Experimental study of the influence of swing grinding parameters on machining surface quality of large-width cam

The swing grinding method is an excellent choice for the manufacturing of the large-width cam to avoid the grinding defects and form splendid surface quality. Since the influence of swing grinding parameters on machining surface quality has not been clearly clarified, experimental study was conducted in the paper. The orthogonal experiment was designed, and based on which, the residual stress, surface roughness, and hardness of large-width cam were analyzed through the combination of quantitative and qualitative methods. The results showed that the grinding depth and the cam speed have a relatively great influence on the residual stress compared with grinding wheel swing frequency and swing amplitude. Moreover, the grinding wheel swing frequency has the greatest influence on the hardness of the cam surface and the increment of it will increase the hardness of the cam.


Introduction
Large-width cam has been widely used in high-power marine diesel engines, due to its well strength, wear-resisting, and service life. The cam is designed with a large width in the axial direction to provide greater force to the valve train. The quenching heat treatment and grinding process on the cam could ensure the requirement of residual stress, surface roughness, hardness, and other machined surface quality. However, during the traditional grinding process, the width of the grinding wheel is usually greater than that of the cam, thus forming a large contact area between the grinding wheel and the cam in the machining process. Therefore, the grinding heat is difficult to be dissipated to lead to grinding burn defects [1]. With the development of modern processing technology, new grinding method has been applied in processing field, such as the high-shear and low-pressure grinding and the swing grinding process. The high-shear and low-pressure grinding method has the ability to improve the processing quality [2] [3]. The swing grinding process could realize the machining of large-width workpiece with narrow grinding wheel by increasing the reciprocating motion of the grinding wheel along its axis. This improvement could reduce the contact area between the grinding wheel and the cam and transfer the grinding heat timely which effectively solves the problems existing in traditional grinding process. However, the application of swing grinding process is limited since the influence of the process parameters, especially the increased swing amplitude and swing frequency, on the machined surface is still not clear. Therefore, it is urgent to study the influence of swing grinding process parameters on surface quality of the large-width cam.
Although there are few works of literature about swing grinding, a lot of research has been conducted on the influence of grinding parameters on surface quality processed by the traditional grinding method. The grinding residual stress plays a major role in deciding the workpiece's surface quality, including the corrosion resistance, the fatigue property, and the reliability [4]. Denkena et al. [5] investigated the influences of grinding with Toric CBN grinding tools on the residual stress and got the conclusion that a good surface finish could be achieved with small cutting grain size, low feed rates, and frontal grinding strategy. Ding et al. [6] studied experimentally the effects of process parameters on the grindability of cast nickel-based superalloys and observed that the grinding depth has the greatest effect on the residual stresses compared with the grinding speed and workpiece speed during creep feed grinding of K424 superalloy with CBN wheel. Wang et al. [7] proposed an analysis model of surface residual stress based on generating grinding method for spiral bevel gears, and they found that the grinding parameters at each point on spiral bevel gear are different under the same grinding conditions (grinding speed, generating speed, and cutting depth) which will affect the distribution of residual stresses on the tooth surface. Ling et al. [8] established a prediction model of grinding residual stress considering initial residual stress generated by straightening process and got a conclusion that the initial residual stress of former process should be taken into account when studying the residual stress of linear guideway induced by grinding process. Hamdi et al. [9] investigated the influence of grinding parameters on the residual stress of steel by simulating the stable grinding stage and the cooling stage after grinding, and they found that the residual stress was positively related to the grinding linear speed.
As an important variable of the grinding quality, the surface roughness is a variable often used to describe the performance of the finished part as well as to evaluate the competitiveness of the overall grinding system [10]. Aiming at the roughness, an improved grinding roughness model was established for cylindrical plunge grinding through analyzing the existing roughness models where the infeed rate of the grinding table was selected as the weighting factor and the model was also used for the optimization algorithm of the grinding procedure [11]. Xu et al. [12] developed a new method for predicting the surface roughness in spherical grinding, and the process parameters that affect spherical surface roughness were theoretically analyzed and experimentally studied. Lin et al. [13] measured the surface roughness during grinding processes on BK7 glass, and the experimental results showed that the values of surface roughness increased with the increment of feed rate and grinding depth and the decrement of wheel speed. To improve the bearing capacity and reliability of face gears, Wang et al. [14] studied the grinding surface roughness based on the grinding machining principle and summarized the influence of grinding parameters on the surface roughness. Yang et al. [15] studied the formation mechanism of aspheric surface roughness and revealed the influence of processing parameters on the surface roughness where the surface roughness was reduced and the aspheric surface roughness distribution tended to be uniform with the increasing grinding wheel speed and reducing grinding depth. Sun et al. [16] extended the models of roughness in metal and ceramic machining to silicon wafer self-rotating grinding through modelling and experimental methods where the model could identify the effects of processing parameters, abrasive grain size, material properties, and grinding mark geometry on roughness quantitatively. Besides the residual stress and the roughness, the hardness also plays an important role to measure the surface integrity and has a significant effect on the fatigue strength and wear resistance of workpieces [17]. Zhou et al. [18] established the one-dimensional analytic model for surface microhardness in terms of grinding speed, grinder work-table feed speed, grinding depth, incline angle, and deflection angle as process parameters in quick-point grinding ceramics through immune algorithms and orthogonal experiment data. Guo et al. [19] integrated the finite element (FE) and cellular automata (CA) approach to explore the distribution and variation of the grinding temperature of the workpiece surface in a grind-hardening process and predicted the hardness of the hardened layer with different grinding parameters. Guo et al. [20] researched the grind-hardening processing through theoretical study and grind experiment. The results showed that the hardness penetration depth increased as the depth of the wheel cut and feeding speed increased.
It can be seen that the research on the swing grinding processing parameters on the surface quality of the cam has not been sufficient so far. Thus, in this paper, the effects of the swing grinding process parameters on the residual stress, the roughness, and the hardness of the cam surface are analyzed through the swing grinding test of the large-width cam. The contribution of this paper is summarized as follows. Firstly, the superiority of swing grinding process compared with the nonswing grinding process in manufacturing the large-width cam is verified experimentally. Then, the analysis results of the surface roughness indicate the limits of traditional evaluation indexes for the roughness which could prompt the research about the new characterization method. Moreover, the research provides practical experience about the manufacturing of the large-width cam so as to extend the application of the swing grinding technology.
The rest of this paper is organized as follows. Section 2 gives the processing principle of swing grinding for camshaft. Section 3 introduces the experimental scheme design and the measuring method of several parameters charactering the machined surface quality. The results of the experiment are discussed in Sect. 4 in which the influences of swing grinding parameters on residual stress, surface roughness, and hardness are analyzed. Finally, some concluding comments are made in Sect. 5.

The processing principle of swing grinding for camshaft
Swing grinding is a processing method that applies low-frequency and high-amplitude reciprocating simple harmonic swings to the grinding wheel along the axis of the grinding wheel compared with conventional grinding. The relative movement of the camshaft and the grinding wheel is shown in Fig. 1. The grinding wheel is programmed to perform a linear reciprocating movement along the Z-axis in addition to the linear movement along the X-axis in conjunction with the rotation of the camshaft. The path of the grinding wheel can be described by g(t) = Msin(2πft) where the symbol t means the grinding time and g(t) is the real-time swing amplitude at time t in the swing grinding process; M and f are the swing amplitude and swing frequency applied to the axis of the grinding wheel, respectively. During the grinding process of the camshaft, the grinding wheel is always externally tangent with the cam contour and the grinding depth is denoted as a p . The grinding wheel is rotated around the coordinate Y-axis with speed V s and the speed of the cam is V w .

Camshaft for experiment
Two camshafts, whose number of the cams was 6, were chosen for the experiment. All of them had the same material status before swing grinding processing. Each cam was marked uniformly as i#-j, which means the jth cam in the ith camshaft. The first camshaft was exhibited in Fig. 2a with numbered cams. In order to study more details of the cam profile and measurement convenience, cam pieces were cut from the four key positions which are the top circle (D), the base circle (J), the lifting section (G1), and the returning section (G2) as Fig. 2b. The material of the cams is 40Cr whose composition and mechanical properties are presented in Tables 1 and 2, separately.

Experimental scheme design
To study the influence of various factors in the grinding process, a group of the orthogonal experiment was designed. The experiments of Group No. 1-No. 9 constitute a fourfactor and three-level orthogonal test with grinding depth a p , cam speed V w , grinding wheel swing frequency f, and grinding wheel swing amplitude M as variables (i.e., L 9 ( 3 4 )).
In addition to the test parameters given in Table 3, grinding fluid was used for cooling and heat dissipation in the grinding process with a pressure 0.3 MPa.
The level values of grinding process parameters in the experiment are comprehensively determined by the grinding    wheel, the machine tool, and the cam material. High-speed grinding is required for the processing of the cam so that CBN grinding wheel is chosen for the grinding process. The diameter of the grinding wheel is selected according to the installation conditions of the machine tool and the parameters recommended by the grinding wheel manufacturer. The cam material limits the grinding depth. The swing grinding parameters are selected according to the performance index of the machine tool and the practical production parameters. The grinding speed V s is usually determined by actual processing parameters in the range of 70 ~ 100 m/s. Overall, the selection range of the grinding parameters is determined by actual processing conditions. According to which the level values of grinding parameters in DOE are determined by the boundaries and the median to maximize the changes of the surface quality with the varying of the grinding parameters.
In the experiment, the swing grinding CNC machine tool was used to grind the camshaft and the physical drawing of the equipment is shown in Fig. 3. The longitudinal micro swing of the Z-axis of the grinding wheel frame is added to grind the camshaft based on the X-C axis coupling linkage of the swing grinding CNC machine tool.

Detecting method of residual stress
The residual stresses of the cam pieces after swing grinding were detected by PROTOIXRD combined residual stress analyzer from Proto Company of Canada based on the X-ray diffraction method as shown in Fig. 4. The instrument has high sensitivity and fast detection speed and thus is suitable for the measurement of various materials which can accurately measure the residual stresses of the cam pieces. The residual stress was measured by the side-incline method, and the incident angle of X-ray tube was set in the range of − 20° ~ 20° which was divided into 7 scanning angles. Two receiving probes A and B were symmetrically placed on both sides of the ray beam, and the swing angle was set as 3°. More specific parameter settings can be seen in Table 4.
To obtain the residual stress distribution on the cam surface along the profile, four points were detected for each cam. These points are located on the middle point of the cam surface which is parallel to the cam axis of the top circle, the base circle, the lifting section, and the returning section, respectively.
Correspondingly, two measuring points in the axial direction were added at each measuring point of the cam plate for the No. 5 experiment and were marked as point P1 and point P3 to obtain the residual stress distribution on the cam surface along the axial direction and profile. The distribution of each measuring point is shown in Fig. 5 where the three measuring points in the axial direction divide the cam into four equal parts in the axial direction. A total of 12 points were detected for the cam in the No. 5 experiment and 4 points for other cams. Residual stresses in the axis direction (X direction) and the direction perpendicular to the axis direction (Y direction) were measured at each test

Measuring method of roughness and hardness
The surface morphology of the cam was observed by OLYMPUS-DSX1000 three-dimensional large DOF (depth of field) digital microscope with a magnification range of 20-7000 X, and the measuring accuracy can reach to the micron level. Three evaluation indexes, the arithmetic average deviation value of the profile Ra, the maximum height of the profile Rz, and the average width of the profile element Rsm, were measured along the axial direction of the cam, as shown in Fig. 6. The surface hardness of the cam was measured by INNO-VATEST FALCON 400 as shown in Fig. 7. The measurement pressure was set as 500N with a hold time of 12 s.

Residual stress distribution on the cam surface
The residual stresses at each measuring point in the X and Y directions are shown in Fig. 8. The horizontal ordinate in Fig. 8 is the measurement point for the cams. The order follows the number of the cams and the measured position of each cam from top circle (D) in an anticlockwise direction. For the cam in the No. 5 experiment, the order is P2 circle, P1 circle, and P3 circle. It can be observed that the residual stress in both X and Y directions is compressive stress. Moreover, the residual stress in the X direction is larger than that of the Y direction which indicates that the resultant direction of residual stress is biased towards the axis direction. This phenomenon is consistent with that of the nonswing grinding process [23].
The mean values of residual stresses at points P1, P2, and P3 on the four positions of the cam profile of the No. 5 experiment are presented in Fig. 9, where R is the resultant force. The upper and lower boundaries are the maximum and    Fig. 9 that the residual stress of swing grinding changes little along the cam axis. However, the residual stress near the cam midline has the largest value in the nonswing grinding process according to Fig. 7 in reference [23].
As the residual stresses are usually positively related to the grinding temperature [7], the difference may be due to the better heat dissipation ability of swing grinding compared with the nonswing grinding process which verifies the superiority of swing grinding process. The mean residual stresses in the X and Y directions and resultant force (R) of the cam at the base circle, the top circle, the rising section, and the returning section are shown in Fig. 10. It can be found that the residual stress at the top circle has the largest mean value, and the mean residual stresses in the transition sections (rising section and returning section) are close to each other in all directions.

The result analysis of residual stress in orthogonal test
The mean residual stress in the orthogonal test for each factor is expressed by K i at the ith (i = 1, 2, 3) level. The range value, which is the difference between the maximum and  Table 5.
The following conclusions can be drawn from Table 5.
(1) Overall, the degree of influence caused by each factor on the residual stress from high to low is grinding depth a p , cam speed V w , grinding wheel swing frequency f, and swing amplitude M, that is to say, the main factors of swing grinding on the residual stress are grinding depth a p and cam speed V w , while the parameters such as grinding wheel swing frequency f and swing amplitude M have relatively little influence on the residual stress. (2) The grinding depth a p has the greatest influence on the residual stress at the base circle, the top circle, and the rising section while the cam speed V w at the returning section. (3) The degree of each influencing factor on the Y direction residual stress of each part of the cam contour surface shows a good consistency where the grinding depth a p and cam speed V w have a greater influence on the residual stress. (4) In the X direction, the grinding depth a p has the greatest influence on the residual stress at the top circle and the rising section. However, the grinding wheel swing  amplitude M and the cam speed V w have the greatest influence at the base circle and the returning section, separately. (5) Under the joint action of residual stress in the X direction and Y direction, the degree of influence caused by each factor on the resultant force R from high to low is the grinding depth a p , the cam speed V w , the grinding wheel swing frequency f, and the grinding wheel swing amplitude M since the amplitude of the residual stress in the X direction is much bigger than it in the Y direction.

Surface roughness at different positions
The three evaluation indexes, Ra, Rz, and Rsm, were normalized and drawn in Fig. 11 following the number of the experiments. It can be observed that they have a certain correlation to some extent.
To further analyze their correlations quantitatively, the correlation coefficients between Ra and Rz, Ra and Rsm, and Rz and Rsm are calculated as 0.7810, 0.3469, and 0.0778, respectively. Check the correlation coefficient test table to get the critical value r 0.01 (34) = 0.4238 under the significance level of 0.01 and the degree of freedom as 34. The results indicate that a significant linear correlation existed only between Ra and Rz.
The correlation coefficients between Ra and Rz, Ra and Rsm, and Rz and Rsm at different positions of the cam contour are listed in Table 6. The critical value r 0.01 (7) is 0.7977 under the significance level of 0.01, and the degree of freedom is 7. It can be found from Table 6 that there is a significant linear correlation between Ra and Rz at the top circle, the rising section, and the returning section. There is no significant linear correlation between Ra and Rsm, so as Rz and Rsm.

The result analysis of surface roughness in orthogonal test
The analysis results of the orthogonal test on surface roughness are shown in Table 5. Conclusions can be drawn from Table 7 as follows. Fig. 11 The normalized three evaluation indexes of roughness (1) In general, the influence degree of each factor on the surface roughness from high to low is the grinding depth a p , the grinding wheel swing amplitude M, the grinding wheel swing frequency f, and the cam speed V w . The main factors affecting the surface roughness of the swing grinding process are the grinding depth a p and the grinding wheel swing amplitude M. (2) For Ra, the influence degree of each factor is the grinding depth a p , the cam speed V w , the grinding wheel swing frequency f, and the grinding wheel swing amplitude M. But the influence on each position of the cam contour surface is different. The main influencing factor is the grinding depth a p at the top circle, the rising sec-tion, and the returning section, which is the grinding wheel swing frequency f in the base circle. (3) For Rz, the influence degree of each factor is the grinding depth a p , the grinding wheel swing amplitude M, the grinding wheel swing frequency f, and the cam speed V w . The main influencing factor is the grinding depth a p at the base circle and the top circle, which is the grinding wheel swing amplitude M in the rising section and the returning section. (4) For Rsm, the influence degree of each factor is the grinding wheel swing amplitude M, the grinding depth a p , the grinding wheel swing frequency f, and the cam speed V w . The main influencing factor is the grind-ing wheel swing amplitude M at the top circle and the rising section, which is the grinding wheel swing frequency f and the grinding depth a p in the base circle and the returning section, separately.
According to the above analysis, it is found that the conclusions based on the three evaluation indexes are different. Therefore, the traditional surface roughness evaluation indexes are not suitable for the swing grinding process.

Influence of swing grinding parameters on hardness
The analysis results of the orthogonal test on hardness are shown in Table 6. It can be concluded from Table 8 that the grinding wheel swing frequency f and the grinding depth a p have a great influence on the hardness of the cam surface. The optimal plan for the swing grinding process is A1B1C3D1 when higher hardness is required for the cam surface.

Conclusion
The influences of the swing grinding process parameters on residual stress, surface roughness, and hardness of largewidth cam are researched experimentally. The main conclusions are listed as follows.
(1) The residual stress of swing grinding process changes little along the cam axis which may be due to the better heat dissipation ability of swing grinding compared with the nonswing grinding process. In general, the grinding depth a p and the cam speed V w have a relatively great influence on the residual stress compared with the grinding wheel swing frequency f and swing amplitude M. (2) Since the main influencing factors at different positions of cam contour on surface roughness based on the three evaluation indexes, Ra, Rz, and Rsm, are diverse, the traditional surface roughness evaluation indexes are not suitable for the swing grinding process. New evaluation index is needed to characterize its surface roughness of swing grinding process. (3) The grinding wheel swing frequency f has the greatest influence on the hardness of the cam surface, and the increment of it will increase the hardness of the camshaft. (4) The fundamentals and the theory induced the observed results from the experiment are worth investigating in our future research.