Introduction

Multi-process processing is a complex and vital process. It is like a relay race in industrial manufacturing. Each step needs to be accurately connected to ensure the high quality of the final product. However, each workpiece clamping and process conversion will bring position errors, which, if not effectively controlled, will affect the accuracy of the product. Pitch error contributes to overall positioning inaccuracies, particularly when measuring dynamic performance during motion.

The alignment and quality of the machine structure are critical for maintaining positioning accuracy. Inaccuracies in the machine structure can lead to significant errors in the final product. This is especially important in demanding components such as aerospace and automotive manufacturing. The precision and capabilities of advanced machine tools, especially five-axis machine tools, play a crucial role in maintaining this accuracy. This paper will discuss the common positioning accuracy problems in multi-step machining and propose feasible solutions.

Influence of position measurement on accuracy in 5-axis machining

Productivity and accuracy play a significant role in attracting new machines in the machining industries. Five-axis CNC machined components significantly increase productivity. They are often able to provide greater metal extraction than 3-axis machined components. Production times could be considerably reduced because of the time needed for reconfigured machines, for example, and by multioperation machines in a single setup. As machining becomes harder, five-axis machining becomes a crucial element.

1. Accuracy of tool path

Description: In five-axis machining, the accuracy of the tool path is crucial because the tool moves not only on the XYZ axis but also on the rotary axes (A and B axes). Precise tool paths ensure the geometry and surface quality of the workpiece.

Impact: If the position measurement system is inaccurate, the tool path will deviate from the predetermined route, resulting in corrugated workpiece surfaces, uneven defects, and even failure to meet the design requirements. Maintaining the tool center point (TCP) ensures precision during different cutter inclinations.

2. Workpiece alignment and positioning

Description: Five-axis machining usually requires multiple machining of the workpiece at different angles and positions, so accurate alignment and positioning of the workpiece, including the two rotary axes, is critical.

Impact: The error will lead to the actual position of the workpiece during each processing being inconsistent with the predetermined position, especially in repeated clamping. If the positioning accuracy is not high, errors will accumulate, ultimately affecting the workpiece’s overall size and shape accuracy.

3. Error Compensation

Description: The position measurement system on the five-axis machine tool can monitor and provide feedback on the position error in real time so that the machine can compensate for the error.

Impact: To maintain machining accuracy, the high-precision position measurement system can effectively detect and compensate for errors caused by temperature changes, machine tool wear, and other factors. For example, by measuring and compensating the tool’s position in real time, dimensional deviations caused by thermal expansion can be significantly reduced.

4. Dynamic response and stability

Description: Five-axis machining usually involves high-speed and high-precision movement, which requires high dynamic response and stability of the machine tool.

Impact: The high-precision position measurement system can provide fast and accurate position information to maintain stability during high-speed movement and avoid machining defects caused by dynamic errors. For example, improving dynamic response and stability can improve the surface quality and consistency of aerospace parts.

5. Inspection of tools and workpieces

Description: Use high-precision probes and sensors to detect the position of the tool and workpiece, ensuring that each step of the machining is carried out precisely.

Impact: For example, in a five-axis machining center, using the probe to detect the actual position and size of the workpiece and compare it with the design size can detect and correct errors in time to avoid subsequent processing problems caused by inaccurate initial positioning.

Positioning accuracy problem in multi-process processing

Repeated positioning accuracy challenges

Definition: Repeated positioning accuracy refers to the workpiece’s ability to return to the same point each time it is reclamped. It is essential for high-precision machining.

Impact: If repeated positioning accuracy is poor, minor errors in each clamping will accumulate at each machining step, eventually leading to severe dimensional deviations. For example, even micrometer errors can lead to assembly difficulties or functional failures when manufacturing a complex aerospace part.

1. Accumulation of position errors

Definition: Machining error refers to the size and shape deviation caused by position error due to inaccurate workpiece positioning or insufficient machine accuracy. These errors will gradually accumulate in multi-process processing.

Impact: The accumulation of errors will affect the accuracy of the final workpiece, resulting in the product not meeting the design requirements and affecting its performance and reliability. For example, in the processing of automobile parts, the accumulation of errors may lead to the correct installation of parts, affecting the safety of the entire vehicle.

Causes of positioning error

Workpiece clamping is unstable.

1. Fixture design: The reasonableness of the fixture’s design directly affects the workpiece’s positioning accuracy. An unstable fixture can cause the workpiece to move or deform during machining.

2. Clamping method: If the workpiece clamping is not firm or the clamping force is not uniform, it will lead to inaccurate positioning. Accurate positioning is crucial, especially for linear axes in CNC machining processes, as it helps mitigate potential errors due to dynamic movement and mechanical transmission limitations. This situation is pronounced in workpieces with complex geometry prone to deviation.

Machine tool accuracy problem

1. Guide wear: The guide rail and slider of the machine tool will be worn due to long-term use, resulting in a decline in motion accuracy. Guide wear will affect the machine tool’s repeated positioning ability and machining accuracy. Additionally, pitch error can contribute to overall positioning inaccuracies, particularly when measuring dynamic performance relative to a user’s point of interest. This includes the relationship between pitch error, offset distances, and resultant Abbe error in the positioning of stages, emphasizing the importance of correcting these errors for improved accuracy. The alignment and quality of the machine structure are critical, as inaccuracies in the machine structure can lead to significant errors in the final product and exacerbate guide wear.

2. Spindle accuracy: The spindle’s vibration and axial and radial clearance can affect the stability of the cutting process, resulting in dimensional errors in the workpiece.

3. Temperature change: The temperature change of the machine can lead to thermal expansion, which in turn affects the machining accuracy. In high-precision machining, it is essential to control the working temperature of the machine tool. Additionally, thermal changes can impact the position acquisition on linear axes, leading to positioning errors and affecting the overall performance of the linear axis.

Workpiece material deformation

1. Thermal deformation: During machining, the heat generated by cutting causes thermal deformation of the workpiece material. This deformation may be significant, especially during long periods of high-load processing. Additionally, motion errors can arise from mechanical inaccuracies, particularly in five-axis machining, affecting position measurement and, thus, the workpiece quality.

2. Cutting force: The force generated during the cutting process causes the workpiece to deform elastically and affects the machining accuracy.

Positioning system problem and error compensation

1. Dowel wear: The wear of dowel pins and dowel blocks can result in the inaccurate alignment of the workpiece during machining. This wear and tear usually occurs gradually and is challenging to detect.

2. Positioning reference problem: the selection of positioning reference is crucial to positioning accuracy. A precise measuring device is essential for accurately determining positional errors in linear axes. Improper reference selection will result in inconsistent workpiece positioning between the various processes.

Process design for precise machining

1. Reference selection: Selecting an inappropriate reference in the process design will affect the final positioning accuracy. For example, choosing an unstable or deformable benchmark can lead to positioning errors. Techniques such as Dynamic Error Compensation can adjust the motion profile in real time, correcting positioning inaccuracies and enhancing overall system performance.

2. Process arrangement: Unreasonable process arrangement will gradually accumulate errors and affect the accuracy of the final workpiece.

Operator skill

1. Clamping technology: Differences in the technical level and experience of the operator during the clamping process may lead to inaccurate positioning of the workpiece, which is crucial for precise machining. Accurate tool positioning and feed drives are essential to achieving high precision in machining, ensuring accurate seam intersections, and maintaining consistent quality in the finished workpiece.

Machine tool operation: The operator’s operating skills on the machine tool will also affect the machining accuracy. Skilled operators can better adjust machine parameters and control the machining process.

The detailed method to solve the problem of positioning accuracy

1. Optimize fixture design

Lifting fixture rigidity

• Design considerations: The fixture design should use highly rigid materials and structures to reduce the fixture’s deformation during processing. Finite element analysis (FEA) can also predict the fixture’s stress and deformation and optimize the design scheme.

• Example: For complex aerospace parts processing, custom multi-point clamps can be used. These clamps can grip the workpiece at multiple contact points, ensuring the workpiece’s stability during processing.

Multi-point clamping technology

• Technical details: Multi-point clamping technology evenly distributes the clamping force through multiple clamping points, reducing local deformation. A pneumatic or hydraulic clamp can more precisely control the clamping force.

• Example: When machining automobile parts, using multi-point fixtures can significantly improve the stability of the workpiece, thus reducing processing errors.

2. Maintain and calibrate the machine regularly

Periodic calibration

• Maintenance Plan: Develop a detailed maintenance and calibration plan for the machine, including regular inspection of the guide, spindle, and servo system. Calibrate machine accuracy using a high-precision laser interferometer to ensure that the accuracy of all moving axes and tables is within permitted tolerances.

• Example: The machine tool’s accuracy is crucial in medical device production, and regular calibration with a laser interferometer ensures that each part meets strict quality standards.

Environmental control

• Control factors: Control the working environment of the machine, including temperature, humidity, and vibration. The air conditioning system keeps the temperature of the machine room stable, and the vibration absorption device reduces the influence of vibration on machining accuracy.

• Example: In semiconductor manufacturing, precise environmental controls ensure that tiny workpiece sizes are kept within micrometer tolerances.

3. Control workpiece deformation

Reasonable cutting parameter

• Parameter adjustment: Select the appropriate cutting speed, feed rate, and cutting depth to reduce the heat and cutting force in the cutting process and the risk of workpiece deformation. Use coolant to effectively lower the temperature of the cutting area.

• Example: When machining superalloy materials, adjusting cutting parameters and using efficient coolant can reduce workpiece deformation due to thermal expansion.

Preprocessed job

• Processing method: Preheat or pretreat the workpiece before processing to reduce deformation during processing. Preheating allows the workpiece material to maintain a stable temperature during processing, thereby reducing thermal deformation.

• Example: Preheating the workpiece can significantly improve machining accuracy in processing aluminum alloy parts, especially in high-load cutting.

4. Use a high-precision positioning system

Select quality components

• Component selection: Use high-precision positioning pins, rotary encoders, and linear encoders to ensure the positioning accuracy of each axis of motion. High-resolution encoders can provide more precise position information.

• Example: In high-precision mold processing, a high-precision rotary encoder can ensure the positioning accuracy of each processing step, thereby improving the mold’s quality.

Real-time monitoring and adjustment

• System integration: Integration of an advanced real-time monitoring system, real-time monitoring of location data, and adjustment through the feedback control system. This can promptly correct any positioning errors during processing.

• Example: In aerospace parts processing, real-time monitoring systems can adjust the tool position automatically to ensure that each part meets strict design standards.

5. Improve the processing technology

Reasonable selection basis

• Benchmark optimization: To ensure consistent positioning of each process, select a stable and easy-to-locate benchmark for processing. The datum selection should be determined according to the geometry of the workpiece and processing requirements.

• Example: In complex mold processing, selecting the appropriate reference can reduce the positioning error and improve the mold’s accuracy and consistency.

Optimized process flow

• Process arrangement: Reasonably arrange processing processes to minimize positioning errors between processes. Ensure that each process can be accurately positioned based on the previous method.

• Example: In producing electronic equipment, optimizing the process flow can reduce the accumulation of errors between processes, ensuring the final product’s high precision and high quality.

6. Train operators

Skill upgrading

• Training Program: This program provides systematic training to operators, including machine tool operation, fixture use, and process optimization. The operators’ skill level is regularly assessed to ensure they can handle complex machining tasks.

• Example: Operator training can reduce operational errors and improve processing accuracy and production efficiency in producing high-precision medical devices.

Standardized operation

• Operating specifications: Develop and implement standardized operating procedures and inspection specifications to ensure the consistency and reliability of the processing process. Use standardized work instructions and checklists.

• Example: Standardized operation processes can improve the consistency of production, reduce human error, and improve overall product quality in the production of auto parts.

Conclusion

The problem of positioning accuracy in multi-process processing is undoubtedly a significant challenge facing the manufacturing industry. However, these challenges drive us to innovate and strive for precision and perfect artistry. Solving the positioning accuracy problem requires a technical breakthrough, a deep understanding of the manufacturing process, and the persistent pursuit of quality.

Imagine a part being passed from one machine to another in a complex process, each step down to the micron level and finally in perfect shape. This is a manufacturing process and a dance of technology and intelligence. Every detail and step in this dance contains the wisdom of engineers’ and artisans’ ingenuity.

Our company focuses on injection molds and precision machining and is committed to providing customers with the highest quality products. We know that positioning accuracy is the key to achieving high quality. Therefore, we use the most advanced measurement and calibration equipment and ensure that every product meets the strict accuracy requirements through continuous optimization of the process flow and operator training.

Suppose you also have unique ideas for improving the accuracy of manufacturing processes or are looking for solutions. In that case, our company will work with you to explore the infinite possibilities of precision machining. Together, let’s move towards a bright future for manufacturing.