The process of transformation and development of our planet has accelerated much more sharply than in previous years. This fact is fully noticeable in our daily routine, in which the digital media and social networks bring an immense amount of information every minute. Being more specific with focus in the manufacturing process and talking about milling, which is a very important machining process for many industry fields, several studies were carried out mainly during the 80’s and 90’s, seeking to establish criteria for the end of tool life, as well, methods that could indicate the ideal time for tool change in real production. Nevertheless, according to an extensive literature review based on publications in manufacturing technology journals, university research and benchmarks with industry developments, it was not possible to find out publications that confirms the usage of automatic definition for the end of tool life under heavy roughing five-axis milling in a production environment. Based on this context, the relevance, originality and contribution of the present work for the scientific community is grounded in developing an automatic tool change process completely independent of the cutting parameters inside a production environment, that can be replicate for many other machining operations by using state of art technologies (Industry 4.0 - The Digital Age), in order to take out of the machine operator's hands the important decision to establish the end of tool life. Aiming this goal, several rough milling experiments were conducted in a real production of hydraulic turbines. The main conclusion was that, in rough milling process, where the admissible flank wear is very high, both, the power consumption of the machine and the cutting temperature at the end of tool life are independent of the cutting conditions (vc and fz) and, consequently, a specific value of power and temperature can be established to determine tool life end.
Research Article
Automatic definition of tool life under heavy roughing five-axis milling process in production environment
https://doi.org/10.21203/rs.3.rs-1247892/v1
This work is licensed under a CC BY 4.0 License
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Five-axis milling
Automatic tool change
End of tool life
Industry 4.0
Competitiveness
As a direct consequence of technological developments, the competitiveness and optimization of machining operations have been studied over the past years in order to understand the representativeness of each variable (labor, cost of acquisition and maintenance of machines, raw material, tool costs and others) involved in this important transformation process. In general, many studies concluded that the direct costs with the tools are responsible for something around 5% of the total cost involved in this operation [1]. However, the inappropriate usage of tools can significantly negatively affect the other variables, causing a high consumption of machine hours and labor, generating high maintenance costs, and causing irreparable damage to the part, that could even generate scraps. As previously mentioned, during the 80's and 90's, several studies were performed about establishing automatic criteria for the end of tool life. However, anyone could indicate an ideal method for tool change in real production, without the participation of the machine operator. To implement this automatic methodology, it is necessary to apply scientific and technological resources based on the operational excellence approach and also Industry I4.0 (sensors, software’s, auxiliary electronic equipment, communication interfaces for data acquisition / processing and others). This procedure provides solid technical basis for an automatic and accurate intervention in respect to the end of tool life criteria, confirming benefits for HSE (Health, Safety and Environment = sustainability aspects), quality, deliveries and costs. Furthermore, it is also very important to mention that this work, in spite of having an automatic procedure to define tool life and replacement, did not use a complex solution, such as neural network or any kind of mathematical model.
The aim of this work is to develop a monitoring system able to automatically define the end of tool life and to perform the automatic tool change in a rough milling operation, completely independent of the cutting parameters inside a production environment and without the participation of the machine operator. In order to develop and implement this very important technological step the experiments were carried out in a five-axis milling operation of blades for hydraulic turbines.
In general, blades for hydraulic turbines are produced in cast stainless steel material and during the milling process of such components (i.e. large size: superficial area around 25m²) more than three (3) tons of chips can be generated in a few tens of machining hours [2].
As contextual information, the energy generated by hydraulic turbines represent around 60% of the Brazilian energy matrix and talking about the whole world, the hydraulic turbines are responsible for something around 30% of the total world energy matrix [3].
The amount of publications referring to topics connected with the automatic tool change and definition for end of tool life under heavy roughing five-axis milling process in production environment is very scarce, which hinders the bibliographical survey and the technological development of five-axis milling in heavy roughing operations with focus in high removal volume (tons of chips). This fact once more demonstrate the relevance and important contribution of the present work which also confirms the originality proposal as a strong development for the scientific and industrial communities that can replicate this approach for further developments in other machining operations, getting as main benefit to minimize the underutilization of tools, which generates unnecessary machine downtime and consequently, several other losses.
Figure 1 shows the main themes investigated during the literature review in order to obtain updated knowledge based on publications in manufacturing technology journals, university research and benchmarks with industry developments.
According to the respective literature review, the key boundary aspects have been observed. Those are presented in the subchapters below.
1.1 The cutting tool
The most important diameter to be considered when selecting cutters with round inserts (toroidal) for mold milling process, aerospace industry and also, in milling process of hydraulic turbine blades (organic shapes) is the outer diameter of the cutters, as it is used as a fundamental parameter for generation of milling path and strategies inside the CAD-CAM-CAV software [4].
The axial depth of cut (ap) is another very important parameter in the milling process and can be defined as the difference between the machined and un-machined surface in the axial direction. The maximum cutting depth is mainly limited by the insert size and machine power. The milled width also known as radial depth of cut (ae) is an extremely relevant parameter for the productivity aspect. By increasing the number of tool teeth (z), the feed rate (vf) can be increased, while the cutting speed (vc) and feed per tooth (fz) remain unchanged [5 - 7].
Other critical factors for the cutting tools in roughing operations are the temperature, power consumption and vibration. In this case, the selection of the inserts have a fundamental contribution in the machining performance and characteristics of high hardness, good wear resistance at high temperatures, combined with toughness are extremely important [8].
As the experiments have focus in roughing operation, the tool toughness is an indispensable property and thus, the insert grade ISO-M35 are commonly used, especially in the rough milling of hydraulic turbine blades. This insert has an aluminum oxide coating with optimized microstructure, applied using the CVD process, combining layers of Titanium Carbonitride and Titanium Nitride (TiCN+Al2O3+TiN), which provide for the tool conditions to work with high cutting speeds [9]. Moreover, due to the fact that the machined material is ductile, the tool geometry must be positive.
1.2 Stainless steels machinability
The machinability of martensitic stainless steels are influenced by the concentration of carbon content, nickel content and metallurgical structure [10]. As in the most materials, increasing hardness typically reduces tool life and machinability. Increasing the carbon content increases the proportion of abrasive chromium carbides in the matrix and, consequently, reduces tool life [11]. The metallurgical factor that has the highest influence on machinability is the proportion of free ferrite in the matrix. Machinability generally increases with free ferrite content [5]. Tool life is related to annealed hardness, which increases with increasing nitrogen content. Increasing the carbon content also increases the work hardening rate that decreases machinability [6]. Abrasive carbon / nitrogen compounds can be formed in the material matrix and, consequently, reduce tool life; these can be controlled by adding titanium or niobium. Hardness increases and machinability decreases with increasing nickel content. In general, stainless steels can contain alloying elements and additives such as sulfur, selenium, tellurium, lead, bismuth and phosphorus intended to improve machinability [10].
1.3 Tool wear and damage
During the milling process, the tool cutting edge is subjected to numerous efforts and friction, which generates wear. This continuous wear on the tool faces is one of the factors that determine the end of tool life [12]. In conventional machining processes, there are several factors, called malfunctions, which determine the necessity of cutting tool replacement. They can be defined as:
a) Breakdown – happens suddenly. Types of damage are breakage, chipping, plastic deformation or built up edge (BUE). Damages are more common in interrupted cutting processes like milling, in which tools are subjected to extreme demands, mainly with regard to thermal and mechanical shocks [6].
b) Wear – continuous and microscopic removal of particles from the tool under the action of the chip and/or the workpiece. If the wear is too high, it may compromise the quality of the machining process and lead to edge breakage. It can occur in both machining conditions, interrupted and continuous cutting [6].
A cutting tool, even if it has enough toughness to withstand the cyclical load and temperature variations that could cause it to malfunction, will not be free from a progressive loss of material, which leads to the necessity of tool replacement [5].
1.4 Considerations for the end of tool life
The precise definition of the end of tool life is very difficult to be made inside the production environment, since it is usually done by the machine operator. The result, in the majority of the cases, is a big waste of tools, due to the difficulty that the machine operator encounters in establishing the end of tool life. Therefore, he (or she) does it very conservatively, in order to not damage the part or to not cause any other damage to the production. The solution for this problem lies in monitoring the tool wear using different indirect methods like vibration sensors, temperatures, acoustic emission, electrical parameters and machining forces, which will provide the necessary subsidies and, based on a scientific approach, indicate the ideal moment for tool change [6].
1.5 Methods for monitoring the end of tool life
The end of tool life can be evaluated through direct and indirect methods. In direct methods, geometry (the element to be controlled is measured) and tool wear are measured using optical system. The indirect method uses the acquisition of measured values of process variables (such as cutting forces, temperatures, electrical parameters, vibration and others) which can be correlated with tool wear [6].
The dimensional laser tool monitoring system is a direct monitoring method that allows the check of wear and/or small damage to the tools during the machining process, which are vital for the safe operation of the system machine-workpiece-tool [13]. The wear measurement is carried out through brief tool travel movements passing through the laser. This is an intrusive monitoring process that generate stops in the machining operation and, consequently, adds unproductive times to the process [6].
In favor of indirect monitoring systems is the fact that it is not necessary to interrupt the process for the control to be carried out. This simultaneity also allows the monitoring of the tool's status in real time, enabling its breakage to be avoided [5]. The most common indirect monitoring systems observed in the literature review are:
Monitoring of electrical parameters: Measuring the electrical parameters of the machine's motor is measuring the cutting efforts. This is because the machine motor, when generating the mechanical power necessary to carry out the machining operation, consumes electrical current in an amount directly proportional to the power and cutting force generated [7]. As the tool wears, the cutting force increases and this increase can be correlated with the tool wear value. The same effect holds for the feed rate drive motor (X-axis), that it is possible to observe the effect of the increased friction force due to the gradual increase in tool wear on the power consumption of t7he machine motor [15, 17].
Temperature monitoring: Every machining operation generates heat during its execution. This fact is mainly due to two factors, which are the friction between tool-workpiece-chip and the internal shear to which the workpiece material is subjected during the formation of chips. The machining process generates a significant amount of heat. The resulting temperatures around the cutting edges have a direct influence on the rate and manner in which tool wear occurs, as well as the friction between the tool and the chip, and also, the tool and the newly formed surface. Radiation measurement methods consist of non-contact thermographic techniques to measure temperature from the thermal energy emitted by the cutting zone. The methods that use these techniques are the most used for measuring the cutting temperature. The main advantages of this technique are the quick response to temperature variation, non-physical contact to carry out the measurement, in addition to allowing the measurement of difficult-to-access objects, for example, in cases where there is some movement that prevents fixing the sensor [14, 16].
Vibration monitoring: Vibration is defined as the movement of a dynamic system around its static position. For a mechanical system to vibrate it is necessary, and sufficient, that it has the capacity to store kinetic energy (which has inertia), elastic potential energy (which is flexible) and a time-dependent external excitation. Consequently, in practice it is very difficult to avoid vibration in machining processes. It usually occurs because of the dynamic effects of manufacturing tolerances, clearances, contacts, friction between machine parts, part material heterogeneities, and due to the effect of unbalanced dynamic forces of rotating and alternative machine components. Again, as the tool wears, the cutting forces increase and, consequently, either the tool or workpiece may increase. In addition to the aforementioned factors, the cutting tool wear increases, vibration amplitude increases in all frequency bands, demonstrating a good correlation between wear and vibration which vary according to the case under study. This fact also justifies the use of the vibration signal in this research [14, 17].
1.6 Case of study - Heavy roughing five-axis milling process in production environment
As already mentioned, to develop and implement the monitoring and automatic change of the worn tool in the shop floor inside the industry, a bottleneck five-axis milling operation in blades for hydraulic turbines was used. This process represent well the end of tool life under heavy roughing five-axis milling process in production environment, because during the milling process of blades for hydraulic turbines (superficial area around 25m²) more than three (3) tons of chips can be generated in a few tens of machining hours [2].
The publications referring to the subject of milling of hydraulic turbine blades are very scarce, which hinders the bibliographical survey and the technological development of five-axis milling of these important components. This condition demonstrates the important contribution of the present work and confirms its originality proposal.
The milling process in hydraulic turbine blades has developed mainly during the last two decades. There are several variants in this process, which aim to give the blade the required geometric shape through the milling operation in machining centers with computer numerical control (CNC) equipped with three, four or five-axis, using in most cases, a cast blade as raw material. Other fabrication concepts using formed sheets are also under development and analysis. However, the traditional and most used manufacturing concept around the world has been the use of cast solution as a raw material for the production of hydraulic turbines blades [2]. Five-axis milling techniques have been widely used in the power generation segments and in the aerospace industry. In part, because of the complexity of the geometries found in turbines and critical aircraft components. Pieces produced in five-axis milling reduce the number of manufacturing steps, avoiding additional assembly’s steps in order to obtain the final product. In the aerospace industry, components that require five-axis milling often have less weight, which is a very important to obtain a good performance. In the heavy mechanics industry where hydraulic turbines for hydroelectric power plants are manufactured, turbine blades are generally large, providing a high amount of chip removal. Thus, five-axis milling reduces manufacturing time and, consequently, the final cost of the product [18].
Further studies concludes that despite the technological advantages of the five-axis milling, the use of this technique is still limited, due to the complexity and difficulties in generating collision-free CNC programs. The most of CAD-CAM-CAV systems have limited functionality for generating milling paths in five-axis. The CNC programmer is often forced to intervene in the tool orientation and manually adjust the respective milling path. In most cases, the CNC programmer uses as base the local region of the surface to be milled with the heights concavity. More advanced CAD-CAM-CAV systems have more customized algorithms for interpolation that optimized the tool movement and orientation [19].
Another very important aspect studied by researchers that must be observed when carrying out five-axis milling is the clearance angle between the cutter and the surface that is commonly called by CAD-CAM-CAV software’s as “lead angle” (details in the section 2). The “lead angle” refers to the angle of inclination from the tool axis in relation to the direction of movement and it is intended to ensure that the back of the cutter does not collide with the back side of the workpiece, which can have a high elevation / curvature (surfaces with different geometric and/or organic shapes) than the front side [20].
This chapter presents all resources used to carry out the milling tests, as well as the description and details of the experimental procedures applied to obtain the results used as basis for the conclusions of this work. Based in these results, it was possible to establish a procedure capable of automatically determining the end of tool life in a five-axis milling for blades of hydraulic turbines inside the production environment and under different cutting conditions. The values of machine power consumption, tool temperature and vibration of the system machine-workpiece-tool were monitored during the different phases of the cutting tests. It is also important to note that the milled width also known as radial depth of cut (ae) is 87.5mm (70% of Da), the axial depth of cut (ap) of 3mm, the machining strategy is concordant cutting direction and clearance angle (lead angle) of 3°, were constant in all tests.
The tests were carried out at VOITH HYDRO Ltda. Company, using a portal machine with CNC Siemens 840 D, maximum rotation capacity of 3,000 rpm and maximum feed rate of 45,000 mm/min (G1). The spindle of the portal machine runs through an alternating current motor that has a continuous rotation variation with a maximum power of 65kW. The machine has independent motors for drives the table (X), transverse axis (Y), RAM (Z) and head axes (A and C). Details are indicated in Fig. 2.
Francis and Kaplan turbine blades were used to carry out the tests in a productive environment. The surfaces of these blades have a total area of approximately 20m². These blades offer robust thicknesses that promote good rigidity and provide good stability for milling operations (Fig. 3).
The raw material of these components is martensitic stainless steel ASTM A743 Grade CA-6NM, whose average hardness is 270 HB (28 HRC). Observing the technical data of this material (Fig. 4), the presence of low carbon content (≤ 0.06% C) can be noted, which is a very important fact to be considered for the insert grade selection [21].
Tab. 1 Voith Hydro Technical material specification according ASTM code (A743 Grade CA-6NM)
Martensitic stainless steels, with high carbon contents, provide difficult machining conditions due to their high hardness, which requires large cutting effort due to the presence of hard and abrasive chromium carbide particles. However, as the stainless steel used in this work has a low carbon content (≤ 0.06% C), the presence of carbides is not a problem for its machinability.
As already applied by Voith in their production process, the insert grade ISO-M35 were used to carry out the tests, supported by a “toroidal” tool holder (round inserts) Da = 125mm and z = 7 (number of inserts). It was assembled on a solid cylindrical shank mandrel with a total length of 145mm that has been fixed in the milling head through an ISO-50 cone (Fig. 4). The ISO-M35 insert has characteristics of high hardness, good wear resistance at high temperatures, combined with toughness [9]. As the operation object of the tests in this study is the roughing operation, the tool toughness is a fundamental property for the tool performance. Each set of inserts (seven pieces) was used only once in order to provide precise conditions for measuring the respective wear.
For monitoring and establishing the end of tool life using criterions based on the power consumption of the electric motor, cutting temperature and vibration of the system machine-workpiece-tool, logical interfaces were developed to support the automatic intervention during milling process. The implementation took place via CNC machine panel based on the application of miscellaneous “M” function, thermographic camera together with a dedicated interface and vibration sensors aid by an external computer.
Monitoring of electrical parameters: The data of electrical parameters (power consumption) of the main motor were monitored and automatically extracted from the CNC machine panel using a text format and converting it to an Excel file in order to generate graphics for a better understanding and visualization of the data (details in the section 3). The signal was measured during the entire machining time. For the definition of tool change based on the power consumption, the configuration of the machine data was carried out in a simple way, since the power consumption is a parameter already monitored by the CNC of the machine in a time window of milliseconds.
Vibration system monitoring: The vibration data monitoring of the system machine-workpiece-tool was monitored based on the standard ISO 20816-1 [22]. According to it, measurements of the vibration speed referring to the vibratory displacement (mm/s) were performed. The vibration sensor used was the VSA/VSE 001- IFM (Fig. 5) and the vibration data throughout the tests were monitored and automatically extracted with the aid of an external computer and IFM software. The data were converted into graphs that are presented in section 3 of this work.
Cutting temperature monitoring: The cutting temperature was monitored during the milling tests using a Fluke Ti400 thermographic camera [23], which was attached to the Z-axis (RAM) fairing of the portal machine (Fig. 6). The temperatures are automatically and continuously taken every three minutes, considering the effective cutting time, as well as the positioning times (passive times). To define the moment for tool change based on the cutting temperature, the thermographic camera acts connected to the CNC machine panel, through a serial port. In this way, the maximum value displayed in the camera monitor window is automatically read and, if the pre-set temperature is reached, the subroutine for automatic change is executed via the miscellaneous “M” function. Fig. 7 presents a schematic illustration about the interface developed for monitoring the cutting temperature. The cutting temperature measurement has focus on the cutting edge of the tool, on the exit position of the cutting edge in contact with the part in each revolution, that is, after the cutting path of each cutter edges in each revolution. Fig. 8 shows the point (ALT) as typical location for temperature measurement. Graphs were also generated and are presented in section 3 of this work.
Monitoring of flank wear (VB): During the course of the tests, the flank wear (VB) was monitored of the insert edges was also carried out by the intervention of the machining engineer every 3 minutes, in order to check the trend of the wears over the milling time. To carry out the wear measurement, it was necessary to stop the milling process (milling head spindle and feed axes) for a few seconds (approximately 30 seconds for each measurement taken). The measurement of flank wear (VB) of the insert edges was performed with the tool attached to the milling head that it was still in the machine, with the aid of a graduated magnifying glass (20x).
As already cited, the tests had focus on the roughing milling operation of blades for hydraulic turbines. Figs. 9, 10 and 11 respectively illustrate the conditions of milled width also known as radial depth of cut (ae), clearance angle (lead angle) of 3° and concordant cutting direction. All those parameters were fixed as constant values in all phases of the milling tests as already mentioned at the beginning of section 2.
2.1 Preliminary tests
To initiate this preliminary phase of the work, a considerable number of worn cutting edges already used in the roughing milling operation of hydraulic turbine blades had their flank wear (VB) measured in the daily production. This operation has the highest volume of material removal of the whole turbine blade production, and it is responsible for something around 90% of the total milling time in this type of component. At this stage, the machine operator used they own criteria to define the end of tool life and the machine operator did not know that they were being monitored in order to avoid any kind of interference on the data collection. The objective of this action was to identify, in a consistent and practical way, the suitability of the criteria applied by the machine operator to define the end of tool life. Thus, a survey of flank wear (VB) was carried out on the edges of all inserts used in production during fifteen (15) working days. The result of this survey will be presented in section 3 of the present work. At this stage of the preliminary tests, there was no simultaneous measurement of power consumption, cutting temperature or vibration of the system machine-workpiece-tool.
2.2 Definitive tests
The definitive milling tests started only after completion of the preliminary test and were divided into three (3) phases. In the first phase of definitive tests the criterion and moment for the end of tool life were still defined by the machine operator, but in parallel, the electrical parameters (power consumption of the main spindle motor), the cutting temperature and vibration of the system machine-workpiece-tool also took place in order to implement a procedure for automatic tool change according to the practical results. On the second phase of definitive tests, already with pre-establish parameters for the end of tool life; the determination occurred automatically without the intervention of machine operator. This second phase of tests aimed to prove the effectiveness of the automatic procedure based on pre-stablish results from the preliminary and first phase of definitive tests. Then, based on the results obtained with the automatic procedure applied in the second phase of definitive tests, the parameters were reassessed, refined and redefined, aiming at the balance between safety, quality and productivity to be applied fully automatically in a productive environment (third phase of definitive tests). Tabs. 2, 3 and 4 respectively present the conditions used in each phase of the definitive tests (vc = cutting speed; fz = feed per tooth; vf = feed rate). The other parameters (ae = 87.5mm; lead angle = 3°, ap = 3.0mm and cutting direction = concordant) were constant.
Tab. 2 Cutting conditions used on the first phase of the definitive tests
Tab. 3 Cutting conditions used on the second phase of the definitive tests
Tab. 4 Cutting conditions used on the third phase of the definitive tests
It is important to emphasize that the parameters for defining the end of tool life must be extremely safe and reliable because, if the tool breaks, it can cause damage to the machine and also, in critical cases, irreparable deviations in the parts.
Figure 15 present a summary of tests (preliminary and definitive) in order to illustrate in a graphical way the steps used for reassessed, refined and redefined the pre-stablish parameters, aiming at the balance between benefits for health and safety, quality, deliveries and costs.
This section presents the results obtained during the tests and their respective analyses for development and implementation of an automatic tool change process inside a production environment completely independent of the cutting parameters and also, taking the important decision to establish the end of tool life without the participation of the machine operator.
3.1 Preliminary tests
This work had as initial action, prior to perform the milling tests, the measurement of a considerable sampling of flank wear (VB) on the edges of the inserts used in the day-to-day of the production environment (preliminary test). The objective of this action was to identify, inside the production environment, how the inserts were being replaced. The values of flank wear (VB) resulted from the criteria applied by the machine operator to define the end of tool life of inserts during day-to-day inside the production. It is important to remember that, to make the decision about the end of tool life, the machine operator used parameters such as power consumption (~40kW) indicated on the potentiometer of the CNC machine panel, noise and the color of the chips produced (dark blue) at different moment of the milling operation in the roughing step. These parameters have been used routinely during the last years of production for this type of component.
During each working day, approximately fourteen (14) inserts were used on average (the cutter has 7 inserts, which means that, each day, two sets of 7 inserts were used). Each insert was positioned in the cutter six times, with the aim of fully utilization of the entire perimeter (circumscribed length) of the cutting edge. Therefore, it was possible to carry out, throughout each working day, the measurement of eighty-four (84 = 14 x 6) flank wears (VB) and consequently, after this fifteen (15) days of monitoring, a total amount of one thousand two hundred and sixty (1260 = 14 x 6 x 15) flank wears (VB) measurements have been done. The summary of this sampling is presented in Fig. 13. In this case, it is important to note that VBmin is the lowest VB value of the fourteen (14) inserts used in a working day and VBmax represents the highest VB value of the fourteen (14) inserts used in one working day.
Analyzing Fig. 13, it is possible to identify a large variation (0.6 to 5.5mm) in the values of flank wear (VB) among the fourteen inserts used in a working day. This large variation in flank wear (VB) confirms that the criteria used by machine operator as mentioned in previous paragraphs were not fully efficient. This indicates that, on several occasions, the end of tool life was inadequately defined through premature changes (small flank wear) and consequently, generating waste (for example: time and costs) in the milling process.
Another important point to be emphasized is that, even with the large variation in flank wear values (VB), at any moment during the process of insert wear monitoring chipping or breakage of the cutting edge were observed. This is a very important indicator, which confirms that even for large flank wear values (sometimes higher than VB = 5mm), the system machine-workpiece-tool is not being put at risk. Accordingly, it can be stated in this initial phase of the present study that the value of flank wear (VB) to be used as pre-establish for the end of tool life completely independent of the cutting parameters, under heavy roughing five-axis milling in a production environment is around five millimeters (5mm).
3.2 Results and analysis of the first phase of definitive tests
Table 5 presents the summary of the results from the eight milling tests when the machine operator still decided for the end of tool life and tool change.
Tab. 5 Flank wear (VB), vibration of the system machine-workpiece-tool, cutting temperature and power consumption when the machine operator decided to change the tool (end of tool life)
3.2.1 Analysis of flank wear monitoring results in the first phase of definitive tests
At this step the machine operator still used they own criteria to establish the end of tool life and at the same time, the parameters of power consumption, vibration of the system machine-workpiece-tool and cutting temperature took place. In this first phase of definitive tests, the machine operator were aware that they were being monitored.
As already described in section 2, the first phase of the definitive tests started only after the previous survey and initial evaluation of the flank wear (VB) in a productive environment (preliminary test) in a period of fifteen working days. At the moment, when the machine operator decided for the end of tool life (inserts), the values of the highest flank wear (VB) were measured and duly registered through photographs of the cutting edge with the aid of a stereoscopic microscope. Fig. 14 shows the highest values of flank wear (VB) among the seven inserts used in each test of this first phase of definitive tests (8 tests = 4 tests + 4 replicas).
Evaluating the inserts showed in Fig. 14, it could be confirmed that the flank wear (VB) was responsible for the end of tool life. No other type of damage such as chipping, breakage and/or crater wear was found in the inserts. Another finding is that the highest flank wear (VB) ranged from 3.5 to 4.3 mm, when the machine operator established the end of tool life.
According to the data described in Fig. 15, it is also important to note that the variation among all inserts (7 cutting edges x 8 tests = 56 cutting edges) of the cutter was between 2.8mm (lowest VB value) and 4.3mm (highest VB value), which results in a variation of approximately 50%.
Comparing the values of flank wear (VB) raised in the preliminary tests (0.6 to 5.5mm) with the results obtained during this first phase of definitive tests (2.8 to 4.3mm) under similar cutting conditions, it can be concluded that the variation was much smaller when the monitoring system was already placed in the machine, but was not the responsible for the definition of the moment to change the tool. In both cases (preliminary test and first phase of definitive tests), the machine operator defined the moment for tool change by himself and this clearly demonstrates the necessity to automate and pre-establish the end of tool life criteria, taking the autonomy of this important decision out of the machine operators’ hands.
Due to the high level of wear, it was not possible to perform an additional analysis to establish the tool wear mechanisms. However, in all inserts it was possible to identify the presence of the workpiece material adhesion on the cutting edge.
Based on the obtained results it is possible to state at this point of the study, that when VB was equal to 4.3mm (highest VB value on the first phase of the definitive tests), the insert did not break. Therefore, a value close to this number can be used as a reference value to define the end of tool life and consequently, the moment for tool change. Then, it remains to detect how representative will be the values of cutting temperature, power consumption and vibration of the system machine-workpiece-tool, when the tool reaches this level flank wear (VB).
Figure 16 presents the curves of VB values measured by the machining engineer with the aid of a graduated magnifying glass (20x), making observations of the highest flank wear (VB) value among the seven cutting tooth in every 3 minutes of cutting.
Analyzing the figure above, it is possible to notice that the wear values were around a few tenths of a millimeter (~0.3mm) during the first minutes of milling. As the test progresses, the wear increased, due to the friction between the cutting edge and the workpiece, which generates a cutting temperature increase and, consequently, a further increase in wear. In general, there was no sudden change in the VB value at any time. However, there were an acceleration of the wear rates in the last three minutes of the tests. Anyhow, there was no chipping or edge breakage.
Considering the variations in the cutting parameters (vc = cutting speed and fz = feed per tooth) between the tests and their respective replicas, it is noted that, in the tests and in their replicas, the machine operator decided for the same end of tool life moment, nevertheless, the flank wear values were different. Evaluating tests 1 and 1’ (vc = 190 m/min. and fz = 1mm), the machine operator decided for the end of tool life in both cases with 21 minutes and the wears were respectively 4.3 and 3.8mm. As for the tests 2 and 2' (vc = 220 m/min. and fz = 1mm) also, tests 4 and 4' (vc = 220 m/min. and fz = 1.15mm) the machine operator decided for the end of tool life in all four tests with 18 minutes, with the wears ranging from 3.6 to 4.2mm. For tests 3 and 3' (vc = 190 m/min. and fz = 1.15mm) the machine operator decided for the end of tool life with 15 minutes in both tests, when flank the wear values were respectively 3.5 and 4.3mm. Therefore, it can be said that in some tests (for example tests 2 and 4) and replicas (1' and 3') the machine operator could have gone further and continued with the milling process without changing the tool.
3.2.2 Analysis of electrical parameters monitoring results in the first phase of definitive tests
Regarding the monitoring of electrical parameters of the portal machine main motor (power consumption = kW), it was automatically extracted from the CNC panel and at this stage of the tests, it was not used to define the moment for tool change, but to evaluate the reliability of the machine operator's decision and also, to verify how this parameter could be used in the next phases of definitive tests in order to support the definition of pre-stablish the basis for the automatic tool change.
The power consumption curves presented in Fig. 17 which were generated during the first phase of definitive tests, showed that the power consumption during the first minutes of milling tests were between 28kW and 34kW. Early in the tool life, there was some influence of cutting conditions on the power consumption. Comparing tests 1 and its replica with tests 2 and its replica, in which the cutting speed (vc) increased by 16% (while maintaining the feed per tooth), the power consumption increased, in average by 6.6%. Comparing tests 3 and its replica with tests 4 and its replica, in which the cutting speed (vc) also increased by 16% (maintaining the same feed per tooth), the power consumption increased among the tests, on average, by 10%.
Comparing tests 1 and its replica, with tests 3 and its replica, which the feed per tooth grew 15% and the cutting speed (vc) remained constant; the power consumption increased 6.6%. Comparing tests 2 and its replica, with tests 4 and its replica, in which the feed per tooth (fz) also increased by 16% and the cutting speed (vc) remained constant, the power increased by 10%. The growth of power consumption values with cutting conditions are compatible with data provided in the literature [6].
As the test progressed, the power consumption grew, showing that wear on the inserts has a direct influence on the power consumption. Furthermore, according to the results presented in this section, the relationship between the power consumption and the end of tool life (inserts) is confirmed, being possible to state that during the last three minutes of all milling tests, the power consumption showed a significant increase, going up from a range between 32kW and 35kW to a range of 39kW and 41kW, which represents an increase around 20%. This fact confirms that power consumption is a good parameter to define the end of tool life (inserts) during the milling process, as the flank wear (VB) and power consumption curves (Figs. 16 and 17) show very similar trends (inclination) and proportional throughout the course of the tests. Based in these aspects it can be confirmed that power consumption is a good parameter to assist the decision about the end of tool life.
The end of tool life varied considerably between the tests and also, between the replicas of the same test, showing once again that the procedure used by the machine operator about the decision regarding the end of tool life it is not adequate. Notwithstanding, the data presented on Fig. 19 also shows that the flank wear values at the end of tool life established by the machine operator varied a lot (test 1 = 4.3mm; test 1' = 3.8mm; test 2 = 3.6mm; test 2' = 4.2mm; test 3 = 4.3mm; test 3' = 3.5mm; test 4 = 3.6mm; test 4' = 4.2mm). However, the end of tool life in relation to the power consumption varied much less, among all tests and the respective replicates (test 1 = 39.5kW; test 1' = 38.5kW; test 2 = 40.0kW; test 2' = 39,0kW; test 3 = 41.0kW; test 3' = 39.5kW; test 4 = 41.0kW; test 4' = 40.5kW. It means that the values presented in the paragraph above indicate a power consumption variation in a range of 6% and it is worth mentioning that the end of tool life in relation to the power consumption is independent of the cutting conditions (vc and fz). This demonstrates that, when wear reaches values as high as those do obtained in the experiments of this work, its influence on power consumption values is so high that overwhelms and hides the influence of the cutting parameters.
The ratio between the power consumption at the end and at the beginning of tool life (Fig. 18) varied greatly between the tests and also, between replicas of the same tests. This variation was not related to the cutting conditions, since tests 2 and 2' (its replica), which used vc = 220m/min, presented a variation of 29% and 34% respectively, while tests 4 and 4' (its replica), with the same value of cutting speed (vc), presented a variation of 24% and 19%, respectively. The small variation of feed per tooth (fz) in these tests would not justify such large percentage discrepancy in the ratio between the end of tool life and beginning of tool life in relation to the power consumption. The same can be said in relation to the tests that used a cutting speed (vc) of 190m/min. Thus, the variation in power consumption over the lifetime does not appear to be an adequate parameter for defining the end of tool life and, consequently, the moment for tool change.
3.2.3 Analysis of temperature monitoring results in the first phase of definitive tests
Figures 19a and 19b show, respectively, the values of the temperatures observed during the first phase of definitive test 1 at the beginning (Fig. 19a: 145.7°C) and at the end of tool life (Fig. 19b: 382.4°C) of the inserts. More precisely, it can be said that these figures show the values of the cutting edge temperature during its idle time in each tool revolution. The cutting temperature that will be used for the indirect analysis of tool wear is that which the edge had immediately after the edge exits its active part, that is, at the point called “ALT” on Figures 19a and 19b.
It is necessary to highlight that various information are displayed in the FLUKE camera monitor (average temperature, minimum temperature, maximum temperature, colors bar vs. temperature graph and others). However, as mentioned in the previous paragraph, the relevant information for our study is the “ALT” point, which informs the maximum temperature at the indicated point. In this type of milling with high removal rates, this parameter (temperature in the cutting region) is not commonly observed by machine operator. With the progress of the test and increase in flank wear levels (VB) in the inserts; there is also an increase in the friction of the tool against to the workpiece, which respectively generates an increase in the cutting temperature as can be seen in Fig. 20.
According the curves show in Fig. 20, the cutting edge temperature values at the beginning of the tool life (three minutes of cutting) ranged between approximately 130°C to 180°C and did not keep a direct relation with the cutting conditions. Furthermore, the tests with heavier conditions (Test 4 and its replica) generated cutting temperatures around 160°C, while the test that generated the highest edge temperature was test 3, which used the lowest cutting speed (vc) among the two (2) tested. To perform a complementary analysis of how the temperature was influenced not only by flank wear (VB), but also, by the cutting parameters, Fig. 21 presents the correlation between the cutting temperature values at the end of tool life, as well as the flank wear (VB) at the same respective moments.
Observing the correlation showed in Fig. 21 between the temperature variation at end of tool life and the flank wear (VB), it is noted that the temperature of the cutting edge in the moment that the machine operator decided to change the tool varied little between tests and its replica, and a little more between all tests. The variation occurred between 344°C and 392°C. This shows that, for flank wear values as high as those obtained in this work (~5mm); the temperature depends much more on the high friction between tool and workpiece than of the cutting conditions.
3.2.4 Analysis of vibration monitoring results in the first phase of definitive tests
Figure 22 shows the vibration values (in velocity) of the system machine-workpiece-tool obtained along all the tests.
Analyzing the Fig. 22, it can be confirmed that with the increase in tool wear, there was an increase of the friction in the cutting area, which generated an increase in cutting forces which and consequently, increased the vibration of the system machine-workpiece-tool.
Based on the results obtained during the tests, it can be confirmed that vibration is influenced by tool wear, as all vibration values at the end of tool life were greater than those at the beginning of tool life. Additionally, it is also possible to verify that vibration in early phase of the tool life is not directly related to the cutting conditions. The heaviest cutting conditions (Test 4 and its replica) generated lower machine vibration at the beginning of the tool life than the tests with the lighter conditions (Test 1 and its replica).
Analyzing the data in Figs. 23, it can be stated that the vibration growth with wear ranged from 60–200% from the beginning to the end of tool lives. The RMS of the vibration values at the end of tool life ranged from 7mm/s to 12mm/s, a much higher variation than what occurred with electrical parameters (power consumption) and cutting temperature. Furthermore, the end of tool life based on the vibration values do not show any correlation with the cutting conditions. The test (4) four (highest vc and fz values) and its replica presented end of tool life vibration RMS values around 8mm/s and test one (1) and its replica (lowest vc and fz values) presented at the end of tool life vibration RMS values between 11mm/s and 12mm/s.
Accordingly, it was not possible to detect any vibration parameter that could accurately indicate that the tool life is near its end. This most likely occurred due to the fact that the machine structure where the vibration sensor was located is very rigid and also allows to detect vibrations coming from other parts of the machine and not only from the milling process.
3.3 Results and analysis of the second phase of definitive tests
Considering the results obtained and presented in the first phase of the definitive tests of this work (section 3.2), new criteria were established for the end of tool life in this second phase of tests. Since no vibration parameter of the system machine-workpiece-tool was found that could accurately indicate the end of tool life, it was decided to monitor only the power consumption of the portal machine's electric motor and the cutting temperature on the outer periphery of the tool until their values were in a level of 40kW and 350°C respectively. The flank wear (VB) should also reach a level of 5mm. It is very important to emphasize that, in this phase of the experiment, if the power consumption of 40kW and/or the temperature of 350°C were reached and the flank wear value (VB) was still less than 5mm, the test should continue until at least 5mm flank wear (VB) was reached, since in both previous phases (preliminary tests and first phase of definitive tests) there was never any breakage of the inserts with flank wear (VB) in the order of 5mm. This means that even in this second test phase of definite tests, conservative parameters are being applied in order to ensure the integrity and safety of the system machine-workpiece-tool.
In this second phase of tests, the focus was on carrying out another replica (second = ") of tests 1, 2, 3 and 4, with the objective of investigating which parameters would be more suitable to define the criteria for the end of tool life in an accurate and secure basis for the productive environment. Additionally, a new test (5 and 5') using a “heavier” cutting condition for the tool was also included, in order to confirm that the pre-established criteria for the end of tool life are also contemplating safely in extreme milling conditions.
It is worth remembering that in this second phase of definitive tests, the determination for the end of tool life occurred according to the pre-established criteria as described in the previous paragraphs (40kW and/or a temperature of 350°C), without the intervention of machine operator in order to confirm the effectiveness of the procedure and removing the autonomy of the machine operator about the decision for the end of tool life.
Tab. 6 Summary of results about flank wear (VB), cutting temperature and power consumption by automatic intervention of the system according pre-stablished criteria
Table 6 Summary of results about flank wear (VB), cutting temperature and power consumption by automatic intervention of the system according pre-stablished criteria
3.3.1 Analysis of flank wear monitoring results in the second phase of definitive tests
It is important to reinforce that the pre-established limits (40kW and/or a temperature of 350°C) were reached in all (6) six tests before flank wear had reached 5 mm and, consequently, the experiments continued up to the moment this value of wear was reached. At the moment of the end of tool life was reached (that is, when the VB value was around 5mm), the power consumption values of the portal machine electric motor, cutting temperature at the outer periphery of the tool and the highest wears of flank (VB) were duly recorded by photographs of the cutting edge with the aid of a stereoscopic microscope.
Figure 24 shows the highest values of flank wear among the seven (7) inserts used in each of the six (6) definitive tests of the second phase.
Evaluating the insert edges showed in Fig. 24, it could be confirmed that, again, flank wear (VB) was responsible for the end of the tool life. No other damage such as chipping, breakage and/or crater wear was found in any of them.
Figure 25 presents a graphic summary with the highest and lowest values of flank wear (VB) among all inserts used in the second phase of definitive tests (7 cutting edges x 6 tests = 42 cutting edges). Additionally, according to the values presented in this respective figure, it is also possible to observe that the standard deviation of flank wear (VB) values is approximately 0.38mm
It is still possible to notice that the variation between the lowest and highest VB value in this second phase of tests it is much smaller compared to the respective values of the first phase of tests (Fig. 15).
In the first phase of the definitive tests, when the machine operator established the end of tool life, the values varied between 2.8mm (lowest VB value) and 4.3mm (highest VB value) with power consumption around 40kW. This fact explains why in this second phase of tests with pre-established criteria of power consumption, cutting temperature and flank wear (VB), with the premise that the tests should continue until reaching at least a flank wear (VB) of 5mm achieved more optimized, stable and homogeneous values.
3.3.2 Analysis of electrical parameters monitoring results in the second phase of definitive tests
During the second phase of definitive tests, the monitoring of the electrical parameters of the portal machine's main motor (power consumption = kW) was monitored directly through the machine's CNC panel. In this second phase of definitive tests, the power consumption was automatically monitored via the portal machine panel and also, used to define the tool replacement moment. The power consumption of 40kW was reached in all tests before the temperature of 350°C and, continuing the test as described in the begging of this section, the end of tool life was defined by the power consumption in a range of 49kW to 52kW and temperatures from 352°C to 389°C. Further details regarding the temperature monitoring will be presented on the next subsection (3.3.3) of this present work. According to the results of the second phase of definitive tests, it was confirmed that the power consumption value at the end of tool life (VB ~ 5mm) is independent of the cutting conditions (vc and fz). This demonstrates that, when wear reaches such high values (~5mm) as those obtained in this present study, its influence on the power consumption values is so high that they end up overlapping the influence of the cutting parameters. The power consumption grew in the range of 40 to 50kW during approximately the last three (3) minutes of testing, following the same behavior of the power curves observed in the first phase of the definitive tests (Fig. 17).
The highest flank wear (VB) values for the end of tool life between the tests (1", 2", 3", 4", 5 and 5') ranged from 5.1mm to 5.5mm (Fig. 26), without edge breakage and power consumption reached values in the level of 50kW. Therefore, the value of 50kW of power consumption can be considered as a safe indicator for the end of the tool life, since higher values of power were reached during the tests, with high VB values (~5mm) without occurring any type of breakage of the inserts.
Additionally, according a summary of results about the flank wear (VB) presented in Fig. 25, considering all inserts (7 edges) at the end of tool life in all six (6) tests, the wears variated from 4.3mm to 5.5mm, and the cutting temperature reached values in a level of 370 / 390°C (Fig. 30).
Another very important consideration is that, even using extreme milling conditions, the power consumption (test 5 = 51kW and 5’ = 52kW) at the end of tool life was very close to the end of tool life based in the power consumption of the other tests (between 49kW and 52kW). This fact confirms the power as a good parameter to be used as criteria for pre-establish the end of tool life regardless of the milling conditions. It is also important to highlight that the 1" and 4'' tests had a higher flank wear value (VB) at the end of tool life, but still a value that did not generate edge breakage (preliminary tests VB ~ 5.5mm).
3.3.3 Analysis of temperature monitoring results in the second phase of definitive tests
During the second phase of the definitive tests, the temperature of the tool's outer periphery was monitored with the support of the system described in section 2 of this work. In this second phase of the definitive tests, the temperature was automatically monitored and also used to define the end of the tool life.
According to the figures presented in Fig. 26, the temperature of 350°C was reached in all tests (1", 2", 3", 4", 5 and 5') with values of the electrical parameters of the main portal machine motor (power consumption) above 40kW. This means, the end of tool life is closer to 350°C than to 40kW of power consumption. Another relevant information is that the end of tool life temperature (when the VB wear exceeded 5mm) varied in most tests between 370°C and 390°C (Fig. 27). As in the first phase of definitive tests, it was possible to confirm that for flank wear (VB) values as high 5mm as those do obtained in this work, the temperature depends much more on the high friction between the tool and workpiece than of the cutting conditions. It was also found that the temperature increased in most tests (1”, 2”, 3” and 4”) from 350°C to ~380°C during the last minutes of the tests, having the similar behavior as the curves showed in Fig. 20, from the first phase of the definitive tests.
Between this and that, the cutting temperature value of 370°C can be considered as a safe indicator for the end of tool life, since higher temperature values (for example 390°C) were reached during the tests, with high values of VB (>5mm) without breaking inserts. Specifically with regard to tests 5 and 5’, the end of tool life temperature ranged between ~350°C and 360°C. Based on these numbers, it was not possible to reach the pre-established temperature of 370°C. This fact confirms again the statement made previously, that the cutting parameters used were very heavy (much above normal production conditions).
Based on the results found in this second phase of definitive tests, the following conditions are proposed for carrying out the final tests: Test 1 (replicas 3 to 6) and Test 4 (replicas 3 to 6), with parameters for the end of tool life: power consumption = 49kW and/or temperature of 370°C, aiming to reach flank wear (VB) in the order of 5.5mm.
3.4 Results and analysis of the third and final phase of definitive tests
Table 7 presents the summary of results of the eight (8) tests in this third and final phase of definitive tests. Pre-established criteria for tool end of tool life and automatic system intervention:
• Power consumption = 49kW;
• Cutting temperature = 370°C;
• Aiming to achieve flank wear (VB) in the order of 5.5mm.
However, when the automatic system indicated the end of tool life, the tool was removed and had its flank wear values measured.
Tab. 7 Summary of results about flank wear (VB), cutting temperature and power consumption by automatic intervention of the system according pre-stablished criteria
3.4.1 Analysis of flank wear monitoring results in the third and final phase of definitive tests
In this third and final phase of definitive tests, the focus was on carrying out test 1 (replicas 3 to 6) and test 4 (replicas 3 to 6) using the automatic method based on power consumption and cutting temperature to define the end of tool life. Accordingly, the determination of the end of tool life occurred completely automatically without the intervention of the machine operator and/or machining engineer, in order to prove the effectiveness of the procedure and remove the autonomy of the machine operator for the decision about the end of tool life.
Evaluating the inserts showed in Fig. 28, it can be confirmed that the flank wear (VB) was responsible for the end of tool life. No other type of damage such as chipping, breakage and/or crater wear was found on them.
Figure 29 presents a graphic summary with the highest and lowest values of flank wear (VB) of all inserts used in the third and final phase of the definitive tests (values of the 7 edges of the tool among the eight (8) tests performed). It is also possible to observe that the standard deviation between flank wear values (VB) was approximately 0.4mm.
Another extremely relevant fact is the variation between the lowest and highest flank wear (VB) value in this third and final phase of definitive tests, which is much smaller compared to the respective values in the first phase of definitive tests (Fig. 18). In the first phase of definitive tests, when the machine operator established the end of tool life, the values vary between 2.8mm (lowest VB value) and 4.3mm (highest VB value), with power consumption in the order of ~40kW. This fact explains why in this third phase of tests with pre-established criteria of power consumption (49kW), cutting temperature (370°C) and flank wear (VB ~5.5mm), it was possible to reach more optimized values (VB > 5mm) and also, with very high levels of flank wear more homogeneous and stable (smaller VB value = 4.9mm and higher VB value = 6.1mm) compared to the tests carried out in the preliminary phase and in the first phase of the definitive tests. Therefore, between the preliminary phase of tests and the final phase of definitive tests, it was possible to increase the power consumption value for the end of tool life from 40kW to 49kW. This represents 25% of increasing in the absolute value of power consumption and, consequently, a better usage not only of inserts, but also of the entire machining system (machine-workpiece-tool).
3.4.2 Analysis of electrical parameters monitoring results in the third and final phase of definitive tests
During the third and final phase of definitive tests, the electrical parameters of the portal machine main motor (power consumption) were also automatically monitored via the machine panel and used to define the precise moment for tool change. The power consumption of 49kW was reached in all tests before the temperature of 370°C. The flank wear (VB) at the end of tool life between all edges (7 edges per test x 8 tests = 56 edges) used in these tests with pre-established end of tool life criteria for tool change in automatic mode, had varied between 4.9mm and 6.1mm (Fig. 29), without breaking the edge. The power consumption reached the pre-established value of 49kW (Fig. 30). Therefore, the value of 49kW of power consumption and temperature of 370°C can be considered as safe indicators for the end of tool life, since power consumption and temperature values higher than these were achieved in previous tests, with high values of VB (>5mm) without breaking the inserts.
According to these results in this final phase of definitive tests, it was confirmed once more that the power consumption at the end of tool life is independent of the cutting conditions. This demonstrates that, when flank wear (VB) reaches values as high as the values obtained in this work (~5mm), its influence on the power consumption values (~50kW) is so high that they end up overlapping the influence of the cutting parameters.
It is still important to emphasize that tests 1 (replicas 3 to 6) and 4 (replicas 3 to 6) presented higher values of flank wear (VB) at the end of tool life according data presented in Fig. 30. However, these values did not generate edge breakage (preliminary tests VB ~ 5.5mm the edge did not break) and also, with a power consumption at the end of the tool life around 50kW.
3.4.3 Analysis of temperature monitoring results in the third and final phase of definitive tests
During the third and final phase of definitive tests, the cutting temperature of the tool's outer periphery was monitored directly from the CNC machine panel with the aid of the system described in section 2 of this work. In this final phase of definitive tests, the cutting temperature was automatically monitored and also, used to define the precise moment for tool change.
Temperatures around 365°C were reached in all tests (test 1: replicas 3 to 6 and test 4: replicas 3 to 6 – Fig. 31) with power consumption values in 49kW. This means that the end of tool life is closer to 360°C and 49kW of power consumption. As in the first phase of definitive tests, it was possible to confirm that for flank wear (VB) values as high (~5mm) as those obtained in this work, the temperature depends much more of the high friction between the tool and workpiece than of the cutting conditions (vc and fz). However, the temperature never reached 370oC, which was the limit established for it. In other words, the power consumption of 49kW was always the parameter, which determined the interruption of the operation and the tool change.
Based on the results obtained in this third and final phase of definitive tests, it is possible to confirm the originality and scientific contribution of this work grounded on the implementation of automatic tool change process using state of art technologies (Industry 4.0 - The Digital Age) within the production environment completely independent of the cutting parameters and also, taking the important decision to establish the end of tool life out of the machine operator's hands. As hard facts of this work it can be state that between the preliminary phase of tests and the final phase of definitive tests, it was possible to implement within the production environment pre-established criteria for the end of tool life with automatic intervention of the system using: power consumption = 49kW, cutting temperature = 370°C and reaching homogeneous flank wear (VB) around 5.5mm, providing not only a better condition of the inserts usage, but also, of the entire milling system (machine-workpiece-tool) within productive and safe limits for operation.
The analysis of the results obtained in this work allows to conclude that, under heavy roughing conditions in five-axis milling process in production environment (case of study: martensitic stainless steel hydraulic blades):
1. Flank wear (VB) was responsible for the end of tool life in all phases of the performed tests. No other type of damage such as chipping or breakage was observed;
2. A wide variation (0.6 to 5.5mm) in flank wear values (VB) values among the 14 inserts used in a working day (two sets of inserts = 7 inserts x 2 assemblies) was found when the machine operator decided the end of tool life. This fact indicates that, on several occasions, the tool end of life was inadequately defined through premature changes (very small flank wear) and consequently, generating waste (time and costs) in the milling process;
3. It was not possible to find any vibration parameter of the system machine-workpiece-tool that could indicate, in a precise way, the tool is near its end of life;
4. Between the preliminary phase of tests and the final phase of definitive tests it was possible to increase the power value for the end of tool life from 40kW to 49kW. This means a 25% increase in the use of inserts and the entire milling system;
5. It was possible to implement the automatic tool change based on pre-established end of tool life parameters (power consumption and/or temperature), taking the decision of the moment for tool change from the machine operator’ hands;
6. After the implementation of the automatic tool change process, using pre-established parameters for end of tool life (power consumption and/or cutting temperature) as criteria, the flank wear (VB) values at the end of tool life moment became much more stable in comparison with tests carried out in previous phases of tests;
7. During the tests, it was confirmed that the power consumption at the end of tool life is independent of the cutting conditions (vc and fz). This demonstrates that, when flank wear reaches such high values (~5mm) as those obtained in the tests, its influence on power consumption (40-50kW) is so high that it ends up overlapping the influence of the cutting parameters;
8. It was possible to confirm in all phases of the definitive tests that for the flank wear (VB) values as high (~5mm) as those obtained in this work, the temperature depends much more of the high friction between the tool and workpiece than of the cutting conditions (vc and fz);
9. During the final phase (third) of definitive tests in a productive environment, it was possible to confirm the effectiveness of the use of the automatic tool change process through the definition of pre-established parameters (power consumption = 49kW and cutting temperature = 370°C) for the end of tool life. Using this method, the tool change was done in a safe, stable and prevented its premature occurrence.
Funding: Not applicable.
Conflicts of interest/Competing interests: Not applicable.
Availability of data and material: No restrictions from the academic and industry point of view about the availability of data and material.
Code availability: Not applicable.
Ethics approval: Not applicable.
Consent to participate: Not applicable.
Consent for publication: As the author of this study, I consent with the respective publication of this work in order to support the scientific and manufacturing community.
Authors' contributions: The respective work has the aim to provide further contributions with the scientific and technological developments in manufacturing process.
- Zonta A (2019) <https://omundodausinagem.com.br/porque-monitorar-a-usinagem>
- Lacerda RS (2014) A contribution to the improvement of the 5-axis milling process of hydraulic turbine blades, Unicamp - São Paulo, Brazil
- Clemente I (2011) 29th Annual Journalists Editors Workshop on Latin America and the Caribbean. FIU – Florida International University
- Sandvik (1994) Modern Metal Cutting – Sandvik Coromant Technical, Editorial Department, Tofters Tryckeri, 10a ed., Sweden
- Machado AR (2015) Material Machining Theory, Blucher publisher - São Paulo, Brazil
- Diniz AE, Marcondes FC, Coppini NL (2013) Materials Machining Technology, Artlibert publisher - São Paulo, Brazil
- Ferraresi D, Ruffino RT, Pallerosi CA (1977) Metal machinig, Artlibert publisher - São Paulo, Brazil
- Trent E, Wright P (2000) Metal cutting, Butterworth – Heinemann, Oxford
- Walter (2020) Technical application manual for milling
- Stephenson DA, Agapiou J (2016) Metal Cutting Theory and Practice -3th edition
- Costa e Silva ALV, Mei PR (2010) Steel and special alloys, 2nd Edtion - Edgard Blücher Ltda - São Paulo, Brazil
- Ost C A (2016) Analysis of surface quality in P20 steel milling with tools with different treatments and lubricant conditions - University of Caxias do Sul
- Blum (2020) Measuring systems for CNC machining centers (blum-novotest.com)
- Silva RHL (2010) Tool wear monitoring in end milling through power and acoustic emission signals with neural networks - Uberlândia, Brazil
- Baek S, Jung S, Park DA (2018) Study on proposal of flank wear criterion by using a built-in current sensor when manufacturing the mold materials in a smart machine tool - International Conference on Metal Material Processes and Manufacturing v. 207
- Abukhshim NA, Mativenga PT, Sheikh MA (2006) Heat generation and temperature prediction in metal cutting: A review and implications for high speed machining. International Journal of Machine Tools and Manufacture, v. 46, n. 7– 8, p. 782–800
- Lauro CH, Brandão LC, Baldo D, Reis RA, Davim JP (2014) Monitoring and processing signal applied in machining processes, (https://doi.org/10.1016/j.measurement.2014)
- Waldrich Coburg (2020) User machining catalog for high performance milling - Germany
- Cope M (2017) The Power Of FIVE. The Definitive Guide to 5-Axis Machining
- Gilles P, Cohen G, Monie F, Rubio W (2013) Torus cutter positioning in five-axis milling using balance of the transversal cutting force, Int J Adv Manuf Technol
- Voith Hydro (2012) Technical Delivery Specification for ASTM A 743 Grade CA - 6NM
- ISO 20816-1 (2016) Vibration monitoring
- IFM (2019) Technical manual of installation, maintenance and operation