Investigation of surface roughness and tool wear in milling of AISI 4340 steel

Investigation of surface roughness and tool wear in milling of AISI 4340 steel has been conducted with a 4-flutes endmill. The relationships between cutting force, surface roughness and cutting parameter were investigated To determine the tool life of the tool. It is required to research the wear of the tool and the roughness of the machining results on the material tested. In this research, the test material used is AISI 4340 steel with a cooling method in the cutting process. The cutting model is slot mill cutting. To determine the wear of the cutting, a variation of the cutting thickness was performed where the selected cutting thickness was 0.5mm, 1mm, and 1.75mm. The tests that have been done are surface roughness testing, tool wear, and tool photos after cutting. The result obtained from the study is the highest level of roughness on the material cutting poses with a cutting thickness of 1.75 mm. In addition, the most optimal wear results on cutting with a cutting depth of 1 mm.


Introduction
CNC milling machining methods continue to be developed in the education, research, and industrial sectors.There have been many articles that discuss the latest methods to support large production results but maintain quality.Research conducted by Rafal Gołębski et al. about the use of new technology to improve product quality by using new hole drilling process methods in the production process on CNC machines [1].Another technology has been carried out by Csaba Felho et al. regarding the surface roughness model by modifying the tool using 3 different types of inserts in one CNC Face milling tool [2].Currently, the manufacturing industry has shifted from conventional to CNC machines with consideration of the demands of market demand quantity, production quality, and the complexity of the manufacturing process in the industry so that CNC machining must be applied [3].
Some research variables that are often encountered are cutting speed, feed rate, cutting depth, and spindle speed.From these parameters, researchers can find out the best parameters in CNC machining.Pooja A Sutar et al. investigated the effect of cutting speed, feed rate, cutting thickness, and type of cooling on the surface roughness of AISI 316L stainless steel [4].In addition, M.A. Sulaiman et al. have investigated the effect of cutting parameters using the minimum amount of cooling by looking at tool wear on the tools used [5].D.L. Zariatin et al. also studied tool wear and surface roughness by looking at variations in spindle speed and cutting speed on CNC machines [6].Hasan Gokkaya researched machine parameters (cutting speed and feed rate) to determine the surface hardness of cutting AA2014 (T4) Alloy material [7].
Several types of tools that are often varied in CNC machining research are tool grade, number of cuts, and the cutting-edge shape.Kaining Shi researched the machining characteristics of magnesium alloy materials using a carbide tool [8].Different types of cutting tools were investigated by MD Nadeem Alam by paying attention to the surface roughness of CNC turning cutting [9].The use of HSS tools has also been investigated by varying the machining and cooling parameters on CNC machines to produce the service life of the tool [10].
AISI 4340 alloy steel material has been studied by looking at the surface roughness of the milling cutting process by comparing cryogenic machining and dry milling [11].Because it is often used in the machinery industry, AISI 4340 steel has known material properties by researching microstructure, mechanical properties, and fractography with the intermediate quenching method [12].
Taking into account previous research, further analysis is needed on the cutting results using a 4flutes endmill tool on AISI 4340 material.The results obtained from this study are photos of tool wear and surface roughness of the cutting results where the research variable is the thickness of the workpiece cutting.In addition, the roughness value of the cutting results can also be known to determine the level of tool wear.

Methodology
The machining process applied was slot milling process.The slot milling cutting process was carried out on the Mazak Vertical Smart 430A S CNC Milling machine as shown in Figure 1.Its using an endmill cutter with a diameter of 8 mm.The number of cutter cuts was 4 flutes.For the selection of cutting tools, Endmill tools were used NACHI GS Mill 4GS8, GS4-R type.The choice of the tool was adjusted to the width of the object of research.The test material in this study was AISI 4340 with a diameter of 1 inch.The mechanical properties of the AISI 4340 test material are shown in Table 1.The clamping of the workpiece uses a vise and a V-block as a tool.Cooling fluid uses coolant Versen Metal Working Oil SL-85C.g

AISI
The test variable is the thickness of the cutting with 3 variations, namely 0.5 mm, 1 mm, and 1.75 mm until it reaches a thickness of 7 mm.Machining parameters are used according to the tool recommendation table, namely, federate 770 mm/min and spindle rotation of 3400 RPM.After the machining process, endmill tool wear will be measured using a digital microscope.The purpose of taking pictures using a digital microscope is to know the location of wear that occurs on the tool.Take photos using 50X magnification on each test sample.After that, the process of measuring the roughness level of the material that has been processed on a CNC machine uses the Mitutoyo SJ-310 Surface roughness.
The measurement of hardness level was conducted by measuring 3 sides of the surface with notation RS: Right Side, Ms: Middle Side and LS: Lift Side as shown in Figure 2. The process of testing the surface roughness is carried out 3 times for each surface where the average roughness value is obtained.

Cutting depth
The comparison of endmill wear measurements shown in this discussion, namely the average wear value of the endmill variable, in which the machining treatment process is carried out with different variations in cutting depth, for wear with a depth of 0.5 mm, in this case, the increase in wear is presented in the graph.From table 2, a graph is made to illustrate the comparison of endmill wear in the cutter wear measurement process, while the resulting graph is shown in figure 3.

Figure 3. Endmill wear on 0.5 mm cutting depth
Based on figure 3 slot results are processed at a depth of 0.5 mm of ingestion on objects with a total time of 175 seconds for each manufacture, the number of cuttings is 14 times repeatedly.so that the endmill wear rate is obtained in slot 1 with the results of 0.263 µm, slot 2: 0.347 µm, slot 3: 0.421 µm, slot 4: 0.450 µm, and slot 5: 0.492 µm.The highest yield is in slot 5 due to long and repeated use of the endmill so that the endmill wear increases.The results of the cutting process specimens with a thickness of 1 mm from the number of cuttings 7 times are shown in table 3. From Table 3 the results obtained are made in a graph to illustrate the comparison of endmill wear in the cutter wear measurement process, while the resulting graph is shown in Figure 4.  Figure 4 shows the endmill wear at a depth of 1 mm resulting from the manufacture of slots processed at a cutting depth of 1 mm with a total time of 87.5 seconds for each manufacture, the number of ingestions of 7 times the repeated cuttings obtained the wear rate in slot 1 with a result of 0.158 µm, slot 2: 0.227 µm, slot 3: 0.241 µm, slot 4: 0.306 µm and slot 5: 0.350 µm.The highest yield is 0.350 µm wear in slot 5.The results of the cutting process specimens with a thickness of 1.75 mm from the number of cuttings 4 times are shown in Table 4. Figure 5 shows the comparison of endmill wear in the cutter wear measurement process  Based on Figure 5, the graph of the wear results of the endmill with a depth of 1.75 mm resulting from the manufacture of slots processed at a cutting depth of 1.75 mm with a total time of 50 seconds for each manufacture, the number of cuttings of 4 times the repeated cuttings obtained the wear rate at the final value in the manufacture of slot 1 with the results 0.259 µm, slot 2: 0.375 µm, slot 3: 0.388 µm, slot 4: 0.398 µm and slot 5: 0.430 µm.for the highest yield was 0.430 µm of wear.

Effect of wear on endmill usage time
From all the data generated about the average endmill wear obtained above, a comparison of tables and graphs can be made to conclude these data.Figure 6 describes a comparison graph of the average tool wear against the time of use or duration of use.Where this study compares the variations in the depth of the cutting used, namely 0.5mm, 1mm, and 1.75mm.The number of slots generated is the same during the process.

Figure 6. Comparison of wear to time of use for each cutting depth
Figure 6 shows that overall wear is increasing with the time of use of each tool.It can be seen that the wear with the highest value on tool 1 with the cutting depth of 0.5 mm is 0.492 m with a time of 875 seconds, wear on tool 2 with the cutting depth of 1 mm is 0.35 m with a time of 437.5 seconds, wear on tool 3 with a cutting depth of 1.75 mm is 0.43 m with a time of 250 seconds.The greatest wear and tear occurred on tool 1 with an ingestion depth of 0.5 mm due to the time and overall process stages of 70 repetitions and the most optimal processing time was on tool 3 with a cutting depth of 1 mm.

Observation of wear area of Endmill
Discussion of the wear area that occurs at each cutting depth.The Flank Wear area which will be seen in the image of the wear area on the Endmill can be seen as follows.Figure 7 shows that the damage that occurred to the Endmill was in the Flank Wear section.The designated part showed that the Endmill was damaged even though the damage that occurred to the Flank Wear was quite small.

Surface Roughness Analysis
After the workpiece is processed by milling CNC using variations in the thickness of the cutting, and the tool has been determined, then the workpiece is measured for its surface roughness using a roughness tester.Before taking measurements of the workpiece surface, the measuring instrument must first be calibrated.The measurement process is carried out on 3 side samples, namely on the upper side surface (a), the right (b), and the left side (c), so 9 samples of work points are obtained where at each point 3 strokes are taken and the average value has been taken.Surface roughness measurement for milling results, the result of which is the Certificate of inspection file format in the attachment.After getting the results of each measurement, then the results are averaged and presented into a table.Table 5 shows the results of the average surface roughness of the samples where there are three sides which roughness is measured.The surface of the right side is measured for side A, the surface of the middle side is measured for side B, and side C is measured for the surface of the left side.The depth of the cutting that became the variation of the study was 0.5 mm, 1 mm, and 1.75 mm.The end mill cutter rotation used is a clockwise rotation.From the 5 slots measured, it can be seen that the deeper the thickness of the cutting for the slot cutting process, the greater the surface roughness value obtained for the left and center side surfaces.The right-side surface has a roughness value of about Ra 0.3 -1.4 at the thickness of 0.5 mm.In addition, the thickness of 1.0 mm has a surface roughness value of Ra 2.0 -2.3.The surface roughness value in the middle for a 0.5 mm depth cutting is Ra 1.0 -1.3.As for the depth of 1.0 mm, the Ra value is 1.2 -1.8. and even deeper at a cutting depth of 1.75 mm, the surface roughness value is Ra 1.0 -2.3.As for the left side of the slot surface, the roughness of Ra is 0.7 -1.3 at a depth of 0.5 mm.For a cutting depth of 1.0 mm, the Ra value is 0.8 -1.8.However, the cutting depth of 1.75 mm has a low roughness value where the result is Ra 0.3 -0.6.This is influenced by the clockwise rotation of the cutter which is adjusted to the cutting angle of the cutter.
Figure 9 shows a graph of the roughness value of the slot cutting results where 3 sides of the surface are measured, namely the right side, the middle side, and the left side.The cutting depth variables were 0.5 mm, 1.0 mm, and 1.75 mm. it can be seen from the graph that the depth of the cutting affects the value of the surface roughness results in the end mill slot cutting.The graph of part (a) of the right-side surface shows almost the same average surface roughness values at the cutting depth of 1.0 mm and  The graph in Figure 10 shows the roughness and wear of the end mill slot cutting results.From the graph, it can be seen that there is a decrease in the level of roughness when the wear value is getting higher.At a cutting thickness of 0.5 mm, the wear rate is the lowest between 1 mm and 1.75 mm cutting depth, but the roughness of the cutting results is the highest among the other depths.As for the cutting depth of 1 mm, it has the greatest wear value but has the lowest roughness level compared to 0.5 mm and 1.75 mm.

Conclusion
Based on the results of research and analysis that has been carried out, it can be concluded that the level of roughness of the keyway produced in the milling process using an endmill cutter tool is very influential from several grooves making, from the results of the research obtained, the roughness changes in the manufacture of the highest level of roughness height in the material cutting process with a cutting thickness of 1.75 mm.In addition, the greatest wear occurs on the tool with a depth of 0.5 mm where the overall process steps are 70 repetitions, and the most optimal is on the tool with a depth of 1 mm.

Figure 8 .
Figure 8. Rough sampling point of endmill cutting results in (a) the right side of the slot cutting, (b) the middle part of the slot cutting, and (c) the left side of the slot cutting

Figure 9 .
Figure 9.The surface roughness according to the surface side; right side (top left), middle side (top right), and left side (bottom).
mm.It is different for the surface roughness value in part (b) of the middle surface where the greater the cutting depth, the greater the surface roughness value.This is influenced by the wear of the chisel used.In part (c) the left surface has a low average roughness value at a depth of 1.75 mm which is influenced by the direction of rotation of the cutter used when cutting.

Figure 10 .
Figure 10.Surface Roughness and wear in end mill slot cutting tools

[ 1 ]
Gołębski R, Piotr S, and Adam G 2019The Use of New Technology to Improve The Quality of Production on a CNC Machine Tool System Safety: Human -Technical Facility -Environment 1 583-90 [2] Felho C, Bernhard K, and János K 2015 Surface Roughness Modelling in Face Milling Procedia CIRP 31 136-41 [3] Novaković B, Mića Đ, Ljiljana R, and James G S 2018 Optimization of Manufacturing Processes Using Modern Automated Cnc Milling Machines Applied Engineering Letters 3 [4] Sutar P A, and Gujar A J 2017 Study the Effect of Machining Parameters on Surface Roughness in CNC Milling of AISI 316L International Journal of Engineering and Technology 10 801-direction of rotation of the cutter used when cutting.

Figure 10 .
Figure 10.Roughness and wear in end mill slot cutting results

Table 2 .
Specimen cutting depth of 0.5 mm

Table 4 .
Specimen cutting depth of 1.75 mm

Table 5 .
The average surface roughness of the right side, center side, and left side of the slot