3.1 Machining strategy analysis
Table 4 shows the average values of Rz together with their deviations. It can be seen how the Rz values obtained for up-milling and down-milling appear in different columns. In general, it can be noted that the values achieved with down-milling strategy are much higher than those of up-milling. This makes sense because in opposition there is a minor force when the tool tooth hits the material. This phenomenon in metal alloys can be favorable, but for composites a lower cutting force can cause the fibers to deform but not break. This may result in a lower final surface quality obtained during machining, which is reflected in values of the table (Fig. 4.a.). These results are consistent with other studies, such as Nguyen et al. [27] and Ozkan et al. [28] in which they note a better machining in CFRP following the up milling strategy. In addition, the fiber pull-out defect appears to a greater extent on the right side of the slot, i.e. with the down milling strategy (Fig. 4.b).
For example, in tool type A, values between 16.42 and 26.00 µm appear in up milling. On the other hand, the Rz reaches values of up to 57.11 um. The deterioration of the surface quality becomes double if the two machining strategies are compared. The same phenomenon occurs with the rest of the tools used in this experiment. Therefore, it seems that for the performance of CFRTP contourmilling maintaining an up-milling strategy provides a higher surface quality.
The highest Rz value is generated by tool B in 1000 rpm and 0.05 mm/tooth test being 115.14 µm. Tool D also produces a very high value of 103.44 um at 1000 rpm and 0.03 mm/tooth. The best surface quality is given to tool A with values such as 16.42–16.57 um. This article focuses on getting good surface quality in conventional CFRTP machining. Therefore, after the previous discussion it has been decided to analyze in detail only the Rz values that are in accordance of machining (up-milling), since, they are the smallest.
Table 4
Rz values obtained in all tests.
Tool | Test | Spindle speed S [rpm] | Feed rate per tooth Fz [mm/tooth] | Average Rz 1 (Up-milling) [µm ± µm] | Average Rz 1 (Down-milling) [µm ± µm] |
A | 1 | 1000 | 0.03 | 17.71 ± 4.10 | 45.31 ± 23.70 |
2 | 1000 | 0.05 | 24.53 ± 3.85 | 51.38 ± 21.25 |
3 | 1000 | 0.07 | 16.57 ± 4.71 | 33.51 ± 16.15 |
4 | 3000 | 0.03 | 16.42 ± 6.52 | 57.11 ± 38.54 |
5 | 3000 | 0.05 | 26.00 ± 8.21 | 46.59 ± 16.64 |
6 | 3000 | 0.07 | 16.54 ± 6.55 | 32.10 ± 4.62 |
B | 1 | 1000 | 0.03 | 24.91 ± 4.26 | 77.74 ± 31.10 |
2 | 1000 | 0.05 | 58.32 ± 12.02 | 115.14 ± 29.08 |
3 | 1000 | 0.07 | 19.82 ± 2.77 | 35.36 ± 12.65 |
4 | 3000 | 0.03 | 22.00 ± 5.35 | 31.88 ± 25.80 |
5 | 3000 | 0.05 | 35.54 ± 6.85 | 42.69 ± 11.73 |
6 | 3000 | 0.07 | 54.78 ± 8.98 | 30.03 ± 6.65 |
C | 1 | 1000 | 0.03 | 36.53 ± 17.53 | 59.90 ± 13.09 |
2 | 1000 | 0.05 | 30.24 ± 8.89 | 58.12 ± 18.65 |
3 | 1000 | 0.07 | 23.72 ± 9.69 | 49.25 ± 28.37 |
4 | 3000 | 0.03 | 40.91 ± 13.98 | 27.29 ± 6.96 |
5 | 3000 | 0.05 | 31.12 ± 9.22 | 36.34 ± 10.53 |
6 | 3000 | 0.07 | 24.03 ± 6.50 | 24.09 ± 4.45 |
D | 1 | 1000 | 0.03 | 21.73 ± 13.30 | 103.44 ± 46.53 |
2 | 1000 | 0.05 | 21.17 ± 5.62 | 63.77 ± 10.69 |
3 | 1000 | 0.07 | 34.03 ± 5.48 | 62.83 ± 17.54 |
4 | 3000 | 0.03 | 37.29 ± 5.84 | 53.71 ± 18.39 |
5 | 3000 | 0.05 | 44.44 ± 7.92 | 62.62 ± 14.52 |
6 | 3000 | 0.07 | 47.53 ± 14.61 | 54.23 ± 21.08 |
E | 1 | 1000 | 0.03 | 28.25 ± 17.18 | 66.03 ± 18.84 |
2 | 1000 | 0.05 | 25.26 ± 8.71 | 62.38 ± 13.43 |
3 | 1000 | 0.07 | 39.20 ± 10.87 | 82.89 ± 15.53 |
4 | 3000 | 0.03 | 35.64 ± 18.09 | 53.52 ± 13.18 |
5 | 3000 | 0.05 | 31.83 ± 6.21 | 37.15 ± 7.96 |
6 | 3000 | 0.07 | 38.44 ± 6.85 | 54.45 ± 6.69 |
1 Average values are based on 6 different measurements. |
3.2 An overview of Rz values measured
Figure 5 shows the Rz values for machining the thermoplastic composite following the up-milling strategy. Each color represents the 6 corresponding trials for each tool. The aim of this subsection is to analyze the results obtained and to define a description of the defects in the tools employed.
In this way, it can be seen how, as discussed in previous section, the lower values of Rz are acquired by tool A. This tool has the biggest evacuation channel of all the tools employed. Chip evacuation is one of the most relevant aspects in composite machining. As mentioned in other studies, such as those carried out by Pereira et al. [29] and Wang et al. [30], a smaller chip channel could cause the chip to get stuck in the tool, causing an increase in the cutting forces generated during machining. The CFRTP used in this article has a low glass transition temperature and is therefore quite influenced by the temperature caused by the machining process. The matrix of composite melts and joins the carbon fiber particles that are removed during machining. This means that the chip generated needs enough space to be evacuated and does not interfere in the final surface quality of composite.
Tools C, D and E, have 8 teeth arranged along the diameter, therefore they have a smaller evacuation channel than tool A. This makes it difficult to remove the chips, which can get stuck in the tool itself. Figure 6 shows the chip stuck in the tool channel at the end of one of the tests. For this reason, these three tools take a higher Rz value than tool A. Tool B is the most particular of 5 tools employed. It contains 11 teeth distributed in a helical way along the tool diameter. This means that the evacuation channels generated between teeth are very small. This is the reason for the randomness of results shown in Fig. 4. Some of the Rz values obtained with tool B are similar to those obtained with the other 4. However, two of them are the highest values of the experimental set.
3.3 Analysis of the surface quality obtained per tool
In this subsection the analysis of the surface quality obtained for each trial is developed. In each graph, the average Rz values together with their deviations are represented. For each combination of parameters, a macrograph of the fiber pull-out generated in that trial has also been included. Above the set of macrographies there are two yellow dashed lines that indicate the point from which the fiber pull-out is generated and the upper limit of this defect.
Figure 7 shows the values obtained for tool A. It can be seen how the highest points of Rz occur with 0.05 mm/tooth for both cutting speeds. In addition, the deviations generated for 1000 rpm are smaller than those for 3000 rpm. Higher cutting speed means higher temperature during the machining process [31]. The temperature causes the adhesion defect to increase, making both the tool and the material susceptible to this defect. This may justify a wider range of values for a cutting speed of 3000 rpm. However, it should be noted that tool A is the one that has obtained the best results from Rz. In particular, a cutting speed of 3000 rpm and a feed rate of 0.03 and 0.07 mm/tooth generate values of 16.42 and 16.54 µm, respectively.
On the other hand, it can be seen that the geometry of tool A is not optimal for achieving a good cosmetic finish. The fiber pull-out defect appears in all the combinations of parameters used, being 1.58 mm the highest value obtained. The top layer of carbon fiber in composite is deformed in the direction of tool rotation but does not break. This tool is not completely designed for machining composite materials. The number of teeth of a tool for composite machining is one of the most influential variables in the process.
Hintze W. et al. [32] in their study agree that an increase in the number of teeth generates poorer results in terms of surface quality, but can improve the pull-out defect or delaminations of composite materials. This discussion matches the results obtained in this experiment. As seen above, tool A is the one that shows the lowest values of Rz.
As mentioned above, tool B has 11 cutting edges distributed in a helical pattern along the tool diameter. This tool geometry is designed to perform the final finish of a contourmilling process on composite materials. The high number of teeth makes the carbon fibers break more easily. This is reflected in the macrographs shown in Fig. 8. Compared to tool A, the fiber pull-out defect generated is much lower and in some cases is even close to 0. In contrast, in the combination of 1000 rpm and 0.05 mm/tooth parameters this defect can be seen significantly. It should be noted that the fiber pull-out coincides with the highest Rz value obtained in this set of trials.
With such small and even insignificant evacuation channels, tool blockage can occur during machining. Fiber particles next to the temperature affected adhesive could consolidate and get stuck between the tool and the material. This can lead to particle adhesion on the machined surface and even to the tool losing its cutting ability.
However, very striking Rz values appear when machining CFRTP with this type of tool. In particular, there are several combinations of parameters that achieve average values between 19.82–24.91 µm with small deviations, similar to those obtained by tool A. On the other hand, this tool contains the highest values of Rz, reaching 70.34 µm. Authors such as Geier N. et al. [22] obtain similar results. With this tool geometry they achieve higher Rz values compared to other tool types.
Tools C, D and E are a hybrid between type A and B. A greater number of cutting edges is known to benefit the machining of composite materials. For this reason, these tools have 8 cutting edges. In addition, an evacuation channel is a positive element that allows the chip not to affect the final machined surface.
Figure 9.a shows the values measured for tool C with a -10° helix angle. It can be seen that for the same cutting speed, Rz values decrease as the feed rate increases. In the same way, the deviations shown follow the same trend as the average values, i.e. they become smaller as feed rate increases. This causes that the final quality obtained is more homogeneous with a greater removal of material.
In general, tool D (Fig. 9.b) and tool E (Fig. 9.c) show similar trends.
There is a higher accuracy of the measured values as the feed rate increases. In addition, in all 3 tools, as the cutting speed increases, the Rz values increase. This effect also occurs with the previous tools. An increase in the cutting speed implies an increase in the process temperature, which can lead to a less clean cut due to the adhesion defect.
The 3 tools are in a similar range of average values. However, it is worth noting the results obtained for tool D (Fig. 9.b) with a combination of 1000 rpm and 0.07 mm/tooth. This combination is the one that generates a lower average value together with a small deviation. The negative helix angle produces the highest Rz values. However, the Rz values obtained for a positive helix angle are smaller. In an up-milling strategy, a positive helix angle makes the cutting edge of the tool to machine in the same direction as the helix angle, which could result in better material removal.
On the other hand, tool C, with a negative helix angle (-10º) generates a decreasing trend as the feed rate increases. On the contrary, the trend in tool E (positive helix angle, 10°) increases as the feed rate also increases.
As regards the fiber pull-out defect, it can be seen how, in general, the high number of cutting edges performs its function. This defect appears in a minor way in the tools that have a helix angle: C and E. In tool C an increase in cutting speed leads to a decrease in carbon fiber pull-out. As for tool E, it can be seen that at low feed rates the results obtained are the best of the set of trials carried out. On the other hand, from a feed rate of 0.07 mm/tooth, it seems that the geometry of the tool makes it difficult to cut these wires.
Tool D, with a helix angle of 0º, has generated fiber pull-out defect in all trials. The helix angle of a tool gives the chip channel a curved shape, in which the detached chips can be evacuated easier. In this case, as could be seen in Fig. 6, the absence of a helix angle has made chip removal difficult during machining. This has caused a blockage of the tool with fiber particles consolidated to the composite matrix.
Shengchao H. et al. [33] study the wear of this type of tools in CFRP machining. They conclude that using this type of tool decreases the cutting force required in the process. Lower cutting force allows higher surface qualities to be achieved. In this article, tool D generates a surface quality similar to the one obtained by tool A when machining at low feed rates. However, tool E, with a positive helix angle (+ 10°) generates less fiber pull-out when the composite is machined at low feed rates.
3.4 Discussion of CFRTP milling
The milling of thermoplastic matrix compounds with different types of tool geometries has led to significant results. In terms of surface quality, the conventional tool with 2 teeth (Tool A), has generated the lowest Rz values obtained in this experiment. Tool B has produced quite different results. It should be noted that this tool geometry is designed to perform the surface finishing of previously machined materials. Therefore, when a complete slot is made with this kind of tool, there may be an unpredictable behavior. Tools C, D and E show slightly higher Rz values than those caused by the conventional tool (A). However, these values have a fairly clear trend.
As for the fiber pull-out defect, it is observed that the conventional tool (A), obtains the worst results. In all the tests carried out with tool A, this defect has been generated in a homogeneous way. As it has been commented along the document, this tool is not designed to perform the machining of composite materials. Therefore, although it generates good surface qualities, it is not able to eliminate the fibers that are found in the upper layer of the material. Something similar has happened with tool D, however, the defect in this tool is caused by the absence of a helix angle. Tool B has generally offered the best results in terms of fiber pull-out. The height of the loose fibers found in the material is very close to 0. Tool C, with a negative helix angle (-10°) provides better results when machining the CFRTP at a high cutting speed. By contrast, tool E, with a positive helix angle (+ 10°), has been affected by the feed rate.
In general, CFRTP milling results in a variety of defects. The use of a thermoplastic matrix makes the glass transition temperature of the material lower than that of a thermosetting resin. Therefore, the thermoplastic material can be more affected by the temperature of the milling process than the CFRP. This is reflected in the surface quality obtained in other experimental studies where the Rz values are much lower than those obtained in this experiment [34]. On the other hand, as in the case of CFRP [35, 36], the milling of this type of material produces defects such as the elimination of the fiber yarns due to the efforts made by the cutting tool.
The machining of this type of material in other non-conventional technologies such as abrasive water jet machining, AWJM, could be interesting in terms of surface quality. In general, the temperature generated in the AWJM is lower than that caused by conventional milling [37]. This can cause the thermoplastic matrix of a composite material to be less affected by machining. However, the selection of an appropriate cutting tool geometry can achieve really good cosmetic defects that the waterjet may not get. Today, cutting tools are manufactured such as B, C, D and E tools, designed to eliminate defects such as fiber pull-out, internal delaminations or die loss during machining.