NC
(Numerical Control, referred to as CNC) refers to the use of discrete digital information to control the operation of machinery and other devices, which can only be programmed by the operator himself
DNC
Direct Numerical Control System (DNC)
A system that uses a general-purpose computer to directly control and manage a group of CNC machine tools for part processing or assembly
CNC
CNC technology application
CNC technology has developed very rapidly, which has greatly improved the productivity of mold processing. Among them, CPU with faster computing speed is the core of the development of CNC technology. The improvement of CPU is not only the improvement of computing speed, but the speed itself also involves the improvement of CNC technology in other aspects. Precisely because CNC technology has undergone such great changes in recent years, it is worthy of our review of the current application of CNC technology in the mold manufacturing industry.
Block processing time and others As CPU processing speed increases and CNC manufacturers apply high-speed CPUs to highly integrated CNC systems, CNC performance has improved significantly. A more responsive, responsive system achieves more than just higher program processing speeds. In fact, a system that can process part programs at a relatively high speed may also behave like a low-speed processing system during operation, because even a fully functional CNC system has some potential problems that may become limitations. Bottleneck of processing speed.
Currently, most mold factories realize that high-speed machining requires more than just a short processing time. In many ways, the situation is similar to driving a race car. Does the fastest car always win the race? Even an occasional spectator of a car race knows that there are many factors besides speed that influence the outcome of a race.
First of all, the driver's knowledge of the track is important: he must know where the sharp turns are in order to slow down appropriately and negotiate the corners safely and efficiently. In the process of processing molds at high feed speeds, the trajectory monitoring technology to be processed in the CNC can obtain information about the appearance of sharp curves in advance, and this function plays the same role.
Similarly, the driver's sensitivity to other driver's actions and uncertain factors is similar to the number of servo feedbacks in the CNC. Servo feedback in CNC mainly includes position feedback, speed feedback and current feedback.
When a driver drives around the track, the consistency of his movements and whether he can brake and accelerate skillfully have a very important impact on the driver's on-the-spot performance. Similarly, the bell-shaped acceleration/deceleration and to-be-processed trajectory monitoring functions of the CNC system use slow acceleration/deceleration instead of sudden speed changes to ensure smooth acceleration of the machine tool.
In addition, racing cars and CNC systems have other similarities. The power of the racing engine is similar to the CNC drive device and motor. The weight of the racing car is comparable to the weight of the moving components in the machine tool. The stiffness and strength of the racing car are similar to the strength and stiffness of the machine tool. The CNC's ability to correct path-specific errors is very similar to a driver's ability to keep a car in its lane.
Another situation similar to the current CNC is that those racing cars that are not the fastest often require drivers with comprehensive skills. In the past, only high-end CNC could ensure high machining accuracy while cutting at high speed. Today, mid- and low-end CNCs have the capabilities to get the job done satisfactorily. Although high-end CNCs offer the best performance currently available, there is also the possibility that the lower-end CNC you are using has the same machining characteristics as a higher-end CNC in the same category. In the past, the factor that limited the maximum feed speed for mold processing was the CNC, but today it is the mechanical structure of the machine tool. When the machine tool is already at its performance limit, better CNC will not improve the performance any further.
Intrinsic characteristics of the CNC system
The following are some basic CNC characteristics in the current mold processing process:
1. Non-uniform rational B-samples of curved surfaces Bar (NURBS) interpolation
This technology uses interpolation along a curve instead of using a series of short straight lines to fit the curve. 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, the powerful CNC also provides five-axis interpolation functions and related features. These properties increase the quality of surface finishes, improve smoother motor operation, increase cutting speeds, and enable smaller part programs.
2. Smaller instruction units
Most CNC systems transmit movement and positioning instructions to the machine tool spindle in units of no less than 1 micron. After taking full advantage of the improvement in CPU processing power, the smallest instruction unit of some CNC systems can even reach 1 nanometer (0.000001mm). After the command unit is reduced by 1000 times, higher processing accuracy can be obtained and the motor can run more smoothly. The smooth operation of the motor allows some machine tools to run at higher accelerations without increasing bed vibration.
3. Bell curve acceleration/deceleration
Also known as S-curve acceleration/deceleration, or crawling control. Compared with the linear acceleration method, this method can achieve better acceleration effect of the machine tool. Compared with other acceleration methods, including linear and exponential methods, the bell-shaped curve method can achieve smaller positioning errors.
4. To-be-processed trajectory monitoring
This technology has been widely used. This technology has many performance differences, making it work in low-end control systems and in high-end control systems. The way of working can be distinguished. Generally speaking, CNC implements program preprocessing through machining trajectory monitoring to ensure better acceleration/deceleration control. Depending on the performance of different CNCs, the number of program blocks required to monitor the trajectory to be processed ranges from two to hundreds, which mainly depends on the minimum processing time of the part program and the acceleration/deceleration time constant. Generally speaking, in order to meet the processing requirements, at least fifteen trajectory monitoring program blocks to be processed are needed.
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, the CNC can control the servo system in a more timely manner, and the CNC's control of the machine tool also becomes more precise.
The functions of the digital servo system are as follows:
1) It will increase the sampling speed of the current loop, coupled with the improvement of the current loop control, thereby reducing the temperature rise of the motor. In this way, not only can the life of the motor be extended, but the heat transferred to the ball screw can also be reduced, thereby improving the accuracy of the screw. In addition, increasing the 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 servo loops, CNCs can obtain more working information of motors and driving devices through communication links. This improves the maintenance performance of the machine tool.
3) Continuous position feedback allows high-precision machining at high speeds. The acceleration of CNC operation speed makes the position feedback rate become a bottleneck restricting the running speed of machine tools. In the traditional feedback method, as the sampling speed of the external encoder of the CNC and electronic equipment changes, the feedback speed is restricted by the signal type. Using serial feedback, this problem will be well solved. Precise feedback accuracy is achieved even when the machine tool is running at very high speeds.
6. Linear motor
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 of GE Fanuc's advanced technologies enable the linear motor on the machine tool to have a maximum output force of 15,500N and a maximum acceleration of 30g. The application of other advanced technologies has reduced the size and weight of machine tools and greatly improved cooling efficiency. All these technological advances give linear motors greater advantages than rotary motors: higher acceleration/deceleration rates; more accurate positioning control, higher stiffness; higher reliability; internal dynamic braking move.
External additional features: Open CNC system
Machine tools using open CNC systems are developing very rapidly. The communication speeds of currently available communication systems are relatively high, resulting in the emergence of various types of open CNC structures.
Most open systems combine the openness of a standard PC with the functionality of a traditional CNC. The biggest benefit of this is that even if the machine tool hardware becomes obsolete, open CNC still allows its performance to change with existing technology and processing requirements. Other functions can be added to Open CNC with the help of other software. These properties can be closely related to mold processing, or they can have little to do with mold processing. Typically, open CNC systems used in mold shops have the following common function options:
Inexpensive network communication;
Ethernet;
Adaptive control capabilities;
Interfaces for barcode readers, tool serial number readers and/or pallet serial number systems;
Ability to save and edit large numbers of part programs;
Collection of stored program control information;
File processing function;
Integration of CAD/CAM technology and workshop planning;
General purpose operating interface.
The last point is extremely important. Because there is an increasing demand for simple-to-operate CNC in mold processing. In this concept, the most important thing is that different CNCs have the same operating interface. In general, operators of different machine tools must be trained separately because different types of machine tools, as well as machine tools produced by different manufacturers, use different CNC interfaces. Open CNC systems create the opportunity for the entire shop to use the same CNC control interface.
Now, machine tool owners can design their own interface for CNC operations even if they do not understand the C language. In addition, the open system controller allows different machine operation modes to be set according to individual needs. This allows operators, programmers and maintenance personnel to configure settings according to their own requirements. When in use, only the specific information they need appears on the screen. Adopting this method can reduce unnecessary page display and help simplify CNC operations.
Five-axis machining
In the process of manufacturing complex molds, the application of five-axis machining is becoming more and more widespread. Using five-axis machining, the number of tooling or/and machine tools required to process a part can be reduced, the number of equipment required for the machining process will be minimized, and the total machining time will also be reduced. CNCs are becoming more and more powerful, allowing CNC manufacturers to offer more five-axis features.
Functions that were only available in high-end CNC in the past are now also used in mid-range products. For those 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 allows five-axis machining to have the following advantages:
Reduces the need for special tools;
Allows tool settings to be set after completing the part program Offset;
Support the design of universal programs so that post-processed programs can be used interchangeably between different machine tools;
Improve the quality of finishing;
< p> can be used for machine tools with different structures, so that it is not necessary to indicate in the program whether the spindle or the workpiece rotates around the center point. Because this will be solved by the parameters of the CNC.We can use the example of ball milling cutter compensation to illustrate why five-axis is particularly suitable for mold processing. In order to accurately compensate for the offset of the spherical milling cutter when the part and tool rotate around the central pivot axis, the CNC must be able to dynamically adjust the tool's compensation amount in the X, Y, and Z directions. Ensuring the continuity of the cutting contact point of the tool is beneficial to improving the quality of finishing.
In addition, the use of five-axis CNC is also reflected in: features related to rotating the tool around the spindle, features related to rotating the part around the spindle, and features that allow the operator to manually change the tool vector.
When the central 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 X, Y, and Z directions. In addition, the original tool diameter offset in the X and Y axis directions is also divided into three components in the X, Y and Z axis directions. Since in cutting engineering, the tool can make feed movements along the direction of the rotation axis, all these offsets must be dynamically updated to account for the continuously changing tool orientation.
Another feature of CNC called "tool center point programming" allows programmers to define the path and center point speed of the tool. The CNC ensures that the tool follows the program through commands for the direction of the rotation axis and linear axis. sports.
This feature prevents the center point of the tool from changing with the change of the tool. This also means that in five-axis machining, the offset of the tool can be directly input like three-axis machining, and it can also be explained through another post-program. Change in tool length. This feature of rotating the spindle to realize the movement of the axis simplifies the post-processing of tool programming.
Using the same function to rotate the workpiece around the central pivot axis, the machine tool can also obtain rotational motion. The newly developed CNC can dynamically adjust fixed offsets and rotating coordinate axes to match the movement of the part. When operators use manual methods to achieve slow feed of machine tools, the CNC system also plays an important role. The newly developed CNC system also allows the axis to slowly feed in the direction of the tool vector, and also allows the direction of the tool tip vector to be changed without changing the tool tip position (see illustration above).
These features allow operators to easily use the 3+2 programming method currently widely used in the mold industry when using five-axis machine tools. However, as new five-axis machining capabilities are gradually developed and accepted, true five-axis mold processing machines may become more common