Surface-Quality Optimisation in Cobalt Ferrite Ultrasonic Elliptical Vibration Cutting of H62 Brass

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors wrote an article entitled: "Surface-Quality Optimisation in Cobalt-Ferrite Ultrasonic Elliptical Vibration Cutting of H62 Brass", which deals with the optimisation of surface quality in ultrasonic elliptical vibration cutting using a tool excited by the magnetostrictive material cobalt ferrite.
Ideas for improvement:
- The tables do not contain standard deviations, confidence intervals or error bars. Without this, it is impossible to assess whether the differences between individual parameters are truly statistically significant
- Only one parameter is changed at a time, while the others are kept constant. This does not allow the interactions between feed, speed, depth of cut and phase to be captured. Discuss this
- For example, the statement that the relative improvement is greatest at a small depth of cut and decreases with increasing depth does not clearly correspond to Table III. In it, on the contrary, the reduction in the cutting direction reaches 41.6% at a depth of 10 μm, which is the highest value in the given table. How is this so? Is it just a misinterpretation?
- it is stated that the phase was varied from 0° to 180° in 30° steps, but the value of 150° is missing from Table IV.
- The reduction of friction, heat and temperature effects is repeatedly mentioned, but the temperature is not measured. These claims should be substantiated
- It is claimed that the tool was changed after twenty cuts and no measurable wear was observed, but they do not show any pictures of the cutting edge or a method of measuring wear. For a diamond tool in brass machining, wear is probably small, but the claim should be substantiated either by a picture or better discussed
- The symbol f is used for both the ultrasonic frequency and in some places for the feed. This can lead to confusion. use a subscript.

Author Response

Comments 1: The tables do not contain standard deviations, confidence intervals or error bars. Without this, it is impossible to assess whether the differences between individual parameters are truly statistically significant.

Response 1: Thank you for this important comment. We agree completely. We have added standard deviations to all data presented in the tables.

Comments 2: Only one parameter is changed at a time, while the others are kept constant. This does not allow the interactions between feed, speed, depth of cut and phase to be captured. Discuss this.

Response 2: Thank you. We agree. We have, accordingly, added an explicit acknowledgement of this limitation  in Section 8 (Page 11, last paragraph) of the revised manuscript. The single-factor design was chosen to resolve the non-monotonic individual response curves (in particular the feed-rate and phi optima) that a five-level Taguchi grid would under-sample. We acknowledge that this design does not capture factor interactions, and a follow-up Taguchi/response-surface campaign that includes interactions between feed, speed, depth and phase has now been identified as future work and added to Section 8.

Comments 3: For example, the statement that the relative improvement is greatest at a small depth of cut and decreases with increasing depth does not clearly correspond to Table III. In it, on the contrary, the reduction in the cutting direction reaches 41.6% at a depth of 10 um, which is the highest value in the given table. How is this so? Is it just a misinterpretation?

Response 3: Thank you very much for catching this. We agree this was an unclear statement. We have, accordingly, rewritten the relevant text (Page 8, paragraph 2, lines 287-298) of the revised manuscript to remove the misinterpretation. The corrected statement now reads: "In the cutting direction the percentage reduction is non-monotonic, in the 22-42 % range across the full 0.5-10 um depth window, with the highest single reduction (41.6 %) occurring at ap = 10 um where the CC surface degrades sharply due to scaly burr formation while the UEVC surface remains comparatively smooth; in the feed direction the reduction grows monotonically from 11.5 % at ap = 1 um to 49.6 % at ap = 10 um, consistent with the suppression of scaly burrs visible on the CC surfaces above ap = 4 um."

Comments 4: It is stated that the phase was varied from 0 deg to 180 deg in 30 deg steps, but the value of 150 deg is missing from Table IV.

Response 4: Thank you for this careful observation. The phi = 150 deg row was indeed inadvertently omitted in the original submission(Page 9, lines 320).

Comments 5: The reduction of friction, heat and temperature effects is repeatedly mentioned, but the temperature is not measured. These claims should be substantiated.

Response 5: Thank you. We agree that this was an over-statement. Cutting temperature was not directly measured in this study; the references to reduced friction-generated heat and reduced cutting temperature are now explicitly identified as inferences from the literature on UEVC of ductile metals (Yang et al., Int. J. Mach. Tools Manuf. 2020; Zheng et al., Int. J. Adv. Manuf. Technol. 2020), where temperature reductions of 15-40 % under comparable cutting conditions have been reported using embedded thermocouples and infrared thermography. The only direct evidence in the present paper is the absence of thermal discolouration on the machined surfaces and the absence of measurable diamond-tool wear after twenty cuts. Direct temperature measurement using a fibre-optic infrared probe co-mounted on the diamond holder is now identified as future work.

Comments 6: It is claimed that the tool was changed after twenty cuts and no measurable wear was observed, but they do not show any pictures of the cutting edge or a method of measuring wear. For a diamond tool in brass machining, wear is probably small, but the claim should be substantiated either by a picture or better discussed.

Response 6: Thank you. We have, accordingly, substantiated this claim in Section 2.2 (Page 5, paragraph 1, lines 170-176) of the revised manuscript and provided supporting micrographs in the Supplementary Material (Figure 2). Figure 2 presents the cutting edge morphology after twenty cutting passes. The inspection was conducted using the same Keyence VK-X1000 laser confocal micro-scope. The inspection was performed in two orthogonal directions on the cutting edge (rake and clearance face) and the apex radius of the nose was extracted from the height map by least-squares circle fit. Across all tools the apex-radius change between before-cut and after-twenty-cuts inspection was below 0.4 um, i.e. below the lateral resolution of the confocal microscope and below 0.2 % of the nominal 0.2 mm nose ra-dius.

Comments 7: The symbol f is used for both the ultrasonic frequency and in some places for the feed. This can lead to confusion, use a subscript.

Response 7: Thank you. We agree. We have, accordingly, disambiguated the notation throughout the revised manuscript (introduced in Section 2.3, Page 5, paragraph 3, lines 202-212, and applied consistently in the text, tables and figure captions): f_us now denotes the ultrasonic working frequency (f_us = 46.4 kHz) and f denotes the feed rate per revolution. Equation (1) for the speed ratio therefore reads K = Vc / (2*pi*f_us*A_y).

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors,

in my opinion, this is a very good manuscript. The research is correctly designed and performed. Furthermore, the presentation is remarkable. Concise, with as few mathematical relationships as possible, but enough to make the presentation easy to understand. Everything in very good English.

The figures are clear and suggestive. The results are clearly stated and well-reasoned.

The conclusion summarizes the research and its results, giving a good general picture of the work. The conclusion also recommends the optimal combination of input parameters for the practical application of the results in industry.

 

However, some questions arise from the way the research was designed and presented:

• Please comment on the possibility of using the Taguchi method to design the experiment and/or reason your choice to sweep the four input parameters consecutively.
• Is the order in which the parameters were chosen to be swept important? If yes, please reason it.
• Another order of sweeping the parameter would have led to the same final result (the optimum combination)?
• For a more intuitive interpretation of the influence of inter-channel electrical phase difference φ between the two pairs is used to control the shape and orientation of the resulting tool-tip ellipse, could it be possible to represent several different phase differences, and the resultant, along with the ellipse shape and orientation? 

Some other specific remaks are displayed in the attached file.

Comments for author File: Comments.pdf

Author Response

Comments 1: Please comment on the possibility of using the Taguchi method to design the experiment and/or reason your choice to sweep the four input parameters consecutively.

Response 1: Thank you for this insightful comment. We agree that a justification is needed. We have, accordingly, added a paragraph in Section 2.3 (Page 5, paragraph 5, lines 213-222) explaining that a Taguchi orthogonal-array design (e.g. L25 with four factors at five levels) would have delivered a main-effects ranking and signal-to-noise ratios. We deliberately chose a single-factor sweep instead because the purpose of this study is not to compress the design space but to map the response curve of each individual factor and, in particular, to resolve the non-monotonic behaviour of the feed rate and of the inter-channel phase difference, which a five-level Taguchi grid would have under-sampled (e.g. the phi = 60 deg optimum sits between two grid points of an L25 design). The single-factor sweep also keeps the speed ratio K constant within each section and therefore decouples the kinematic regime from the parameter under study.

Comments 2: Is the order in which the parameters were chosen to be swept important? If yes, please reason it.

Response 2: Thank you. We have, accordingly, added a clarification in Section 2.3 (Page 6, paragraph 2, lines 223-232) of the revised manuscript. In a strict single-factor design the four sweeps are independent of one another because, in each sweep, the other three parameters are fixed at the same reference value. The order in which the four sweeps are run in the laboratory therefore has no effect on the final identified optimum (f = 1 um/rev, Vc = 50-100 mm/s, ap = 1-2 um, phi = 60 deg). It does affect the amount of intermediate exploration required: we ran feed-rate first because the kinematic relation Ra approx f^2/(8r) gives an a-priori estimate of the optimal feed; we ran phi last because it is the strongest single lever and we wanted to apply it at the already-optimised (f, Vc, ap) reference point.

Comments 3: Another order of sweeping the parameters would have led to the same final result (the optimum combination)?

Response 3: Yes. As detailed in our reply to the previous comment and as now stated explicitly in Section 2.3 (Page 6, paragraph 4, lines 226-232), running the same four sweeps in any other order would have led to the same optimum combination, only via a different intermediate path; the optimum is determined by the response surfaces themselves and not by the sequencing of the experiments.

Comments 4: For a more intuitive interpretation of the influence of inter-channel electrical phase difference phi between the two pairs used to control the shape and orientation of the resulting tool-tip ellipse, could it be possible to represent several phase differences, and the resultant, along with the ellipse shape and orientation?

Response 4: Thank you for this excellent suggestion. We agree that relevant illustrations are missing to demonstrate the effect of the electrical phase difference φ between the two vibrators, which governs the shape and orientation of the elliptical tool tip trajectory. We will supplement the diagrams showing different phase differences, combined motion results as well as the corresponding shape and orientation of ellipses in our future work.

Reviewer 3 Report

Comments and Suggestions for Authors

Title: Surface-Quality Optimisation in Cobalt-Ferrite Ultrasonic El- 2 liptical Vibration Cutting of H62 Brass

 

Manuscript ID：coatings-4343289

 

 

This study investigates the optimization of key process parameters in a cobalt-ferrite (CoFe₂O₄) magnetostrictive ultrasonic elliptical vibration cutting (UEVC) tool for improving the surface quality of H62 brass. Experiments showed that feed rate, cutting speed, cutting depth, and especially inter-channel phase difference significantly affect surface roughness, with the best performance achieved at a 60° phase difference. Compared to conventional cutting, the proposed UEVC system reduced surface roughness by up to 57%, demonstrating its strong potential as a low-cost alternative for precision machining.

• Please divide Figure 1 into two separate subfigures as Figure 1(a) and Figure 1(b), and provide appropriate labels/captions for each part to improve clarity and readability.
• Why was this material selected in the study? Please explain the rationale for its selection.
• The experimental setup should be added to the manuscript. Including this information would be beneficial for readers and would improve the reproducibility and clarity of the study.
• In addition, the imaging device used in the study should also be included in the experimental setup section.
• Please explain why Ra was selected as the surface roughness parameter in this study. The justification for choosing Ra instead of other roughness parameters should be clarified.
• Please provide more detailed basis and reference for selecting cutting parameters and levels. Please specify. Aren't the cutting parameters too low? Can these parameters be used in industrial conditions?
• What is the experimental uncertainty?
• 3D images obtained from the surface roughness measurement device can be included in the manuscript. This would improve the clarity of the results and potentially increase the paper’s visibility and citation impact.
• What are the standards used in the tests and measurements?
• The obtained results should be compared with the existing literature and discussed using appropriate scientific terminology. Rather than simply stating that a parameter increased or decreased, the underlying phenomenon and mechanisms responsible for the observed behavior should be clearly explained.
• The conclusion section should be shortened and presented in bullet-point format with more study-specific findings. This would improve clarity and better highlight the main contributions of the work.

Author Response

Comments 1: Please divide Figure 1 into two separate subfigures as Figure 1(a) and Figure 1(b), and provide appropriate labels/captions for each part to improve clarity and readability.

Response 1: Thank you for pointing this out. We agree with this comment. Therefore, we have split Figure 1 into two sub-figures: Figure 1(a) assembled dual-bending UEVC tool and Figure 1(b) UEVC cutting device. The new caption and sub-labels have been inserted in Section 2.1 (Page 3, immediately after the Figure 1 caption, paragraph 3, line 113).

Comments 2: Why was this material selected in the study? Please explain the rationale for its selection.

Response 2: Thank you for raising this point. We agree that a clear rationale is needed. Accordingly, we have added a dedicated paragraph in Section 2.2 (Page 4, paragraph 2, lines127-135) that justifies the choice of H62 brass on three grounds: (i) it is a representative ductile non-ferrous engineering alloy widely used in precision fluid-handling and electronic components, where surface roughness directly affects sealing performance and electrical contact resistance; (ii) its excellent machinability with a single-crystal diamond cutter avoids confounding the UEVC-related roughness reduction with diamond-tool graphitisation wear that would otherwise dominate on ferrous workpieces.

Comments 3: The experimental setup should be added to the manuscript. Including this information would be beneficial for readers and would improve the reproducibility and clarity of the study.

Response 3: Agreed. We have, accordingly, expanded Section 2.2 (Page 3, paragraph 1, lines 117-126) Cutting tests are carried out on a custom-built five-axis precision lathe. The X/Y/Z linear stages have a 0.5 μm displacement resolution, and the U/V rotary stages a 0.05° angular resolution. The UEVC tool is mounted on the moving platform with the dia-mond cutter aligned to the spindle axis, and the workpiece (H62 brass, hardness 80 HB) is held by a precision collet on the spindle. The two-channel ultrasonic supply (RIGOL DG1022V signal generator + Aigtek ATA-3082 power amplifier) feeds the two channels of the tool through series-resonance compensation capacitors selected so that the input impedance of each coil is reduced to below 1.2 mΩ at the working frequency. A current probe and a voltage probe are used to confirm that the excitation amplitude stays at 1.5 A throughout each test.

Comments 4: In addition, the imaging device used in the study should also be included in the experimental setup section.

Response 4: Thank you. We have added the full imaging-device description in Section 2.2 (Page 4, paragraph 4, lines 145-149) of the revised manuscript: " Imaging device, measurement standards and uncertainty. Surface topography is acquired with a Keyence VK-X1000 laser confocal microscope (violet 408 nm laser, 50x objective, 1 nm vertical and 0.5 um lateral resolution, X-Y stage repeatability +/- 0.2 um). Each 250x250 um area is captured with a 0.05 um z-step and processed using the manufacturer's MultiFileAnalyser software."

Comments 5: Please explain why Ra was selected as the surface roughness parameter in this study. The justification for choosing Ra instead of other roughness parameters should be clarified.

Response 5: Thank you for this comment. We have, accordingly, added a justification in Section 2.2 (Page 4, paragraph 4, lines 149-159) explaining that Ra was selected because (i) it is the most commonly reported parameter in the UEVC literature and therefore enables direct cross-study comparison, (ii) it is the parameter specified in the industry acceptance standard ISO 1302 for finished functional surfaces, and (iii) for the type of periodic micro-pattern produced by UEVC, Ra is sufficiently discriminative. We also note that Rq, Rz and Sa were recorded as well and lead to the same qualitative ranking of the operating points (now included in the Supplementary Material).

Comments 6: Please provide more detailed basis and reference for selecting cutting parameters and levels. Please specify. Aren't the cutting parameters too low? Can these parameters be used in industrial conditions?

Response 6: Thank you. This is an important question. We have, accordingly, added a dedicated paragraph in Section 2.2 (Page 5, paragraph 2, lines 177-194) that explains this study adopts a four-factor parameter window including a feed rate of 0.5–20 μm/rev, a cutting speed of 50–375 mm/s, a cutting depth of 0.5–10 μm, and a phase difference of 0–180 deg, where the selected feed and depth values can cover the transi-tion from ductile cutting to residual groove formation predicted by the classical kine-matic relation Ra ≈ f²/(8r) for diamond cutters with a nose radius of 0.2 mm [8], the adopted cutting speed range encompasses the speed-ratio threshold K = 1 correspond-ing to the 46.4 kHz tool used in this work with a critical cutting speed of approximate-ly 542 mm/s, and the set phase difference covers the complete geometric envelope of the tool-tip elliptical motion ranging from the straight-line trajectory at 0 deg and 180 deg to the circular trajectory at 90 deg. All selected parameter levels are within the ul-tra-precision finishing range, which corresponds to the mature industrial operating regime for brass diamond cutting, where the typical industrial surface roughness Ra is controlled within 0.1–1.5 μm and the feed rate is limited below 5 μm/rev [9]. Accord-ingly, the recommended operating parameters of the developed tool, namely the feed rate of 1 μm/rev, cutting speed of 50–100 mm/s, cutting depth of 1–2 μm, and phase difference of 60 deg, are highly applicable to the industrial diamond-turning finishing process of H62 brass, while high-throughput roughing machining requires upgraded tools with larger tool-tip amplitude or higher working frequency, as further discussed in Section 7.2.

Comments 7: What is the experimental uncertainty?

Response 7: Thank you. We have, accordingly, added the experimental-uncertainty values in Section 2.2 (Page 6, paragraph 4, lines 159-161): for each operating point the reported Ra is the arithmetic mean of three independent 250x250 um measurement areas. The pooled sample standard deviations are sigma_cut = 0.03 um and sigma_feed = 0.04 um; the corresponding expanded uncertainties (k = 2) are U_cut = +/- 0.06 um and U_feed = +/- 0.08 um. With these uncertainties the UEVC-vs-CC differences in Tables I-IV are well above the noise floor and statistically significant at the 95 % confidence level.

Comments 8: 3D images obtained from the surface roughness measurement device can be included in the manuscript. This would improve the clarity of the results and potentially increase the paper's visibility and citation impact.

Response 8: Thank you for the suggestion. Representative 3D surface-height maps obtained with the Keyence VK-X1000 are now included in the Supplementary Material as Figures and a sentence has been added in Section 2.2 (Page 4, paragraph 4, lines 161-163) pointing to them; they visually confirm the suppression of feed-direction grooves and scaly burrs reported in Sections 3-6.

Comments 9: What are the standards used in the tests and measurements?

Response 9: Thank you. We have, accordingly, added in Section 2.2 (Page 4, paragraph 4, lines 149-158) the standards used: roughness is evaluated in accordance with ISO 4287:1997 (geometrical product speci-fications - surface texture: profile method) and ISO 25178-2:2012 (areal method), using a Gaussian filter with lambda_c = 0.08 mm cut-off. The arithmetic mean roughness Ra was selected as the headline metric because it is the most commonly reported parame-ter in the UEVC literature and therefore enables direct cross-study comparison, the parameter specified in the industry acceptance standard ISO 1302 for finished func-tional surfaces, and sufficiently discriminative for the type of periodic micro-pattern produced by UEVC. Additional parameters including Rq, Rz and Sa were also recorded, and these parameters yielded the same qualitative ranking of the operating points. For each operating point, the reported Ra value is the arithmetic mean of three measure-ments.

Comments 10: The obtained results should be compared with the existing literature and discussed using appropriate scientific terminology. Rather than simply stating that a parameter increased or decreased, the underlying phenomenon and mechanisms responsible for the observed behavior should be clearly explained.

Response 10: Thank you for this constructive comment. We have, accordingly, added a comparison paragraph in Section 7.3 (Page 10, paragraph 6, lines 370-390) that the 57% reduction in cutting-direction roughness achieved at a phase difference of 60 deg using the CoFe₂O₄-driven UEVC tool in this study is consistent with, and reaches the upper level of the roughness reduction ranges reported in previous studies regarding the UEVC machining of ductile metals based on piezoelectric and Ter-fenol-D drives, where a 40%–55% roughness reduction was achieved for tungsten al-loys , a 30%–50% reduction for hardened steel , and a 35%–60% reduction for tungsten carbide . Mechanically, the prominent surface quality improvement is primarily at-tributed to the periodic tool-chip separation behavior, which interrupts the accumula-tion of secondary plastic flow at the tool-chip interface and reverses the friction direc-tion in each vibration cycle, thereby jointly inhibiting the generation of feed-direction burrs and decreasing the average normal force acting on the cutting edge. The optimal machining performance obtained at the phase difference of 60 deg is mainly due to the oblique elliptical tool-tip trajectory that minimizes the re-engagement velocity along the cutting direction, as analyzed ; this phenomenon has not been independently in-vestigated for CoFe₂O₄-driven UEVC tools in previous studies but is consistent with the kinematic simulation results reported in existing literature . Different from previous studies that merely demonstrated the reduction of surface roughness Ra induced by ultrasonic vibration cutting, this study further identifies the geometric characteristics of the tool-tip elliptical trajectory as the core physical factor dominating the machining improvement effect, which provides a universal design principle for the structural and parameter optimization of dual-bending electronically phased UEVC tools.

Comments 11: The conclusion section should be shortened and presented in bullet-point format with more study-specific findings. This would improve clarity and better highlight the main contributions of the work.

Response 11: Thank you. The Conclusion (Section 8) has been rewritten in bullet-point form on Page 12 with six study-specific bullets covering (i) A single-crystal diamond + CoFe2O4 dual-bending UEVC tool reduces the cut-ting-direction Ra of H62 brass by 16-45 % across feed (0.5-20 um/rev), 12.6-38.7 % across speed (50-375 mm/s), 22-42 % across depth (0.5-10 um), and up to 57 % via phase-angle optimisation, with respect to paired conventional cutting. (ii) The inter-channel phase difference is the strongest single lever; the optimum is phi = 60 deg, at which the tool-tip trajectory is an oblique ellipse whose major axis is tilted by approx 50-55 deg above the cutting direction. (iii) The 60 deg optimum is explained kinematically by the minimisation of the cut-ting-direction re-engagement velocity, which in turn minimises the normal force at the chip root. (iv) The UEVC advantage is gated by the speed ratio K = Vc / (2 pi f_us A_y); for the pre-sent tool the K = 1 threshold is at Vc approx 542 mm/s, so the productive operating window for finishing is Vc <= 100-200 mm/s. (v) Recommended operating point on H62 brass with the present tool: f = 1 um/rev, Vc = 50-100 mm/s, ap = 1-2 um, phi = 60 deg, giving Ra approx 1.21 um (cut) / 1.47 um (feed) against 2.82 / 3.73 um for paired CC (57 / 61 % reduction), and (vi) Future work: extension to harder ductile alloys, brittle-mode trials, and fac-tor-interaction Taguchi/response-surface study coupled with an on-line force-based closed-loop phase controller.