of the groove and damages the workpiece surface.

The influence of the geometry is particularly evident when comparing different chip breaker profiles. Ground, round chip-breaker profiles often have only a minimal effect on the chip, which flows largely uncontrolled and forms long ribbons. Geometries with pronounced chip breaking profiles, on the other hand, produce short, controlled and highly deformed chips. This controlled chip formation is the basis for stable machining processes. Particularly with stainless materials such as 1.4305, the correct geometry determines process reliability and tool life. Even at cutting speeds of around 100 m/min and feeds of approximately 0.12 mm/rev, chip formation has a significant influence on the machining result.

Trochoidal grooving

Trochoidal grooving expands the possibilities of conventional grooving processes. Instead of machining the entire groove in a full-depth cut, the tool moves along the workpiece following a superimposed path. This strategy reduces the load on the tool. At the same time, chip evacuation is improved because smaller chip cross-sections are produced. This method offers particular advantages for deep grooving or when processing materials that are difficult to machine. Typical process parameters include, for example, cutting speeds of around 280 m/min and feeds of up to 0.6 mm/rev. Minimum depths of cut of approximately 0.7 mm and maximum depths of cut up to 1.5 mm are employed. The cutting angle typically ranges between 40° and 60°. Smaller engagement angles lower the radial forces. At the same time, the thermal load on the cutting edge is reduced. The process is particularly suitable for modern, high-performance machining and achieving high metal removal rates.

The importance of modern coatings

The coating of modern grooving inserts influences cost-effectiveness and process reliability. It reduces wear, minimises friction between the chip and the tool, prevents built-up edges and enables higher cutting parameters. At the same time, a suitable coating improves wear detection and increases process stability. Numerous influencing factors must be taken into account when developing tool coatings. They include the chemical interaction between the tool and the workpiece material, the cutting edge radius, the coating process and the mechanical properties of the coating. However, the interplay of all these elements remains critical. If the geometry, substrate and coating are not compatible, internal stresses arise within the coating. These can improve hardness and toughness, but can also cause cracking or spalling.

A coating adapted to the process influences tool life. However, the coating must not be considered in isolation. The interplay of the entire machining process remains key. Machine stability, workpiece clamping, tool holder, cooling strategy and cutting parameters directly affect the performance of the insert. Practical trials show that significant increases in tool life are possible simply through the use of adapted coatings. At the same time, unsuitable combinations of geometry, coating and process parameters quickly lead to premature wear. Process design always requires a holistic view of the entire machining system.

Cooling during the grooving process

Cooling plays a crucial role in grooving. It reduces the thermal stress on the cutting edge, while the coolant actively supports chip removal from the cutting zone. High temperatures are generated, particularly when grooving stainless steel. Practical studies reveal differences between various cooling strategies. When external coolant supply is low pressure, severe wear often occurs on the rake face. If the coolant pressure is increased, wear is significantly reduced.
Systems that direct coolant to the rake face achieve excellent results. Modern tool holders with integrated coolant supply enable targeted cooling directly to the cutting zone. This reduces the thermal stress on the cutting edge. Furthermore, chip evacuation is improved. Modern toolholder systems supply the rake and flank faces precisely via integrated cooling channels. Direct coolant supply lifts the chip early and transports it out of the groove in a controlled manner. The tool holders usually feature universal coolant connections via slots or G1/8” ports.

It is not only the coolant supply, but also the composition of the coolant that influences the machining process. Even small changes in the oil content of an emulsion can have a significant impact on tool life. Practical examples show that increasing the oil content from 11 per cent to 13 per cent when machining Inconel 718 can double tool life. Particularly with high-temperature-resistant materials, a higher lubricant content lessens the friction between the chip and the tool. This reduces temperature and wear, whilst simultaneously improving chip formation.

Parting off in the X and Y axes

Parting-off is the final machining step at the main spindle. Errors in this process lead directly to scrap or damage to the component.
Traditionally, parting-off is performed along the X-axis. In this case, the cutting forces act perpendicular to the tool holder. As the distance between the cutting edge and the holder increases, the overhang and hence the bending moment grows and the stress and susceptibility to vibration rise. When parting-off via the Y-axis, the direction of the resulting cutting force changes. A majority of the force is absorbed by the rigid machine structure, whilst only a small proportion is directed against the feed mechanism. The resulting force is transmitted into the tool holder at an angle of approximately 30 degrees. Consequently, the effective tendency to bend is significantly reduced. In addition, chip evacuation is improved, as chip discharge is directly downwards into the machine bed.

The different force ratios directly affect the load on the insert seat. Assuming forces of 2000 N perpendicular to the cutting edge and 400 N radially, parting off along the X-axis results in a significantly higher load due to the high bending moment.
With Y-axis parting, this load is reduced by around 30 per cent. Consequently, processes run more smoothly and with reduced vibration. Practical examples show that switching from X-axis to Y-axis parting improves both tool life and process stability.
In some applications, tool life has been more than doubled while keeping cutting parameters unchanged. At the same time, the noise level is reduced. However, Y-parting requires that the machine and control system support the necessary axis travels and machining strategies.

Grooving of complex contours

Complex contours, such as multi-tooth profiles, place stringent demands on the process strategy. One option is to create the contour conventionally, step by step, using a grooving tool. In this case, individual grooves are programmed and executed sequentially. This method is particularly suitable for prototypes or small batches. For large production runs, however, the programming and machining effort increases considerably. Alternatively, form grooving tools enable the entire contour to be produced in a single machining operation using a specially designed form-profiling edge that creates the entire geometry in one go. The advantages are high repeatability and short machining times. However, the large length of the cutting edge engaged at any one time generates high machining forces, that can cause vibration, surface marks and dimensional deviations.

Tangential profile grooving extends the classic form grooving process with a significantly more stable strategy. The insert is typically seated at an angle of approximately 45° in the insert seat. The tool does not enter the workpiece radially, but is guided tangentially past the workpiece. In this process, the individual elements of the contour are created sequentially. In contrast to radial form grooving, the entire cutting edge width does not engage with the material simultaneously. Measurements taken with force measuring systems show clear differences between the two methods. In conventional form grooving, cutting forces of almost 6,000 N can occur. In tangential profile grooving, the forces are significantly lower. The reduced load ensures smoother cutting processes, less machine stress and lower noise levels. Furthermore, the surface finish of the turned part is improved.

Conclusion

Horn is regarded as a specialist in cost-effective and reliable grooving. Optimised cutting edge geometries, modern coatings, targeted cooling and high machine stability are crucial for stable processes. Optimised chip formation and internal cooling significantly improve chip removal, surface quality and tool life. Processes such as trochoidal grooving or Y-axis parting-off reduce forces, increase process stability and enhance cost-effectiveness.