A combined process of electrochemical machining and slotting for semi-blind hole involute internal splines enhanced by a new cathode design

A combined process of electrochemical machining and slotting for semi-blind hole involute internal splines enhanced by a new cathode design | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article A combined process of electrochemical machining and slotting for semi-blind hole involute internal splines enhanced by a new cathode design Yi Huang, Cong Fang, Yan Xu, Senlin Gong, Xi Wang This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4244272/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 20 Feb, 2026 Read the published version in The International Journal of Advanced Manufacturing Technology → Version 1 posted You are reading this latest preprint version Abstract Machining complex internal structures in high-hardness materials presents a significant challenge. Conventional cutting methods often suffer from issues such as extensive tool wear and prolonged production cycles. This research proposes a combined process that integrates roughing electrochemical machining (ECM) with a precision slotting to achieve fast involute internal spline shaping of intricate high-hardness components. To enhance the process stability of the spline ECM process, a curved cathode design tailored for ECM is introduced. Flow field simulations demonstrate that the proposed curved cathode design leads to a relatively uniform distribution of electrolyte flow in the machining area. A dedicated ECM fixture and system are established for conducting experiments. The obtained ECM outcomes substantiate the commendable capacity of the optimized curved cathode to accomplish high precise and efficient machining and shaping of splines. Consequently, the optimized curved cathode design exhibits substantial potential for widespread application in the large-scale production of intricate internal cavity components composed of challenging-to-machine materials. Electrochemical machining (ECM) Involute internal spline Cathode design Flow field simulation Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1. Introduction In recent years, there has been growing interest in high-strength, durable involute internal spline key components for use in aeromechanical transmission systems [ 1 , 2 ]. These components excel in efficiently transmitting larger loads and torque, thereby reducing wear and extending operational lifespans [ 3 – 5 ]. However, due to the challenging machining characteristics of Ni-based, Ti-based or Cr-Co-Mo-based alloys [ 1 , 6 ], particularly when combined with intricate internal cavity configurations like semi-blind holes, crafting these spline parts remains a significant challenge. The prevailing method for producing precise high-hardness involute internal spline components in relevant industries currently relies on a complete slotting process, which suffers from issues such as extensive tool wear and prolonged production cycles [ 6 ]. Electrochemical machining (ECM) has emerged as a cost-effective and vital option in the aerospace sector, particularly in the roughing stage [ 6 – 8 ]. ECM offers numerous advantages, including the elimination of tool wear, irrespective of a component's high hardness and complex structure, along with high metal removal rates [ 9 – 13 ]. However, achieving high precision in machining semi-blind hole involute internal splines can be challenging with ECM alone due to the relatively limited machining precision. To address this challenge, a combined process that integrates roughing ECM with a precision slotting is proposed to achieve the involute internal spline shaping for intricate high-hardness components [ 15 ]. During the ECM phase, the use of a well-designed cathode is crucial for ensuring a smooth ECM process [ 16 – 19 ]. This research introduces a specialized cathode design tailored for ECM applied to semi-blind hole involute internal splines within high-hardness materials. This combined process and proposed cathode design demonstrate significant potential for widespread adoption in the large-scale production of complex internal cavity components crafted from challenging-to-machine materials. 2. Proposed combined process In semi-blind hole involute internal splines, the spline profiles remain consistent along the axis, featuring dimensions larger than those of the lower section. The unique semi-blind hole structure involves a two-step spline-forming process. This paper concentrates on the initial step, which involves the efficient machining of all initial spline contours using axial ECM. This step provides the necessary allowances for the subsequent finishing slotting process. Figure 1 illustrates the axial ECM process for semi-blind hole involute internal splines. In this procedure, the workpiece is connected to the positive pole of the DC power supply, while the tool cathode is connected to the negative pole and moved toward the workpiece at a specific feed rate. During the preparatory stage (Fig. 1 a), fresh electrolyte is introduced from the bottom and exits from the top, establishing a favourable and unenergized ECM initial state. In the processing stage (Fig. 1 b), as power is activated and the cathode continuously advances axially, the ECM gap forms, and the fresh electrolyte engages in electrochemical reactions, resulting in by-products such as electrolytic sludge. This by-product contributes to the formation of contaminated electrolyte, which exits from the top. The entire process continues until all initial spline profiles have been electrochemically machined. In the final stage (Fig. 1 c), as the cathode moves toward the tool withdrawal area, the power is deactivated, indicating the completion of the spline ECM process. Subsequent to the initial spline profiles being machined via spline axial ECM, with an adequate allowance area in section A-A (Fig. 1 d and e), the second step involves achieving precision tolerance ranges by forming the involute internal spline contours through the finishing slotting process. 3. Design evolution of proposed cathodes To enhance the process stability of spline axial ECM, the cathode structure and shape are systematically analysed and proposed, as illustrated in Fig. 2 . Initially, the conventional cathode design is considered, represented as a 1 b 1 c 1 ~ a 2 b 2 c 2 (Fig. 2 a). This design involves inwardly reducing the target involute internal spline contour ABC by 0.5 mm, with the first 0.3 mm serving as a slotting finishing allowance and the subsequent 0.2 mm defining the ECM side machining gap size. However, this unoptimized design is susceptible to short-circuiting due to an uneven electrolyte distribution in the ECM processing gap region. In response to this challenge, a commonly proposed cathode design, denoted as a 1 b 1 c 1 ~ Ma 2 c 2 (Fig. 2 b), is introduced. This design involves the modification of the initial conventional cathode working teeth into a chamfered structure. Nonetheless, challenges persist in ECM machining, especially in the tooth root circle forming region of the spline profiles. To further enhance the structure, a "three-side feeding" design approach [ 20 ] is adopted in the working teeth region of the curved proposed cathode, labelled as a 1 b 1 c 1 ~ NS (Fig. 2 c). This design exhibits a gradual transition from the point to the initial conventional cathode working teeth profile section. Figure 2 (d) presents a detailed structure design of the working teeth within the curved proposed cathode at the z -plane. The thickness of the cathode working teeth area is determined to be 2.2 mm, influenced by the tool withdrawal area with a height of 4 mm. This region comprises a 2 mm front variable section zone ( LNRS ) and a 0.2 mm equal section zone ( a 1 b 1 c 1 ~ RNL ) at the rear. The curved structure ( LNRS ) is formed by connecting all lines ( N 1 S 1 , N 2 S 2 , N 3 S 3 …N n−2 S n−2 , N n−1 S n−1 , N n S n ) parallel to line NS . This segment undergoes a gradual expansion from a point to the initial spline profile, ensuring a seamless ECM transition from point to surface and uniform electrolyte flow distribution in the ECM processing gap region. The 0.2 mm rear section corresponds to the equal section of the initial conventional cathode working teeth profile, facilitating precise alignment of the front ECM-formed contours. Moreover, non-working areas of the cathode and the rod body are subsequently shielded with an insulation layer to protect the internal non-machined area of the semi-blind hole from electrolytic corrosion. 4. Validation and discussion 4.1 Electrolyte flow distribution in ECM processing gap region Achieving a uniform and stable distribution of electrolyte flow within the ECM processing gap region is essential for the successful operation of spline axial ECM [ 21 ]. High-speed electrolyte flow serves the crucial role of efficiently removing by-products generated within the narrow ECM processing gap region, thereby preventing issues like electrolyte starvation and cavitation, which can potentially lead to short circuits. The distribution of the electrolyte flow field in ECM process is simulated using the COMSOL Multiphysics software. Several key assumptions are made regarding the behaviour of the electrolyte fluid: (a) The electrolyte is considered incompressible, displaying constant Newtonian behaviour. It is assumed to be free from air bubbles and solid particles, maintaining consistent dynamic viscosity and density; (b) any energy loss within the electrolyte caused by temperature and concentration changes is disregarded; (c) the fluid adheres to the principles of mass and momentum conservation. Given the complex geometry of the electrolyte fluid flow path in the ECM processing gap region, the flow field simulation employed the k-𝜔 turbulence model, based on Navier-Stokes equation [ 22 – 24 ]: $$\frac{\partial }{{\partial }_{t}}\left(\rho k\right)+\nabla \bullet \left(\rho kV\right)=\nabla \bullet \left[\left(\mu +{\sigma }_{k}{\mu }_{t}\right)\nabla k\right]+{P}_{k}-\rho {\beta }^{*}{f}_{{\beta }^{*}}(\omega k-{\omega }_{0}{k}_{0})+{S}_{k}$$ 1 $$\frac{\partial }{{\partial }_{t}}\left(\rho \omega \right)+\nabla \bullet \left(\rho \omega V\right)=\nabla \bullet \left[\left(\mu +{\sigma }_{\omega }{\mu }_{t}\right)\nabla \omega \right]+{P}_{\omega }-\rho \beta {f}_{\beta }({\omega }^{2}-{{\omega }_{0}}^{2})+{S}_{\omega }$$ 2 where 𝜌 is the electrolyte density, V is the mean velocity, 𝜇 is the dynamic viscosity, 𝑘 is the kinetic energy, 𝜔 is the specific dissipation rate. In actual flow field simulations, the electrolyte dynamic viscosity is 1.005 \(\times\) 10 −3 Pa \(\bullet\) s, the density is 1.089 \(\times\) 10 3 kg/m 3 , the inlet pressure is 1.1 MPa and the outlet pressure is 0.7 MPa. Figure 3 provides insight into the distribution of electrolyte flow within the ECM processing gap region, comparing two cathode designs. Sections A and B depict the longitudinal and transverse velocity distributions of the electrolyte within the ECM processing gap region. In Fig. 3 (a) (section A), the electrolyte flow encounters an obtuse turn followed by a right-angle turn before entering the ECM processing gap region, resulting in high resistance, slow flow, inadequate electrolyte supply, and the potential for short circuits. Section B shows an uneven flow velocity, rapidly decreasing from non-machined areas to the involute tooth root circle when using the conventional cathode. This unevenness is attributed to the elongated and intricate spline profile flow path. Conversely, in Fig. 3 (b) (section A), the variable-section cathode working teeth simplify the flow path, reducing resistance, and enhancing electrolyte supply for spline axial ECM. Correspondingly, section B exhibits a more uniform and less steep flow velocity gradient, facilitating successful spline axial ECM. 4.2 Spline axial ECM experiments Figure 4 illustrates the experimental setup for spline axial ECM and its associated fixture. The ECM experimental system (Fig. 4 a) consists of a CNC machine, an operational platform, a fixture, a DC power supply, electrolyte tanks and a filtration system. It can be divided into four functional units: (a) Fresh Electrolyte Supply: This unit ensures a continuous supply of clean electrolyte to the ECM processing gap region, maintaining a high inlet pressure of 1.1 MPa. A vertical centrifugal pump is used for by-product removal; (b) DC Power Supply: The DC power supply unit is responsible for powering the ECM process. It connects the positive pole to the lower fixture, energizing the workpiece, and connects the negative pole to the tool cathode; (c) CNC Machine Operation: In this unit, the CNC machine controls the tool cathode feed rate and direction with Z-axis movement. It also manages the initial ECM location and fixture positioning using X and Y-axis movements; (d) Contaminated Electrolyte Filtration and Circulation: This unit incorporates a contaminated electrolyte tank with varying screen heights and porosities, as well as filters of different pore sizes. These components are designed to intercept ECM by-products, including hard-to-dissolve electrolyte sludge and bubbles. Additionally, this unit plays a role in cooling and recycling the filtered electrolyte. The fixture components include an upper fixture, a lower fixture, a tool cathode, electrolyte tubes, a Bakelite adapter, a toolholder, and a marble base (Fig. 4 b). The workpiece is positioned within the lower fixture. The tool cathode is linked to the upper fixture through O-ring sealing and securely fastened to the lower fixture with four bolts, allowing vertical movement without any electrolyte leakage. Fresh electrolyte enters the lower fixture through the electrolyte inlet tube and exits from the upper end after ECM process. The marble base provides insulation between the fixture and the CNC machine table, while the Bakelite adapter serves as both an insulator and a heat-resistant barrier to withstand the elevated ECM temperatures. The relevant ECM experiments of semi-blind hole involute internal splines are carried out to assess process stability and accuracy using two different cathodes. The key process parameters, including the electrolyte solution, machining voltage, and cathode feed rate, are summarized in Table 1 . Table 1 Process parameters of ECM experiments Process parameters Values or conditions Cathode material Cathode types SUS 304 Conventional and proposed Test piece material 15Cr14Co12Mo5Ni2WA Test piece hardness HRC 48–52 Cathode feed depth, mm 39.5 Initial ECM surface gap, mm 0.5 Electrolyte solution 8% NaNO 3 Machining voltage, V 12 Cathode feed rate, mm/min 3.0, 3.1, 3.2, 3.3, 3.4, 3.5 Electrolyte temperature, \(℃\) 25 \(\pm\) 3 Electrolyte inlet pressure, MPa 1.1 Electrolyte outlet pressure, MPa 0.7 Figure 5 illustrates the machining current during spline axial ECM with two different cathodes at a cathode feed rate of 3.2 mm/min. The machining current for the curved proposed cathode demonstrates remarkable consistency, with minimal fluctuations around 168 A at equilibrium, indicating a stable process with no short circuits. In contrast, the conventional cathode exhibits erratic and pronounced current fluctuations, frequently resulting in rapid growth followed by short circuits due to inadequate supply and uneven distribution of electrolyte within the ECM processing gap region. To further confirm the process stability of the curved proposed cathode and enhance the accuracy of spline formation, the spline axial ECM has been performed at varying cathode feed rates, as illustrated in Fig. 6 . It is evident that as the cathode feed rate escalates from 3.0 mm/min to 3.5 mm/min, the machining current rises from approximately 157 A at equilibrium to around 182 A. It maintains a consistent stability throughout the entire ECM process, with no discernible fluctuations. Simultaneously, the ECM side machining gap size decreases from 0.20 mm to 0.11 mm, indicating a higher precision in spline formation achieved by the negative mirror of the curved proposed cathode on the workpiece. However, when the cathode feed rate was increased to 3.6 mm/min or beyond, frequent short circuits were observed. In Fig. 7 (a)(b), the schematic presentation of the left and right tooth flanks areas of the target semi-blind hole involute internal spline workpiece is provide. To assess the accuracy of the process four random sets of left and right tooth flanks areas are selected from the 19 spline profiles that were machined by ECM using two different cathodes. These measurements encompass tooth profile and helical deviation, and they were obtained during the spline axial ECM process with a constant cathode feed rate of 3.2 mm/min. Comparing the results, it is observed that the proposed curved cathode exhibits: (a) An average tooth profile deviation of 177 µm, representing a 15% reduction compared to the conventional cathode's 208.3 µm; (b) An average tooth helical deviation of 19.8 µm, showing a reduction of 13.5% compared to the conventional cathode's 22.9 µm. It is worth noting that some measurement results for the conventional cathode are missing due to short circuits. Figure 8 shows the workpieces after spline axial ECM and finishing slotting. The target deviations for the workpiece are 21 µm for the maximum tooth profile and 10 µm for the maximum tooth helical. Utilizing the curved proposed cathode for spline ECM (Fig. 8 a) yielded successful results, allowing for subsequent finishing slotting. The final finishing slotting outcome (Fig. 8 b) met accuracy requirements, with a maximum tooth profile deviation of 18.6 µm and a maximum tooth helical deviation of 7.7 µm. Significantly, the combined roughing ECM and finishing slotting process took only about 45 minutes, a substantial time-saving compared to the approximately 2 hours required for traditional full slotting process. 4. Conclusion This study introduced a combined process that integrates roughing ECM with precision slotting, achieving a remarkable 62.5% increase in machining speed while maintaining exceptional stability and precision. The results include a maximum tooth profile deviation of 18.6 µm and a maximum tooth helical deviation of 7.7 µm for axial ECM of semi-blind hole involute internal splines in challenging-to-machine materials. This approach holds significant potential for widespread application in the mass production of intricate internal cavity components crafted from high-hardness materials. Declarations Competing interests The authors declare no competing interests to disclose. Funding information This work was supported by the National Key Research and Development Project (2020YFB1713503) and the Fundamental Research Funds for the Central Universities under Grant (No. 20720190009). Author contributions All authors were involved in the conceptualization and design of the study. Yi Huang and Xu Yan performed the cathode design, fluid simulation, data collection and result analysis. Yi Huang and Senlin Gong were responsible for the system construction, experimental operation and result measurement. Yi Huang and Cong Fang wrote the first draft of the manuscript, and all authors reviewed and revised previous versions of the manuscript. All authors read and approved the final manuscript. Acknowledgements The authors would like to thank the China Scholarship Council (CSC) and Prof. Wang from Xiamen University for his novel idea and great support, and the authors for their useful suggestions for this research. References Han X, Chen L, Hu X, Hua L, Chai F (2023) Microstructure and mechanical property evolution mechanisms of 15Cr14Co12Mo5Ni2WA aviation gear steel during cold rotary forging. 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J Mater Process Technol 275:116323 Cite Share Download PDF Status: Published Journal Publication published 20 Feb, 2026 Read the published version in The International Journal of Advanced Manufacturing Technology → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4244272","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":291773220,"identity":"abda1d26-a0b2-4c98-b5c4-7dc3be86a2f4","order_by":0,"name":"Yi Huang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAw0lEQVRIiWNgGAWjYBACA3YeBmYGBgk5IJvxAANDAhFamMFaLIwZgBRJWioSG4jWYs7Me/B2YZtEer9E/oEDHyrSGPjbu/Hrs2zmS7ae2SaRO3NGMsPBGWdyGCTOnN2A32GHecykeYFaNtxIZjjM21bBYABkE6Ul3YBkLQlQLTlEaTG25jknYTiz57EB0C9pPIT9crzH8DZPWZ08P3viwwcfKpLl+Nt78WsBAQlGNgSHh6BysBaGP0SpGwWjYBSMgpEKAF8HQxA5cC+iAAAAAElFTkSuQmCC","orcid":"","institution":"Xiamen University","correspondingAuthor":true,"prefix":"","firstName":"Yi","middleName":"","lastName":"Huang","suffix":""},{"id":291773221,"identity":"1e462b11-0279-48af-a25f-eb15cc550861","order_by":1,"name":"Cong Fang","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Cong","middleName":"","lastName":"Fang","suffix":""},{"id":291773222,"identity":"f71c92c4-afd6-45f3-baf3-14b8f0cff8ff","order_by":2,"name":"Yan Xu","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Yan","middleName":"","lastName":"Xu","suffix":""},{"id":291773223,"identity":"20ea0bcf-0102-4cd4-891d-dadd1df1086e","order_by":3,"name":"Senlin Gong","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Senlin","middleName":"","lastName":"Gong","suffix":""},{"id":291773224,"identity":"e146a389-7338-4688-8b84-a950bbafcbc6","order_by":4,"name":"Xi Wang","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Xi","middleName":"","lastName":"Wang","suffix":""}],"badges":[],"createdAt":"2024-04-10 00:19:04","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4244272/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4244272/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s00170-025-17366-x","type":"published","date":"2026-02-21T00:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":54951261,"identity":"ec430ef5-7984-4ad0-bafb-49b881088eb3","added_by":"auto","created_at":"2024-04-19 05:40:16","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":148635,"visible":true,"origin":"","legend":"\u003cp\u003eThe spline axial ECM process, (a) preparatory stage; (b) processing stage; (c) final stage; (d) section view of A-A in (b) for the cathode and formed workpiece forming section; (e) partial view of a spline tooth area.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-4244272/v1/c9c447aad73e1d168c7fd3ce.png"},{"id":54951262,"identity":"d31e0837-d1c2-4b3c-8196-f0981d3ca774","added_by":"auto","created_at":"2024-04-19 05:40:16","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":142946,"visible":true,"origin":"","legend":"\u003cp\u003eThe target involute internal spline contour \u003cem\u003eABC\u003c/em\u003e and design evolution of proposed cathodes, (a) Stage 1: initial conventional cathode \u003cem\u003ea\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eb\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e\u003cem\u003ec\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e~a\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eb\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003ec\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e; (b) Stage 2: common proposed cathode \u003cem\u003ea\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eb\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e\u003cem\u003ec\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e~Ma\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003ec\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e; (c) Stage 3: curved proposed cathode \u003cem\u003ea\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eb\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e\u003cem\u003ec\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e~NS\u003c/em\u003e; (d) the detailed structure design of the working teeth within the curved proposed cathode at the \u003cem\u003ez-\u003c/em\u003eplane.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-4244272/v1/90f2bdcab57a7edc8d67b76c.png"},{"id":54951264,"identity":"881868ad-f5f0-467e-b8e6-56c8b53eb054","added_by":"auto","created_at":"2024-04-19 05:40:16","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":229570,"visible":true,"origin":"","legend":"\u003cp\u003eThe electrolyte flow distribution within the ECM processing gap region using two cathodes, (a) conventional cathode; (b) curved proposed cathode; section A is longitudinal and section B is transverse ECM processing gap region.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-4244272/v1/61feb427543f85d27d85dce4.png"},{"id":54951260,"identity":"c27f56e0-2edf-4ba2-ad24-43ffaec4202f","added_by":"auto","created_at":"2024-04-19 05:40:16","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":582466,"visible":true,"origin":"","legend":"\u003cp\u003e(a) The assembled spline axial ECM experimental system; (b) The geometric model and object of ECM fixture.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-4244272/v1/84ea025d5df1381fc7f624b0.png"},{"id":54951266,"identity":"ddb8e593-4d1b-45b2-a084-adedaab476be","added_by":"auto","created_at":"2024-04-19 05:40:16","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":44117,"visible":true,"origin":"","legend":"\u003cp\u003eThe machining current for spline axial ECM with two cathodes at a cathode feed rate of 3.2 mm/min.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-4244272/v1/750c341f68c13fa9f703d56a.png"},{"id":54951655,"identity":"f1db23db-1af9-4b29-9b2f-2848e3ac92f6","added_by":"auto","created_at":"2024-04-19 05:48:16","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":71636,"visible":true,"origin":"","legend":"\u003cp\u003eThe machining current for spline axial ECM with proposed curved cathode at varying cathode feed rates.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-4244272/v1/6b8199bbb815b3483335feb2.png"},{"id":54951263,"identity":"aa8fd95a-ed43-44f7-84d1-e95caa666383","added_by":"auto","created_at":"2024-04-19 05:40:16","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":73253,"visible":true,"origin":"","legend":"\u003cp\u003e(a) The target semi-blind hole involute internal spline workpiece; (b) the left and right tooth flanks areas in a spline tooth; (c) the machining deviation in spline axial ECM with two cathodes at a cathode feed rate of 3.2 mm/min.\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-4244272/v1/665734e186280cccf0b0a7b7.png"},{"id":54951267,"identity":"c67b8702-2aad-424c-a047-1e33f382f077","added_by":"auto","created_at":"2024-04-19 05:40:16","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":166665,"visible":true,"origin":"","legend":"\u003cp\u003eWorkpieces machined after (a) spline axial ECM; (b) slotting.\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-4244272/v1/a8ded3ae3355d2e01d3eff80.png"},{"id":103270980,"identity":"3fc8e4e9-479c-4aea-9c64-5fe0025cbe21","added_by":"auto","created_at":"2026-02-23 22:04:32","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2270147,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4244272/v1/ac9611cf-5974-40c8-89ae-719317ab6390.pdf"}],"financialInterests":"","formattedTitle":"A combined process of electrochemical machining and slotting for semi-blind hole involute internal splines enhanced by a new cathode design","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eIn recent years, there has been growing interest in high-strength, durable involute internal spline key components for use in aeromechanical transmission systems [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. These components excel in efficiently transmitting larger loads and torque, thereby reducing wear and extending operational lifespans [\u003cspan additionalcitationids=\"CR4\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. However, due to the challenging machining characteristics of Ni-based, Ti-based or Cr-Co-Mo-based alloys [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e], particularly when combined with intricate internal cavity configurations like semi-blind holes, crafting these spline parts remains a significant challenge. The prevailing method for producing precise high-hardness involute internal spline components in relevant industries currently relies on a complete slotting process, which suffers from issues such as extensive tool wear and prolonged production cycles [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eElectrochemical machining (ECM) has emerged as a cost-effective and vital option in the aerospace sector, particularly in the roughing stage [\u003cspan additionalcitationids=\"CR7\" citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. ECM offers numerous advantages, including the elimination of tool wear, irrespective of a component's high hardness and complex structure, along with high metal removal rates [\u003cspan additionalcitationids=\"CR10 CR11 CR12\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. However, achieving high precision in machining semi-blind hole involute internal splines can be challenging with ECM alone due to the relatively limited machining precision. To address this challenge, a combined process that integrates roughing ECM with a precision slotting is proposed to achieve the involute internal spline shaping for intricate high-hardness components [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. During the ECM phase, the use of a well-designed cathode is crucial for ensuring a smooth ECM process [\u003cspan additionalcitationids=\"CR17 CR18\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. This research introduces a specialized cathode design tailored for ECM applied to semi-blind hole involute internal splines within high-hardness materials. This combined process and proposed cathode design demonstrate significant potential for widespread adoption in the large-scale production of complex internal cavity components crafted from challenging-to-machine materials.\u003c/p\u003e"},{"header":"2. Proposed combined process","content":"\u003cp\u003eIn semi-blind hole involute internal splines, the spline profiles remain consistent along the axis, featuring dimensions larger than those of the lower section. The unique semi-blind hole structure involves a two-step spline-forming process. This paper concentrates on the initial step, which involves the efficient machining of all initial spline contours using axial ECM. This step provides the necessary allowances for the subsequent finishing slotting process.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e illustrates the axial ECM process for semi-blind hole involute internal splines. In this procedure, the workpiece is connected to the positive pole of the DC power supply, while the tool cathode is connected to the negative pole and moved toward the workpiece at a specific feed rate. During the preparatory stage (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea), fresh electrolyte is introduced from the bottom and exits from the top, establishing a favourable and unenergized ECM initial state. In the processing stage (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb), as power is activated and the cathode continuously advances axially, the ECM gap forms, and the fresh electrolyte engages in electrochemical reactions, resulting in by-products such as electrolytic sludge. This by-product contributes to the formation of contaminated electrolyte, which exits from the top. The entire process continues until all initial spline profiles have been electrochemically machined. In the final stage (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec), as the cathode moves toward the tool withdrawal area, the power is deactivated, indicating the completion of the spline ECM process.\u003c/p\u003e \u003cp\u003eSubsequent to the initial spline profiles being machined via spline axial ECM, with an adequate allowance area in section A-A (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed and e), the second step involves achieving precision tolerance ranges by forming the involute internal spline contours through the finishing slotting process.\u003c/p\u003e"},{"header":"3. Design evolution of proposed cathodes","content":"\u003cp\u003eTo enhance the process stability of spline axial ECM, the cathode structure and shape are systematically analysed and proposed, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Initially, the conventional cathode design is considered, represented as \u003cem\u003ea\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eb\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e\u003cem\u003ec\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e\u0026thinsp;\u003cem\u003e~\u0026thinsp;a\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eb\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003ec\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea). This design involves inwardly reducing the target involute internal spline contour \u003cem\u003eABC\u003c/em\u003e by 0.5 mm, with the first 0.3 mm serving as a slotting finishing allowance and the subsequent 0.2 mm defining the ECM side machining gap size. However, this unoptimized design is susceptible to short-circuiting due to an uneven electrolyte distribution in the ECM processing gap region.\u003c/p\u003e \u003cp\u003eIn response to this challenge, a commonly proposed cathode design, denoted as \u003cem\u003ea\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eb\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e\u003cem\u003ec\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e\u0026thinsp;\u003cem\u003e~\u0026thinsp;Ma\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003ec\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb), is introduced. This design involves the modification of the initial conventional cathode working teeth into a chamfered structure. Nonetheless, challenges persist in ECM machining, especially in the tooth root circle forming region of the spline profiles.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo further enhance the structure, a \"three-side feeding\" design approach [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e] is adopted in the working teeth region of the curved proposed cathode, labelled as \u003cem\u003ea\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eb\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e\u003cem\u003ec\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e\u0026thinsp;\u003cem\u003e~\u0026thinsp;NS\u003c/em\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec). This design exhibits a gradual transition from the point to the initial conventional cathode working teeth profile section. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(d) presents a detailed structure design of the working teeth within the curved proposed cathode at the \u003cem\u003ez\u003c/em\u003e-plane. The thickness of the cathode working teeth area is determined to be 2.2 mm, influenced by the tool withdrawal area with a height of 4 mm. This region comprises a 2 mm front variable section zone (\u003cem\u003eLNRS\u003c/em\u003e) and a 0.2 mm equal section zone (\u003cem\u003ea\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eb\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e\u003cem\u003ec\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e\u0026thinsp;\u003cem\u003e~\u0026thinsp;RNL\u003c/em\u003e) at the rear. The curved structure (\u003cem\u003eLNRS\u003c/em\u003e) is formed by connecting all lines (\u003cem\u003eN\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003e1\u003c/em\u003e\u003c/sub\u003e, \u003cem\u003eN\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e, \u003cem\u003eN\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e\u0026hellip;N\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u0026minus;2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u0026minus;2\u003c/em\u003e\u003c/sub\u003e, \u003cem\u003eN\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u0026minus;1\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u0026minus;1\u003c/em\u003e\u003c/sub\u003e, \u003cem\u003eN\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e) parallel to line \u003cem\u003eNS\u003c/em\u003e. This segment undergoes a gradual expansion from a point to the initial spline profile, ensuring a seamless ECM transition from point to surface and uniform electrolyte flow distribution in the ECM processing gap region. The 0.2 mm rear section corresponds to the equal section of the initial conventional cathode working teeth profile, facilitating precise alignment of the front ECM-formed contours. Moreover, non-working areas of the cathode and the rod body are subsequently shielded with an insulation layer to protect the internal non-machined area of the semi-blind hole from electrolytic corrosion.\u003c/p\u003e"},{"header":"4. Validation and discussion","content":"\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e4.1 Electrolyte flow distribution in ECM processing gap region\u003c/h2\u003e \u003cp\u003eAchieving a uniform and stable distribution of electrolyte flow within the ECM processing gap region is essential for the successful operation of spline axial ECM [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. High-speed electrolyte flow serves the crucial role of efficiently removing by-products generated within the narrow ECM processing gap region, thereby preventing issues like electrolyte starvation and cavitation, which can potentially lead to short circuits. The distribution of the electrolyte flow field in ECM process is simulated using the COMSOL Multiphysics software. Several key assumptions are made regarding the behaviour of the electrolyte fluid: (a) The electrolyte is considered incompressible, displaying constant Newtonian behaviour. It is assumed to be free from air bubbles and solid particles, maintaining consistent dynamic viscosity and density; (b) any energy loss within the electrolyte caused by temperature and concentration changes is disregarded; (c) the fluid adheres to the principles of mass and momentum conservation. Given the complex geometry of the electrolyte fluid flow path in the ECM processing gap region, the flow field simulation employed the k-\u0026#120596; turbulence model, based on Navier-Stokes equation [\u003cspan additionalcitationids=\"CR23\" citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\frac{\\partial }{{\\partial }_{t}}\\left(\\rho k\\right)+\\nabla \\bullet \\left(\\rho kV\\right)=\\nabla \\bullet \\left[\\left(\\mu +{\\sigma }_{k}{\\mu }_{t}\\right)\\nabla k\\right]+{P}_{k}-\\rho {\\beta }^{*}{f}_{{\\beta }^{*}}(\\omega k-{\\omega }_{0}{k}_{0})+{S}_{k}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\frac{\\partial }{{\\partial }_{t}}\\left(\\rho \\omega \\right)+\\nabla \\bullet \\left(\\rho \\omega V\\right)=\\nabla \\bullet \\left[\\left(\\mu +{\\sigma }_{\\omega }{\\mu }_{t}\\right)\\nabla \\omega \\right]+{P}_{\\omega }-\\rho \\beta {f}_{\\beta }({\\omega }^{2}-{{\\omega }_{0}}^{2})+{S}_{\\omega }$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u0026#120588; is the electrolyte density, \u003cem\u003eV\u003c/em\u003e is the mean velocity, \u0026#120583; is the dynamic viscosity, \u0026#119896; is the kinetic energy, \u0026#120596; is the specific dissipation rate. In actual flow field simulations, the electrolyte dynamic viscosity is 1.005\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\times\\)\u003c/span\u003e\u003c/span\u003e10\u003csup\u003e\u0026minus;3\u003c/sup\u003e Pa\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\bullet\\)\u003c/span\u003e\u003c/span\u003es, the density is 1.089\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\times\\)\u003c/span\u003e\u003c/span\u003e10\u003csup\u003e3\u003c/sup\u003e kg/m\u003csup\u003e3\u003c/sup\u003e, the inlet pressure is 1.1 MPa and the outlet pressure is 0.7 MPa.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e provides insight into the distribution of electrolyte flow within the ECM processing gap region, comparing two cathode designs. Sections A and B depict the longitudinal and transverse velocity distributions of the electrolyte within the ECM processing gap region. In Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e (a) (section A), the electrolyte flow encounters an obtuse turn followed by a right-angle turn before entering the ECM processing gap region, resulting in high resistance, slow flow, inadequate electrolyte supply, and the potential for short circuits. Section B shows an uneven flow velocity, rapidly decreasing from non-machined areas to the involute tooth root circle when using the conventional cathode. This unevenness is attributed to the elongated and intricate spline profile flow path. Conversely, in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e (b) (section A), the variable-section cathode working teeth simplify the flow path, reducing resistance, and enhancing electrolyte supply for spline axial ECM. Correspondingly, section B exhibits a more uniform and less steep flow velocity gradient, facilitating successful spline axial ECM.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e4.2 Spline axial ECM experiments\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e illustrates the experimental setup for spline axial ECM and its associated fixture. The ECM experimental system (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea) consists of a CNC machine, an operational platform, a fixture, a DC power supply, electrolyte tanks and a filtration system. It can be divided into four functional units: (a) Fresh Electrolyte Supply: This unit ensures a continuous supply of clean electrolyte to the ECM processing gap region, maintaining a high inlet pressure of 1.1 MPa. A vertical centrifugal pump is used for by-product removal; (b) DC Power Supply: The DC power supply unit is responsible for powering the ECM process. It connects the positive pole to the lower fixture, energizing the workpiece, and connects the negative pole to the tool cathode; (c) CNC Machine Operation: In this unit, the CNC machine controls the tool cathode feed rate and direction with Z-axis movement. It also manages the initial ECM location and fixture positioning using X and Y-axis movements; (d) Contaminated Electrolyte Filtration and Circulation: This unit incorporates a contaminated electrolyte tank with varying screen heights and porosities, as well as filters of different pore sizes. These components are designed to intercept ECM by-products, including hard-to-dissolve electrolyte sludge and bubbles. Additionally, this unit plays a role in cooling and recycling the filtered electrolyte.\u003c/p\u003e \u003cp\u003eThe fixture components include an upper fixture, a lower fixture, a tool cathode, electrolyte tubes, a Bakelite adapter, a toolholder, and a marble base (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb). The workpiece is positioned within the lower fixture. The tool cathode is linked to the upper fixture through O-ring sealing and securely fastened to the lower fixture with four bolts, allowing vertical movement without any electrolyte leakage. Fresh electrolyte enters the lower fixture through the electrolyte inlet tube and exits from the upper end after ECM process. The marble base provides insulation between the fixture and the CNC machine table, while the Bakelite adapter serves as both an insulator and a heat-resistant barrier to withstand the elevated ECM temperatures.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe relevant ECM experiments of semi-blind hole involute internal splines are carried out to assess process stability and accuracy using two different cathodes. The key process parameters, including the electrolyte solution, machining voltage, and cathode feed rate, are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eProcess parameters of ECM experiments\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"2\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eProcess parameters\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eValues or conditions\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCathode material\u003c/p\u003e \u003cp\u003eCathode types\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSUS 304\u003c/p\u003e \u003cp\u003eConventional and proposed\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTest piece material\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e15Cr14Co12Mo5Ni2WA\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTest piece hardness\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eHRC 48\u0026ndash;52\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCathode feed depth, mm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e39.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eInitial ECM surface gap, mm\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eElectrolyte solution\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e8% NaNO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMachining voltage, V\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCathode feed rate, mm/min\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3.0, 3.1, 3.2, 3.3, 3.4, 3.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eElectrolyte temperature, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(℃\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e25\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\pm\\)\u003c/span\u003e\u003c/span\u003e3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eElectrolyte inlet pressure, MPa\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eElectrolyte outlet pressure, MPa\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.7\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e illustrates the machining current during spline axial ECM with two different cathodes at a cathode feed rate of 3.2 mm/min. The machining current for the curved proposed cathode demonstrates remarkable consistency, with minimal fluctuations around 168 A at equilibrium, indicating a stable process with no short circuits. In contrast, the conventional cathode exhibits erratic and pronounced current fluctuations, frequently resulting in rapid growth followed by short circuits due to inadequate supply and uneven distribution of electrolyte within the ECM processing gap region.\u003c/p\u003e \u003cp\u003eTo further confirm the process stability of the curved proposed cathode and enhance the accuracy of spline formation, the spline axial ECM has been performed at varying cathode feed rates, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. It is evident that as the cathode feed rate escalates from 3.0 mm/min to 3.5 mm/min, the machining current rises from approximately 157 A at equilibrium to around 182 A. It maintains a consistent stability throughout the entire ECM process, with no discernible fluctuations. Simultaneously, the ECM side machining gap size decreases from 0.20 mm to 0.11 mm, indicating a higher precision in spline formation achieved by the negative mirror of the curved proposed cathode on the workpiece. However, when the cathode feed rate was increased to 3.6 mm/min or beyond, frequent short circuits were observed.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(a)(b), the schematic presentation of the left and right tooth flanks areas of the target semi-blind hole involute internal spline workpiece is provide. To assess the accuracy of the process four random sets of left and right tooth flanks areas are selected from the 19 spline profiles that were machined by ECM using two different cathodes. These measurements encompass tooth profile and helical deviation, and they were obtained during the spline axial ECM process with a constant cathode feed rate of 3.2 mm/min. Comparing the results, it is observed that the proposed curved cathode exhibits: (a) An average tooth profile deviation of 177 \u0026micro;m, representing a 15% reduction compared to the conventional cathode's 208.3 \u0026micro;m; (b) An average tooth helical deviation of 19.8 \u0026micro;m, showing a reduction of 13.5% compared to the conventional cathode's 22.9 \u0026micro;m. It is worth noting that some measurement results for the conventional cathode are missing due to short circuits.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e shows the workpieces after spline axial ECM and finishing slotting. The target deviations for the workpiece are 21 \u0026micro;m for the maximum tooth profile and 10 \u0026micro;m for the maximum tooth helical. Utilizing the curved proposed cathode for spline ECM (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea) yielded successful results, allowing for subsequent finishing slotting. The final finishing slotting outcome (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eb) met accuracy requirements, with a maximum tooth profile deviation of 18.6 \u0026micro;m and a maximum tooth helical deviation of 7.7 \u0026micro;m. Significantly, the combined roughing ECM and finishing slotting process took only about 45 minutes, a substantial time-saving compared to the approximately 2 hours required for traditional full slotting process.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eThis study introduced a combined process that integrates roughing ECM with precision slotting, achieving a remarkable 62.5% increase in machining speed while maintaining exceptional stability and precision. The results include a maximum tooth profile deviation of 18.6 \u0026micro;m and a maximum tooth helical deviation of 7.7 \u0026micro;m for axial ECM of semi-blind hole involute internal splines in challenging-to-machine materials. This approach holds significant potential for widespread application in the mass production of intricate internal cavity components crafted from high-hardness materials.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eCompeting interests\u003c/h2\u003e \u003cp\u003eThe authors declare no competing interests to disclose.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFunding information\u003c/h2\u003e \u003cp\u003eThis work was supported by the National Key Research and Development Project (2020YFB1713503) and the Fundamental Research Funds for the Central Universities under Grant (No. 20720190009).\u003c/p\u003e\u003ch2\u003eAuthor contributions\u003c/h2\u003e \u003cp\u003eAll authors were involved in the conceptualization and design of the study. Yi Huang and Xu Yan performed the cathode design, fluid simulation, data collection and result analysis. Yi Huang and Senlin Gong were responsible for the system construction, experimental operation and result measurement. Yi Huang and Cong Fang wrote the first draft of the manuscript, and all authors reviewed and revised previous versions of the manuscript. All authors read and approved the final manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eThe authors would like to thank the China Scholarship Council (CSC) and Prof. Wang from Xiamen University for his novel idea and great support, and the authors for their useful suggestions for this research.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eHan X, Chen L, Hu X, Hua L, Chai F (2023) Microstructure and mechanical property evolution mechanisms of 15Cr14Co12Mo5Ni2WA aviation gear steel during cold rotary forging. 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Chin J Aeronaut 34(2):28\u0026ndash;53\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRajurkar KP, Zhu D, McGeough JA, Kozak J, De Silva A (1999) New developments in electro-chemical machining. CIRP Ann 48(2):567\u0026ndash;579\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRajurkar KP, Sundaram MM, Malshe AP (2013) Review of electrochemical and electrodischarge machining. Procedia CIRP 6:13\u0026ndash;26\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKlocke F, Klink A, Veselovac D, Aspinwall DK, Soo SL, Schmidt M, Schilp J, Levy G, Kruth JP (2014) Turbomachinery component manufacture by application of electrochemical, electro-physical and photonic processes. CIRP Ann 63(2):703\u0026ndash;726\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eChen X, Qu N, Li H, Xu Z (2015) Pulsed electrochemical micromachining for generating micro-dimple arrays on a cylindrical surface with a flexible mask. 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CIRP Ann 69(1):157\u0026ndash;160\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJianshe Z, Feng W, Zhuang L, Xiangli Z, Weimin G, Zongjun T (2016) Flow field design and process stability in electrochemical machining of diamond holes. Chin J Aeronaut 29(6):1830\u0026ndash;1839\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhang J, Zhao C, Qu N, Shen Z (2022) Enhancement of surface quality in the electrochemical milling of 316L using a novel cathode structure. Precis Eng 78:134\u0026ndash;145\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLi J, Wang D, Zhu D, He B (2020) Analysis of the flow field in counter-rotating electrochemical machining. J Mater Process Technol 275:116323\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Electrochemical machining (ECM), Involute internal spline, Cathode design, Flow field simulation","lastPublishedDoi":"10.21203/rs.3.rs-4244272/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4244272/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eMachining complex internal structures in high-hardness materials presents a significant challenge. Conventional cutting methods often suffer from issues such as extensive tool wear and prolonged production cycles. This research proposes a combined process that integrates roughing electrochemical machining (ECM) with a precision slotting to achieve fast involute internal spline shaping of intricate high-hardness components. To enhance the process stability of the spline ECM process, a curved cathode design tailored for ECM is introduced. Flow field simulations demonstrate that the proposed curved cathode design leads to a relatively uniform distribution of electrolyte flow in the machining area. A dedicated ECM fixture and system are established for conducting experiments. The obtained ECM outcomes substantiate the commendable capacity of the optimized curved cathode to accomplish high precise and efficient machining and shaping of splines. Consequently, the optimized curved cathode design exhibits substantial potential for widespread application in the large-scale production of intricate internal cavity components composed of challenging-to-machine materials.\u003c/p\u003e","manuscriptTitle":"A combined process of electrochemical machining and slotting for semi-blind hole involute internal splines enhanced by a new cathode design","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-04-19 05:40:11","doi":"10.21203/rs.3.rs-4244272/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"6c6b85a1-26ff-41cd-a007-645933a520a9","owner":[],"postedDate":"April 19th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-02-23T22:04:22+00:00","versionOfRecord":{"articleIdentity":"rs-4244272","link":"https://doi.org/10.1007/s00170-025-17366-x","journal":{"identity":"the-international-journal-of-advanced-manufacturing-technology","isVorOnly":false,"title":"The International Journal of Advanced Manufacturing Technology"},"publishedOn":"2026-02-21 00:00:00","publishedOnDateReadable":"February 21st, 2026"},"versionCreatedAt":"2024-04-19 05:40:11","video":"","vorDoi":"10.1007/s00170-025-17366-x","vorDoiUrl":"https://doi.org/10.1007/s00170-025-17366-x","workflowStages":[]},"version":"v1","identity":"rs-4244272","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4244272","identity":"rs-4244272","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}