NC
(Numerical Control, referred to as CNC), refers to the use of discrete digital information to control the operation of mechanical devices, which can only be programmed by the operator himself
CNC
CNC Technology Applications
CNC technology has developed rapidly, which has greatly improved the productivity of mold processing. Among them, the faster CPU is the core of CNC technology development. The improvement of CPU is not only the improvement of computing speed, but also the speed itself involves the improvement of other aspects of CNC technology. It is precisely because CNC technology has undergone such great changes in recent years that it is worth reviewing the current application of CNC technology in the mold manufacturing industry.
Block processing time and other CNC performance has been significantly improved due to the increase in CPU processing speed and CNC manufacturers applying high-speed CPUs to highly integrated CNC systems. Faster and more sensitive systems achieve more than just higher program processing speeds. In fact, a system that can process part processing programs at a relatively high speed may also be like a low-speed processing system during operation, because even a fully functional CNC system has some potential problems that may become a bottleneck limiting processing speed. Most mold shops now realize that high-speed machining requires more than just short processing times. In many ways, the situation is similar to driving a race car. Does the fastest car win the race? Even a casual race watcher knows that there are many factors other than speed that influence the outcome of the race. First, the driver's knowledge of the track is important: he must know where the sharp turns are so that he can slow down appropriately and pass the corners safely and efficiently. In the process of machining molds at high feed rates, the CNC's pending trajectory monitoring technology can obtain information about the occurrence of sharp curves in advance, which plays a similar role. Similarly, the driver's sensitivity to the actions of other drivers and uncertain factors is similar to the number of servo feedbacks in the CNC. Servo feedback in the CNC mainly includes position feedback, velocity feedback and current feedback. When the driver drives around the track, the consistency of the action, whether he can brake and accelerate skillfully, etc., have a very important impact on the driver's on-the-spot performance. Likewise, the bell-shaped acceleration/deceleration and monitoring of the path to be processed in a CNC system use slow acceleration/deceleration instead of sudden speed changes to ensure smooth acceleration of the machine tool.
Beyond this, there are other similarities between racing cars and CNC systems. The power of a racing car engine is similar to the drive unit and motor of a CNC, the weight of a racing car can be compared to the weight of the moving parts in a machine tool, and the stiffness and strength of a racing car are similar to the strength and stiffness of a machine tool. The ability of a CNC to correct for specific path errors is very similar to the ability of a driver to control the car within the lane.
Another similarity with current CNCs is that those racing cars that are not the fastest often require well-rounded drivers. In the past, only high-end CNCs could achieve high machining accuracy while cutting at high speeds. Today, the functions of mid-range and low-end CNCs may also be able to complete the work satisfactorily. Although high-end CNCs have the best performance currently available, it is also possible that the low-end CNC you use has the same machining characteristics as the high-end CNC in the same product range. In the past, the factor limiting the maximum feed rate of mold processing was the CNC, today it is the mechanical structure of the machine tool. A better CNC will not improve performance when the machine tool is already at its performance limit.
Intrinsic Features of CNC Systems
Here are some of the basic CNC features currently used in mold processing:
1. Non-Uniform Rational B-Spline (NURBS) Interpolation of Curves and Surfaces
This technology uses interpolation along a curve rather than fitting the curve with a series of short straight lines. The application of this technology has become quite common. Many CAM software currently used in the mold industry provide an option to generate part programs in NURBS interpolation format. At the same time, powerful CNCs also provide five-axis interpolation capabilities and related features. These features improve the quality of surface finishing, improve the smoothness of motor operation, increase cutting speeds, and make part processing programs smaller.
2. Smaller Instruction Units
Most CNC systems transmit motion and positioning instructions to the machine tool spindle in units of no less than 1 micron. After taking full advantage of the increase in CPU processing power, the minimum instruction unit of some CNC systems can even reach 1 nanometer (0.000001mm). After the instruction unit is reduced by 1000 times, higher machining accuracy can be achieved, which can make the motor run more smoothly. The smooth operation of the motor allows some machine tools to run at higher acceleration without increasing the vibration of the bed.
3. Bell curve acceleration/deceleration
Also known as S-curve acceleration/deceleration, or creep control. Compared with using linear acceleration, this method can achieve better acceleration effect of machine tools. Compared with other acceleration methods, including linear and exponential methods, the bell curve method can achieve smaller positioning errors.
4. Processing trajectory monitoring
This technology has been widely used and has many performance differences that distinguish the way it works in low-end control systems from that in high-end control systems. In general, CNC uses machining trajectory monitoring to pre-process the program to ensure better acceleration/deceleration control. Depending on the performance of the different CNCs, the number of program blocks required for machining trajectory monitoring ranges from two to hundreds, which mainly depends on the minimum machining time of the part program and the time constant of acceleration/deceleration. Generally speaking, at least fifteen blocks of monitoring programs for the processing trajectory are required to meet the processing requirements.5. Digital servo control The development of digital servo systems is so rapid that most machine tool manufacturers choose this system as the servo control system for machine tools. After using this system, CNC can control the servo system more timely, and CNC control of machine tools becomes more precise.The role of digital servo system is as follows: 1) The sampling speed of the current loop will be increased, and the improvement of current loop control will reduce the temperature rise of the motor. In this way, not only can the life of the motor be extended, but also the heat transferred to the ball screw can be reduced, thereby improving the accuracy of the screw. In addition, the faster sampling speed can also increase the gain of the speed loop, which helps to improve the overall performance of the machine tool.2) Since many new CNCs use high-speed sequences to connect to the servo loop, the CNC can obtain more working information of the motor and drive device through the communication link. This can improve the maintenance performance of the machine tool.3) Continuous position feedback allows high-precision processing at high-speed feed. The faster CNC operation speed makes the rate of position feedback a bottleneck restricting the operation speed of the machine tool. In the traditional feedback method, the feedback speed is restricted by the signal type as the sampling speed of the external encoder of the CNC and electronic equipment changes. With serial feedback, this problem will be well solved. Even if the machine tool runs at a very high speed, precise feedback accuracy can be achieved.6. Linear motors In recent years, the performance and popularity of linear motors have been significantly improved, so many machining centers have adopted this device. To date, Fanuc has installed at least 1,000 linear motors. Some advanced technologies of GE Fanuc have enabled the maximum output force of the linear motors on machine tools to be 15,500N and the maximum acceleration to be 30g. The application of other advanced technologies has reduced the size of the machine tools, reduced the weight, and greatly improved the cooling efficiency. All these technological advances make linear motors more advantageous than rotary motors: higher acceleration/deceleration rates; more accurate positioning control, higher rigidity; higher reliability; internal dynamic braking.External additional features: open CNC system Machine tools using open CNC systems are developing very rapidly. The communication speeds of the currently available communication systems are relatively high, resulting in a variety of open CNC structures. Most open systems combine the openness of a standard PC with the functionality of a traditional CNC. The biggest benefit is that even if the machine hardware is outdated, the open CNC allows its performance to change with current technology and processing requirements. With the help of other software, other functions can be added to the open CNC. These functions can be closely related to mold processing or have little to do with mold processing. Generally, the open CNC system used in the mold shop has the following common function options: low-cost network communication; Ethernet; adaptive control function; interface for connecting bar code readers, tool serial number readers and/or pallet serial number systems; the ability to save and edit a large number of part programs; the collection of stored program control information; file handling functions; CAD/CAM technology integration and workshop planning; common operation interface.The last point is extremely important. Because mold processing has an increasing demand for CNCs that are easy to operate. In this concept, it is most important that different CNCs have the same operation interface. Generally speaking, operators of different machine tools must be trained separately because different types of machine tools and machines produced by different manufacturers use different CNC interfaces. Open CNC systems create the opportunity to use the same CNC control interface throughout the shop. Now, machine owners can design their own interface for CNC operation even if they don't know C language. In addition, open system controllers allow different machine operation settings to be set according to individual needs. This allows operators, programmers and maintenance personnel to set up according to their requirements. When in use, only the specific information they need appears on the screen. This approach can reduce unnecessary page displays and help simplify CNC operation. Five-axis machining Five-axis machining is becoming more and more widely used in the process of manufacturing complex molds. Using five-axis machining, the number of tooling and/or machine tools required to machine a part can be reduced, the number of equipment required for the machining process will be minimized, and the total machining time will be reduced. CNC functions are becoming more and more powerful, which allows CNC manufacturers to offer more five-axis features. Features that were once only available on high-end CNCs are now also used in mid-range products. For manufacturers who have never used five-axis machining technology, the application of these features makes five-axis machining easier. Applying current CNC technology to five-axis machining gives five-axis machining the following advantages: Reduce the need for special tools; Allow tool offsets to be set after the part program is completed; Support the design of universal programs so that post-processed programs can be used interchangeably between different machine tools; Improve the quality of finishing; Can be used for machine tools of different structures, so there is no need to specify in the program whether it is the spindle or the workpiece that rotates around the center point. Because this will be solved by the CNC parameters. We can use the example of compensation for spherical milling cutters to illustrate why five-axis is particularly suitable for mold processing. When the part and the tool rotate around the central axis, in order to accurately compensate for the offset of the spherical milling cutter, the CNC must be able to dynamically adjust the tool compensation amount in the X, Y, and Z directions. Ensuring the continuity of the tool contact point is conducive to improving the quality of finishing. In addition, the uses of five-axis CNC are also manifested in: characteristics related to rotating the tool around the spindle, characteristics related to rotating the part around the spindle, and characteristics that allow the operator to manually change the tool vector. When the center axis of the tool is used as the rotation axis, the original tool length offset in the Z-axis direction will be divided into components in the three directions of X, Y, and Z. In addition, the original tool diameter offset in the X and Y-axis directions is also divided into components in the three directions of X, Y, and Z. Since the tool can feed along the rotation axis in the cutting process, all these offsets must be dynamically updated to account for the continuously changing tool position. Another feature of CNC called "tool center point programming" allows programmers to define the path and center point speed of the tool. CNC ensures that the tool moves according to the program through commands in the rotation axis and linear axis directions. This feature makes the center point of the tool no longer change with the change of the tool, which also means: in five-axis machining, the tool offset can be directly input like three-axis machining, and the change of tool length can be accounted for by post-programming again. This motion characteristic of the axis by rotating the spindle simplifies the post-processing of tool programming. Using the same function, the machine tool can also obtain rotational motion by rotating the workpiece around the central axis. New CNCs can dynamically adjust fixed offsets and rotating axes to match part motion. CNC systems also play an important role when operators manually slow feed the machine. New CNC systems also allow axes to slowly feed in the direction of the tool vector and change the direction of the tool tip vector without changing the tool tip position (see the illustration above). These features make it easy for operators to use the 3+2 programming method currently widely used in the mold and die industry when using five-axis machines. However, as new five-axis machining capabilities are gradually developed and accepted, true five-axis mold and die machines may become more common.
