Tool Path Strategy and Cutting Process Monitoring in Intelligent Machining

Front. Mech. Eng. 2018, 13(2): 232–242 https://doi.org/10.1007/s11465-018-0469-y

RESEARCH ARTICLE

Ming CHEN, Chengdong WANG, Qinglong AN, Weiwei MING Tool path strategy and cutting process monitoring in intelligent machining

Abstract Intelligent machining is a current focus in improving the offset cutter path strategy by selecting the advanced manufacturing technology, and is characterized proper entrance and exit conditions can increase the by high accuracy and efficiency. A central technology of volume of the removed metal and improve tool life [13]. In intelligent machining—the cutting process online monitor- a past study, Zhou et al. [14] proposed a tool path for ing and optimization—is urgently needed for mass down-milling and the progressive radial depth of cut with production. In this research, the cutting process online consideration of the soft edge. Fan et al. [15] established a monitoring and optimization in jet engine impeller modified algorithm tool path strategy in the finish milling machining, cranio-maxillofacial surgery, and hydraulic of the blade surface, and demonstrated its effectiveness via servo valve deburring are introduced as examples of numerical simulation and a practical impeller test. Gao intelligent machining. Results show that intelligent tool et al. [16] established an intelligent process planning path optimization and cutting process online monitoring method based on feature-based history machining data. are efficient techniques for improving the efficiency, The proposed approach can effectively accumulate quality, and reliability of machining. machining experience, improve the efficiency and quality of process planning, and enhance the level of automation Keywords intelligent machining, tool path strategy, and intelligence. Chen et al. [17] presented a model process optimization, online monitoring parametric process plan based on the feature parameters of parts, and proposed a solution for the automated process planning of part families. The established model can fulfill 1 Introduction actual requirements of various industries as well as demonstrate effective system performance and rapid Process optimization and online monitoring are key factors response. Experimental and theoretical results regarding for improving efficiency and quality in intelligent machin- the dynamic process behavior also reveal the relevance of ing [1,2]. Cutting process configuration, which includes these influences with respect to machining performance tool path generation and cutting parameters setting, is and workpiece quality [18]. crucial in machining complicated structure parts [3–6]. Online monitoring of the cutting process has been However, such configuration largely depends on experi- carried out for the observation and prediction of tool wear ences over an extended period of time, hindering the and breakage and the abnormal defects of a machined efficiency and quality of machining [7–10]. With the surface [19–21]. Using acceleration sensors to evaluate development of intelligent machining, the optimization of tool breakage in the cutting process, Ratava et al. [22] cutting process configuration during actual production has demonstrated the feasibility of estimating tool deflection, become more accessible [11,12]. The novel approach of which is involved in the main cutting force for detecting the chipping and small fracture of the tool edge. Frangos Received March 26, 2017; accepted May 21, 2017 [23] developed a model to draw conclusions for the ✉ uncertain quantities of interest in a practical problem in an Ming CHEN ( ), Qinglong AN, Weiwei MING oil field. The proposed model was incorporated in the School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Bayesian framework to allow for the optimal estimate of E-mail: [email protected] the location and amount of cuttings transported along the wellbore in real time. Chengdong WANG In the current work, the online monitoring and School of Mechanical and Electric Engineering, Soochow University, Suzhou 215021, China optimization of the cutting process in jet engine impeller Ming CHEN et al. Intelligent tool path strategy and cutting process monitoring 233 machining, cranio-maxillofacial surgery, and hydraulic The average taper angle of the conical space formed by servo valve deburring are introduced as examples of two adjacent blades can be defined as intelligent machining. d – d ¼ – 1 2 1 ¼ : t 2tan l 13 27°, (1) 2 r 2 Intelligent cutting tool path planning for a d jet engine impeller where 1 is distance between the left and right adjacent blades, d2 is the top spacing, and lr is the blade radial fi length. 2.1 Dif culties in machining the jet engine impeller Generally, the cutting process encounters the following difficulties: The integral impeller of the micro-turbojet engine was 1) The overall and local feature size is minimal (material studied in this section. Figure 1 illustrates a three- removal rate of 61.57%), which is not conducive for dimensional model of the integral impeller with a fi cutting tool selection and tool path planning. simpli ed internal wheel hub structure. The integral 2) The overall impeller structure is complex, with thin impeller has 23 blades, with an impeller diameter of 88 and twisted leaves. Thus, it is prone to produce interference mm, wheel diameter of 44 mm, blade radial length (lr)of l in the process. 22 mm, and axial length of ( a) 22 mm. 3) Given the small tool diameter and the considerable extension length in the entire impeller finish process, the machining process is prone to vibration because of the uneven cutting margin, resulting in cutting tool or blade deformation rather than meeting the accuracy requirements.

2.2 Tool path planning strategy

The cutting strategy is the principle upon which the tool path is calculated. The said strategy can be categorized into Fig. 1 Three-dimensional model of the impeller 2.5-axis machining, three-axis machining, and multi-axis machining. Furthermore, the processing stage can be The geometrical features of the impeller blades are classified into the roughing and the finishing strategies, shown in Fig. 2. The minimum distance (d1min) between whereas the tool path can be divided into the following the left and right adjacent blades was about 1.37 mm, the stages: Parallel processing, contour machining, projection most minimal spacing (d2min) was roughly 6.49 mm, and processing, radiographic processing, reference processing, the minimum blade thickness (tmin) was around 0.7 mm. spiral processing, and processing and clearance angle Furthermore, the blade and hub junction (blade root) milling. The overall impeller computer numerical control transition fillet (rmin) was about 0.55 mm, and blade (CNC) cutting process strategy includes 3 plus 2 axis chamfering around the front and rear approximately ranged model area removal, surface projection processing, stream- from 0.3 to 0.5 mm. line processing, and clearance angle machining strategy, as

Fig. 2 Geometrical features of the impeller blades 234 Front. Mech. Eng. 2018, 13(2): 232–242 shown in Fig. 3. The term “safety height” refers to the height of the cutting tool into or out of the tool path, which covers the safety plane to the height of the uppermost surface of the workpiece. A certain height is set to ensure the safe travel of the tool during the air cutting movement, thus preventing the collision between the tool and the workpiece surface. Different feed rates can be defined according to the position of the tool, which can help improve the running efficiency of the tool path. For the entire impeller parts machining, the cylinder can be used to define the space for tool changing safety during machining, as shown in Fig. 4. The impeller work piece is located inside the space covered by the cylinder, thus avoiding collision between the tool and the blade surface.

Fig. 5 Leads and links

in turn, improves the stability of the process. The cutter axis strategy is a method of cutter axis vector control while in motion. In three-axis or ﬁxed-axis processing, the cutter axis is often set to a ﬁxed direction, whereas cutter axis control strategies in multi-axis machining include the forward/roll, from the point toward, from the line toward, and from the curve toward directions. Figure 6 shows the blade path generated using the forward/ roll strategy.

Fig. 3 Surface projection machining strategy

Fig. 6 Tool axis strategy (forward/roll)

After the above elements are set, the tool path can be solved. Tool path evaluation is conducted through calculation. If the tool path scheme is not feasible, a more applicable tool path scheme can be identified by Fig. 4 Safe height clearance adjusting the cutting strategy, the safe height, the cutting-in and cutting-out connections, and the tool axis strategy. The cut-in, cut-out, and connections define the path at which the tool path discontinuities start, end, and link two 2.3 Process simulation and the integral impeller machining tool paths, respectively. As shown in Fig. 5, setting the cut- test in and cut-out functions helps reduce the occurrence of defects (e.g., over-cutting or blanking directly into the In the computer aided manufacturing (CAM) software blank). This ensures the continuity of the tool path and environment, CNC tool path simulation can be applied to prevents sudden changes of tool in direction or load, which identify the material removal method in the cutting Ming CHEN et al. Intelligent tool path strategy and cutting process monitoring 235 process, the motion of the axes on the machine tool, and As shown in Fig. 8, the finishing surface morphology of the occurrence of a collision within the system. However, blades is machined using a different tool inclination angle obtaining the real surface of the material through ψ for the CAM tool path planning. Figure 8 also illustrates machining simulation is difficult. Furthermore, as a that the tool inclination angle ψ has a significant effect on processing object, the whole impeller has a narrow and the machining quality of the blade surface. Apparent over- deep flow channel, and the space between adjacent leaves cutting of the machined surface of the blade was observed is very limited. In the actual process, due to the existence of with the ψ of 88°. Additionally, the machined surface is various system errors within the process system, the tool over-cut as ψ decreases. The surface roughness of the blade moving routine easily intervenes with the processed surface is increased partly because the cutting thickness is surface, thus affecting the quality of surface processing. too small when ψ is 82°. Thus, ψ was increased Such problems are effectively identified by using the CNC appropriately, and through the cutting experiment, the cutting test, which facilitates effective amendments to tool final choice for the tool angle was 80°. At this point, the path planning, enhances the planning accuracy and blade surface does not over-cut and surface roughness is tolerance, and ensures the smooth progress of the formal not increased. cutting. Therefore, before processing the Ni-based super- Based on the results of the cutting test described above, alloy impeller, aluminum alloy (6063, LD31) blades were the tool path of the cutter position and the cutter axis vector used for the prototype machining experiments to correct were adjusted and restrained in the whole impeller CAM the unreasonable geometry of the CAM tool path planning. programming process. Issues regarding the micro-inte- Figure 7 shows the surface contact area of the taper grated impeller CNC cutting process and processing milling cutter during the side milling process. V1 is the interference were also successfully solved. The overall normal vector of the cutter and the machined surface tool path planning for each step of the impeller after contact point. The cutter axis vector is V2, and ψ is the debugging and the actual machining results are presented angle of V1 and V2. As shown in Fig. 7, the tool inclination in Fig. 9. ψ changes the contact area between the tool and the The obtained nickel-based superalloy overall impeller workpiece surface; moreover, the shape of the contact area according to the proposed whole cutting process is shown is related to the curvature of the machined surface. If ψ is in Fig. 10. Clearly, the proposed cutting process can too large, then the cutting thickness increases, leading to completely avoid defects, such as the interference between over-cutting. Conversely, if ψ is too small, then the cutting the blade root and the hub, effectively improving the thickness decreases, causing the material surface slip force surface quality of the whole impeller. and the plow force to increase because the cutting thickness is below the minimum required. When the cutting force and the burr are increased, the surface quality 3 Intelligent cutting process in of the workpiece becomes unsatisfactory. As the blade is a cranio-maxillofacial surgery free-form surface, the surface curvature of the points becomes inconsistent. Therefore, calculating the inclina- 3.1 Force tactile feedback based on virtual reality (VR) tion ψ of the cutter with theoretical calculation in CAM technology path planning is a difficult task. In this study, appropriate ψ values will be determined by cutting experiments. In this section, the cutting force tactile feedback based on virtual reality (VR) technology is studied. The real cranio- maxillofacial model data were obtained through a computed tomography (CT) scan. The 3D data of bone and soft issue anatomy were obtained by mixing median filtering, medical image interpolation, and segmentation and extraction of the target tissues [24–26]. Then, the anatomy was reconstructed using marching cubes algorithm. Due to the vast amounts of mesh model triangles, we applied the algorithm through a combination of vertex clustering to obtain moderate size and quality for the grid model, thereby meeting real-time display and further model rendering processing needs. Continuously reconstructing and drawing the surface model during the cutting or drilling process is necessary; hence real-time force feedback is difficult to guarantee using an ordinary computer. Volume rendering is suitable for reconstructing the CT images of multiple organs, which is helpful for Fig. 7 Schematic of milling for tapered cutter observing the spatial relationship between organs or 236 Front. Mech. Eng. 2018, 13(2): 232–242

Fig. 8 Tool-path commissioning for blade machining

Fig. 9 Results of tool-path commissioning

with 0.5 mm unit size to ensure the visual fidelity effect and force feedback. After the final craniofacial soft and hard tissue triangular mesh model was obtained, the normal two-dimensional digital photo texture was added to the craniofacial soft tissue model. Figure 11 illustrates the facial model. To accurately reflect the surgical site environment and the operation of surgical instruments in the construction of the patients’ mouth during the model surgery, the force between the surgical instrument and the cranio-maxillofa- Fig. 10 Integrated impeller part cial model is fed back to the surgeon through the force feedback system. The accurate modeling and visualization lesions and normal tissues, hence its significance in of surgical instruments and the relative position of said practical clinical application. The skin, tongue, and teeth instruments and the cranio-maxillofacial models are of the patient are constructed by using surface rendering. necessary to achieve real-time collision detection between The osteotomy model and the drilling model of the planned the instruments and the model and to calculate and path in the bone tissue were constructed by a voxel model feedback the size of the force and its direction. Ming CHEN et al. Intelligent tool path strategy and cutting process monitoring 237

Fig. 11 Anatomical tissue deformation: Real-time simulation experiment photos and screenshots

3.2 Intelligent cutting of cranio-maxillofacial bone 3.2.1 Drilling force analysis The experimental material used was neat pig cranial jaw, as displayed in Fig. 12. The experiments were conducted on a As shown in Fig. 15, the measured drilling force is a three-axis micro-cutting platform built in the laboratory, time-varying curve in the micro drilling process. Note that for which the maximum rotational speed of the spindle the axial force of the drill in a drilling cycle is similar to could reach 300000 r/min. The Kistler 9256C2 plate that of “M,” and can be divided into four stages. dynamometer and FLIR SC6X5 A615 thermal imager 1) The drill bit contacts the material from Point A, and were used to measure the cutting force and temperature then the drill bit cuts further into the workpiece. The during the drilling process. The experimental tool was a drilling force increases and reaches maximum at Point B,at diamond-coated carbide micro-drill with a diameter of 1.5 which time the entire drill tip has fully entered the bone mm (Fig. 13). The entire experimental system is shown in material. Given that the cortical bone layer of the selected Fig. 14. All the equipment parameters were selected porcine bone is very thin, the drill tip remains in the according to the device manual, which considered the cortical bone for a very short period of time. Subsequently, physical properties of the sample jaw bone. the drill tip enters the cancellous bone layer, and the drilling force diminishes rapidly. 2) At Point C, the drill bit has completely entered the cancellous bone, and the drilling force is maintained at a low level. Bone itself is a non-uniform composite material, and cancellous bone mineral density within the bone can change. Therefore, the axial force also exhibits a certain ﬂuctuation in the CD stage. 3) The drill tip enters the cortical bone stage. Similar to the AB process, the axial force rises rapidly and reaches its peak as the drill tip completely enters the cortical bone. Then the drill tip breaks through the cortical bone and the axial force drops rapidly. Fig. 12 Porcine jaw bone 4) At Point F, the drill has completely penetrated the cortical bone and the axial force drops to zero.

3.2.2 Drilling temperature analysis

As demonstrated by the measurements of the thermal imager during the experiment, changes occur in the temperature of the cutting area (Fig. 16). Where N is the spindle speed, f is the feed rate. The variation curve shows a similar inverted “V” shape to the one from the cutting force. The change is also divided into the following stages. First, the beginning of the drill bit gradually cut into the bone, followed by the rapid rise in the temperature of the cutting area as the drill completely cut into the peak. Fig. 13 Carbide micro-drilling Second, the drill completely entered the workpiece to 238 Front. Mech. Eng. 2018, 13(2): 232–242

Fig. 14 Experimental set-up of jaw micro-drilling

retreat of the drill bit and then gradually returned to room temperature.

4 Online monitoring of the intelligent cutting process

4.1 Deburring of the hydraulic servo valve

The hydraulic servo valve has been widely used in Fig. 15 Change in the axial force during the drilling process hydraulic servo system for its following advantages: Simple structure, considerable output power per unit volume, reliable operation, and good dynamic performance. The said valves involved in this study are commonly utilized in the aerospace industry, which has extremely high processing accuracy requirements for key parts [27–29]. The servo valve key parts of the important parts of the processing quality is the bottleneck, which resulting in servo valve processing facing short-term situation. The spool, a key component of the hydraulic servo valve, achieves electric liquid conversion. Spool throttling working edge grinding is the critical process of servo valve manufacturing. As the axial dimension is unstable, their micron size difference usually brings small burrs and Fig. 16 Change curve of the cutting temperature during the results in low accuracy in the lap test and difﬁculty in drilling process machining. Therefore, we must examine the precision grinding technology, select reasonable means to remove achieve a stable cutting state, and the surface temperature burrs, improve the existing precision grinding process, and of the bone rapidly declined. Finally, the temperature further improve the parts’ machining quality and consis- measurement area exhibited a slight ﬂuctuation after the tency. Ming CHEN et al. Intelligent tool path strategy and cutting process monitoring 239

Currently, servo valve spool parts are mainly used for degree of work after deburring. In the deburring process, the processing of manual precision cylindrical grinding when the carbide strips, the metallographic sandpaper with machines. In spool processing, the last procedure involves the working side is not vertical. A problem occurs with the grinding (face grinding) of the working edge, as shown in micro-cutting (up to 5mm) of the working edge, which Fig. 17. First, the manual grinding wheel was chosen causes the spool to be scrapped directly. The manual according to the size of the two models of the distance working edges of the front and back of the burr are shown between the two different widths. The width of the in Fig. 19. grinding wheel was generally 5 to 10 mm, parallel to the degree of parallelism within 0.001 mm, with about 1° concave angle, and rounded corners of R0.5 mm. Then, the spool was ﬁxed to the front and rear tips of the cylindrical grinder, which were moved horizontally through the guide rails on the working side and were controlled by the grinding edge, such that the horizontal feedrate ensured the overlap size. The slight burrs generated in the grinding process (including the slight deformation of the plastic during the machining of the material) were removed for processing on the deburring device [28]. Afterwards, polished hard alloy strips and shredded shavings with gold-faced sandpaper were repeatedly squeezed with their inner and outer faces, as shown in Fig. 18. Following the removal of the burr, the amount of overlap was measured; if it failed to meet the requirements, the process was repeated until the overlap was achieved.

Fig. 17 Schematic diagram of the processing of the slide valve working edge

Fig. 19 Microscopic images before and after deburring

3) The “cutting tool relieving” phenomenon occurs in the grinding process, due to the existing grinding wheel abrasive, binder, and organization (among other reasons). Tool relieving results in roughness and burr, making it difﬁcult to control the problem. The phenomenon indicates Fig. 18 Method of burr removal. (a) Pressing with metallic a difference in the size of the axial grinding of the spool sandpaper; (b) pressing of the outer ring with an alloy bar and the consistency of the grinding dimension. The automatic deburring set-up shown in Fig. 20 has The above method for processing the precision working successfully achieved deburring at the hydraulic servo edge of the spool presents the following problems: valve. 1) Working edge grinding is inefﬁcient, usually requir- ing at least three times of repeated grinding, deburring, and 4.2 Equipment for deburring the spool working edge in the lap measurement. hydraulic servo valve 2) The manual deburring process readily breaks the work side integrity. The 3D inspection of the working The various components of the equipment were designed edges under super-depth microscopy shows an incomplete based on the above requirements, in accordance with the 240 Front. Mech. Eng. 2018, 13(2): 232–242

Fig. 20 Main components of the automatic deburring device system speciﬁc function each part belongs to, namely, the shows sharp edges without any burrs. The working area of precision feed system, the motion control system, the the servo valve spool was tested using a confocal three- automatic tooling system, the special deburring tools and dimensional proﬁler to obtain the working edge appear- power supply, the control cabinet, and other ancillary ance, as shown in Fig. 22. equipment.

4.3 Deburring the spool working edge in the hydraulic servo valve based on online monitoring

Figure 21 demonstrates the actual effect of the automatic glitch removal device after removing the burr. In Fig. 21, the working surface of the servo valve spool (magniﬁed 200 times using the optical microscope) shows a grinding depth of 10 mm for the end face. According to the previous simulation and the experiment, the predicted burr has a height of 29 mm and a width of 10 mm. As can be seen in Fig. 21, through face grinding and online deburring, the optional spool of the working edges of the three areas

Fig. 22 Effect of deburring the working edge

The working radius is less than 2 mm, and the end angle is 90.1° (39.7° + 50.4°). After testing all 100 samples in this study, all of the working edges were cut at right angles to the edges without burrs. The reject rate was 0. The ﬁllet Fig. 21 Schematic diagram of grinding machining and online radius and edge angle of the working edge is shown in deburring device Fig. 23. Clearly, the burr is eliminated. Ming CHEN et al. Intelligent tool path strategy and cutting process monitoring 241

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