In machine automation machining, the accuracy control technology is based on the multidimensional principle.
In terms of the principle of error compensation, the geometric errors generated in the manufacturing and assembly process of machine tools are accurately calculated and compensated through the establishment of error models, such as polynomial error models.
Usually, in high-precision machining, the positional accuracy compensation can be as accurate as ±0.002 mm.
In terms of tool path planning, according to the machining contour and process requirements, the tool path is planned using the equal residual height method, and the feed is calculated by the formula to ensure that the machined surface roughness reaches the standard of 0.8 to 1.6 μm, and the residual height is calculated by the following formula:
Where: h is the residual height; f is the feed; r is the tool radius.
In terms of the control system, advanced digital control technology is adopted to realize precise control with G code programming, such as G01 X_Y_Z_F_ instruction for linear interpolation movement, which precisely controls the movement trajectory of the tool in three-dimensional space, and the positioning accuracy of each axis can reach ±0.001 mm.
These principles work together to control mechanical automation machining accuracy effectively. They address different aspects to meet the processing needs of various precision parts.
Mechanical automation processing accuracy of the factors affecting
1.Precision of the equipment itself
Equipment as the core carrier of mechanical automation processing, its own accuracy plays a decisive role in processing accuracy.
The geometric accuracy of the machine tool is the basis, such as the spindle rotary accuracy.
If there is radial runout, axial runout and other errors in the rotary process of the spindle, it will lead to deviation in the relative position between the tool and the workpiece.
Turning, for example, when the radial runout of the spindle is ±0.01 mm, the machined outer surface will show roundness error. This error affects the cylindricity, which is a key indicator of shape accuracy.
The transmission accuracy of the screw-nut pair is equally important. The pitch error of the screw will proportionally affect the machining size of the workpiece.
According to the cumulative formula of pitch error, in long-distance transmission, small pitch errors are continuously superimposed. This ultimately causes the workpiece’s length dimension deviation to exceed the allowable range, affecting machining accuracy.
2.Machining process system
The machining process system involves multiple links such as tools, fixtures and workpiece clamping. Among them, tool wear will change the shape and size of its cutting edge.
For example, in milling, when the wear of the back face of the milling cutter reaches 0.3 mm, the cutting force changes, resulting in an increase in the roughness of the machined surface of the workpiece and affecting the contour dimensional accuracy.
The positioning accuracy of the fixture determines the accuracy of the initial position of the workpiece during machining.
If the manufacturing error of the fixture positioning element is ±0.05 mm, the position of the workpiece in the machining coordinate system will deviate after clamping, which will make the positional accuracy of the machined hole system fail to meet the design requirements.
In the process of workpiece clamping, too much clamping force may lead to deformation of the workpiece, such as thin-walled sleeve parts, too much clamping force will cause elastic deformation.
3.Material Characteristics
The characteristics of raw materials are the influencing factors of machining accuracy that cannot be ignored.
(1) Uneven hardness of the material will lead to fluctuation of cutting force.
In the cutting process, when the tool encounters an area of higher hardness, the cutting force increases and the tool gives way, making the machined size smaller;
Conversely, in the area of lower hardness, the cutting force decreases and the machining size becomes larger.
(2) The coefficient of thermal expansion of the material has a significant impact on machining accuracy.
In the grinding process, due to the grinding heat so that the temperature of the workpiece increases, if the workpiece material thermal expansion coefficient of 1.2 × 10-5 ℃ -1, then the temperature rises 50 ℃, the length of 100 mm of the workpiece will be due to thermal expansion elongation of 0.06 mm, the size of the cooling contraction, resulting in dimensional accuracy errors.
4.External environmental factors
External environmental factors mainly include temperature, humidity and vibration.
In terms of temperature, changes in the temperature of the machining shop will cause thermal deformation of the machine tool and workpiece.
When the temperature change of the workshop is ± 5 ℃, the thermal expansion or contraction phenomenon will lead to changes in the geometry of the machine tool guideway, which in turn affects the straightness of the workpiece and other shape accuracy.
The impact of humidity on machining accuracy is mainly reflected in the corrosion of metal materials, humid environments will accelerate the corrosion of tools and workpieces, shorten the life of tools and change the surface quality of the workpiece.
Vibration is an important factor affecting machining accuracy, if the vibration frequency of other equipment or the external environment of the vibration frequency and the inherent frequency of the machining system is close to the resonance will be triggered, thus affecting the machining accuracy.
Application Measures of Accuracy Control Technology in Mechanical Automation Processing
1.Design and implementation of open-loop and closed-loop control systems
In mechanical automation processing, the design of the open-loop control system focuses on the one-way transmission of instructions.
Taking the CNC machine tool as an example, the CNC device sends instruction signals to the drive device according to the pre-set machining program, and the drive device controls the motor according to the signals to drive the table or tool to move.
Open-loop control system structure is relatively simple, low cost, the key to its design is to determine the appropriate control parameters, such as pulse equivalent.
Pulse equivalent refers to the displacement generated by each pulse signal to move machine tool parts. It generally ranges from 0.001 to 0.01 mm. The exact value depends on machining accuracy requirements.
In the implementation process, to ensure that the pulse signal output from the CNC device is stable and accurate.
The closed-loop control system introduces a feedback link. For example, in precision grinding machine processing, a displacement sensor is installed on the machine table. It detects the actual position of the table in real time. This position information is fed back to the control system.
The control system adjusts the output signal according to the deviation of the feedback signal from the preset value.
The common control algorithm is the proportional-integral-derivative control algorithm, in which Kp, Ki, Kd are the proportional, integral and differential coefficients respectively.
Reasonable adjustment of these three coefficients can effectively improve the control accuracy of the system.
In the implementation of closed-loop control system, the accuracy and response speed of the sensor is a key factor, such as high-precision scale measurement accuracy of up to ± 1 μm, can meet the requirements of high-precision processing.
2.Real-time monitoring and adaptive control
Real-time monitoring is crucial in automatic machining.
Taking turning processing as an example, the cutting force sensor can be used to monitor the cutting force in real time.
The size of the cutting force is closely related to tool wear, cutting parameters and other factors, when the cutting force exceeds a certain threshold, indicating that the processing state is abnormal.
The adaptive control strategy is adjusted according to the real-time monitored data.
On the one hand, when tool wear is monitored, the adaptive control system can adjust the cutting parameters. If tool wear increases the cutting force, the system can reduce the feed appropriately to ensure machining accuracy.
On the other hand, for different workpiece materials, the adaptive system can adjust the machining strategy. It does this according to the hardness, toughness, and other characteristics of the material.
3.Application of intelligent optimization algorithm in precision control
In the precision control of mechanical automation processing, genetic algorithm has a unique application.
Genetic algorithm is an optimization algorithm based on biological evolution theory. In tool path optimization, the tool path is represented as a chromosome. Each node on the path acts as a gene.
With the help of defining the fitness function to evaluate the advantages and disadvantages of the tool path, the fitness function can include factors such as machining accuracy, machining time, etc. During the operation of the algorithm, the tool path is continuously optimized through selection, crossover and mutation operations.
Simulated annealing algorithm can also be used for precision control.
Taking the surface roughness control of parts as an example, the surface roughness is taken as the objective function.
The simulated annealing algorithm simulates the annealing process of solid matter to find the global optimal solution in the search space.
During the execution of the algorithm, the inferior solution is accepted according to a certain probability to avoid the algorithm falling into the local optimum.
For example, in grinding processing, the simulated annealing algorithm can optimize grinding parameters. This effectively reduces surface roughness and improves machining accuracy.
4.Integration of CNC Programming and Online Measuring System
The integration of CNC programming and online measurement system can significantly improve the precision of automatic mechanical processing.
In terms of CNC programming, the CNC code developed by the International Organization for Standardization is used.
For example, G code is used to control the movement of the machine tool, and M code is used to control the auxiliary functions of the machine tool.
During the programming process, tool paths, cutting parameters, etc. should be precisely planned.
The online measurement system obtains the dimensional information of the workpiece in real time during the machining process.
Using the coordinate measuring instrument, its measurement accuracy can reach ± 0.001 mm, the data obtained from online measurement is fed back to the CNC programming system, and the CNC programming system corrects the subsequent machining program according to the measurement results.
When milling complex surfaces, the online measurement system measures the dimensional deviation of the machined part. The CNC programming system then adjusts the cutting depth and feed of the tool based on the deviation. This helps improve the machining accuracy.
Mechanical automation machining accuracy control examples
1.Example background and processing requirements
The case focuses on the automated machining of automobile engine block.
As a key component of the engine, the machining accuracy of the automobile engine block directly affects the performance and reliability of the engine. The material of the cylinder block is aluminum alloy, which has strict requirements on dimensional accuracy, shape accuracy and positional accuracy.
In terms of dimensional accuracy, each bore tolerance should be controlled within ±0.03 mm. The cylindricity of the cylinder barrel should be within ±0.002 mm. In terms of shape accuracy, the flatness error should be less than ±0.05 mm.
In terms of positional accuracy, the coaxiality error between the cylinder holes is required to be controlled at ±0.01 mm to ensure smooth piston movement and reduce engine vibration and noise.
2.Accuracy control program design
For the accuracy of the equipment itself, the selection of high-precision CNC machine tools, its spindle rotary accuracy of ± 0.001 mm, screw pitch accuracy error control in ± 0.000 5 mm.
In the machining process system, diamond coated cutting tools are used, and the tool wear is controlled below 0.1 mm to improve cutting stability;
Design of special high-precision fixtures, positioning accuracy of ± 0.005 mm, to prevent deformation of the workpiece clamping.
For the characteristics of raw materials, hardness testing of aluminum alloy materials before machining, screening materials with small hardness deviation.
In the process of machining using coolant to accurately control the temperature of the workpiece, the coefficient of thermal expansion so that the dimensional changes are controlled to ± 0.01 mm.
3.Effect evaluation and experience summary
After actual machining verification, the cylinder bore size deviation is controlled at ±0.02 mm, the cylindricity is ±0.001 5 mm, the flatness error is ±0.04 mm, and the coaxiality error of each cylinder bore is ±0.008 mm, which all meet the design requirements.
The machining results show that the comprehensive consideration of various factors to develop the accuracy control program is effective.
In the subsequent processing, continue to optimize the maintenance cycle of the equipment to ensure stable equipment accuracy, and regularly detect tool wear, timely replacement of tools.
At the same time, strengthen quality control of raw materials. Stabilize the material characteristics. Strictly control environmental parameters. Reduce external interference. Further improve precision of mechanical automation processing. Enhance product quality and production efficiency.
Precision control technology in mechanical automation processing development trend
1.The integration of intelligent manufacturing and Internet of Things technology
(1) With the help of sensor networks, real-time collection of processing equipment operating parameters, such as temperature, vibration, torque, etc., transmitted to the central control system.
For example, a high-precision temperature sensor is installed in the machine tool spindle. It monitors temperature changes in real time. This is done according to the heat conduction model. It helps prevent deviations in machining accuracy caused by thermal deformation.
(2) Utilize industrial Ethernet to realize data interaction among equipments, and improve the accuracy and timeliness of data transmission according to the standard protocol of open platform communication unified architecture.
(3) Construct an intelligent decision-making system to automatically adjust the machining parameters according to the collected data and the preset machining model, and accurately adjust the feed rate and cutting depth.
2.Application of digital twin technology
(1) Construct a virtual model of the physical entity, including the machine structure, tool geometry, workpiece material properties, etc.. Through the finite element analysis model, simulate the stress and strain distribution in the machining process, and predict the workpiece deformation in advance.
(2) Synchronize the real-time data during machining, such as tool position, cutting force, temperature, etc., and map them to the virtual model. According to the data synchronization protocol, keep the virtual and reality highly consistent.
(3) Optimize the machining process using a virtual model. Search for the best combination of machining parameters, such as spindle speed and feed. Use a multi-objective optimization algorithm. This helps improve machining accuracy and surface quality.
3.Precision control innovation under the concept of green manufacturing
(1) Adopt dry cutting or micro-lubrication cutting technology.
Replace the traditional cutting fluid with vegetable oil-based cutting fluid. Ensure lubrication between the tool and the workpiece during cutting. This is done according to the lubrication performance evaluation index of the cutting fluid. It helps maintain machining accuracy.
(2) Optimize the machining process.
Through the establishment of cutting force and energy consumption models, the minimum energy consumption is set as the objective function. The particle swarm optimization algorithm is used to find the optimal cutting parameters. This reduces energy consumption while ensuring precision.
(3) Recycling and reuse of processing waste.
Based on the analysis of material composition, physical and chemical methods are used to separate and purify the waste materials. These methods reduce the cost of raw materials. They also ensure the recycled materials meet processing accuracy standards.
Conclusion
With the integration of intelligent manufacturing and Internet of Things technology, the application of digital twin technology and the depth of the green manufacturing concept, the mechanical automation machining accuracy control technology will realize the development of more intelligent, precise and environmentally friendly, provide strong support for the transformation and upgrading of manufacturing industry, and continuously improve product quality and production efficiency.