From the current situation of CNC machine tool use in most enterprises, the selection of cutting parameters is a major problem plaguing CNC machining. Currently, there are no standards for selecting CNC machining cutting parameters, and their selection is greatly influenced by individual technical skills. This is because during the cutting process, factors such as changes in workpiece material hardness, different cutting depths, tool wear, and changes in cutting fluid flow will all cause the actual cutting conditions to deviate from the ideal state. Furthermore, this deviation is not kept at a stable level. To avoid or reduce the impact of this deviation on the tool and the quality of the machined workpiece, CNC programmers often adopt relatively conservative parameter settings in actual work to cope with complex and variable cutting processes. Even in enterprises that use CNC machine tools extensively, there are prominent problems such as low levels of process management and a lack of NC machining process guidance documents and NC cutting parameters for typical parts. Practice has proven that correctly and reasonably selecting cutting parameters and optimizing the cutting process is crucial for ensuring product quality, improving productivity, reducing manufacturing costs, ensuring the safe operation of CNC machine tools, and improving overall efficiency. 1. Technical Challenges of CNC Machining Optimization The role of optimizing NC programs can be summarized as: machining time optimization, machining path optimization, cutting condition optimization, etc. Machining time optimization is one of the most common issues encountered in production. For example, consider milling the part shown in Figure 1 using a φ12mm end mill. Figure 1 shows that the material removal rate differs between the long and short sides during machining. That is, given a specific feed rate, the cutting load on the cutter axis varies. When milling the short side, the material removal rate is high, and the cutting load is large; while when milling the long side, the material removal rate is low, and the cutting load is small. Since the cutter axis can effectively remove material from the short side (assuming a cutting load of 100%), it doesn't utilize its full power when milling the long side, leaving room for improved production efficiency. To improve machining efficiency, programmers can consider maintaining a relatively high cutting load on the cutter axis when milling all four sides. For example, setting the feed rate for milling the short side to 120mm/min and the feed rate for milling the long side to 160mm/min can shorten the milling time. For simple two-dimensional contour machining, to maintain a good level of equal-volume material removal, the feed rate for different line segments can be manually programmed. However, in more complex two-dimensional machining, the workload of programmers in considering how to allocate feed rates is very large. Even if different feed rates are edited separately in the NC program, the actual cutting load of the tool axis will fluctuate greatly. It would be very difficult to manually allocate different feed rates for each cutting program segment in the NC program of a three-dimensional part. The machining process of CNC machine tools is limited by the machining shape and cutting conditions. Thus, at each machining stage, the speed and feed rate set by the program must be used, without enough flexibility to adapt to the dynamic changes during machining. In fact, cutting conditions tend to change dynamically due to the following reasons. These reasons include: (1) The surface of the workpiece is often uneven, whether it is a billet or bar stock, forging or casting. (2) The tool wears gradually during machining. (3) The different materials between workpieces and the uneven hardness of the material within the workpiece produce hard spots and soft spots. (4) The shape and size of the workpiece change. (5) The different cooling effects during machining produce changes in surface hardness. The purpose of studying cutting parameters and optimizing machining tool trajectories is to enable CNC machine tools to achieve the best machining efficiency and to make the product have the best machining quality. Cutting parameters are related to factors such as tool life and machine tool parameters (spindle speed, power, torque). Considering these factors, to ensure machining safety, programmers have no choice but to adopt the most conservative cutting parameters, leading to reduced machining efficiency. Conversely, to shorten machining time, programmers have to set larger machining parameters, which can damage the tool, workpiece, and machine tool. No matter how optimized CNC programs are, they cannot take into account the dynamic changes during machining and are far from meeting the need to adjust cutting parameters in real time according to actual cutting conditions. 2. OMAT Machine Tool Adaptive System Increasing market competition demands drastically reduced costs, requiring producers to reduce unnecessary cost expenditures, including: machining time, machine tool maintenance, upkeep costs, tooling costs, and long lead times. Real-time optimization technology used in adaptive systems is indispensable in solving these economic problems. A highly efficient adaptive control system achieves real-time complete optimization of the machining process based on continuously detected cutting parameters, maximizing the potential of the CNC machine tool and ensuring the highest tool life. In the early 1990s, OMAT began applying adaptive control technology to CNC machining, combining it with its metalworking expert system. The most significant feature of the O-MAT machine tool adaptive system is its ability to achieve real-time optimal feed based on most parameters of a specific spindle and workpiece material. These parameters can be input or obtained from an external tool catalog. The operator does not need to know the specific load limits; the internal expert system has already determined the load limits for each tool. If the OMAT adaptive controllers (OptiMil-XL for milling, OptiTurn-XL for turning, and OptiDrill-XL for drilling) are directly installed on the CNC machine tool, they monitor the cutting conditions in real time and automatically adjust the feed rate of each step to the most suitable value. This ensures a constant cutting load, which is calculated considering variations in cutting conditions, thereby minimizing machining time and maximizing the capabilities of the tool and machine tool. O-MAT's OptiMonitor-XL continuously monitors the cutting process and only activates (stops or issues an alarm) in case of overload. In addition, the 0MAT machine tool adaptive system allows NC programmers to be bolder and set the feed rate as if using a new tool. The feed rate is adjusted accordingly based on the degree of tool wear. Therefore, the system can automatically monitor the tool during machining and apply the monitoring results to the feed. At the same time, the system also allows the operator to replace worn tools in a timely manner, so as not to cause tool breakage or premature tool replacement. The 0MAT machine tool adaptive system can not only detect tool wear, but also protect the tool, reduce scrap and reduce repetitive work time by monitoring the spindle load and adjusting the feed rate accordingly. When a sudden overload occurs during cutting, the alarm system will sound an alarm and automatically stop the machine tool if necessary. 3. OptiMil-XL milling machine function The following uses the OptiMil-XL milling machine as an example to explain the working principle and usage of the OptiMil-XL milling machine. (1) Working principle of the OptiMil-XL milling machine The OptiMil-XL milling machine (hereinafter referred to as the milling machine) stores the cutting parameters for each step. These cutting parameters are identified by a unique machining code. Cutting parameters are grouped by operation. For each tool that will be controlled or monitored by the milling machine, prepare the parameters required for the machining operation. These parameters can be input into the milling machine's memory via the keyboard on the milling machine, corresponding to the NC program, setting new operations or selecting existing operations. Put the milling machine into automatic mode, and then run the NC program on the machine tool. In this mode, the milling machine is ready to receive commands from the NC program to activate its adaptive control function or monitoring characteristics. (2) Main characteristics of the milling machine Figure 2 illustrates the milling process controlled by the milling machine. During the milling process controlled by the milling machine, when the cutting state exceeds the normal range, the cutting feed rate is reduced to below the value set by the machining program, and the machine tool is stopped when necessary to avoid damage to the cutting tool, workpiece and machine tool. When the cutting state allows time to be saved, the milling machine increases the feed rate to be higher than the value set by the machining program. The final result is equivalent to obtaining an optimal average feed rate, which is more efficient than the feed rate set by the machining program. In the case of multi-axis machine tools, the milling machine continuously detects the load on each spindle and then adjusts the feed rate according to the spindle with the largest load. (3) Two operating methods of the milling machine ① Preset method (default method). When using this method, the expert system inside the milling machine uses the user-input operating parameters to calculate the maximum allowable load value for each step of the tool and achieves this load by continuously optimizing the actual feed rate throughout the tool process. ② Training method This method includes two stages: "learning" and "relearning" for special cases, including fixture problems, special tools, using old tools, and special workpiece materials not included in the milling machine's material library. In the learning stage of each step of the tool, the milling machine does not perform adaptive feed rate control. According to the feed rate set by the program, the system only monitors the load change and records the maximum load that can be achieved in this step of the tool. This learned maximum load value is then applied in the "relearning" stage (see Figure 3), so that the feed rate under adaptive control is equal to 100% of the feed rate set by the program at the place where the load is detected to be the largest. As a result, the cutting speed is higher than the feed rate set by the program when the load is relatively light. At the maximum load point, the milling machine will cut at the feed rate set by the program. 4. Experimental conclusions In recent years, we have conducted cutting tests on different parts. The experimental data proves that: in roughing, the amount of workpiece blank removed is large and the precision requirements of the parts are generally relatively low. The optimization effect of using the milling machine is better. In finishing, since the precision requirements of the parts have been taken into account, the machining allowance is relatively uniform and the cutting load does not change much. The optimization effect may not be very obvious. Generally speaking, in roughing and semi-finishing, when the material cutting amount changes, the material hardness changes, or the workpiece surface changes significantly, adaptive control is very effective. Under typical cutting conditions, the machining cycle can be shortened by 10% to 40% depending on the machining situation. When using the milling machine, we believe that the following points should also be noted: (1) First, we must start with the selection of tools. It is recommended to use advanced tools such as carbide tools as much as possible. Because the allowable cutting speed of high-speed steel tools and carbide tools of the same specification is tens of times different, and the spindle speed is also much different. From the perspective of tool life, high-speed steel tools have a relatively low lifespan. It is very difficult to improve the machining efficiency of machine tools by selecting high-speed steel tools. (2) Generally speaking, the parameter values ​​recommended by the tool supplier are an upper limit for selection. Exceeding the recommended parameter values ​​will shorten the tool life and increase the tool cost; if the values ​​are too low, the tool performance will not be fully utilized. (3) The CNC machine tool spindle has power torque characteristics. At a specific speed, the machine tool outputs constant torque, and the output power gradually increases. After reaching that speed, the machine tool outputs constant power. When using a milling machine, once the tool cutting parameters are determined, it is necessary to verify whether the cutting parameters are within the range of power and torque that the machine tool can output. The power torque characteristics of the milling machine output should match the characteristics of the optimized spindle motor to prevent overload.