NC
Numerical control (NC) refers to the use of discrete digital information to control the operation of mechanical devices, which can only be programmed by the operator.
CNC
CNC technology application
The development of CNC technology has been quite rapid, which has greatly improved the productivity of mold processing. The faster processing speed of the CPU is the core of this development. Improvements in the CPU not only increase processing speed, but also influence other aspects of CNC technology. It is precisely because of these significant changes in CNC technology in recent years that it is worthwhile to conduct a review of its current application in the mold manufacturing industry.
Program block processing time and other factors: Due to the increased processing speed of CPUs and the application of high-speed CPUs to highly integrated CNC systems by CNC manufacturers, CNC performance has significantly improved. Faster, more responsive systems achieve more than just higher program processing speeds. In fact, a system capable of processing part machining programs at a fairly high speed may still operate like a slow-speed system during operation because even fully functional CNC systems have potential problems that can become bottlenecks limiting machining speed.
Most mold manufacturers now realize that high-speed machining requires more than just shorter processing times. In many ways, this is similar to driving a race car. Does the fastest race car always win? Even a casual racer knows that many factors besides speed influence the outcome.
First, a driver's understanding of the track is crucial: they must know where the sharp turns are so they can slow down appropriately and navigate the curves safely and efficiently. Similarly, in high-feed-speed die machining, the machining trajectory monitoring technology in CNC machining can pre-detect sharp curves, serving a similar purpose.
Similarly, a driver's responsiveness to the actions of other drivers and unpredictable factors is similar to the frequency of servo feedback in CNC machining. Servo feedback in CNC machining mainly includes position feedback, speed feedback, and current feedback.
When a driver is navigating a racetrack, the fluidity of their movements and their ability to brake and accelerate skillfully have a significant impact on their performance. Similarly, the bell-shaped acceleration/deceleration and machining trajectory monitoring functions of a CNC system use gradual acceleration/deceleration instead of sudden speed changes to ensure smooth acceleration of the machine tool.
Besides these similarities, racing cars and CNC systems share other characteristics. The power of a racing car's engine is analogous to the drive unit and motor of a CNC machine; the weight of a racing car is comparable to the weight of moving parts in a machine tool; and the rigidity and strength of a racing car are similar to the strength and rigidity of a machine tool. The ability of a CNC machine to correct specific path errors is remarkably similar to a driver's ability to keep the car within the lane.
Another situation similar to the current state of CNC is that race cars, which are not the fastest, often require drivers with comprehensive skills. In the past, only high-end CNC machines could maintain high machining accuracy while performing high-speed cutting. Today, mid-range and low-end CNC machines can also perform tasks satisfactorily. While high-end CNC machines offer the best performance currently available, it's possible that a low-end CNC machine you're using might have the same machining characteristics as a high-end CNC machine in its class. In the past, the limiting factor for the maximum feed rate in mold machining was the CNC machine; today, it's the machine tool's mechanical structure. When a machine tool has reached its performance limits, a better CNC machine will not further improve performance.
Inherent characteristics of CNC systems
The following are some basic CNC characteristics in the current mold processing process:
1. Non-uniform rational B-spline (NURBS) interpolation of curves and surfaces
This technique uses curve interpolation instead of a series of short straight lines to fit the curve. Its application is already quite widespread. Many CAM software programs currently used in the mold-making industry offer an option to generate part programs in NURBS interpolation format. Meanwhile, powerful CNC systems offer five-axis interpolation capabilities and related features. These features improve surface finish quality, enhance motor smoothness, increase cutting speed, and reduce part machining programs.
2. Smaller command units
Most CNC systems transmit motion and positioning commands to the machine tool spindle in units no smaller than 1 micrometer. By fully utilizing the processing power of the CPU to enhance this advantage, some CNC systems can even achieve a minimum command unit of 1 nanometer (0.000001 mm). Reducing the command unit by a factor of 1000 results in higher machining accuracy and smoother motor operation. Smoother motor operation allows some machine tools to operate at higher accelerations without increasing bed vibration.
3. Bell curve acceleration/deceleration
Also known as S-curve acceleration/deceleration, or crawling control. Compared to linear acceleration, this method allows the machine tool to achieve better acceleration. Compared to other acceleration methods, including linear and exponential methods, the bell curve method achieves smaller positioning errors.
4. Monitoring of the processing trajectory
This technology is widely used and exhibits numerous performance differences, distinguishing its operation in low-end control systems from that in high-end systems. In general, CNC preprocesses the program through machining trajectory monitoring to ensure superior acceleration/deceleration control. Depending on the performance of different CNC machines, the number of program blocks required for machining trajectory monitoring ranges from two to hundreds, primarily depending on the shortest machining time of the part program and the acceleration/deceleration time constant. Generally, at least fifteen machining trajectory monitoring program blocks are needed to meet machining requirements.
5. Digital Servo Control
Digital servo systems have developed so rapidly that most machine tool manufacturers have chosen them as the servo control system for their machines. Using this system, CNC machines can control the servo system more promptly, and the CNC's control over the machine tool becomes more precise.
The functions of a digital servo system are as follows:
1) Increasing the sampling speed of the current loop, coupled with improvements in current loop control, reduces motor temperature rise. This not only extends motor life but also reduces heat transferred to the ball screw, thereby improving screw accuracy. Furthermore, the increased sampling speed improves the gain of the speed loop, all of which contribute to enhancing the overall performance of the machine tool.
2) Because many new CNC machines use high-speed sequences connected to servo loops, they can obtain more operational information about motors and drives through communication links. This improves the maintainability of the machine tool.
3) Continuous position feedback allows for high-precision machining even with high-speed feeds. The increased processing speed of CNC machines has made the position feedback rate a bottleneck limiting machine tool speed. In traditional feedback methods, the feedback speed is constrained by the signal type, as the sampling speed of the CNC and the external encoder of the electronic equipment changes. Serial feedback effectively solves this problem. Even at very high machine tool speeds, precise feedback accuracy can be achieved.
6. Linear motor
In recent years, the performance and popularity of linear motors have significantly improved, leading to their adoption in many machining centers. To date, Fanuc has installed at least 1,000 linear motors. GEFanuc's advanced technologies enable linear motors on machine tools to achieve a maximum output force of 15,500 N and a maximum acceleration of 30 g. Other advanced technologies have reduced the size and weight of machine tools and greatly improved cooling efficiency. All these technological advancements give linear motors a significant advantage over rotary motors: higher acceleration/deceleration rates; more accurate positioning control; higher rigidity; higher reliability; and internal dynamic braking.
External Additional Features: Open CNC System
Machine tools employing open CNC systems are developing rapidly. Currently available communication systems offer high communication speeds, leading to various types of open CNC architectures. The vast majority of open systems combine the openness of a standard PC with the functionality of a traditional CNC. The biggest advantage of this is that even if the machine tool hardware becomes outdated, the performance of an open CNC can still be adapted to current technology and machining requirements. Furthermore, additional functions can be added to an open CNC using other software. These functions can be closely related to mold processing or less so. Typically, open CNC systems used in mold workshops offer the following commonly used function options:
Inexpensive network communication;
Ethernet;
Adaptive control function;
An interface is available for connecting to barcode readers, tool serial number readers, and/or tray serial number systems;
The ability to save and edit programs for a large number of parts;
The collection of stored program control information;
File processing functions;
Integration of CAD/CAM technologies and workshop planning;
A universal user interface.
This last point is extremely important. The demand for easy-to-operate CNC machines is increasing in mold making. The most crucial aspect of this concept is that different CNC machines share the same user interface. Generally, operators of different machine tools must be trained separately because different types of machine tools, and even machine tools from different manufacturers, use different CNC interfaces. Open CNC systems create the opportunity for the entire workshop to use a single CNC control interface.
Now, even machine tool owners without C programming knowledge can design their own interfaces for CNC operation. Furthermore, the open system controller allows for different machine operation modes to be set according to individual needs. This allows operators, programmers, and maintenance personnel to configure settings to their own requirements. During operation, only the specific information they need appears on the screen. This approach reduces unnecessary page displays and helps simplify CNC operation.
Five-axis machining
The application of five-axis machining is becoming increasingly widespread in the manufacture of complex molds. Using five-axis machining reduces the number of tooling and/or machine tools required to machine a part, minimizing the amount of equipment needed for the machining process and reducing overall machining time. The increasing capabilities of CNC machines allow CNC manufacturers to offer more five-axis features.
Features previously only available in high-end CNC machines are now found in mid-range products. For manufacturers who have never used five-axis machining technology before, these features simplify the process. Applying current CNC technology to five-axis machining offers the following advantages:
Reduce the need for specialized tools;
Allows tool offset to be set after the part program is completed;
It supports the design of universal programs, so that the post-processed programs can be used interchangeably between different machine tools;
Improve the quality of finishing;
It can be used on machine tools with different structures, so there is no need to specify in the program whether the spindle or the workpiece is rotating around the center point, as this will be handled by the CNC parameters.
We can use the example of ball end mill compensation to illustrate why five-axis machining is particularly suitable for mold making. As the workpiece and tool rotate around the pivot axis, to accurately compensate for the ball end mill's offset, the CNC must be able to dynamically adjust the tool compensation amount in the X, Y, and Z directions. Ensuring continuous tool contact improves the quality of finishing.
In addition, the applications of five-axis CNC are also reflected in: characteristics related to the rotation of the tool around the spindle, characteristics related to the rotation of the part around the spindle, and characteristics that allow the operator to change the tool vector manually.
When the tool's centerline is used as the axis of rotation, the original tool length offset in the Z-axis direction will be divided into components in the X, Y, and Z directions. Additionally, the original tool diameter offsets in the X and Y axes will also be divided into components in the X, Y, and Z directions. Since the tool can feed along the rotation axis during cutting, all these offsets must be dynamically updated to account for the continuously changing tool orientation.
Another CNC feature known as "tool center point programming" allows programmers to define the tool path and center point speed. The CNC uses commands along the rotary and linear axes to ensure the tool moves according to the program. This feature means that the tool's center point no longer changes with the tool itself. This also means that in five-axis machining, the tool offset can be directly input as in three-axis machining, and changes in tool length can be specified through a post-programming step. This feature, which achieves axis motion by rotating the spindle, simplifies the post-processing of tool programming.
Using the same function, the machine tool can also achieve rotary motion by rotating the workpiece around the central axis. The newly developed CNC can dynamically adjust the fixed offset and rotary axes to match the movement of the part. The CNC system also plays an important role when the operator uses manual mode to achieve slow feed of the machine tool. The newly developed CNC system also allows the axis to feed slowly along the direction of the tool vector, and allows the direction of the tool vector to be changed without a change in the tool tip position (see the illustration above).
These features allow operators to easily utilize the 3+2 programming method currently widely used in the mold industry when using five-axis machining centers. However, as new five-axis machining functions gradually develop and become more accepted, true five-axis mold machining centers may become more common.
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