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The quality of the milled surface affects the performance of the affiliated workpiece, since it plays a vital role in determining the precision of the geometry and duration of service time. In this paper, a modified convolution recurrent neural network (CRNN) is proposed to effectively predict the surface quality of the end milling workpiece. First, the validated features of milling force data in the machining process are extracted based on the proposed artificial network model. Second, a modified CRNN model is constructed by merging residual neural network with the help of bidirectional long- and short- term memory as well as attention mechanism. Third, the model’s weight is optimised according to the changes in the loss function and directional propagation principle, which significantly improves the effectiveness of the proposed model. Finally, the actual experiment is carried out on a 5-axis milling centre to validate our model. Also, the surface quality predicted by the CRNN model is in good accordance with the experimental result. In our experiment, an accuracy of 98.35% is achieved, which is a significant improvement compared to the classic CRNN method. Keywords: surface quality prediction; deep learning model; convolutional recurrent neural network; end milling 1 Introduction ‘Industry 4.0’ is significantly considered as the fourth industrial revolution , in which smart factories and intelligent manufacturing play a significant part. Besides new concepts of Internet of Things (IoT) , digital twins (DT)  and cloud manufacturing (CM) , the fundamental aspects of industry still heavily rely on tra- ditional production procedures  such as casting, moulding, welding and machining. Since machining is vital for precision engineering and complex workpieces, much effort has been directed at improving the performance Corresponding author. Email address: email@example.com ISSN 2444-8656 doi:10.2478/amns.2021.2.00213 Open Access. © 2022 Guan et al., published by Sciendo. This work is licensed under the Creative Commons Attribution alone 4.0 License. 70 Guan et al. Applied Mathematics and Nonlinear Sciences 8(2023) 69–80 of NC machining. The quality of the milled surface affects the performance of the affiliated workpiece, since it greatly determines the precision of the geometry and duration of service time. To develop advanced manufactur- ing towards intelligent factories, surface quality prediction is of great research focus . Comprehensive related approaches and studies have been reported on surface profile and the roughness of machined products [7, 8]. Generally, many features, such as cutting condition, tool status, material properties and process parameters (tool geometry, cut depth, feed rate and spindle) are considered to have a significant influence on the surface rough- ness. As the determination mechanism of surface roughness is complicated, no simple closed-form model could accurately predict the surface finish. Ending milling, which accounts for a large proportion of milling, is universally used in moulding, aerospace and automobile industries . The surface quality is not only an important index to measure whether the work- piece meets the requirements but also the primary goal of monitoring the machining process and parameters’ optimisation. Therefore, it is essential to predict the quality before actual machining is performed, to decrease the time for boring parameter trials and corrections, and reduce the cost of the manpower. Lee et al.  proposed a high-speed milling surface roughness simulation approach, which adopted acceleration signals instead of cutting force signals for studying high-speed milling surface roughness. In the research, a statistical milling model was established for analysing the influence of spindle deformation and vibration on roughness. Mizugaki  and Kim , respectively, discussed the effect of the blunt radius of the blade on the surface roughness in the study of the tool path of the milling process. Then, Singh et al.  studied the bearing steel with higher hardness and used tools with different radii and bevel angles. The experimental results show that the feed rate has the most significant influence on the surface quality, while the bevel angle has the least influence on the machined surface. Quinsat et al.  believed that the processing strategy and parameters are related to the surface quality. The process parameters are obtained from the angle and the required surface quality, and the machining processing is simulated and tested. Results show that the importance of parameter design did achieve the required processing surface through parameters set strategy. Different cutting conditions make different influencing facts, and thus no closed-form model could achieve accurate predicting, because different materials and different parameters, as well as different machine tools, can alter the cutting condition. Thus, direct statistical simulation or single effect analysis might not fulfil the requirement for predicting the surface quality. Other researchers used evolutions, artificial intelligent algorithms or deep network for surface quality pre- diction. Zain et al.  used genetic algorithm to analyse the radial clearance angle of the milling cutter and considered the influence of feed and cutting depth on roughness, and finally found the optimal surface under the combination of high speed, low feed rate and radial clearance angle. Cosres et al.  used a non-contact displacement measurement system to record the movement of the tool during processing and the problem of tool vibration at high frequencies, and derived the surface topography after processing. Using traditional inspection equipment to inspect the surface quality of processed workpieces takes a lot of time, and it is impossible to predict the surface quality during processing. To be able to predict the surface quality during the machining process and adjust the machining process parameters in time to improve the machining quality, an intelligent detection method is urgently needed. Sahith Reddy Madara et al.  used an artificial neural network to model and predict surface roughness in abrasive waterjet cutting of Kevlar 49. Girish Kant et al.  developed an optimisation model by coupling the ANN and GA. In addition, evolution algorithm and artificial intelligence methods make great application prospects. At present, in the machining process, various digital signals are generally obtained directly through sensors, but the collected data are always contaminated with noises, which seriously hinders the identification of the actual wear status of the tool, and thus could not reflect the surface quality. In this work, a modified convolutional recurrent neural network (CRNN) is applied to the prediction of surface quality. First, the validated features of milling force data in the machining process are extracted based on the proposed artificial network model. Then, a modified CRNN model is constructed by combining residual neural network and bidirectional long- and short- term memory as well as attention mechanism. The weight coefficients in the model are optimised according to the change of loss function and directional propagation principle, which greatly improves the effectiveness of Prediction of surface quality in end milling 71 the proposed model. Finally, the real experiment is carried out on a 5-axis milling centre to validate the proposed approach. 2 Construction of modified CRNN model 2.1 The classic CRNN network Convolutional neural network (CNN) and recurrent neural network (RNN) are the main components of CRNN. CNN consists of multiple convolutional layers, pooling layers and fully connected layers. To deal with the enormous calculation, gradient loss and over-fitting involved during the back propagation process, a rectified linear unit (ReLU) function will be used. This could provide rapid convergence speed. Then RNNs are adopted for modelling dynamic changes in time series. The details about classic CRNN can be found in Li et al. ; this study used CRNN for predicting combustion states of a rotary kiln. The classical CRNN architecture combines the advantages of deep CNN and RNN, and gives a good result; further, it shows that CRNN makes for a great application prospect. Also, CRNN network also has the characteristics of both CNN and RNN networks. 2.2 The modified CRNN network Due to the over-fitting phenomenon in the deep learning network in CRNN in the feature extraction process, and the insufficient recognition of the full-position context information by the convolutional network, based on the original CRNN, residual neural network, the bidirectional long- and short-term memory and attention mechanism are applied to build a modified CRNN model. 2.2.1 Components of modified CRNN Most conventional CNN models pile up the convolutional layers to deepen the network to elevate the accu- racy of the recognition. However, as the network structure deepens (resulting in redundancy of the convolutional layers), problems come to light. On the one hand, gradient loss or disappearance results in the inefficiency of updating the gradient to the previous network layers through back error propagation, causing parameters to not be updated. On the other hand, the model accuracy appears saturated and the accuracy declines rapidly, which means that the network degrades. The deep residual network is proposed to solve these problems. In this work, DenseNet is mainly introduced, discussed and applied to the CRNN framework. The CNN network performs well on discovering the local spatial features, while it cannot reflect the temporal relation information of the signals in time series. The CRNN structure proposed in this paper aims to increase the depth of CNN network structure based on the framework of machining surface quality recognition algorithm, so as to improve its capability of deep feature extraction. Meanwhile, the structure of residual network is added to avoid the phenomenon of over-fitting in the process of feature extraction of the deep network. At the same time, BILSTM and attention mechanism are combined to extract deep features, which makes up for the lack of a general convolutional network to recognise the complete location context information. Eventually, we integrate features through the fully connected layer and output the final status recognition result after softmax. In this paper, the machined surface quality is divided into three categories according to the value range of surface roughness value: good, medium and poor. The whole CRNN model framework is shown in Table 1. The final network structure is shown in Figure 1. The inputs are three-direction cutting force signals. Since the sampling frequency in this experiment is 2500 Hz, the data collected in the experiment are enhanced through down-sampling to obtain more data samples. Finally, the input dimension of the network is set as (10,000,1). The input passes through the convolutional layer, pooling layer, DenseNet residual layer and BILSTM at- tention layer to complete the identification of machining surface quality. The structure of the whole network is shown in Table 1. The convolution kernel size is reduced, which can, on the one hand, improve the robustness of the network, and on the other hand, reduce the computational cost. Meanwhile, to adapt to the number of the 72 Guan et al. Applied Mathematics and Nonlinear Sciences 8(2023) 69–80 output channels in the convolutional layer, a constant factor ranging (0,1) is added after the output of the dense block to adjust the number of output channels. Table 1 The structure of modified CRNN Layers Size DenseNet(4_2) Convolution 5000 Conv 1*7, stride = 2 Pooling 2500 Maxpool 1*3, stride = 2 Dense Block 1 3000 (1*2 conv, 1*1 conv)*4 Transition 3000 1*1 conv Block 1500 1*2 average pool, stride = 2 BILSTM 725 BiLSTM, unit = 128 Attention 725 Atten = 128 Global pooling 128 Average pool Fully connect 64 Fully connected Output 3 Softmax Fig. 1 Diagram of machining surface quality prediction model based on modified CRNN network 2.2.2 Training of modified CRNN After the network structure is determined, the process of surface quality recognition based on CRNN has also been determined. First, the signal of the original milling force is down-sampled to obtain the expanded data. Then, the data are normalised. Finally, the data are divided into training set and testing set with the ratio of 8:2, the training samples are input into the original CRNN network with four training control mechanisms and the training network enables it to have the fitting ability of machining surface quality recognition. An early stopping mechanism ensures the network will not overfit. Eventually, recognition ability and effect of the trained CRNN network are verified on testing set. The process flow of the model is shown in Figure 2. After the prediction of the machining surface quality CRNN model and the training process, the weights in the model will be updated and optimised through the loss function and the principle of direction propagation, resulting in the gradual improvement of recognition ability. Figure 3 shows the changes of loss value and accuracy of the entire network in the training and testing sets. With the training process, loss value decreases continuously on both the training and testing sets, while the accuracy rate increases. Finally, the convergence is realised, and the prediction accuracy of the CRNN model on the verification set reaches 98.35%, which confirms the applicability and accuracy of the CRNN network proposed in this paper. To verify the ability of the machining surface quality recognition and the effect of different CRNN deep networks, the DenseNet and BILSTM structure in CRNN deep network are separated and trained, respectively. The input is uniformly set as the force signal in the machining process, the output is set as three categories of surface quality and the test set is used for verification. The result of loss function on the testing set is shown Prediction of surface quality in end milling 73 Fig. 2 Flow diagram of machining surface quality prediction model based on modified CRNN network (A) Loss function curve (B) Accuracy curve Fig. 3 CRNN model indicators for surface quality prediction Fig. 4 The loss function of different algorithms varies with the number of iterations 74 Guan et al. Applied Mathematics and Nonlinear Sciences 8(2023) 69–80 Fig. 5 The accuracy curve of different algorithms on the verification set Table 2 Comparison of prediction accuracy and operation time of different network Structure Accuracy Iterative convergence Proposed CRNN DenseNet(4_2) BILSTM 98.35% 97.68% 72.96% 15 18 22 in Figure 4, while the result of accuracy on testing set is shown in Figure 5. As is shown in Figures 4 and 5, in contrast with the rapid convergence and high accuracy of the prediction results of the CRNN, the loss of the DenseNet model on the testing set is more volatile, and the accuracy on the testing set is lower than that on the CRNN, although the DenseNet model can eventually converge. BILSTM is obviously not as effective at this task as the other two network structures. According to the variation trend of loss value in Figure 5, it can be seen that DenseNet network can learn and extract the features of signals in the spatial domain, while BILSTM can extract the features of input signals in the temporal dimension and fit the correlation between various features through the attention mechanism. However, only these two parts of the network structure will cause the problem of insufficient model fitting ability, and thus it is necessary to combine the two effectively. The training results of the three networks are compared as shown in Table 2. The proposed CRNN can learn more related characteristics compared with ordinary multi-layer neural net- works. At the same time, full correlation analysis is carried out on the extracted features through the BILSTM network with attention mechanism, so as to realise the accurate recognition of the machining surface quality. Based on the data in Table 2, the effectiveness of the proposed CRNN network on the surface quality prediction can be verified. 3 Experimental results and discussion 3.1 Equipment setup This work aimed to investigate the effect of cutting parameters and conditions on surface roughness param- eters, such as cutting tool, stiffness of machine tool and material. In our experiment, the workpiece material was 42CrMo steel with a rectangular shape of 65 mm × 140 mm × 40 mm. The steel has good corrosion resistance and high hardness of 30 HRC. Therefore, its surface quality is challenging to guarantee during machining pro- cess. Table 3 shows the chemical composition of 42CrMo steel and Figure 6 shows the shape of the workpiece. Table 3 Workpiece material composition Composition C Si Mn P S Cr Mo Wt(%) 0.38–0.43 0.15–0.35 0.75–1.00 ≤ 0.035 ≤ 0.040 0.80–1.10 0.15–0.25 Prediction of surface quality in end milling 75 Fig. 6 42CrMo steel workpiece As shown in Figure 7, a series of dry milling experiments were conducted on Starrag LX051, which has a maximum of 18,000 rpm, rated torque of 181 Nm, positioning accuracy of 0.004 mm and repeat positioning accuracy of 0.002 mm. The cutting tool used for the experiments is a detachable end mill. The diameter of the shank is 16 mm. The insert on the shank is square-shouldered milling cutters (PKT11T7308-PM). During milling, a Spike 1.2 rotary force measuring instrument was used to measure the three-way force. The roughness was measured by the SJ-210 surface roughness tester, whose accuracy can reach 0.001 µ m. Fig. 7 Experimental setup The orthogonal experiment method  is used to replace the enormous amount times of tests with only a partial part while still expressing the whole situation. This investigation is carried out upon surface roughness with different machining parameter combinations. To improve the experimental efficiency, this research work chooses the spindle speed, the axial cutting depth and the feed per tooth as the influencing factors, and each factor has three levels. Three levels were specified for each factor, as indicated in Table 4. The orthogonal array chosen was L7, which has seven rows corresponding to the number of parameter combinations, with three columns at three levels as shown in Table 5, in which A, B and C represent the machining parameters. The surface morphology of v fi e groups is described in Table 6, and pictographic representation of the surface morphology is also provided in Figure 8. 76 Guan et al. Applied Mathematics and Nonlinear Sciences 8(2023) 69–80 Table 4 Experiment factor and level Level Factor Spindle speed S (rpm) Cutting depth ap (mm) Feed per tooth fz (mm/tooth) 1 3200 0.5 0.0625 2 6400 1.5 0.125 3 8000 2 0.25 Table 5 Designed orthogonal experiment Index A B C 1 1 1 1 2 1 2 2 3 1 3 3 4 2 1 2 5 2 2 3 6 2 3 1 7 3 1 3 Table 6 Process parameters of cutting experiment samples Index Feed per tooth (mm) Surface topography Surface roughness 7 0.05 Figure 8(A) 0.567 2 0.0625 Figure 8(B) 0.612 3 0.125 Figure 8(C) 0.706 8 0.2 Figure 8(D) 1.025 5 0.25 Figure 8(E) 1.395 (A) Index 7 (B) Index 2 (C) Index 3 (D) Index 8 (E) Index 5 Fig. 8 Surface topography samples Prediction of surface quality in end milling 77 Fig. 9 Surface roughness prediction model (A) Experiment #3 (B) Experiment #4 (C) Experiment #5 (D) Experiment #6 Fig. 10 Comparison of CRNN deep network model prediction value and experimental value 78 Guan et al. Applied Mathematics and Nonlinear Sciences 8(2023) 69–80 3.2 Analysis of surface roughness results The range of processed surface quality can be accurately predicted by the constructed deep network, and in real application scenarios, it is often necessary to accurately identify and predict the surface roughness more accurately, so as to accurately obtain the quality results of each process to ensure that each process is within the accuracy requirements. Based on the quality recognition model in the previous section, using the extracted internal features to build a regression model, mapping output to surface roughness value, that is, the input is the milling force signal during machining, and the output is the surface roughness prediction result. The specific process is shown in Figure 9. It involves replacing the softmax structure of the last layer of the CRNN network with a regression model to build a surface roughness prediction model during processing and using MSE mean square error loss function as objective function. When conducting the surface roughness prediction experiment, since it is necessary to predict the quality of the workpiece during the entire processing process, the training and test sets need to use a complete processing experiment data individually, and the roughness prediction model in this section is based on the previous one. For the classification model in this section, the model parameters before the softmax layer of the model in the previous section are migrated as the trained weights. Only three sets of data are used to fine-tune the regression model. The experiment selects three sets of data of 1, 2 and 7 as the training set, and four sets of data of 3, 4, 5 and 6 as the test set to verify the prediction effect of the model. The abscissa represents the number of processing cycles, and the ordinate represents the surface quality value. The real surface quality value after secondary processing is used as the standard, and the result of comparison with the model output is shown in Figure 10. The results show that even if there is a certain error between the model prediction and the real situation, the CRNN model proposed in this paper can meet the repeatability and accuracy requirements of the surface quality prediction in the general processing process. 4 Conclusion A modified CRNN was proposed to predict the surface quality of end milling. The proposed CRNN network could solve the over-fitting problem during the feature extraction process through optimisation of the conver- gence speed and loss function. The predicted values were compared with the actual experimental results, and the predicted values demonstrate a good accordance with the real cutting. 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Applied Mathematics and Nonlinear Sciences – de Gruyter
Published: Jan 1, 2023
Keywords: surface quality prediction; deep learning model; convolutional recurrent neural network; end milling
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