Design and Analysis of a Desktop Multi-Axis Hybrid Milling-Filament Extrusion CNC Machine Tool for Non-Metallic Materials

Round 1

Reviewer 1 Report

There are two things that I considerate are needed for publication:

1. justify and explain the advantages of having a 6th degree of freedom. A more exhaustive analysis should be included of the benefits of having an additional DoF overcomes the complexity and additional considerations that it requires in comparison with robust solutions of five axes that can be founded and extensively used in the industry. How is the proposed solution better than the standard settings of AC (rotary tilt table) or an AB (rotary tilt head)?

2. Conventionally, this type of article is expected to have experimental tests that can support the proof of concept; at least, that's how I remember recent articles of this scope. It is good to have simulation tests, but at least a simple part that could be manufactured with this machine must be included, and the analysis of the process. 

The article is well-written and has a good presentation. 

Author Response

The authors would like to thank the reviewer for the time and effort put in this review and the constructive remarks. Reviewer's comments in bold, authors' response in plain text.

1. justify and explain the advantages of having a 6th degree of freedom. A more exhaustive analysis should be included of the benefits of having an additional DoF overcomes the complexity and additional considerations that it requires in comparison with robust solutions of five axes that can be founded and extensively used in the industry. How is the proposed solution better than the standard settings of AC (rotary tilt table) or an AB (rotary tilt head)?
The authors would to thank the reviewer very much for this well-put comment. It is true, that as far as machine tools is concerned, 5-axis CNC machines are the norm. However, there is a lot of proven work on 6 DoF robots that perform Additive or Subtractive Manufacturing or both, see L.203-216 in revised manuscript. The following text was added in Section 2, last paragraph, L. 225-230: "Compared to the most common 5-axis HMTs, introduction of the 6th axis offers the flexibility of 6 DoF robots in multi-material production with multiple manufacturing heads, but at improved machine stiffness and rigidity. This is because the motion mechanisms are divided in different rests, instead of working in a dependent sequential way, which induces higher possibility of vibrations during manufacturing."

2. Conventionally, this type of article is expected to have experimental tests that can support the proof of concept; at least, that's how I remember recent articles of this scope. It is good to have simulation tests, but at least a simple part that could be manufactured with this machine must be included, and the analysis of the process.
As stated in the revised manuscript in Section 7, L.690-693: "Based on the analysis results, a HMT prototype is being built, after the completion of the presented design study. With the completion of the prototype, it will be benchmarked against process parameters and cutting force profiles found in literature and the numerical results obtained in this study." The physical machine tool is not yet ready, so no actual test part can be manufactured. This work/study is on the design of the machine, and it is the predecessor of following work, which will extensively present the actual working capabilities of the machine (with test cases on different application fields of high-added value sectors, as briefly presented in L696-733). At the time being, with this manuscript we present the rationale and the systematic method of designing the machine and choosing components/subsystems based on set specifications, imposed by the test cases presented in Section 7.
The following test was included in Section 8. Conclusions (L.766-768): "This study presents the rationale and the systematic method of designing the machine tool, and choosing proper components/subsystems based on set specifications, which are imposed by high-added value parts."

Reviewer 2 Report

In this paper, the design and structure analysis of a new desktop multi-axis hybrid machine tool was presented,which combined CNC machining center and 3D printer, and its feasibility was analyzed by static, dynamic and harmonic response finite element. The structure of the paper is clear, however, it still needs some minor revisions:

1.Figure 3 is repeated with the analysis part below and is recommended to be deleted.

2.It is suggested to use the same format for marking (a) and (b) of all pictures in the article.

3.In section 4.2.1, paragraph 2, page 10, there is too little comparison between direct drive and lead screw. A comparison table needs to be added.

4.Figure 7 (a) and figure 8 should mark the position of each coordinate axis.

5.Page 19, the first paragraph mentioned that the Z-axis deformation is very low. The specific size of the deformation is need to specify, making the analysis more convincing.

6.Page 22, in the first paragraph, the design adopts 24,000rpm spindle. However, harmonic response analysis only use 7200rpm, which is not comprehensive. It is suggested to add multiple groups of parameters for analysis.

7.Figure 11-15 are not clear enough and could be replaced with higher quality ones.

In this paper, the design and structure analysis of a new desktop multi-axis hybrid machine tool was presented,which combined CNC machining center and 3D printer, and its feasibility was analyzed by static, dynamic and harmonic response finite element. The structure of the paper is clear, however, it still needs some minor revisions:

1.Figure 3 is repeated with the analysis part below and is recommended to be deleted.

2.It is suggested to use the same format for marking (a) and (b) of all pictures in the article.

3.In section 4.2.1, paragraph 2, page 10, there is too little comparison between direct drive and lead screw. A comparison table needs to be added.

4.Figure 7 (a) and figure 8 should mark the position of each coordinate axis.

5.Page 19, the first paragraph mentioned that the Z-axis deformation is very low. The specific size of the deformation is need to specify, making the analysis more convincing.

6.Page 22, in the first paragraph, the design adopts 24,000rpm spindle. However, harmonic response analysis only use 7200rpm, which is not comprehensive. It is suggested to add multiple groups of parameters for analysis.

7.Figure 11-15 are not clear enough and could be replaced with higher quality ones.

Author Response

The authors would like to thank the reviewer for the time and effort put in this review and the constructive remarks. Reviewer's comments in bold, authors' response in plain text.

1.Figure 3 is repeated with the analysis part below and is recommended to be deleted.
Fig.3 depicts the actual designed machine frame, not the whole machine as seen in Fig. 9, or the simplified model for FE analysis shown in Fig. 10. For clarity and text continuity reasons, the authors would like to keep Fig.3. 

2.It is suggested to use the same format for marking (a) and (b) of all pictures in the article.
Fig.5 was updated to match the marking of all the other figures.

3.In section 4.2.1, paragraph 2, page 10, there is too little comparison between direct drive and lead screw. A comparison table needs to be added.
The following text was added at the end of the 1st paragraph of 4.2.1: "Direct drives seems a very good alternative for quick and direct axis movement for AM processes, where the applied external forces are small as there is no mechanical contact between the tool and the part. However, its use in a HMT would mean that this would operate for cutting operations also, thus working conditions would be far from preferable, risking mechanical machine failure. Besides, direct drive systems are costly, in a magnitude of few thousand € per unit, and their operation and control complex, resulting in significantly increasing the proposed machine costs, which is intended to be kept low." Besides, since direct drive systems vary a lot in technology, it is not safe to create a comprehensive and compact list comparing between direct drive and power screws. Another point is that direct drive systems are all in one, whereas the respective mechanical systems would need a number of components: a screw, its nut and nut housing, at least one linear guideway and the respective block, and a plate joining the block to the nut housing.

4.Figure 7 (a) and figure 8 should mark the position of each coordinate axis.
Fig7 a & b were revised to show all the axes of the machine. In the authors' opinion, there is no need to update Fig.8, since all axes appear now in Fig 7 (a & b).

5.Page 19, the first paragraph mentioned that the Z-axis deformation is very low. The specific size of the deformation is need to specify, making the analysis more convincing.
The following text was entered and revised after the 1st sentence of the 1st paragraph (L.568-573 in the revised manuscript): "Deformation in the most critical areas of the machine tool when stationary, is under 0.1um in the X-axis beam with the spindle in the middle of the X axis and under 0.2um in the support beam of B axis, which holds the table and A and C axis subsystems. Thus, overall deformation under the weight is very low, and considered negligible, as it does not affect the accuracy of machine tool."

6.Page 22, in the first paragraph, the design adopts 24,000rpm spindle. However, harmonic response analysis only use 7200rpm, which is not comprehensive. It is suggested to add multiple groups of parameters for analysis.
24,000 rpm is the maximum rotational speed of the spindle. This doesn't mean that all operations/processes are carried out with this spindle speed. The implemented spindle speed depends on machine, part material and tool properties and specifications, e.g recommended feed per tooth, fz, recommended cutting velocity, Vc, based on part material hardness etc. The process parameter values and the corresponding cutting force profile used in the manuscript were obtained by literature, so as to have a reference for machine benchmarking, at least with the presented numerical method. As stated in the manuscript, the machine is currently being built. As soon as it is ready, we will benchmark it against the results in literature. The following sentence was included in the revised manuscript (L.687-689): "With the completion of the prototype, it will be benchmarked against process parameters and cutting force profiles found in literature and the numerical results obtained in this study."

7.Figure 11-15 are not clear enough and could be replaced with higher quality ones.
Print screens from the FEM software do not allow for resizing the fonts and using the full image does not help in grouping the pictures, which makes the manuscript compact and concise. The authors think that the color coding, along with the numerical values presented in the respective legends of the figures, are clear enough.  Also, the important values are given in the tables and the text of the manuscript. High resolution images are used that allow zooming.

Reviewer 3 Report

Please see the attached file.

Comments for author File: Comments.pdf

Author Response

The authors would like to thank the reviewer for the time and effort put in this review and also for his nice comments.

Here are responses to reviewer's points (reviewer's comments in bold):
- Title is Ok
Thank you.

- High number of references (90)
Thank you.

- High quality figures
Thank you.

- The formatting of all tables must be reviewed. I suggest decreasing the font size.
Font size of all tables was reduced to 9 from the original 10 and a spacing of 3pts was inserted between rows. In Tables 4 & 5, bullets were deleted and a table line was added to seperate the rows.

- The header of Table 3 and Table 10 must be improved.
Table 3 and 10 captions were revised.

- The caption of Figure 11 must include the unit of displacement after 2.39 x 10-1 (mm)
Added. Fig.15 caption was also revised.

- The unit of temperature is 275ºC and not 275oC (please, see lines 179, 566 and 644.
Thank you for pointing these out. Corrected. 

- Line 562: "kw" must be replaced by "kW"
Thank you for pointing these out. Corrected. 

- Line 576: "&" must be replaced by "and"
Thank you for pointing this out. Corrected. 

- How were the cutting parameters selected? (please, see line 22, line 618)
The sentence was revised as follows: "The above process parameters and cutting model was were obtained by references [71-73]". Process parameter values were taken by references 71-73.

- Line 620: "f_z" must be replaced by "fz"
Thank you for pointing these out. Corrected. 

- In Line 625, it is written Therefore, the proposed machine design is considered to be safe from harmonic loading. Why?
Sentence added in L.627-629

Reviewer 4 Report

I would review a multitude of end-of-line scripts. I indicate a few, it is not advisable to leave a single syllable: 2, 14, 20, 31, 40, 41, ….

I find the article very interesting and convenient.

Author Response

The authors would like to thank the reviewer for the time and effort put in this review and also for his nice comments.

The hyphenation was inserted by the editing team of Machines, it wasn't applied in the original manuscript. If the editor wants, we, the authors, can change that back.

Reviewer 5 Report

This article presented a very interesting issue, but it is necessary to provide revision according to the comments:

1) in the state-of-the-art section, you should also present the design possibilities of CNC machines from non-conventional materials., eg. Hatala, M., et al (2020). New trends of mineral composite application in the production of CNC machine tools. TEM Journal, 9(3), 977.; or Poklemba, R., et al. (2020) Design and investigation of machine tool bed based on polymer concrete mixture. International Journal of Simulation Modelling (IJSIMM), 19(2).

2) table 2 – if you want to provide information about costs, it is necessary to determine the values

3)  Table 8 is missing, it is probably only bad numbering

4) the presentation of results (tables 7,9,10) – it is not correctly mathematically expressed (the symbol “e” is necessary to replace by real numbers)

5 ) conclusion part – please add the contribution to practice and science in a few bullets – provide a summary of your results

Author Response

The authors would like to thank the reviewer for the time and effort put in this review, his kind words and the constructive remarks. Reviewer's comments in bold, authors' response in plain text.

1) in the state-of-the-art section, you should also present the design possibilities of CNC machines from non-conventional materials., eg. Hatala, M., et al (2020). New trends of mineral composite application in the production of CNC machine tools. TEM Journal, 9(3), 977.; or Poklemba, R., et al. (2020) Design and investigation of machine tool bed based on polymer concrete mixture. International Journal of Simulation Modelling (IJSIMM), 19(2).
Thank you for your comment. The authors cannot see the direct relevance of the proposed article to the specific study, as they refer to an alternative method of producing composite material parts for CNC machines. The study, though, is about the machine layout and design. In the authors' opinion, the 90 used references should be enough. 

2) table 2 – if you want to provide information about costs, it is necessary to determine the values
In Table 2 there are no prices. In Table 3, where there are indicative prices, the currency is determined in the first row, last column cell, specifically "Cost (US$/kg)" is written. Please, check and acknowledge this.
 
3)  Table 8 is missing, it is probably only bad numbering
Table 8 is not missing, it is located right after Table 7. The table is shared in P.20 &21, figure legend in L.565 in the revised manuscript.

4) the presentation of results (tables 7,9,10) – it is not correctly mathematically expressed (the symbol “e” is necessary to replace by real numbers).
Using e as the symbol for powers of 10 is a well known symbolism, especially in numerical analysis. And there is no specific mention of this in the Guides for authors (Section 6.2 Measurements and Units in https://www.mdpi.com/authors/layout). If the editors want us to change this, we will gladly do this. 

5 ) conclusion part – please add the contribution to practice and science in a few bullets – provide a summary of your results
The following text was added in L.766-772 of the revised manuscript: "In static analysis, even when applying 100 N three dimensional force at the tip of the table holding a 100 N part, the table rim displacement is less that 0.24 mm. Natural frequency analysis showed that the back strut of the machine can reach 4 mm compressive deformation when the spindle operates at 21585.60 rpm, so this should be avoided, unless this is more supported. However, Harmonic Response Analysis proved that maximum deformation is lower than 0.06 mm during operation, which is an excellent result for the proposed low-cost HMT structure."

Round 2

Reviewer 5 Report

The article is suitable for publication in  present form.