Sustainable Milling of Ti-6Al-4V: Investigating the Effects of Milling Orientation, Cutter′s Helix Angle, and Type of Cryogenic Coolant

Round 1

Reviewer 1 Report

This paper presents an analysis of the milling Ti6Al4V sustainability studying the influence of the cutting tools orientation, the helix angle and lubri-cooling techniques. This procedure is considered a field of interest due to the necessity of more eco-friendly manufacturing solutions. The work presents many data and several experimental tests, however the manuscript must be rewritten and improved before being adequate for publication. 

Major comments: 

The introduction must be rewritten and the literature review and cites must be improved. The following lines covers some points/critical examples to be improved:

First paragraph (lines 36-43): Lack of references, there are many studies in the last decade around different lubri-cooling techniques for thermoresistant superalloys. The author presented many limitations in a repetitive manner. Lines 44-50): it is presented a brief introduction to MQL, but once again it is missing some references and links with cryogenic or conventional coolant (there are some works comparing with both of them). Advantages and disadvantages? Lines 80-85: The explanation is not clear, it is not easy to be followed. Lines 103-104: "the latter was found to have improved...." this sentence has no sense Line 112: review the English verb tenses, if it is spoken about a research performed by another author should be past simple. "a study....was reported"

Interesting references:

Pereira O, Urbikaín G, Rodríguez A, Fernández-Valdivielso A, Calleja A, Ayesta I, López de Lacalle LN. (2017). Internal cryolubrication approach for Inconel 718 milling. Procedia Manufacturing, 13, 89-93.

Pusavec F, Deshpande A, Yang S, M’Saoubi R, Kopac J, Jr OWD, Jawahir I. (2014). Sustainable machining of high temperature nickel alloy – Inconel 718: part 1. – predictive performance models. Journal of Cleaner Production, 81, 255-269.

Pereira O, Martín-Alfonso JE, Rodríguez A, Calleja A, Fernández-Valdivielso A, López de Lacalle LN. (2017). Sustainability analysis of lubricant oils for minimum quantity lubrication based on their tribo-rheological performance. Journal of Cleaner Production, 164, 1419-1429.

Regarding the experimental work:

Figure 1 matches better in the subsection 2.1 About variables to be controlled it is missing the conventional coolant (wet machining) lines 191-195: it is recommended an extra explicative figure  Subsection 2.3: it is not clear the control performed to avoid the dry ice during the cutting process. This is a real problem that some authors control using different devices.

Related to the experimental results: it is not presented as easy read.

 Tool wear: If the dry ice issue is not controlled, the comparison with the LN2 it is not a real application. It is missing the scientific explanations about the obtained results, try to avoid sentences as “it seems that….” They must be reordered and clarify. They are presented in a confuse way, this is the surface roughness average is presented out of the subsection named surface roughness. The tool wear subsection mixes different parameters instead of the tool wear in depth.

Discussion and conclusions.

Must be clarify and it requires consistency in the order of the presented ideas and must be improved.

Comments for author File: Comments.pdf

Author Response

Metals-676525

Reviewer 1

This paper presents an analysis of the milling Ti6Al4V sustainability studying the influence of the cutting tools orientation, the helix angle and lubri-cooling techniques. This procedure is considered a field of interest due to the necessity of more eco-friendly manufacturing solutions. The work presents many data and several experimental tests, however the manuscript must be rewritten and improved before being adequate for publication. 

Major comments: 

Comment 1:

The introduction must be rewritten and the literature review and cites must be improved. The following lines covers some points/critical examples to be improved:

First paragraph (lines 36-43): Lack of references, there are many studies in the last decade around different lubri-cooling techniques for thermoresistant superalloys. The author presented many limitations in a repetitive manner.

Response:

Thank you very much for your contributive suggestions. Regarding the first paragraph (lines 36-43), the authors have just discussed the general difficulties offered by the titanium alloy during its machining. The authors have not yet started talking about any lubri-cooling techniques at this point. The detailed discussion and literature review regarding lubri-cooling techniques for thermoresistant superalloys starts from the second paragraph. I guess the respected reviewer has hinted toward adding a few references to the first paragraph. In compliance, the authors have added two more references (numbered 2 and 3) related to the difficulties faced in machining titanium alloys.

Comment 2:

Lines 44-50): it is presented a brief introduction to MQL, but once again it is missing some references and links with cryogenic or conventional coolant (there are some works comparing with both of them). Advantages and disadvantages? Lines 80-85: The explanation is not clear, it is not easy to be followed.

Response:

The opening paragraphs just provide introduction to the coolants (MQL, LN₂, and CO₂). The detailed descriptions, including advantages / disadvantages, and pertinent literature review are comprehensively provided in Section 1.1. Nevertheless, the authors have added two more references (number 4 and 5) regarding benefits of using MQL in machining processes in the second paragraph.

The explanation of the text in the lines 80 – 86 has now been improved.

Comment 3:

Lines 103-104: "the latter was found to have improved...." this sentence has no sense Line 112: review the English verb tenses, if it is spoken about a research performed by another author should be past simple. "a study....was reported"

Response:

The suggested corrections have now been accommodated in the manuscript.

Comment 4:

Interesting references:

Pereira O, Urbikaín G, Rodríguez A, Fernández-Valdivielso A, Calleja A, Ayesta I, López de Lacalle LN. (2017). Internal cryolubrication approach for Inconel 718 milling. Procedia Manufacturing, 13, 89-93.

Pusavec F, Deshpande A, Yang S, M’Saoubi R, Kopac J, Jr OWD, Jawahir I. (2014). Sustainable machining of high temperature nickel alloy – Inconel 718: part 1. – predictive performance models. Journal of Cleaner Production, 81, 255-269.

Pereira O, Martín-Alfonso JE, Rodríguez A, Calleja A, Fernández-Valdivielso A, López de Lacalle LN. (2017). Sustainability analysis of lubricant oils for minimum quantity lubrication based on their tribo-rheological performance. Journal of Cleaner Production, 164, 1419-1429.

Response:

First two references have now been included in the manuscript.

Comment 5:

Regarding the experimental work:

Figure 1 matches better in the subsection 2.1 About variables to be controlled it is missing the conventional coolant (wet machining) lines 191-195: it is recommended an extra explicative figure 

Response:

Figure 1 has now been moved to the Subsection 2.1. Wet machining (emulsion cooling) is out of the scope of this work as it has now been established that cryogenic coolants yield better performance than the conventional flood coolant. This study mainly focuses on the mutual comparison of the performances of the two kinds of cryogenic coolants and micro-lubrication. Extra explicative figure has now been added as Figure 2.

Comment 6:

Subsection 2.3: it is not clear the control performed to avoid the dry ice during the cutting process. This is a real problem that some authors control using different devices.

Response:

The authors would like to clarify that “dry ice” is not used in our experiments. We have rather used compressed CO₂ gas, which produces a cooling effect on throttling. The CO₂ gas (compressed / uncompressed) is a fluid and behaves very much like LN₂ and MQL. On the other hand, dry ice is a crystalline SOLID of CO₂, which was obviously not encountered at any stage of this study.

Comment 7:

Related to the experimental results: it is not presented as easy read.

 Tool wear: If the dry ice issue is not controlled, the comparison with the LN2 it is not a real application. It is missing the scientific explanations about the obtained results, try to avoid sentences as “it seems that….” They must be reordered and clarify. They are presented in a confuse way.

Response:

As described before, dry ice is not encountered at any stage of the experiments. What we think the respected reviewer is referring to is CO₂ snow, which is altogether different from dry ice. Furthermore, the CO2 snow does not develop onto the surface of the fast rotating tool or the hot regions of the work close to the cutting area. It is only formed on the exit nozzle, work holding device and work surface far away from the cutting region.

Scientific explanations regarding tool wear are comprehensively provided in the second and third paragraphs of Subsection “Tool Wear”. The confusing sentences such as “it seems that….” Have now been removed from the manuscript.

Comment 8:

this is the surface roughness average is presented out of the subsection named surface roughness. 

 Response:

The figure showing the results of surface roughness is now moved into the subsection named surface roughness. 

Comment 9:

The tool wear subsection mixes different parameters instead of the tool wear in depth.

Response:

This subsection is actually not mixing different parameters. The focus throughout the subsection remains on tool wear only. The first paragraph describes the findings visible from the data graph. The second and third paragraphs describe the technicalities behind the tool wear related observations. The last two paragraphs describe, in detail, the tool wear mechanisms and their dependences on the tested levels of the predictors. Thus, the whole subsection focuses on tool wear only.

Comment 10:

Discussion and conclusions.

Must be clarify and it requires consistency in the order of the presented ideas and must be improved.

Response:

Both “Discussion” and “Conclusions” sections have been revised and improved for clarity and consistency.

Reviewer 2 Report

Authors present an interest experimental study about the Milling of Ti-6Al-4V alloy under some cutting conditions (Milling Orientation, Cutter’s Helix Angle and Type of Cryogenic Coolant). The paper is globally well written. Some sections can be regrouped (e.g. about specific cutting energy and cutting force). The section about Surface Roughness can be improved by adding e.g. hardness and depth of the subsurface affected by cutting.

Give cutting conditions in a table. Give a schematic representation of cutting configurations. Remarks about cutting energy and cutting forces: Give the measured power, the non-cutting power and the effective cutting power for each cutting condition (please present this in a table) Please, calculate the effective cutting power and then the specific cutting energy from measured cutting force components Compare the effective cutting power and the specific cutting energy obtained from the two methods. It is highly interesting to analyze the chip morphology with respect to the Milling Orientation, Cutter’s Helix Angle and Type of Cryogenic Coolant. The surface hardness and depth of the subsurface affected by cutting are some parameters allowing the analysis of the effect of cutting parameters on the surface integrity. It is highly interesting to add this, in addition to the surface roughness (Ra), to analyze deeply the surface integrity. Please give more details about the process cost. How the process cost was calculated?

Author Response

Metals-676525

Reviewer 2

Authors present an interest experimental study about the Milling of Ti-6Al-4V alloy under some cutting conditions (Milling Orientation, Cutter’s Helix Angle and Type of Cryogenic Coolant). The paper is globally well written.

Comment 1:

Some sections can be regrouped (e.g. about specific cutting energy and cutting force). Remarks about cutting energy and cutting forces: Give the measured power, the non-cutting power and the effective cutting power for each cutting condition (please present this in a table) Please, calculate the effective cutting power and then the specific cutting energy from measured cutting force components Compare the effective cutting power and the specific cutting energy obtained from the two methods.

Response:

The sections related to specific cutting energy (3.3) and cutting forces (3.4) are now combined into a single subsection (3.3). Table 2 has now been added in Subsection 3.3 that provides the detail regarding the measured power, the non-cutting power and the effective cutting power for each cutting condition. However, the calculation of cutting power based on the machining forces is not included because of the following reason:

Cutting power in a machining process is the product of the cutting force component and the cutting speed. In the case of side- and end-milling, determination of the cutting force component from the three measured force components (Fx, Fy, and Fz) is very difficult. This is so, because the cutting force component experienced by a helical milling cutter not only depends upon the three orthogonal force components but also on the shear plane angle and tool geometry (tool’s helix / rake angle). As determination of the shear plane angle for different cutting conditions is very difficult, the cutting force component cannot be evaluated with an acceptable level of accuracy.

Furthermore, the title of the manuscript suggests that the focus of the work is on enhancing sustainability of the milling process and the comparison of the cutting power values based on electric current and cutting forces won’t serve this purpose. Therefore, we feel this effort would not be worthy enough. Nonetheless, the authors have provided the observations regarding the correlations between the measured force components and the cutting power at the end of the sub-section 3.3.

Comment 2:

Give cutting conditions in a table. Give a schematic representation of cutting configurations.

Response:

The needful is done in the form of Table 1 and Figure 2.

Comment 3:

The section about Surface Roughness can be improved by adding e.g. hardness and depth of the subsurface affected by cutting. It is highly interesting to analyze the chip morphology with respect to the Milling Orientation, Cutter’s Helix Angle and Type of Cryogenic Coolant. The surface hardness and depth of the subsurface affected by cutting are some parameters allowing the analysis of the effect of cutting parameters on the surface integrity. It is highly interesting to add this, in addition to the surface roughness (Ra), to analyze deeply the surface integrity.

Response:

We agree that work surface integrity and chip morphology are important characteristics. We have included the data regarding surface roughness but not that related to machined surface hardness or subsurface characteristics. We would like to apologize as this requirement cannot be fulfilled at this stage partially because some of the machined surfaces were re-machined (after completing the surface tests) during the following runs in order to conserve the work material.

Comment 4:

Please give more details about the process cost. How the process cost was calculated? 

Response:

The procedure for calculating the process cost is comprehensively described in the FOURTH paragraph of sub-section 2.3. It is clearly detailed that the process cost for each run is evaluated as summation of the five components: (1) tooling; (2) direct electricity consumption; (3) overhead; (4) equipment’s depreciation; and (5) cutting fluid’s consumption cost. Each component is also effectively elaborated.

The authors could easily have included the actual cost data in respect of all the five cost components evaluated for all the 24 experimental runs in the manuscript but eventually decided against it because the resulting table would take a huge space of the manuscript and, quite likely, the audience won’t be interested in going through the fine “dollar” details of the cost elements. The audience, rather, is expected to be more interested in getting the overall cost comparisons for the 24 runs, which we have already provided in sub-section 2.4. 

Reviewer 3 Report

Incorrect marking of cutting speed. Cutting speed is given as low “v” and index “c”, it shoud be v_c (line 185). Why the authors decided on cutting speed v_c = 100 m/min and v_c = 175 m/min. The cutting speeds were determined on the basis of a literature review or of preliminary testes were selected. Please explain why and how? Incorrect marking of parameter Ra, ”a” is a lowercase letter not a index, please correct (line 199 and in whole text and in figure 5 I would suggest adding a standard deviation to the bars in Figure 5 (despite the fact that the Ra parameter is the average of 4 measurements), this will allow a slightly broader view of the obtained values and allow for estimating the impact of the factors studied on the Ra parameter. I suggest using the components of the total cutting force as F_f (feed force), F_p (passive force), F_c (main cutting force) instead of F_x, F_y, F_z. Feed rate should be given in “mm/min” and not in “m/min”, please replace (line 210). The surface roughness was measured using a contact device. What was the sampling length (i.e. what type of filter was used), please give this information. The information given in text (line 238) are incorrect. Why nothing is mentioned in the methodology that Analysis of Variance (ANOVA) will be performed. The rest of the work contains information on the results obtained from the Analysis of Variance (ANOVA). Please added information in the methodology. Looking at figure 6a, it seems that the measurement of surface roughness was not possible, whether the chips were removed, what percentage of the 24 obtained in the experiment was similar to surface 6a. Please replace the Bars with MPa (line 438 and line 447). Why the figure 8 is out of chapter 3.4? In the chapter 3 (results) and 4 (Discussion on Milling Sustainability) is no references to literature. I propose that the authors compare their results with the work carried out so far regarding milling of the Ti6AL4V titanium alloy or the cooling metods (cryogenic cooling CO₂ snow, LN₂ or MQL).

Author Response

Metals-676525

Reviewer 3

Comment 1:

Incorrect marking of cutting speed. Cutting speed is given as low “v” and index “c”, it shoud be v_c(line 185).

Response:

Correction is accomplished.

Comment 2:

Why the authors decided on cutting speed v_c = 100 m/min and v_c = 175 m/min. The cutting speeds were determined on the basis of a literature review or of preliminary testes were selected. Please explain why and how?

Response:

The levels of the cutting speed were determined on the basis of screening tests. The pertinent details are now added to the second paragraph of sub-section 2.2.

Comment 3:

Incorrect marking of parameter Ra, ”a” is a lowercase letter not a index, please correct (line 199 and in whole text and in figure 5 I would suggest adding a standard deviation to the bars in Figure 5 (despite the fact that the Ra parameter is the average of 4 measurements), this will allow a slightly broader view of the obtained values and allow for estimating the impact of the factors studied on the Ra parameter.

Response:

The corrections regarding Ra are done throughout the manuscript including Figure 6 (previously Figure 5). Error bars (standard deviation) have now been added to Figure 6 (previously Figure 5).

Comment 4:

I suggest using the components of the total cutting force as F_f (feed force), F_p (passive force), F_c (main cutting force) instead of F_x, F_y, F_z. Feed rate should be given in “mm/min” and not in “m/min”, please replace (line 210).

Response:

This matter has already been touched in response to the first comment of Reviewer 2 and the relevant explanation is provided in the last paragraph of sub-section 3.4. We try to further explain as follows:

The matter of converting the orthogonal components (F_x, F_y, F_z) of the machining forces as measured by the dynamometer into another system of forces comprising of cutting, feed, and passive forces is not straightforward for milling as is for a turning processes. The process of side- and end-milling involves complex mechanics because of the helical arrangements of the multipoint cutting tool. The exact determination of the cutting, feed, and passive force components for a peripheral milling depends also on the tool geometry (rake/helix angles) and shear plane angle.

The authors would prefer not to get involved into the mechanics and complexities of determining the force components as the main focus of the manuscript is to enhance sustainability of the milling process.

Comment 5:

The surface roughness was measured using a contact device. What was the sampling length (i.e. what type of filter was used), please give this information. The information given in text (line 238) are incorrect.

Response:

Sampling length for each measurement is 4 mm and Gaussian Profile Filter (ISO 11562) is used for finding the Ra values. This information is added to the manuscript (third paragraph of sub-section 2.3).

The information given in text (line 238 – now 265) has now been corrected.

Comment 6:

Why nothing is mentioned in the methodology that Analysis of Variance (ANOVA) will be performed. The rest of the work contains information on the results obtained from the Analysis of Variance (ANOVA). Please added information in the methodology.

Response:

Thank you very much for highlighting this deficiency. ANOVA has now been introduced in Section 2 (just before sub-section 2.4).

Comment 7:

Looking at figure 6a, it seems that the measurement of surface roughness was not possible, whether the chips were removed, what percentage of the 24 obtained in the experiment was similar to surface 6a.

Response:

That’s true. The adhered chips were very gently removed before taking the Ra measurements. This was the severest case of chip adhesion to the work surface of all the 24 tests. We would say that all the tests involving up-milling and cryogenic coolant (LN2 or CO2) – 8 of the 24 – showed this tendency but not of the same intensity as the one shown in image (a).

Comment 8:

Please replace the Bars with MPa (line 438 and line 447).

Response:

The needful is done. The readings of 1.013 bar and 6 bar have been converted to 0.1013 MPa and 0.6 MPa, respectively.

Comment 9:

Why the figure 8 is out of chapter 3.4?

Response:

Following the merger of sub-sections 3.3 and 3.4 into sub-section 3.3, Figure 8 (now 9) has now automatically been placed in sub-section 3.3.

Comment 10:

In the chapter 3 (results) and 4 (Discussion on Milling Sustainability) is no references to literature. I propose that the authors compare their results with the work carried out so far regarding milling of the Ti6AL4V titanium alloy or the cooling metods (cryogenic cooling CO₂ snow, LN₂ or MQL).

Response:

Thank you very much for a very contributive suggestion. The authors have regarded this deficiency and have worked to eliminate it. Now, no less than a dozen references have been cited at various places in Sections 3 (Experimental Results) and 4 (Discussion) for the sake of comparison of the results of this work with the previously published reports.

Round 2

Reviewer 2 Report

In Table 2, the total cutting power (Ptotal) and the effective cutting power (Pcut) are the same?!!! Calculate the effective cutting power and then the specific cutting energy from measured cutting force components. Compare the effective cutting power and the specific cutting energy obtained from the two methods. Give more details about the process cost. How the process cost was calculated?

Author Response

Metals-676525

Reviewer 2 (Round 2)

Comment 1:

In Table 2, the total cutting power (P_total) and the effective cutting power (P_cut) are the same?!!!

Response:

Our apologies for the grave mistake. The values for P_total were wrongly entered in the column. We mistakenly entered the values of P_cut in that column. The correct values have now been entered in P_total (shown in Blue).

Comment 2:

Calculate the effective cutting power and then the specific cutting energy from measured cutting force components. Compare the effective cutting power and the specific cutting energy obtained from the two methods.

Response:

The following table puts forward the comparison of the effective cutting power obtained from the two methods (electric power measurement and cutting forces):

It can be seen that the estimation of cutting power through the F_x, F_y, and F_z data yields percentage errors of 58%, 36%, and 31%, respectively. The error values are too high to permit the inclusion of the comparison in the manuscript. The technical reason behind the mismatch is provided below:

Cutting power in a machining process is the product of the cutting force component and the cutting speed. In the case of side- and end-milling, determination of the cutting force component from the three measured force components (Fx, Fy, and Fz) is very difficult. This is so, because the cutting force component experienced by a helical milling cutter not only depends upon the three orthogonal force components but also on the shear plane angle and tool geometry (tool’s helix / rake angle). As determination of the shear plane angle for different cutting conditions is very difficult, the cutting force component cannot be evaluated with an acceptable level of accuracy.

Furthermore, the title of the manuscript suggests that the focus of the work is on enhancing the sustainability of the milling process and the comparison of the cutting power values based on electric current and cutting forces won’t serve this purpose. Therefore, the authors have decided not to include this table in the manuscript.

Comment 3:

Give more details about the process cost. How the process cost was calculated?

Response:

More details about the process cost are now added to the fourth paragraph of sub-section 2.3 (from Line 271 to Line 295).

Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

It is difficult to understand how the process cost was calculated by reading the text (section 2.3). Please give formula used for this

Author Response

Metals-676525

Reviewer 2 (Round 3)

Comment 1:

It is difficult to understand how the process cost was calculated by reading the text (section 2.3). Please give formula used for this

Response:

The needful is done. Kindly see lines 295 – 303.