How do general factories optimize the precision machining proces

Table of Contents

Due to various reasons such as funding, general enterprises can’t be equipped with world-leading equipment and cutting tools. Therefore, they need to start from the process to change the problems of low processing equipment level, weak processing process control ability, and difficult surface quality assurance, which can no longer meet the precision and intelligence needs of modern machinery.

To improve the processing quality and precision of metal parts, it is urgent to apply CNC technology to optimize the process.

This article intends to analyze the concept and principle of CNC technology, analyze the difficulties faced by the current precision processing technology of metal parts, and propose relevant strategies for using CNC technology to achieve process upgrades, to provide a reference for general enterprises to improve the level of mechanical metal parts processing.

Concept and principle of CNC technology

 

CNC technology is an automation technology based on modern electronic computer technology and servo systems, which uses digital equipment to implement programmed control of the movement of machine tools and other processing equipment. Its basic principle is to compile CNC instruction codes containing process information and load them into the control system of CNC equipment. During the processing, the control system interprets and executes these instruction codes line by line according to the pre-defined program, controls the movement posture of the machine tool in real-time, and completes the processing task.

The CNC technology system mainly comprises three parts: CNC equipment, actuators, and feedback devices. CNC equipment is the “brain” of the entire system, in which CNC instruction codes are stored; the actuators are the “limbs”, which receive control instructions from CNC equipment, convert them into mechanical movement, and realize the processing of workpieces; the feedback device plays a “perception” role, and feeds back the actual movement status of the actuator to the CNC equipment to form a closed-loop control to ensure the accuracy of the processing process.

Compared with traditional processing technology, the precision of metal parts processing using CNC technology can be improved by one order of magnitude, and the surface roughness Ra can reach 0.1 μm. This is mainly because the CNC system can realize continuous variable control of machining parameters such as spindle speed and feed rate according to the complex machining program compiled in advance, and dynamically adjust the relative position relationship between the tool and the workpiece.

For example, when turning a linkage surface, the spindle speed can fluctuate according to the sine law and the feed rate can change according to the exponential law through CNC programming. This kind of precision control which is difficult to achieve with traditional technology reliably guarantees the shape and position tolerance and surface quality indicators of the parts.

Another major advantage of the CNC system is the real-time monitoring of the machining process. Modern CNC systems integrate various sensors, which can perform online detection of parameters such as spindle power, nozzle temperature, and vibration acceleration.

Table 1 lists the typical functional modules and functions of CNC systems.

Current status and existing problems of precision machining of metal parts in general factories or enterprises

 

1. Low level of machining equipment

 

The general precision machining industry for metal parts faces the problems of equipment shortage and backward technology. Some enterprises even use machine tools and cutting tools that are still based on the technology level of the 1980s, and the degree of automation is not high. These traditional machine tools have insufficient dynamic and static rigidity, and the radial runout and axial runout of the spindle are both greater than 15 μm. The positioning repeatability of the engraving machine is less than 0.01 mm, which seriously restricts the machining quality.

It is difficult for these backward equipment to achieve precise control of cutting parameters in precision machining. Taking spherical grinding machine machining as an example, the linear speed of the grinding wheel contacting the workpiece surface should be kept constant under ideal conditions, to obtain a uniform distribution of removal.

However, due to the large positioning error and poor transmission rigidity of some low-level linkage mechanisms, the actual linear speed of the grinding wheel fluctuates greatly, resulting in uneven grinding heat-affected zone and corrugated surface defects.

The accuracy of feedback components such as grating rulers and capacitive sensors makes it difficult to reach the micron or even nanometer level, which seriously restricts the effect of position and speed loop control.

The closed-loop bandwidth is also generally less than 100 Hz, making it difficult to achieve high dynamic response. Therefore, when machining complex three-dimensional surfaces, due to the limitations of the trajectory interpolation and control functions of the CNC system, there is a large deviation between the actual motion trajectory of the machine tool and the ideal trajectory, resulting in the inability to guarantee the surface shape and position accuracy.

2. Weak control capability of the machining process

 

In the process of precision machining of metal parts, real-time monitoring and control are crucial to ensure machining quality. However, due to the limitations of sensor measurement and testing technology, it is difficult for most small and medium-sized machining companies to achieve online process detection, which leads to the setting of process parameters relying on experience and blindness, and the quality of machining depends on the experience and skills of the operator.

Even CNC machining faces similar problems. Taking CNC turning machining as an example, closed-loop control of information such as spindle power, vibration acceleration, and tool wear should be achieved under ideal conditions. However, most of the current solutions are simple open-loop control, which sets the cutting speed based on experience and cannot respond to and optimize the real-time status.

Another prominent problem is the low level of automation and imperfect auxiliary facilities. A large number of operations still rely on manual labor, such as measurement, clamping, tool replacement, etc. This not only reduces efficiency but also increases the risk of quality problems caused by human factors. The lack of effective auxiliary systems such as temperature control, cleaning, and dust protection makes the machining process susceptible to changes in workshop environmental conditions.

For example, metal chips and splashing of cutting fluids can contaminate optical components, and dust can enter electrical terminals to produce pitting-like electro-corrosion, which can lead to product quality defects. According to incomplete statistics, more than 50% of machine tool product quality problems are caused by environmental conditions and human factors.

3. Tool performance makes it difficult to meet processing requirements

 

Tool performance is one of the key factors that determine the quality of precision machining of metal parts. Ordinary carbide tools have a high cost-effectiveness, but poor edge sharpness and wear resistance. Taking precision boring processing as an example, the processing length of a single hole of an ordinary carbide boring head is less than 50 mm, and the drilling and grinding ratio is only 100~200;

The drilling and grinding ratio of advanced microcrystalline carbide boring heads and diamond boring heads can reach 300~500, and the processing length of a single hole is greater than 80 mm, which directly affects the quality of deep countersinking of metal parts. Another obvious shortcoming is the large geometric error of the tool. This is mainly due to the limited process control capability of the air flotation method to produce carbide substrates.

It is normal for ordinary carbide drills to have axial runout ≥50 μm and radial runout ≥35 μm, which is difficult to meet the precision machining requirements of IT7-IT8. In contrast, some leading companies use precision pressure control technology to achieve high-precision control of the air float metallurgical process, so that the axial runout of carbide tools can be controlled within 20 μm.

At present, carbide conical spiral drill bits can only achieve a three-slot opening structure at most. Some leading large tool manufacturers have achieved a six-slot opening design, which greatly improves the blade strength of the tool and extends its life.

In addition, the surface of general tools only has simple traditional hard coatings such as TiN and TiCN. These coatings are mainly used to improve wear resistance, and the improvement of welding resistance is not obvious. The newly developed composite gradient jet coating (FGST) technology can achieve significant welding resistance. With the application of FGST-coated tools, the cutting speed of high-speed milling steel can be increased by 2 times. This coating technology is extremely rare in general companies.

4. Surface quality is difficult to guarantee

 

The surface quality of metal parts is difficult to guarantee, mainly reflected in the following three aspects:

First, the background environment has large vibration and noise. Most factory workshops have poor environmental conditions, and equipment operation and road traffic vibration interference are serious. This will be transmitted to the tool contact area through the machine tool foundation and processing system. According to actual measurements, the total value of ground vibration acceleration in ordinary workshops can reach 50~80 m/s2, and the stress excitation during high-speed milling is as high as 100 m/s2 or more, which has seriously exceeded the allowable range of precision processing equipment.

Strong vibration will cause the tool to have contact intermittent, squeeze out periodic dents on the surface of the workpiece, and it impossible to obtain a smooth processing surface. In addition, background noise such as air compressor noise will also cause serious interference with the monitoring of acoustic emission sensors, making processing monitoring ineffective.

Second, chip formation and outflow are blocked. During high-speed and high-efficiency processing, a large number of high-temperature long chips accumulate in the cutting area. These chips that are not discharged in time will pass through the tool again, rubbing deep pits on the surface of the workpiece, especially in spherical grinding.

In addition, relevant studies have found that inappropriate chip blank shapes can also cause intermittent chip splashing, contaminating the machined surface with a good finish.

Third, there are insufficient online quality detection and control methods. At present, many metal-cutting workshops are extremely lacking in online quality detection and control systems, which is in sharp contrast to the digital workshops in developed countries.

In most cases, the measurement of finish, roughness, and geometric dimensional errors is still achieved through offline manual measurement, which is inefficient and unreliable. Offline measurement cannot implement closed-loop control of the machining process, which makes it difficult to detect and eliminate surface defects in time, and quality problems can only be rectified through repeated processing and rework.

Strategy for optimizing precision machining of metal parts using CNC technology

 

1. Use high-end CNC machine tools to improve machining accuracy

 

As an important platform for realizing automated machining, the accuracy level of CNC machine tools directly determines the machining quality of parts. Upgrading CNC machine tools is the primary strategy to improve the level of metal parts machining equipment.

Specifically, the focus should be on increasing the technical research and development and introduction of high-end equipment such as horizontal machining centers, compound machining machines, and five-axis linkage machining centers. These new CNC machine tools are equipped with a fully digital control system, which is an order of magnitude better than traditional machine tools in terms of positioning accuracy, repeatability, dynamic and static stiffness, etc.

For example, the positioning repeatability accuracy of the worktable of the 5-axis horizontal milling machine newly developed by Sharp can reach 2 μm, which meets the requirements of high-speed and high-efficiency machining. In addition, from the perspective of system integration, CNC machine tools have realized the matching optimization design of the spindle drive, feed drive, and control system, effectively reducing the influence of error superposition between components.

In terms of control, the new generation of CNC systems has high-speed and high-precision motion control capabilities. The interpolation cycle of mainstream CNC systems can reach 0.5 ms, and the position loop bandwidth can reach 500 Hz, which can accurately track the spindle speed and position.

This fast-response control system can effectively suppress the coupling vibration between the axes of the machine tool, ensure the dynamic stiffness of the system, and is suitable for high-efficiency processing. In addition, the modeling and interpolation of the workpiece’s starting position and motion trajectory are becoming more and more accurate. For example, the improved polynomial interpolation is used to replace the traditional linear interpolation algorithm to improve the contour trajectory control accuracy by more than 30%.

When processing complex spatial surfaces, this high-precision trajectory control can ensure that the workpiece geometry and surface quality indicators meet the design requirements. The new sensors integrated into the CNC machining system can also provide real-time feedback on the process status to ensure machining stability.

Modern CNC machine tools can be equipped with acoustic emission sensors to monitor the shedding and wear of tool particles, power sensors to monitor cutting resistance, vibration sensors to detect abnormal vibrations, etc. Once the measured value exceeds the preset range, the CNC system will automatically implement corresponding strategies such as limited depth cutting and intermittent cutting to reduce risks. This closed-loop monitoring and control based on multiple sensors reliably ensures the safety and stability of the machining process.

2. Use a CNC system to realize closed-loop control of the machining process

 

As the “brain” of metal cutting machining, the upgrading and transformation of its software and hardware functions is the key to realizing process control and optimization. At present, high-end CNC systems are equipped with powerful process models and multi-sensor data fusion algorithms, which can accurately predict process conditions such as cutting force, chip shape, tool wear, etc., and accurately calculate the optimal cutting parameters on this basis to guide the optimization control of the machining process.

For example, the system can dynamically adjust the spindle speed and feed speed according to the measured real-time cutting resistance value using an adaptive control strategy to keep the cutting force in the target range, avoid overload, and ensure machining stability. Using this closed-loop control technology based on model and sensor feedback, the surface roughness of the part can be reduced from the original Ra3.2 μm to Ra1.6 μm.

In addition, the introduction of additive manufacturing technology provides the possibility for the functional expansion of CNC systems. Modern CNC systems can be equipped with additive equipment for metal and non-metal materials. Based on the calculation and optimization results, laser cladding or additive methods are selected to construct a functional gradient layer on the surface of the workpiece to replace traditional machining.

This integrated manufacturing method can reasonably regulate the surface layer structure and properties, and achieve non-destructive processing and surface enhancement of the main body of the part. For example, laser cladding of WxC or TiC particles significantly improves the surface hardness and wear resistance of aluminum alloy parts. This type of in-situ additive manufacturing technology is expected to solve the problem of difficulty ensuring the surface quality of metal parts, avoiding expensive electroplating or spraying processes.

 

3. Development of high-performance CNC cutting tools

 

As a “tool” in the metal cutting process, the performance of the tool directly affects the processing quality. High-speed and high-efficiency CNC machining places higher requirements on the performance indicators of the tool. Therefore, it is urgent to develop special high-performance tools based on the characteristics of the CNC machining process.

One of the directions worth paying attention to is customized tools, that is, customized design based on objective demand characteristics and subjective habits. The structural dimensions, bottom blade shape, coating material, and other characteristics of this type of tool will be personalized according to the workpiece material, cutting parameter range, and operator usage habits of the machining task.

Such “exclusive customization” can maximize the performance of the tool and improve the efficiency of CNC machining. Compared with standardized batch tools, customized tools can improve the processing efficiency by more than 15%, and do not require the operator’s skill adaptation and experience accumulation.

Another frontier field is thin film cutting tools, the principle of which is to deposit a superhard film on the tool substrate using methods such as physical vapor deposition to obtain ultra-high surface hardness and heat resistance. Typical cutting ceramic films such as aluminum nitride (AlN) and silicon carbide (SiC) can remain chemically inert and exhibit extremely high wear resistance at high temperatures of 1,000°C.

Compared with traditional milling cutters, aluminum nitride film tools increase cutting speed by 200% and extend life by 300% when used for high-speed machining, significantly improving machining efficiency. The use of the above two types of high-performance CNC special cutting tools will greatly promote intelligent manufacturing and transformation and upgrading of the metal cutting industry.

4. Integrated CNC technology to achieve online surface treatment

 

The quality of the workpiece surface is directly related to the performance of the parts and is an important indicator for measuring the processing quality. Therefore, the integration and optimization of surface treatment using CNC technology is an important way to ensure surface quality. Specifically, the CNC system can be used to effectively control online testing and composite processing to ensure surface quality.

For example, laser speckle detection technology is used to obtain parameter information such as surface roughness and residual stress of the workpiece. After these feedback signals are input into the CNC system, the subsequent processing strategy can be dynamically adjusted to eliminate surface defects in a targeted manner. If excessive local residual stress is detected, the CNC system will automatically call rolling or low-stress grinding processes to accurately trim the problem area until the surface residual stress returns to normal.

This online regulation based on sensor feedback reliably optimizes surface quality. Digital closed-loop control not only improves the autonomy of the processing system but also reduces the need for rework and manual intervention. In addition, by programming and controlling the switching of different processes and processing heads, the CNC system can integrate multiple surface treatment functions on one device.

Typical composite technology integration includes:

1) Firstly, high-speed boring or turning is performed to obtain the matrix contour, and then programming control is switched to electrolytic grinding to eliminate surface stress;

2) After obtaining the surface texture by wire cutting or wire rolling, the small-angle flat hob is automatically replaced to achieve micro-nano surface hardening and smoothing;

3) Firstly, DQ precision forging is performed to obtain the basic shape, and then the surface material performance gradient design is achieved by controlling laser cladding.

This integrated processing mode of in-situ process conversion can effectively shorten the production process and improve the refinement of surface quality. It is an important embodiment of digital design and manufacturing concepts in the field of metal processing and is worthy of active promotion and application.

Conclusion

 

In general, the innovative application of CNC technology and systems by general enterprises is an important way to achieve intelligent upgrading of the industry.

Specifically, the use of high-precision CNC machine tools can significantly improve processing accuracy; the monitoring and optimization functions of the CNC system can achieve precise control of the processing process; the development of customized and thin film tools can meet the new needs of CNC machining for tools; integrated laser additives and other new technologies can revolutionize the surface quality problem.

It can be foreseen that with the rapid development of CNC equipment in the manufacturing industry, metal parts processing companies, especially in the field of aviation equipment manufacturing, will achieve an accelerated transition from traditional processing to CNC processing.

This will not only greatly improve product accuracy and performance, but will also greatly promote the intelligent transformation of the industry.

Looking to the future, the innovation and application of CNC technology and systems will still be the strategic focus of enhancing the core competitiveness of enterprises.