[Abstract]: This paper analyzes the role and structural function of multi-track spatial cams in high-speed cigarette packaging machines, identifies machining challenges, and proposes corresponding solutions. Applying theoretical and practical experience in machining, a scientifically sound, operable, and economical multi-track spatial cam process design scheme is developed. Through process trial production and trial operation of the new product YB47 high-speed cigarette packaging machine (550 packs/min), the scientific validity, rationality, reliability, and economy of the multi-track spatial cam process design scheme are verified. This lays a foundation for exploring the development and application of high-precision, complex-structured multi-track spatial cams in ultra-high-speed cigarette packaging machines.
[Keywords]: High-speed multi-track spatial cam, CNC program, four-axis four-linkage high-speed cigarette packaging machine; Process design and exploration of multi-track spatial cam; Zhu Yiru, Technology Development Department, Shanghai Tobacco Machinery Co., Ltd., Mechanical Transmission
1. Preface
One of the most important applications of spatial cams is their use as intermittent indexing in automated machinery, primarily for converting continuous circular motion into intermittent circular motion, i.e., as a spatial cam indexing mechanism. Spatial cam indexing mechanisms are widely used in various high-speed cigarette packaging machines. With the rapid development of cigarette packaging machine technology, in addition to improvements in packaging form and quality, one of the most important parameters of new packaging machines has been a significant increase in packaging speed. From the original medium-speed cigarette packaging machines below 500 packs/min to the current high-speed machines above 500 packs/min, ultra-high-speed cigarette packaging machines of 800 packs/min have been developed abroad. The packaging action of high-speed cigarette packaging machines is mainly achieved through the high-speed motion of multi-track spatial cam indexing mechanisms. Therefore, multi-track spatial cams, as key components, play an extremely important and crucial role in the indexing mechanism of high-speed cigarette packaging machines. The development of high-speed cigarette packaging machine technology is closely and inevitably linked to spatial cams, especially the application of multi-track spatial cam indexing mechanisms, which cannot be replaced by other mechanisms. Our company's new product, the YB47 high-speed cigarette packaging machine (500 packs/min), adopts the multi-track spatial cam indexing mechanism of GD's X3000 (700 packs/min) cigarette packaging machine. The high-precision spatial cam, as a component of a high-pair mechanism, presents entirely new requirements for process design due to its complex manufacturing process.
2. Process Design of Multi-Trajectory Spatial Cam
2.1 Comparison and Analysis of Multi-Track Spatial Cam Structures <br />When the speed n of a spatial cam is greater than 500 r/min, it is considered a high-speed spatial cam. The higher the speed of the spatial cam indexing mechanism, the greater the dynamic load, and therefore the greater the inertial force, impact force, and vibration, which greatly affects the indexing accuracy and motion transmission smoothness of the mechanism. Figures 1 and 2 show the YB45.11.084-48 spatial cam in the YB45 (400 packs/min) cigarette packaging machine and the 1BBG41002400 spatial cam in the YB47 (550 packs/min) cigarette packaging machine, respectively. The following differences are observed through comparison:
a. The rotational speeds are n=400r/min and n=550r/min, respectively, with the rotational speed increasing by 37.5%.
b. High-speed spatial cams use dual tracks (A track, B track) and dual rollers to transmit motion, while medium-speed spatial cams use a single track and a single roller to transmit motion.
c. The accuracy of the medium-speed spatial cam trajectory to the positioning datum is ±0.020mm, and the accuracy of the high-speed spatial cam trajectory to the positioning datum is ±0.015mm. The dimensional accuracy between the two trajectories of the medium-speed spatial cam is ±0.015mm, and the dimensional accuracy between the two trajectories of the high-speed spatial cam is ±0.010mm.
d. Because the dual tracks of the high-speed spatial cam are distributed vertically on the track surface (see Figure 1E-E section), the machining depth of the track surface is more than 3 times that of the medium-speed spatial cam.
d. High-speed spatial cams have lower structural rigidity than medium-speed spatial cams and are more prone to deformation.
2.2 Machining Analysis of Multi-track Spatial Cams 2.2.1 Current Machining Status The multi-track spatial cam indexing mechanism is a high-pair mechanism, where the cam and roller have line contact during operation. After long-term high-speed operation, the cam track surface suffers severe wear, requiring the cam material to undergo carburizing and hardening treatment to achieve a hardness of HRC58-62. Therefore, the cam track needs grinding. For example, the YB45.11.084-48 spatial cam in the YB45 cigarette packaging machine and the 1BBG41002400 spatial cam in the YB47 are made of 20CrMnTi, with a heat treatment of T215-S0.5-C58. Dedicated CNC machining equipment without grinding capabilities cannot meet the machining requirements of spatial cams; therefore, multi-track spatial cams mainly rely on imports or outsourcing for machining.
2.2.2 Selection of Machining Method and Equipment <br />The machining of spatial cams is usually carried out using a four-axis, four-linkage CNC boring and milling machine or a cam CNC milling and grinding machine. The selection of machine tool is determined based on factors such as the cam's structure, trajectory accuracy, heat treatment condition, machining clamping method, machining efficiency, and machining cost. Figures 3 and 4 show the clamping methods of the cam on the two machine tools. Figure 3 shows the machining clamping method of a four-axis, four-linkage CNC boring and milling machine. Since the cam is always in a cantilevered state during machining, it is prone to vibration under the action of cutting force, making it difficult to guarantee the accuracy and surface roughness of the cam trajectory. Because the spatial cam profile surface of our company is currently a collection of contact lines that are instantaneously transformed into the cam coordinate system, it is difficult to establish a three-dimensional mathematical model. Therefore, tool compensation cannot be used to machine spatial cams on CNC boring and milling machines. Therefore, the diameter of the tool used in finishing the cam trajectory must be the same as the diameter of the roller. Even slight tool wear will fail to meet the machining requirements, resulting in a large tool consumption. Table 1 compares the accuracy parameters and machining capabilities of CNC boring and milling machines and CNC cam milling and grinding machines. Table 1 shows that the spindle power of the CNC boring and milling machine is relatively low (the WF72C has the highest spindle power among our company's CNC boring and milling machines). Furthermore, due to the clamping method, the cutting amount during machining is very small, ranging from 1 to 2 mm/tool, resulting in a long production cycle. Therefore, the machining cost of using a four-axis, four-linkage CNC boring and milling machine is high. Figure 4 shows the machining clamping method of the cam milling and grinding machine. It uses a one-clamp, one-support clamping method to ensure that the cam is always in a good stress state during machining, thereby eliminating vibration caused by cutting forces and ensuring the accuracy and surface roughness requirements of the cam trajectory. This machine tool has both milling and grinding functions. As shown in Table 1, compared with the four-axis, four-linkage CNC boring and milling machine, its positioning accuracy is higher and its spindle power is greater, so the cutting amount is also greatly increased, reaching 8 to 15 mm/tool for different materials. Another feature of the machine tool is that the CNC operating system has an automatic tool or grinding wheel compensation function. This means that it can process any tool or grinding wheel with a diameter smaller than the roller diameter, and worn tools or grinding wheels can be reused after sharpening or correction, thus significantly reducing processing costs. Considering clamping methods, machine tool accuracy, processing performance, and processing capabilities, only the FSK25S CNC cam milling and grinding machine, currently the most advanced in the world manufactured by KOPP in Germany, can process multi-track spatial cams that meet the design requirements of high-speed cigarette packaging machines.
Figure 3 CNC boring and milling machine
Figure 4 Cam Milling Machine
Table 1. Comparison of accuracy parameters and machining capabilities of CNC boring and milling machines and CNC cam milling machines.
Machining equipment axis positioning accuracy, repeatability, machining method, cooling method, spindle power, cam milling machine SFK25S, xyz≤0.005
mm
C≤±5sec xyz≤0.003
mm
C≤±3sec Milling and grinding high pressure constant low temperature 11kw
CNC boring and milling machine
WF72C xyz≤0.012
mm
C≤±15sec xyz≤0.006
mm
C≤±8sec, milling conventional cooling 7.5kw
2.2.3 Machining Software The FSK25S CNC cam milling and grinding machine is a state-of-the-art, high-precision cam machining machine tool manufactured by K0OP GmbH in Germany, possessing both milling and grinding functions. Its CNC system adopts the latest Siemens 840D system, and the machining software is the company's newly developed, state-of-the-art parametric programming cam machining software (K0PPS0FTWARE). It automatically verifies whether the trajectory formed by the fitted cam parameters meets the machining requirements and automatically optimizes and smooths the cam trajectory, ensuring that the machined trajectory fully meets the design requirements in terms of accuracy. As shown in Figures 5 and 6, during the parametric CNC programming process, only the necessary technical parameters need to be input, and the machine can automatically calculate and display parameters such as the rotation direction, angle, and spindle feed rate of the spatial cam trajectory. This not only improves the accuracy and reliability of the program but also greatly facilitates CNC programming. The software's greatest advantage lies in its significant improvements and development of the machine tool's CNC operating system on the Siemens 840D CNC system. This operating system features automatic tool or grinding wheel compensation when machining spatial cams, essentially solving the problem of tool or grinding wheel incompensation during CNC boring and milling machine machining of spatial cams. In other words, there are no strict requirements on the diameter of the tool or grinding wheel used for machining spatial rib cams or spatial groove cams; as long as the diameter is smaller than the roller diameter, it can be used for machining.
Figure 5
Figure 6
2.2.4 Processing Deformation and Control Measures <br />Analysis of the structure (see Figure 1) and main dimensions of the 1BBG41002400 spatial cam in the YB47 high-speed cigarette packaging machine reveals that, due to the machining depth of 37mm on the trajectory surface, the thickest part of the rib formed by the two trajectory surfaces is 18mm and the thinnest part is about 12mm. The cross-sectional thickness of the cam in the groove (E-E section) is uneven, making it a easily deformable part from a heat treatment perspective. Controlling the deformation of this type of part, especially during machining and heat treatment, is extremely difficult. The material utilization rate of this part is only 33%, and the machining cutting volume is large. Particularly after rough machining of the cam trajectory, the stress distribution in the part's structure is uneven, and stress concentration easily occurs in local areas. This leads to greater deformation after carburizing and hardening during heat treatment, making it difficult to guarantee the dimensional requirements and processing control of subsequent processes. Therefore, a stress-relieving heat treatment process must be added after rough machining of the cam trajectory surface to reduce or even eliminate the internal stress of the material, thereby minimizing processing deformation to the greatest extent. Because the cam is a deformable part, the deformation after heat treatment and hardening is mainly concentrated in the cam groove. Using a salt bath furnace for hardening ensures uniform heating during the process and reduces oxidation and decarburization. This results in a uniform surface hardness of the cam after hardening. It is crucial that the cam be suspended vertically during hardening, meaning the cam groove is vertical during heating, to control the amount of deformation. If heated horizontally, the cam groove will be horizontal, which can easily cause shrinkage or expansion of the cam groove opening after hardening, leading to excessive deformation that makes subsequent machining impossible. Therefore, the amount of deformation after hardening should be controlled within a pre-set machining allowance to effectively control the deformation of the cam after machining and heat treatment, while also ensuring the quality of the heat treatment.
2.2.5 Selection of Grinding Wheel Material <br />The selection of grinding wheel material directly affects the dimensional accuracy, surface roughness, and machining efficiency of the cam trajectory surface. The FSK25S cam milling grinder features high-speed, precision, and high-efficiency grinding, with a spindle speed of 50,000 r/min and a grinding wheel linear speed of up to 104.72 m/s. In high-speed grinding, the grinding removal rate of CBN (cubic boron nitride) grinding wheels can reach 2000 mm/s, while that of ordinary grinding wheels is 500–1000 mm/s. Using BCN (cubic boron nitride) grinding wheels not only allows for the processing of various materials but also significantly improves the grinding quality of the trajectory surface in high-speed grinding, reduces grinding force, achieves smaller dimensional and shape errors, improves machining accuracy, and allows for the processing of multiple parts in a single dressing cycle, thus resulting in a long grinding wheel service life.
2.2.6 Determination and Control of Key Processes and Machining Allowances <br />The scientific, advanced, rational, operable, and economical nature of process design is a comprehensive reflection of process design capabilities and levels. The determination of key processes and the setting of machining allowances not only play a crucial role in connecting processes but are also an extremely important process design link in ensuring the precision requirements of parts. Through structural analysis and deformation prediction of the 1BBG41002400 spatial cam in the YB47 high-speed cigarette packaging machine, the stress relief, carburizing, and hardening heat treatment processes were identified as key processes and are subject to focused control. This aims to minimize or even eliminate the internal stress of the part structure generated after rough machining, thereby effectively controlling the impact of part deformation on subsequent machining processes. In the carburizing and quenching heat treatment processes, emphasis is placed on using a salt bath furnace for hardening and the requirement that the cam must be vertically suspended during hardening to ensure uniform heating of the part, control the deformation of the cam groove, reduce oxidation and decarburization, and ensure uniform surface hardness of the cam. The machining allowance is determined by fully considering the special characteristics of the part structure, process requirements, and economic efficiency, while referring to the process standards. Excessive machining allowance can easily lead to cutting deformation and stress concentration in the part, and is also uneconomical. However, insufficient machining allowance makes it difficult to guarantee the machining allowance requirements for milling and grinding processes after heat treatment due to deformation, resulting in part scrap. Therefore, the relationship between critical processes and machining allowance is that heat treatment controls deformation and ensures the machining allowance for subsequent processes, while an appropriate machining allowance can compensate for the deficiencies of heat treatment; thus, the two are complementary. Considering all factors, the finishing (milling) allowance for the cam trajectory is determined to be 1–1.5 mm, and the grinding allowance is 0.3–0.35 mm.
3. Determination of process design scheme
The scientific nature, advancement, rationality, operability, and economy of the process design scheme are inevitably related to whether the part can ultimately meet the design requirements. Based on the analysis of factors such as the structure, technical requirements, machining and clamping methods, machining equipment, deformation conditions, heat treatment measures, machining allowance, production cycle, and machining costs of the spatial cam, the basic process design scheme for the spatial cam is determined as follows: Forging → Normalizing → Turning → Quenching and Tempering → Turning → Milling → CNC Milling (Rough Milling of Cam Track) → Stress Relief → Turning → CNC Milling (Fine Milling of Cam Track) → Carburizing → Scribing → Drilling → Salt Bath Hardening, Sandblasting → Internal Grinding → Surface Grinding → 10,000 Grinding → CNC Grinding (Fine Grinding of Cam Track) → Fitting.
4. Trial fabrication of multi-track spatial cam
Process trial production is a process of evaluating the process design scheme for important parts, adjusting the process design scheme, and verifying the accuracy of the process problems and solutions identified in the machining analysis. During the process trial production, the focus is on controlling key processes and the relationship between machining allowance and deformation state to ensure the machining allowance of the cam trajectory in each process. Strict testing is conducted on the deformation generated by key processes (heat treatment for stress relief and carburizing, quenching). Table 2 shows the record of the cam trajectory deformation after heat treatment. The results show that the deformation condition conforms to the above machining analysis prediction, and the deformation is completely controlled within the expected requirements. After the cam machining is completed, all technical parameters are inspected and found to meet the design requirements, indicating that the process trial production was a complete success.
Table 2. Record of Deformation Amount During Heat Treatment of Cam Track
Machining allowance after heat treatment, deformation amount after stress relief: 1-1.5mm, 0.3-0.45mm
Carburizing and salt bath quenching: 0.3–0.35 mm, 0.1–0.15 mm
5. Economic Comparison Analysis and Exploration of Future Space Cam Technology Design
Due to limitations in processing equipment and software, the spatial cams in the X1 and X2 cigarette packaging machines were previously mainly imported or outsourced. During the development and trial production of the YB47 high-speed cigarette packaging machine, the domestic production of multi-track spatial cams was successfully achieved for the first time. This filled the gap in the company's reliance on imports during the prototype development and trial production stage of new products. It not only met design requirements in terms of processing precision but also freed the company from GD's long-standing control over processing technology and production cycles, giving the company complete control over production planning and assembly cycles. More importantly, the successful domestic production of multi-track spatial cams not only has significant practical value for the successful trial development of the YB47 high-speed cigarette packaging machine but also holds profound historical significance for the development of ultra-high-speed cigarette packaging machines, laying the foundation for the process design and processing of more complex and precise multi-track spatial cams. The successful domestic production of spatial cams has also significantly reduced production costs. Table 3 compares the costs of imported and domestically produced spatial cams (taking the YB45.11.084-48 spatial cam in the ZB45 cigarette packaging machine as an example). In 1997, the import price of the YB45.11.084-48 spatial cam was 1861.03 German marks, equivalent to RMB 10235.67 at the exchange rate at the time. The domestic cost was RMB 2626.97, only 25.66% of the import price, saving RMB 7608.87 per cam compared to the import price. Based on an annual production of 35 sets of X2 cigarette packaging machines, with one package per machine, this single cam alone could save the company RMB 266310.45 and foreign exchange 65136.05 German marks annually. The 1BBG41002400 multi-track spatial cam in the new YB47 high-speed cigarette packaging machine (550 packs/min) is priced at approximately 850 euros, equivalent to RMB 13,132.5 at the current exchange rate. The cost of domestic production is RMB 3,058.18, only 23.29% of the imported price, saving RMB 10,074.82 per cam compared to the imported price. Based on the current production capacity of 5 sets of YB47 cigarette packaging machines per year, with two packs per machine, the company can save RMB 100,743.2 and 8,500 euros in foreign exchange annually. With the continuous increase in market demand, its economic benefits will become even more significant.
Table 3. Cost Comparison of Imported and Domestically Produced Spatial Cams
Part Number Import Price (converted to RMB) Self-manufacturing Cost Savings Cost Saving Rate
YB45.11.084—48 1861.03DM 10235.67 2626.97 yuan 7608.87 yuan 74.34%
1BBG41002400 Approximately 850 Euros 13132.5 Yuan 3058.18 Yuan 10074.82 Yuan 76.71%
6. Conclusion <br />This paper uses a combination of theoretical and practical methods to comprehensively analyze the structure and machining of the 1BBG41002400 multi-track spatial cam in the new product YB47 high-speed cigarette packaging machine. Through process trial production, the process design scheme was verified, and the machining difficulties of the multi-track spatial cam were solved. Trial operation of the new product proved that all parameters fully met the design requirements. For the first time, the domestic production of imported key components was achieved during the new product development and trial production stage. This not only reflects the improvement and strengthening of our company's domestic production capabilities but also lays the foundation for process design and high-speed grinding of more complex high-speed, high-precision multi-track spatial cams. However, further exploration is needed to continuously improve the machining quality and efficiency of multi-track spatial cams. Future research will focus on the mechanism of high-speed grinding and optimizing the process parameters to obtain better grinding effects and improve grinding quality and machining efficiency.
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