Topology Optimization of a Milling Cutter Head for Additive Manufacturing

Abstract

The rapid growth of the machining market and advancements in additive manufacturing (AM) present new opportunities for innovative tool designs. This preliminary study proposes a design for additive manufacturing (DfAM) approach to redesign a milling cutter head in 17-4 PH stainless steel by integrating topology optimization (TO) and internal coolant channel optimization, enabled by laser powder bed fusion (LPBF). An industrial eight-insert milling cutting tool was reimagined with conformal cooling channels and a lightweight topology-optimized structure. The design process considered LPBF constraints and was iteratively refined using computational fluid dynamics (CFD) and finite element analysis (FEA) to validate fluid flow and structural performance. The optimized milling head achieved approximately 10% weight reduction while improving stiffness (reducing maximum deformation under load from 160 μm to 151 μm) and providing enhanced coolant distribution to the cutting inserts. The results demonstrate that combining TO with internal channel design can yield a high-performance and lightweight milling tool that leverages the freedom of additive manufacturing. As proof of concept, this integrated CFD–FEA validation approach under DfAM guidelines highlights a promising pathway toward superior cutting tool designs for industrial applications.

1. Introduction

Machining remains one of the most critical manufacturing processes globally, with increasing demand for high-performance materials and components in several sectors, such as aerospace, automotives, and energy. Milling, in particular, is responsible for nearly 20–30% of global metal cutting operations [1,2]. Innovations in cutting tool design have become essential to remain competitive and meet modern industrial requirements [3,4]. The design of the milling cutter head plays a decisive role in machining efficiency and surface finish, influencing chip evacuation, dynamic stiffness, thermal behavior, tool life, and productivity [5,6].

The selected eight-insert milling cutter head was chosen due to its industrial relevance and its performance limitations under high-speed machining conditions, particularly related to thermal loads and insert wear [7,8]. In such applications, inadequate heat dissipation often leads to premature tool degradation, reduced surface quality, and increased tool replacement frequency [8,9]. This makes it an ideal candidate for a performance-oriented redesign using additive manufacturing (AM) to showcase how conformal cooling and topology optimization (TO) can address performance degradation related to insufficient internal fluid management.

Traditional milling tool designs are constrained by the geometric limitations of subtractive manufacturing [5,10], and are typically produced by subtractive or casting methods, which are constrained by manufacturability limits, especially in achieving internal cooling channels or mass-optimized load-bearing features [11,12]. These limitations have prompted the adoption of AM to redesign tooling systems, offering greater freedom in geometrical complexity, weight reduction, and internal functionality [13,14].

Among the metal AM processes, laser powder bed fusion (LPBF) stands out for its ability to produce intricate parts with high precision and good mechanical properties [15,16,17]. However, LPBF also imposes specific constraints, such as the need to minimize unsupported overhangs and to ensure the depowdering of enclosed channels, which must be addressed during the design phase to ensure manufacturability and functionality [18,19]. While LPBF can produce geometrically complex components with high resolution, its dimensional accuracy remains a constraint for functional tooling applications. Typical tolerances for LPBF parts range from ±50 to 100 μm, depending on build orientation, feature size, and process parameters [20,21].

Internal cooling channels are especially important in milling tools with multiple inserts, where heat accumulates rapidly during machining. Traditional manufacturing techniques often limit the complexity and positioning of these channels [22,23]. AM, specifically LPBF, offers geometric freedom to embed conformal cooling paths directly into the tool structure, enabling uniform coolant distribution and directionality to each insert edge, thus enhancing thermal management [24,25]. This is crucial for extending tool life, improving machining stability, and ensuring dimensional accuracy in machined components [26].

Design for additive manufacturing (DfAM) strategies must be systematically integrated into the product development cycle to address these issues. Following the principles of DfAM, engineers can exploit AM’s capabilities to develop advanced tool geometries, such as topology-optimized structures and conformal internal coolant channels [27,28]. This integrated approach reduces material usage and allows for better cutting insert cooling during machining, thereby contributing to improved tool life [3,29].

Recent advancements in computational design have enabled the coupling of TO with LPBF, allowing designers to strategically remove non-load-bearing material and reinforce critical load paths, which results in lighter, tougher, and more efficient components [14,17,20]. Similarly, conformal cooling channels integrated into AM tooling have demonstrated enhanced cutting zone cooling, contributing to reduced thermal loads and extended tool life [2,30,31].

TO is a well-established field with various algorithmic families designed to determine optimal material distribution under constraints [32,33]. Common approaches include density-based methods such as solid isotropic material with penalization (SIMP), used in this study through nTop’s gradient-based solver. This method assigns continuous pseudo-densities to finite elements [32,34]. Other key strategies include homogenization methods, level-set techniques (often combined with topological derivatives), evolutionary approaches such as ESO and BESO, and the ground structure method for discrete truss-like structures. While advanced methods such as SEMDOT and moving morphable components (MMCs) offer promising multi-resolution or multi-material design capabilities [34,35], SIMP was selected in this work due to its computational efficiency, robust convergence in constrained geometries, and suitability for LPBF constraints (e.g., overhang angle, powder removal, and support minimization) [32]. This made SIMP the most practical and reliable choice for integrating DfAM criteria into optimizing a complex industrial tool [36,37].

The integration of structural optimization and advanced coolant flow design presents a promising opportunity for improving the performance of milling cutter heads through AM [10,38]. By adopting a DfAM strategy that incorporates both computational fluid dynamics (CFD) and finite element analysis (FEA), it is possible to optimize mechanical behavior and thermal management holistically [29,39,40]. This approach enables the development of milling tools that not only reduce mass and enhance stiffness but also provide effective and targeted coolant delivery to each cutting insert edge [41,42,43].

Despite this potential, few studies have explored the simultaneous application of TO and conformal cooling in industrial-scale tools fabricated via LPBF [44]. There is a particular gap in the literature regarding tools made from 17-4 PH stainless steel, a material widely used in cutting tools due to its favorable strength, corrosion resistance, and post-build heat treatment [45,46].

This study presents the redesign of an industrial milling cutter head that integrates topology optimization and advanced internal coolant channels tailored for LPBF using 17-4 PH stainless steel. The optimization process was guided by DfAM principles and validated through FEA and CFD simulations. Developed in collaboration with a tooling manufacturer, the strategy illustrates the disruptive potential of AM in producing high-performance, application-specific machining tools. To the authors’ knowledge, this is one of the first studies to combine TO and conformal cooling in redesigning an industrial milling cutter head explicitly engineered for LPBF with 17-4 PH stainless steel.

2. Materials and Methods

2.1. Original Milling Cutter Head and Materials

The reference component selected for redesign is a milling head with indexable cutting inserts developed by Palbit S.A. (Aveiro, Portugal) for face and shoulder milling applications. The original design (Figure 1) includes eight symmetrically arranged cutting inserts, each served by an internal radial coolant channel, with a limited and non-conformal original cooling configuration. The tool used is the industrial reference 063A20190-08-08-022040 [47], with a nominal diameter of 63 mm, commonly used in high-performance operations. The cutting inserts used in simulation and thermal assessment correspond to XPET 100304 PDSR-MP [48], which are suitable for general-purpose steel and cast-iron milling. The milling head is conventionally machined from Toolox 44, a pre-hardened tool steel valued for its high stiffness, wear resistance, and machinability combination. These characteristics made it a suitable baseline for evaluating improvements enabled by additive manufacturing (AM), while also illustrating the potential of a material transition to LPBF-compatible 17-4 PH stainless steel.

The baseline geometry selected for this study corresponds to a commercial-grade 8-insert milling head used in high-performance milling applications. The tool’s compact size, insert configuration, and known thermal performance challenges under continuous use made it a practical and strategically relevant subject for dual optimization. Moreover, its geometry offers sufficient complexity to demonstrate the advantages of conformal cooling and LPBF-specific DfAM strategies.

For AM, 17-4 PH stainless steel was selected due to its high strength, good corrosion resistance, and compatibility with LPBF processing.

Table 1. Mechanical properties of the original (Toolox 44) and redesigned (LPBF 17-4 PH) milling head materials [49,50].

	Toolox 44 (Original)	17-4 PH Steel (LPBF)
Young’s modulus (GPa)	210	202
Poisson’s ratio	0.28	0.275
Density (g/cm3)	7.85	7.86

provides a comparison of the mechanical properties of both materials. These mechanical properties were input parameters for the redesign’s FEA and TO stages.

The selected milling cutter head is widely used in industry and exhibits thermal performance limitations during prolonged high-speed operation. Its compact size and standardized geometry enable effective benchmarking of design improvements through LPBF. Each insert was initially served by a straight radial cooling channel, limiting thermal extraction and prompting the integration of conformal cooling. Additionally, the original Toolox 44 material, while suitable for traditional manufacturing, is not ideal for LPBF due to crack sensitivity and hardenability issues, justifying the transition to 17-4 PH stainless steel.

2.2. Simulation Setup and Boundary Conditions

Structural simulations were conducted using FEA to evaluate performance and guide optimization. A static analysis was carried out on the original design under a representative load case derived from typical face milling conditions. Each insert was assumed to experience a cutting force of approximately 9000 N (based on the defined cutting parameters) when machining a modified DIN 40 CrMnNiMo 8-6-4 quenched steel workpiece at a cutting speed of 250 m/min, an axial depth of cut of 10 mm, a radial width of cut of 37.8 mm (60% of the cutting diameter), and 0.3 mm/tooth of feed. Including a safety factor 1.5, a total load of 13,500 N was applied.

The force was uniformly distributed across the seating faces of the inserts. The component was constrained on two opposing faces of the central mounting groove to simulate the tool–holder interface. Material behavior was modeled as linear elastic using the values in Table 1. This setup was applied to the original and optimized designs for consistent comparison.

Coolant channel performance was analyzed using CFD simulations, assuming a steady-state flow of water-based coolant under typical supply pressure. The fluid was modeled as incompressible and Newtonian. CFD was used in an iterative process to evaluate channel flow uniformity, pressure drop, and velocity distribution.

2.3. Design and Optimization Strategy

The redesign strategy combined two distinct stages. The coolant channel optimization aimed to improve coolant delivery to the rake face of each insert. The original straight-drilled channels were replaced with complex conformal channels developed using nTop (New York, NY, USA) [51]. Several geometries were created and analyzed in CFD to evaluate flow behavior. Key design goals were to ensure laminar flow, minimize turbulence, and target the highest thermal load zones near the cutting edges. Once the coolant channels were finalized, the bulk of the milling cutter head was topology-optimized. All critical functional regions—including insert pockets, screw holes, the central bore, and coolant paths—were preserved from the original geometry. The domain available for optimization was divided into one-eighth of the full component to reduce computational cost, exploiting the part’s symmetry. After optimization, the segment was replicated around the whole geometry and smoothed to ensure continuity.

A 45° overhang constraint was applied to ensure LPBF manufacturability without extensive support structures. A displacement constraint (200 μm max under the given load) was included to guarantee mechanical stability. The objective of optimization was to minimize mass while meeting these constraints. nTop’s gradient-based solver was used to generate the final design.

A combined FEA and CFD approach was employed to evaluate the redesigned milling cutter head, which was aligned with a DfAM-driven optimization workflow. Structural and thermal–fluid simulations were performed separately and iteratively to guide design refinements. The FEA analysis was conducted using nTop (nTopology Inc., New York, NY, USA), utilizing its built-in gradient-based solver. The mechanical behavior of the topology-optimized structure was assessed under quasi-static loading. Boundary conditions included a fixed constraint at the tool holder interface and a vertical compressive load of 2 kN applied at the insert seating regions. Material properties for LPBF 17-4 PH stainless steel were defined according to Table 1. The model assumed linear elastic behavior and did not account for dynamic tool–workpiece interactions or vibration. The CFD simulations were performed using Ansys Fluent (ANSYS, Inc., Canonsburg, PA, USA) to analyze the redesigned conformal coolant channels. The analysis assumed steady-state laminar flow of water at 25 °C. An inlet velocity of 1 m/s was prescribed and applied uniformly over the inlet area. A 0 Pa relative pressure was set at the outlet, positioned sufficiently far from the flow path exit to avoid local back-pressure effects. Constant thermophysical properties were assumed. This preliminary feasibility study did not consider phase change, cavitation, turbulence, or erosion phenomena. This modeling strategy was intended to provide a first-order assessment of flow uniformity and thermal extraction potential in the redesigned channels. Results from both FEA and CFD simulations were used to refine the internal channel geometry and bulk structure iteratively.

While the models offer valuable quantitative insights into deformation, stiffness, and coolant distribution, they do not yet incorporate machining-induced heat generation, thermal–mechanical coupling, or cyclic loading conditions. Future work will address these limitations, including experimental validation and in-process monitoring.

3. Results and Discussion

3.1. Coolant Flow Analysis

The iterative redesign of the internal coolant channels was central to improving thermal performance in the milling head. The original straight-drilled channels were replaced with curved conformal geometries developed specifically for AM. Two versions of the channel geometry were evaluated using CFD to assess flow direction, velocity profiles, and pressure losses.

Version 1 (Figure 2) introduced a smooth downward-bending curvature immediately after coolant entry, preserving the original flow alignment. While this configuration directed coolant toward the insert, CFD (Figure 3) revealed a localized velocity spike at the bend, causing separation from the inner wall and creating turbulent recirculation zones. These flow disruptions reduced the uniformity of the coolant, reducing the cooling ability on the insert face.

Version 2 (Figure 4) addressed this by increasing the cross-sectional area and using a more gradual curvature. This design change effectively reduced local velocity peaks, smoothing the flow and reducing pressure loss. Figure 4a,b visually compare these two versions, with streamlines in Version 2 showing improved coverage of the insert edge.

Version 2 (Figure 5) introduced finer outlet angle and bend radius adjustments. The flow was more uniformly distributed across the jet’s cross-section and more precisely toward the insert rake face. The pressure drop from inlet to outlet was reduced by approximately 12% compared to Version 1, and the flow field was predominantly laminar, minimizing shear-induced turbulence.

These results confirm that DfAM-oriented channel optimization, coupled with CFD feedback, can substantially enhance fluid delivery. The use of nTop enabled quick geometric iterations, and the final solution demonstrates that integrating simulation into the early stages of design development is crucial for leveraging AM’s design freedom into actual functional benefit.

3.2. Topology Optimization and Structural Performance

The TO process produced lightweight, rib-reinforced geometry while preserving all functional constraints (insert pockets, central bore, coolant pathways). The algorithm selectively removed material from low-stress regions, particularly the peripheral body and between inserts, while retaining and reinforcing high-load areas near the hub and cutting interfaces.

The final design (Figure 6) adhered to all imposed constraints, including the 45° overhang angle and a maximum allowable displacement of 200 μm. The optimized structure features truss-like internal ribs and arching connections between the insert blocks and the central hub, forming self-supporting and organically contoured geometry.

Quantitatively, the final design reduced part mass from approximately 3.5 kg to 3.15 kg—an approximately 10% reduction. Static FEA showed that under the simulated 13.5 kN worst-case load, the maximum displacement decreased from 160 μm in the original design to 151 μm in the optimized version. This corresponds to a 5.6% improvement in stiffness, despite the significant reduction in mass.

The Von Mises stress analysis of the TO component indicates a satisfactory stress distribution. Although the maximum stress value of 1170 MPa is attributed to a singularity, the effective peak stress is approximately 820 MPa and remains localized at the clamping interface, consistent with the original design. The analysis indicates that other component regions demonstrate significantly reduced stress concentrations. This is due to the strut-like geometry, which efficiently transfers loads to the base and reduces bending deformation in the outer sections.

Figure 7 compares the deformation fields of both the original and optimized designs. The original part exhibits broader deflection, particularly around unsupported radial regions, whereas the optimized version displays more localized and efficient structural response. These results validate the TO as an effective strategy to reduce mass without compromising and even slightly enhancing mechanical performance.

3.3. Combined CFD and FEA Validation

A distinguishing feature of this work is the concurrent optimization of structural and thermal–fluidic performance, validated through both FEA and CFD. Notably, the design process accounted for the structural consequences of large internal voids (coolant channels), comparing the original component (Figure 8a) with the TO component (Figure 8b). By integrating these geometries as preserved domains in TO, sufficient surrounding material was retained and reinforced to ensure mechanical integrity.

This dual-domain validation showed the following:

• Structurally, the component meets the imposed 200 μm displacement constraint, while slightly outperforming the original (Figure 8a) in stiffness, where the maximum displacement is 160 μm. The maximum displacement for the TO component (Figure 8b) is 151 μm.
• Thermally, the coolant flow reaches each insert more evenly and efficiently, with reduced pressure losses and minimal turbulence.

This synergy is made possible by the design freedom of LPBF and the advanced modeling capabilities of nTop, which allowed the integration of AM-specific constraints, like build angles, support minimization, and de-powderability, into the design workflow.

The result is a milling head that is lighter, stiffer, and better cooled than its traditionally manufactured predecessor.

3.4. Industrial and Manufacturing Implications

From a practical standpoint, the redesigned milling head offers several industrial benefits:

• Lower inertial mass improves dynamic spindle response. It reduces vibration during high-speed operations, which means a possible positive impact on tool life and control of the quality of machined surfaces.
• Improved coolant delivery can lead to better chip evacuation, reduced thermal loading, and extended insert life.
• Optimized material usage reduces production cost per part and contributes to more sustainable manufacturing.

This component is designed specifically for LPBF in 17-4 PH stainless steel. All features, including fillet radius and channel outlets, were configured to avoid excessive support structures and allow straightforward powder removal. Post-processing consists of stress relief and precipitation hardening, compatible with the alloy and geometry.

Furthermore, close collaboration with an industrial partner (Palbit S.A.) ensured that real-world constraints, such as assembly compatibility, coolant pressure ratings, and machining dynamics, were embedded into the design logic. The success of the simulation-based prototype provides confidence that the part is not only manufacturable but also industrially deployable with minimal downstream adaptation.

3.5. Work Summary

This work demonstrates the successful application of DfAM principles to redesign a complex and performance-critical machining component. Integration of TO and internal channel optimization within a unified LPBF-compatible design process yielded the following results:

• A ~10% reduction in mass;
• A ~6% increase in stiffness (lower maximum displacement);
• A ~12% reduction in coolant pressure drop;
• Significantly improved coolant flow uniformity.

These improvements were realized without sacrificing manufacturability or violating industrial constraints. The redesigned milling head offers enhanced performance and efficiency, supporting the broader adoption of AM for tooling and functional components.

This approach can be extended to other tooling geometries, such as drill bodies, collet chucks, or even hybrid cutting tool molds, where integrated structural and thermal performance is critical.

4. Conclusions and Future Work

This preliminary study comprehensively redesigned an industrial milling cutter head by leveraging additive manufacturing (AM) design freedoms and advanced optimization techniques. The main findings and contributions are summarized below:

• Integrated DfAM Strategy: A dual-optimization approach combining topology optimization (TO) for structural efficiency with CFD-guided internal coolant channel design was implemented. The methodology was tailored for laser powder bed fusion (LPBF), incorporating AM-specific constraints such as overhang angles, powder evacuation paths, minimum wall thicknesses, and support minimization. These constraints were embedded into the TO setup using manufacturability filters. The final design demonstrates the ability of DfAM principles to generate geometries that are simultaneously efficient, complex, and LPBF-compatible.
• Enhanced Mechanical and Thermal Performance: The topology-optimized design achieved an approximate 10% mass reduction compared to the reference component, contributing to lower inertial loads during high-speed machining. Despite the mass reduction, the structure showed improved rigidity, with maximum deflection decreasing from 160 μm to 151 μm, which is a 5.6% increase in stiffness. Concurrently, CFD simulations demonstrated improved coolant flow distribution and a ~12% reduction in pressure drop. The redesigned conformal cooling channels achieved direct, balanced fluid delivery to each insert edge, improving thermal extraction at the cutting interface and potentially extending insert life and process stability.
• Holistic Simulation Validation: The integration of FEA and CFD allowed a multidomain evaluation of the component, ensuring that mechanical strength and thermal–fluidic functionality were optimized simultaneously. This is a critical advancement from prior studies, which have addressed structural optimization in isolation. The combination of mechanical and thermal validation within the same design loop ensured that neither performance domain was compromised.
• Industrial Relevance and Manufacturability: The case study was developed with a cutting tool manufacturer using a real-world geometry compatible with commercial LPBF systems. The final design employed 17-4 PH stainless steel and incorporated minimal internal overhangs and self-supporting channels to ease powder removal and post-processing. Attention was given to building orientation, internal accessibility, and platform anchoring to ensure manufacturability without excessive support generation. The optimized head also shows the potential to reduce machining-induced vibrations and extend tool life.

While promising, this work remains a preliminary computational study, and experimental validation is necessary to confirm the predicted performance gains fully.

The future work will focus on and include the following:

• Physical fabrication of the optimized milling head via LPBF using 17-4 PH stainless steel, applying tuned process parameters to ensure dimensional accuracy and metallurgical integrity.
• Structural validation, including static loading tests and modal analysis, to assess stiffness, global deformation, and vibrational performance under operating conditions.
• Thermo-mechanical verification, incorporating thermal imaging and flow visualization to experimentally validate the CFD-based predictions regarding pressure drop and cooling distribution.
• Machining performance trials, aimed at evaluating tool wear, chip formation, cutting forces, and surface finish in comparison with the original component.
• Benchmarking studies directly compare the LPBF-built component with traditionally manufactured tools regarding machining performance and operational stability.

Additionally, the design and validation methodology developed here will be extended to other multi-functional tool geometries and integrated into broader industrial AM workflows. These efforts will support the scalable adoption of DfAM strategies for next-generation tooling solutions in high-performance applications.