Parametric Analysis of Macro-Geometrical Deviations in Dry Turning of UNS A97075 (Al-Zn) Alloy

Comment 1

The introduced algebraic model is useful. However, the reliability if it is not perfectly explained. Explain why the goodness of fit (coefficient of determination, line 316) was considered as acceptable when the values vary between 0.6 and 0.7. Or instead of theoretical explanation the authors should mention a few studies from high-rank journals in which this value was accepted. Perhaps more data could result in a higher R2 coefficient or the refinement of the experimental conditions (e.g. considering the vibration or surface layer properties) may decrease the variation of the outcome values.

Taken Actions

The authors thank the reviewer for his appreciation of the article. In reference to the models reliability and their comparison with other studies, currently there is a lack of studies (for this alloy, under dry conditions) that relate cutting parameters and geometrical deviations. However, the works in this regard (referenced along the text, namely reference 9 and 37) do not show the numerical values about the model fit (R² or similar). Notwithstanding, their dispersion levels seems to be very similar to those obtained in the experimental data of the present work.

Although the mathematical model does not present an excellent fit, it is not the purpose of the present work to present a general function for every cutting condition. In the present state of this research, the obtained mathematical models are used to analyze general trends and allow the comparison with other models.

The aim of the authors is not to obtain a general model that relates the studied output variables as a function of the cutting parameters, but to offer models that may facilitate the understanding of the physical phenomena that occurs, under the studied conditions and, as a result, facilitate the comprehension of the obtained experimental results of this work.

As a result, we agree with the reviewer. Although these models are not general and valid for all the machining conditions, they are valid within the range of applied cutting parameters, the used tool and cutting conditions. It is our intention, due to the complexity of the problem, to increase, step by step, the number of variables, in order to obtain a more complete model. This fact has been clarified in the text.

Comment 2

Concerning the grammar and style of the paper; it can be stated that there are few errors (mainly grammar) in it. A final revision by a native speaker is recommended

Taken Actions

Text grammar has been revised and several changes have been applied

Comment 1

Please, attach the picture of the cutting insert used and describe the cutting edge geometry (primarily tip radius).

Please, attach the picture of the whole workpiece in the machine to show the workpiece fixing in the tailstock.

Taken Actions

A new image has been included where the tool cutting edge geometry and the specimen in the machine are shown.

Comment 2

Theoretical PAR, resp. CYL should be calculated (scallop value for given tool tip radius vs. depth of cut and feed) for the selected feed levels (mm/r). If the theoretical values of the scallop exceed the requirements of the ISO 1143:2010, such feeds are not relevant for the study.

Taken Actions

We agree with the assessment of the reviewer. The corresponding scallop value calculations have been made. For the most unfavourable case, f = 0.2 mm/r, the scallop value obtained is 6.27 µm, which does not exceed the requirements of the ISO 1143:2010 (20 µm). Globally, the scallop values calculated for the feed range implemented in the study (0.05-0.2 mm/r) are between 1-6 µm.

Comment 3

Comment on the cutting force (feed and radial component) dependency on cutting speed and feed is recommended. This will allow in the following chapters of the paper a more relevant explanation of the effect of workpiece stiffness on the geometrical deviations.

Taken Actions

The reviewer refers to a series of variables that we have not directly included in the study. However, they are indirectly present through the study of the feed (f) and the cutting depth (a_p). As a_p is constant, F_c mainly depends on f. In addition, studies in this regard [8,34,36] show that f is the predominant parameter that influences the F_c obtained, for the 2000 and 7000 alloy series (machined under dry conditions).

Due to the cutting tool geometry, the major cutting edge angle is of 66.5˚ (compatible with the specimen single-pass machining), the force axial component is dominant over the radial component. Therefore, the bending effect on sample has been limited. A clarification has been introduced in the text.

Comment 4

Chapter 3.2: It would be helpful to include also evaluation of RON, R_mean and J_mean dependency on the section position 1 – 6. Including the R_mean and J_mean values could help in understanding if larger structural deviation of the workpiece occur in the sections which are more distant from the chuck.

Taken Actions

The corresponding graphs of the roundness evolution as a function of the cutting parameters, for each section, have been added and commented in Results and Discussion. Since the circular run-out can be obtained as a combination of roundness and concentricity, the observation of both graphs allows d the concentricity values trend.

Comment 5

Figure 9 c): Large scatter of CRO values at each section of the workpiece for different feed levels may indicate significantly elevated level of vibration. Was the cut still stable (no chatter) in all cases? If not, the data should be excluded.

Taken Actions

Indeed, the authors agree with the reviewer that there are high vibration values, as discussed in the results. However, one of the objectives of this study is not the analysis of ideal finishing conditions, but to study a wider cutting parameters range, considered between finishing and roughing, that it may be interesting for some industrial applications.

Comment 6

Please confirm. Conclusions: Slenderness of the workpiece (specimen) should exactly be expressed and compared with the values of the referenced experiments from the literature to define the validity of the new results (trends of the geometrical deviations).

Taken Actions

The reviewer’s comment has been considered to introduce changes in the text and include the slenderness values of the tested specimens.

Comment 7

Conclusions, line 378 – 379: Rather a different interpretation can be delivered to support the explanation of the feed effect and position dependent bending stiffness of the workpiece; namely, an increased impact of feed on CRO has been observed in the sections more distant from the chuck.

Taken Actions

The authors agree with the reviewer’s suggestion. The conclusions have been extended to include it.

Comment 1

Abstract: in my opinion the sentence: "Despite its importance, there is a lack of research that analyses them as a function of the cutting parameters." is untrue, since there are a lot of researches regarding the analysis of deviations occurring during turning process. Maybe put the impact on some gaps in relation to cutting of slender workpieces made of Al-Zn alloys?

Taken Actions

The reviewer’s suggestion has been taken into account and the text has been changed to be more accurate.

Comment 2

In a current form, the introduction section could be not concise for the reader. Introduction is expected to have an extensive literature review followed by an in-depth and critical analysis of the state of the art. If you avoided reference overkill/run-on, i.e. do not use more than 3 references per sentence. If you need to use more, make sure you state the key relevant idea of each reference. I suggest add information to better describe what other researchers have done in this area. Each one of the cited references within the body of the paper should be discussed individually and explicitly to demonstrate their significance to the study. Moreover, the Authors have missed recent researches regarding the surface integrity - dynamic implications in machining processes, as well as some papers regarding the application of environmentally friendly/MQCL cutting processes. Thus the following studies should be included in the Introduction section:

1. Machined surface roughness including cutter displacements in milling of hardened steel. Metrology and Measurement Systems 18(3), (2011) pp. 429-440.

2. Effects of extreme pressure and anti-wear additives on surface topography and tool wear during MQCL turning of AISI 1045 steel. Journal of Mechanical Science and Technology 32(4), (2018) pp. 1585-1591

3. Ecological trends in machining as a key factor in sustainable production – A review. Journal of Cleaner Production. 218, (2019) pp. 601-615.

4. Surface quality and topographic inspection of variable compliance part after precise turning. Applied Surface Science 434 (2018), pp. 91-101.

Taken Actions

The introduction has been expand identifying the study that the contribution of the authors of the references mentioned. Also, the studies suggested have been taken into account and introduced in the text.

Comment 3

Materials and methods: please provide the details regarding the geometry of the applied tool (WC-Co insert-ISO DCMT 11T308-14 IC20), since the parameters as: rake, flank angles, corner radius, influentially affect the forces during cutting and thus the obtained surface integrity.

Taken Actions

A new image has been included where the tool cutting edge geometry is shown.

Comment 4

Materials and methods: please provide the technical specification of the employed dial gauge. 

Taken Actions

The technical specification of the employed dial gauge has been included

Comment 5

3. Results and Discussion: the dependencies depicted in Figs. 6a,6b reveal low changes in function of feed. Moreover, the error bars show also high deviations of the results. Therefore it is advised to carry out the statistical significance test in order to reveal the statistical influence/no-inlfuence of "f" on PAR and STR parameters.

Taken Actions

The authors agree with the reviewer comment. The experimental results have not shown a clear trend in the whole range of tested cutting parameters. Notwithstanding, a higher dependence on feed and cutting speed has been observed in comparison with the rest of considered studies (performed in specimens with lower slenderness).  A significant variation can be appreciated when considering higher cutting speed combined with high feed. These aspects have been clarified in the explanation of the results.

Comment 6

3. Results and Discussion: Why Authors carried out the investigations in a range of cutting speeds corresponding to the intense BUE formation? 

Taken Actions

The authors agree with the reviewer’s comment. However, it must be pointed that, although low cutting speeds are not recommended for machining aluminum alloys, these alloys are often hybridized with other materials in which these low cutting speeds are required, such as Fiber Metal Laminates, FML (CFRP +Al + Ti). In addition, this fact allows the comparison with previous studies on the geometrical deviations performed in the same cutting parameters range. Several references have been included in this regard.

Comment 7

3.5. Parametric Models for Macro-Geometrical Deviations: In my opinion, the formulation of experimental regression models for the investigated data seems to be inappropriate, since they have relatively low accuracy of fitting. According - for example - to figure 13b, the differences between the modelled and calculated values can exceed the 70%. Thus, it is not providing any important and reliable data. Moreover, the investigated deviation parameters in function of feed reveal some significant variations. Therefore, it is very risky to describe them by a simple power function models.  

Taken Actions

Although the mathematical model does not present an excellent fit, it is not the purpose of the present work to present a general function for every cutting condition. In the present state of this research, the obtained mathematical models are used to analyze general trends and allow the comparison with other models (usually obtained by experimental data regressions). As a result, we agree with the reviewer. Although these models are not general and valid for all the machining conditions, they are valid within the range of applied cutting parameters, the used tool and cutting conditions. It is our intention, due to the complexity of the problem, to increase, step by step, the number of variables, in order to obtain a more complete model. This fact has been clarified in the text.

Comment 8

The analysis of results in a current form is not providing the scientific discussion between obtained results and phenomena responsible for these dependencies. The Authors are mainly describing the trends in the figures and sometimes claim that the results are affected by vibrations. However, it was not supported by any data... During the cutting processes the geometrical surface texture is mainly affected by displacements/vibrations in machining system caused by cutting forces. Thus, it is advised to extend this analysis, considering the measured/predicted forces and their effect on the obtained deviations.

Moreover, the discussion on relation between the machinability of Al-Zn alloys and the obtained results was also omitted (e.g. the effect of material's mechanical/thermal properties on forces, BUE and other factors influencing the formation of geometrical deviations during machining). It should be emphasized that this aspect is especially important in relation to publication in journal devoted to analysis of metallic materials! 

Taken Actions

We agree with the reviewer that the inclusion of additional variables will improve the overall knowledge of these alloys dry machining behaviour. In fact, the analysis of the wear, the chip geometry and other variables have already been addressed, such as shown in references [9, 17, 29-30,37]. In fact, our main research line is focused on the integral parametric analysis, including the influence of cutting parameters on several mechanical properties of these alloys, such as fatigue behaviour, allowing a general vision of the machinability of these alloys. Given the complexity of addressing this study simultaneously, the purpose of the present work is to focus on the influence of the cutting parameters on geometric deviations. These studies will be completed in the future, incorporating the rest of the variables, as the reviewer rightly suggests.

Comment 1

It is important to point out that there are also intermediate solutions that show good results as a compromise between surface integrity, cutting forces and tool wear, like Minimum Quantity Lubrication and Cryogenic cooling. I suggest including also a brief analysis of the state of the art of such technology in the introduction of the paper.

Taken Actions

The authors agree with the reviewer. A brief analysis and some references have been included in the introduction section. The introduction has been expanded identifying different studies related with other friendly machining techniques as Minimum Quantity Lubrication and Minimum Quantity Cooling Lubrication.

Comment 2

Please provide more details about the experimental tests, such as the machine used for the test and the number of samples machined, in the paper is repeated many times “several samples” but the credibility of the research would profit from a number.

Taken Actions

One sample has been machined for each cutting condition set. In spite of this, for each geometrical deviation exposed, between six (STR and PAR) to twelve (RD, CRO and CON) measures were taken. Additionally, six sections and twelve generatrix were considered to evaluate the possible results dispersion. This was clarified in the text.

Comment 3

The results reported are quite interesting and the effect of the process parameters on the surface characteristic are well described and motivated. It is not mandatory in this paper, but it will be interesting to compare the results measured during these tests with at least a test in the same condition BUT using lubrication. Maybe this will be the base for another publication.

Taken Actions

The authors agree with the reviewer comment. The bibliography evaluated barely expose macrogeometrical deviations considering other friendly manufacturing techniques, especially in aluminium alloys machining operations. The authors purpose is to continue in this investigation line taking in to account other variables that can affect the macrogeometrical deviations as tool geometry, slender workpiece or lubrication as rightly commented the reviewer.

Comment 4

Please report the equation of the response surfaces created to generate figure 14.

Taken Actions

Please, note that equations are indirectly exposed in the paper. Equation 3 shows the general form of the model and Table 4 shows the values of the different constants calculated. Authors consider that this way to present the results facilitate the understanding of cutting conditions influence in the geometrical deviations studied.

Comment 1

Theoretical PAR, resp. CYL should be calculated (scallop value for given tool tip radius vs. depth of cut and feed) for the selected feed levels (mm/r).

Taken Actions

We would like to apologize cause the scallop value was calculated and the validation pf the feed-rate range was justified to the reviewer in the first revision round but not integrated in the article. The justification has been introduced in the text (line 222, track changes document, marked in yellow)

Comment 2

Comment on the cutting force (feed and radial component) dependency on cutting speed and feed rate is recommended. 

Taken Actions

The explanation has been expanded in the text (line 167 and 313, track changes document, marked in yellow)

Comment 3

Conclusions: I have found mentioning “20 times less slender” specimens – I am not sure about that meaning (line 446)? By my previous comment I ment to calculate the ratio Length/Diameter.

Taken Actions

The ratio Length/diameter was exposed in line 148. In spite of this, the slenderness calculation has been better indicated to clarify the result (line 148, track changes document, marked in yellow). Additionally, in conclusions, “20 times less slender” are referred to a comparison with other research, where the specimens tested present higher rigidity. So, according to the specimens tested in the works referenced, the samples used in these works present a relation Length/diameter around 20 times less.

Comment 1

The Authors have carefully revised their manuscript in accordance with reviewer's remarks. Therefore, in my opinion its scientific quality has been increased and thus it can be accepted for publication.

Taken Actions

The authors would like to thank the reviewer for their comments, making possible the improvement of the article proposed for publication.