Analysis and Optimization of Factors Affecting the Machining Dimensional Accuracy of Machining Centers
Abstract: This paper thoroughly explores various factors that affect the machining dimensional accuracy of machining centers and divides them into two categories: avoidable factors and irresistible factors. For avoidable factors, such as machining processes, numerical calculations in manual and automatic programming, cutting elements, and tool setting, etc., detailed elaborations are made, and corresponding optimization measures are proposed. For irresistible factors, including workpiece cooling deformation and the stability of the machine tool itself, the causes and influence mechanisms are analyzed. The aim is to provide comprehensive knowledge references for technicians engaged in the operation and management of machining centers, so as to improve the control level of the machining dimensional accuracy of machining centers and enhance product quality and production efficiency.
I. Introduction
As a key equipment in modern machining, the machining dimensional accuracy of machining centers is directly related to the quality and performance of products. In the actual production process, various factors will affect the machining dimensional accuracy. It is of great significance to deeply analyze these factors and seek effective control methods.
As a key equipment in modern machining, the machining dimensional accuracy of machining centers is directly related to the quality and performance of products. In the actual production process, various factors will affect the machining dimensional accuracy. It is of great significance to deeply analyze these factors and seek effective control methods.
II. Avoidable Influencing Factors
(I) Machining Process
The rationality of the machining process largely determines the machining dimensional accuracy. On the basis of following the basic principles of the machining process, when machining soft materials such as aluminum parts, special attention should be paid to the influence of iron filings. For example, during the milling process of aluminum parts, due to the soft texture of aluminum, the iron filings generated by cutting are likely to scratch the machined surface, thus introducing dimensional errors. To reduce such errors, measures such as optimizing the chip removal path and enhancing the suction of the chip removal device can be taken. Meanwhile, in the process arrangement, the allowance distribution of rough machining and finish machining should be reasonably planned. During rough machining, a larger cutting depth and feed rate are used to quickly remove a large amount of allowance, but an appropriate finish machining allowance, generally 0.3 – 0.5mm, should be reserved to ensure that the finish machining can achieve a higher dimensional accuracy. In terms of fixture usage, besides following the principles of reducing clamping times and using modular fixtures, the positioning accuracy of the fixtures also needs to be ensured. For example, by using high-precision locating pins and locating surfaces to ensure the positional accuracy of the workpiece during the clamping process, avoiding dimensional errors caused by the deviation of the clamping position.
The rationality of the machining process largely determines the machining dimensional accuracy. On the basis of following the basic principles of the machining process, when machining soft materials such as aluminum parts, special attention should be paid to the influence of iron filings. For example, during the milling process of aluminum parts, due to the soft texture of aluminum, the iron filings generated by cutting are likely to scratch the machined surface, thus introducing dimensional errors. To reduce such errors, measures such as optimizing the chip removal path and enhancing the suction of the chip removal device can be taken. Meanwhile, in the process arrangement, the allowance distribution of rough machining and finish machining should be reasonably planned. During rough machining, a larger cutting depth and feed rate are used to quickly remove a large amount of allowance, but an appropriate finish machining allowance, generally 0.3 – 0.5mm, should be reserved to ensure that the finish machining can achieve a higher dimensional accuracy. In terms of fixture usage, besides following the principles of reducing clamping times and using modular fixtures, the positioning accuracy of the fixtures also needs to be ensured. For example, by using high-precision locating pins and locating surfaces to ensure the positional accuracy of the workpiece during the clamping process, avoiding dimensional errors caused by the deviation of the clamping position.
(II) Numerical Calculations in Manual and Automatic Programming of Machining Centers
Whether it is manual programming or automatic programming, the accuracy of numerical calculations is of crucial importance. During the programming process, it involves the calculation of tool paths, the determination of coordinate points, etc. For example, when calculating the trajectory of circular interpolation, if the coordinates of the center of the circle or the radius are calculated incorrectly, it will inevitably lead to machining dimensional deviations. For programming complex-shaped parts, advanced CAD/CAM software is needed to carry out accurate modeling and tool path planning. During the use of the software, the geometric dimensions of the model should be ensured to be accurate, and the generated tool paths should be carefully checked and verified. Meanwhile, programmers should have a solid mathematical foundation and rich programming experience, and be able to correctly select programming instructions and parameters according to the machining requirements of the parts. For example, when programming drilling operations, parameters such as drilling depth and retract distance should be accurately set to avoid dimensional errors caused by programming errors.
Whether it is manual programming or automatic programming, the accuracy of numerical calculations is of crucial importance. During the programming process, it involves the calculation of tool paths, the determination of coordinate points, etc. For example, when calculating the trajectory of circular interpolation, if the coordinates of the center of the circle or the radius are calculated incorrectly, it will inevitably lead to machining dimensional deviations. For programming complex-shaped parts, advanced CAD/CAM software is needed to carry out accurate modeling and tool path planning. During the use of the software, the geometric dimensions of the model should be ensured to be accurate, and the generated tool paths should be carefully checked and verified. Meanwhile, programmers should have a solid mathematical foundation and rich programming experience, and be able to correctly select programming instructions and parameters according to the machining requirements of the parts. For example, when programming drilling operations, parameters such as drilling depth and retract distance should be accurately set to avoid dimensional errors caused by programming errors.
(III) Cutting Elements and Tool Compensation
The cutting speed vc, feed rate f, and cutting depth ap have significant impacts on the machining dimensional accuracy. Excessive cutting speed may lead to intensified tool wear, thus affecting the machining accuracy; excessive feed rate may increase the cutting force, causing workpiece deformation or tool vibration and resulting in dimensional deviations. For example, when machining high-hardness alloy steels, if the cutting speed is chosen too high, the cutting edge of the tool is prone to wear, making the machined size smaller. Reasonable cutting parameters should be determined comprehensively considering various factors such as workpiece material, tool material, and machine tool performance. Generally, they can be selected through cutting tests or by referring to relevant cutting manuals. Meanwhile, tool compensation is also an important means to ensure machining accuracy. In machining centers, tool wear compensation can real-time correct the dimensional changes caused by tool wear. Operators should adjust the tool compensation value in a timely manner according to the actual wear situation of the tool. For example, during the continuous machining of a batch of parts, the machining dimensions are regularly measured. When it is found that the dimensions are gradually increasing or decreasing, the tool compensation value is modified to ensure the machining accuracy of subsequent parts.
The cutting speed vc, feed rate f, and cutting depth ap have significant impacts on the machining dimensional accuracy. Excessive cutting speed may lead to intensified tool wear, thus affecting the machining accuracy; excessive feed rate may increase the cutting force, causing workpiece deformation or tool vibration and resulting in dimensional deviations. For example, when machining high-hardness alloy steels, if the cutting speed is chosen too high, the cutting edge of the tool is prone to wear, making the machined size smaller. Reasonable cutting parameters should be determined comprehensively considering various factors such as workpiece material, tool material, and machine tool performance. Generally, they can be selected through cutting tests or by referring to relevant cutting manuals. Meanwhile, tool compensation is also an important means to ensure machining accuracy. In machining centers, tool wear compensation can real-time correct the dimensional changes caused by tool wear. Operators should adjust the tool compensation value in a timely manner according to the actual wear situation of the tool. For example, during the continuous machining of a batch of parts, the machining dimensions are regularly measured. When it is found that the dimensions are gradually increasing or decreasing, the tool compensation value is modified to ensure the machining accuracy of subsequent parts.
(IV) Tool Setting
The accuracy of tool setting is directly related to the machining dimensional accuracy. The process of tool setting is to determine the relative positional relationship between the tool and the workpiece. If the tool setting is inaccurate, dimensional errors will inevitably occur in the machined parts. Selecting a high-precision edge finder is one of the important measures to improve the accuracy of tool setting. For example, by using an optical edge finder, the position of the tool and the edge of the workpiece can be accurately detected, with an accuracy of ±0.005mm. For machining centers equipped with an automatic tool setter, its functions can be fully utilized to achieve rapid and accurate tool setting. During the tool setting operation, attention should also be paid to the cleanliness of the tool setting environment to avoid the influence of debris on the accuracy of tool setting. Meanwhile, operators should strictly follow the operating procedures of tool setting, and take multiple measurements and calculate the average value to reduce the tool setting error.
The accuracy of tool setting is directly related to the machining dimensional accuracy. The process of tool setting is to determine the relative positional relationship between the tool and the workpiece. If the tool setting is inaccurate, dimensional errors will inevitably occur in the machined parts. Selecting a high-precision edge finder is one of the important measures to improve the accuracy of tool setting. For example, by using an optical edge finder, the position of the tool and the edge of the workpiece can be accurately detected, with an accuracy of ±0.005mm. For machining centers equipped with an automatic tool setter, its functions can be fully utilized to achieve rapid and accurate tool setting. During the tool setting operation, attention should also be paid to the cleanliness of the tool setting environment to avoid the influence of debris on the accuracy of tool setting. Meanwhile, operators should strictly follow the operating procedures of tool setting, and take multiple measurements and calculate the average value to reduce the tool setting error.
III. Irresistible Factors
(I) Cooling Deformation of Workpieces after Machining
Workpieces will generate heat during the machining process, and they will deform due to the thermal expansion and contraction effect when cooling after machining. This phenomenon is common in metal machining and is difficult to completely avoid. For example, for some large aluminum alloy structural parts, the heat generated during machining is relatively high, and the size shrinkage is obvious after cooling. To reduce the impact of cooling deformation on the dimensional accuracy, coolant can be reasonably used during the machining process. The coolant can not only reduce the cutting temperature and tool wear but also make the workpiece cool evenly and reduce the degree of thermal deformation. When selecting the coolant, it should be based on the workpiece material and machining process requirements. For example, for aluminum part machining, a special aluminum alloy cutting fluid can be selected, which has good cooling and lubricating properties. In addition, when performing in-situ measurement, the influence of cooling time on the workpiece size should be fully considered. Generally, the measurement should be carried out after the workpiece has cooled to room temperature, or the dimensional changes during the cooling process can be estimated and the measurement results can be corrected according to empirical data.
Workpieces will generate heat during the machining process, and they will deform due to the thermal expansion and contraction effect when cooling after machining. This phenomenon is common in metal machining and is difficult to completely avoid. For example, for some large aluminum alloy structural parts, the heat generated during machining is relatively high, and the size shrinkage is obvious after cooling. To reduce the impact of cooling deformation on the dimensional accuracy, coolant can be reasonably used during the machining process. The coolant can not only reduce the cutting temperature and tool wear but also make the workpiece cool evenly and reduce the degree of thermal deformation. When selecting the coolant, it should be based on the workpiece material and machining process requirements. For example, for aluminum part machining, a special aluminum alloy cutting fluid can be selected, which has good cooling and lubricating properties. In addition, when performing in-situ measurement, the influence of cooling time on the workpiece size should be fully considered. Generally, the measurement should be carried out after the workpiece has cooled to room temperature, or the dimensional changes during the cooling process can be estimated and the measurement results can be corrected according to empirical data.
(II) Stability of the Machining Center Itself
Mechanical Aspects
Loosening between the Servo Motor and the Screw: The loosening of the connection between the servo motor and the screw will lead to a decrease in transmission accuracy. During the machining process, when the motor rotates, the loosened connection will cause the rotation of the screw to lag or be uneven, thus making the movement trajectory of the tool deviate from the ideal position and resulting in dimensional errors. For example, during high-precision contour machining, this loosening may cause deviations in the shape of the machined contour, such as non-compliance with requirements in terms of straightness and roundness. Regularly checking and tightening the connection bolts between the servo motor and the screw is a key measure to prevent such problems. Meanwhile, anti-loose nuts or thread locking agents can be used to enhance the reliability of the connection.
Loosening between the Servo Motor and the Screw: The loosening of the connection between the servo motor and the screw will lead to a decrease in transmission accuracy. During the machining process, when the motor rotates, the loosened connection will cause the rotation of the screw to lag or be uneven, thus making the movement trajectory of the tool deviate from the ideal position and resulting in dimensional errors. For example, during high-precision contour machining, this loosening may cause deviations in the shape of the machined contour, such as non-compliance with requirements in terms of straightness and roundness. Regularly checking and tightening the connection bolts between the servo motor and the screw is a key measure to prevent such problems. Meanwhile, anti-loose nuts or thread locking agents can be used to enhance the reliability of the connection.
Wear of Ball Screw Bearings or Nuts: The ball screw is an important component for realizing precise movement in the machining center, and the wear of its bearings or nuts will affect the transmission accuracy of the screw. As the wear intensifies, the clearance of the screw will gradually increase, causing the tool to move erratically during the movement process. For example, during axial cutting, the wear of the screw nut will make the positioning of the tool in the axial direction inaccurate, resulting in dimensional errors in the length of the machined part. To reduce this wear, good lubrication of the screw should be ensured, and the lubricating grease should be regularly replaced. Meanwhile, regular precision detection of the ball screw should be carried out, and when the wear exceeds the allowable range, the bearings or nuts should be replaced in a timely manner.
Insufficient Lubrication between the Screw and the Nut: Insufficient lubrication will increase the friction between the screw and the nut, not only accelerating the wear of components but also causing uneven movement resistance and affecting the machining accuracy. During the machining process, a crawling phenomenon may occur, that is, the tool will have intermittent pauses and jumps when moving at a low speed, making the machined surface quality worse and the dimensional accuracy difficult to guarantee. According to the machine tool’s operation manual, the lubricating grease or lubricating oil should be regularly checked and supplemented to ensure that the screw and the nut are in a good lubrication state. Meanwhile, high-performance lubricating products can be selected to improve the lubrication effect and reduce friction.
Electrical Aspects
Servo Motor Failure: The failure of the servo motor will directly affect the motion control of the tool. For example, a short circuit or open circuit of the motor winding will cause the motor to be unable to work normally or have an unstable output torque, making the tool unable to move according to the predetermined trajectory and resulting in dimensional errors. In addition, the encoder failure of the motor will affect the accuracy of the position feedback signal, causing the machine tool control system to be unable to precisely control the position of the tool. Regular maintenance of the servo motor should be carried out, including checking the electrical parameters of the motor, cleaning the motor’s cooling fan, and detecting the working state of the encoder, etc., to timely discover and eliminate potential fault hazards.
Servo Motor Failure: The failure of the servo motor will directly affect the motion control of the tool. For example, a short circuit or open circuit of the motor winding will cause the motor to be unable to work normally or have an unstable output torque, making the tool unable to move according to the predetermined trajectory and resulting in dimensional errors. In addition, the encoder failure of the motor will affect the accuracy of the position feedback signal, causing the machine tool control system to be unable to precisely control the position of the tool. Regular maintenance of the servo motor should be carried out, including checking the electrical parameters of the motor, cleaning the motor’s cooling fan, and detecting the working state of the encoder, etc., to timely discover and eliminate potential fault hazards.
Dirt Inside the Grating Scale: The grating scale is an important sensor used in the machining center to measure the position and movement displacement of the tool. If there is dirt inside the grating scale, it will affect the accuracy of the grating scale’s readings, thus making the machine tool control system receive incorrect position information and resulting in machining dimensional deviations. For example, when machining high-precision hole systems, due to the error of the grating scale, the position accuracy of the holes may exceed the tolerance. Regular cleaning and maintenance of the grating scale should be carried out, using special cleaning tools and cleaners, and following the correct operation procedures to avoid damaging the grating scale.
Servo Amplifier Failure: The function of the servo amplifier is to amplify the command signal issued by the control system and then drive the servo motor to work. When the servo amplifier fails, such as when the power tube is damaged or the amplification factor is abnormal, it will make the servo motor run unstably, affecting the machining accuracy. For example, it may cause the motor speed to fluctuate, making the feed rate of the tool during the cutting process uneven, increasing the surface roughness of the machined part, and decreasing the dimensional accuracy. A perfect machine tool electrical fault detection and repair mechanism should be established, and professional electrical repair personnel should be equipped to timely diagnose and repair faults of electrical components such as the servo amplifier.
IV. Conclusion
There are numerous factors affecting the machining dimensional accuracy of machining centers. Avoidable factors such as machining processes, numerical calculations in programming, cutting elements, and tool setting can be effectively controlled by optimizing process schemes, improving programming levels, reasonably selecting cutting parameters, and accurately setting tools. Irresistible factors such as workpiece cooling deformation and the stability of the machine tool itself, although difficult to completely eliminate, can be reduced in their impact on machining accuracy by using reasonable process measures such as the use of coolant, regular maintenance and fault detection and repair of the machine tool. In the actual production process, operators and technical managers of machining centers should fully understand these influencing factors and take targeted measures for prevention and control to continuously improve the machining dimensional accuracy of machining centers, ensure that product quality meets requirements, and enhance the market competitiveness of enterprises.
There are numerous factors affecting the machining dimensional accuracy of machining centers. Avoidable factors such as machining processes, numerical calculations in programming, cutting elements, and tool setting can be effectively controlled by optimizing process schemes, improving programming levels, reasonably selecting cutting parameters, and accurately setting tools. Irresistible factors such as workpiece cooling deformation and the stability of the machine tool itself, although difficult to completely eliminate, can be reduced in their impact on machining accuracy by using reasonable process measures such as the use of coolant, regular maintenance and fault detection and repair of the machine tool. In the actual production process, operators and technical managers of machining centers should fully understand these influencing factors and take targeted measures for prevention and control to continuously improve the machining dimensional accuracy of machining centers, ensure that product quality meets requirements, and enhance the market competitiveness of enterprises.