NC
(Numerical Control, digital control, referred to as numerical control), 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.
CNC
CNC technology application
The development of CNC technology is quite rapid, which greatly improves the productivity of mold processing, and the CPU with faster operation 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. It is precisely because of such great changes in CNC technology in recent years that it is worthwhile for us to make a review of the current application of CNC technology in the mold manufacturing industry.
picture
Program block processing time and others Due to the increase in CPU processing speed and the application of high-speed CPUs to highly integrated CNC systems by CNC manufacturers, the performance of CNC has been significantly improved. Faster, more responsive systems enable more than just higher program processing speeds. In fact, a system that can process a part program at a fairly high speed may behave like a low-speed processing system because even a fully functioning CNC system has some potential problems that may become a bottleneck that limits the processing speed.
At present, most mold factories realize that high-speed machining requires more than short processing time of machining programs. In many ways, the situation is similar to driving a race car. Does the fastest car always win the race? Even an occasional spectator knows that there are many factors besides speed that affect the outcome of a race.
First of all, the driver's knowledge of the track is very important: he must know where there are sharp turns, so that he can slow down appropriately to pass the corner safely and efficiently. In the process of processing molds at high feed rates, the track monitoring technology to be processed in CNC can obtain the information of sharp curves in advance, and this function plays the same role.
Likewise, the driver's sensitivity to other drivers' actions and uncertainties 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 the driver drives around the track, the consistency of the action, whether the brakes and acceleration can be skillful, etc. have a very important impact on the driver's on-the-spot performance. Likewise, the bell-shaped acceleration/deceleration and track-to-process monitoring functions of the CNC system use slow acceleration/deceleration instead of sudden speed changes to ensure smooth acceleration of the machine tool.
Beyond that, there are other similarities between racing cars and CNC systems. The power of the racing car engine is similar to the driving device and motor of the CNC, the weight of the racing car can be compared with the weight of the moving components in the machine tool, and the rigidity and strength of the racing car are similar to the strength and rigidity of the machine tool. The ability of a CNC to correct for certain path errors is very similar to the ability of a driver to keep a car within the lane.
Another situation similar to the current CNC is that those cars that are not the fastest often require well-rounded drivers. In the past, only high-end CNC could guarantee high machining accuracy while cutting at high speed. Today, mid-range and low-end CNCs have the capability to do the job satisfactorily. While the high-end CNC has the best performance available today, there is also the possibility that the low-end CNC you are using has the same processing characteristics as a high-end CNC in its class. In the past, the factor that limited the maximum feed rate of mold processing was CNC, but today it is the mechanical structure of the machine tool. Where a machine tool is already at its performance limit, a better CNC won't make it any better.
Intrinsic Characteristics of CNC System
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
The technique uses interpolation along a curve rather than a series of short straight lines to fit the curve. The application of this technology is quite common. Many of the CAM software currently in use in the tooling industry offer an option to generate part programs in NURBS interpolation format. At the same time, the powerful CNC also provides five-axis interpolation function and related features. These properties improve the quality of surface finishing, improve the smoothness of motor operation, increase cutting speeds, and enable smaller part programs.
2. Smaller instruction unit
Most CNC systems transmit motion and positioning commands to the machine tool spindle in units of no smaller than 1 micron. After making full use of the advantages of CPU processing power improvement, the minimum instruction unit of some CNC systems can even reach 1 nanometer (0.000001mm). After the command unit is reduced by 1000 times, higher machining accuracy can be obtained, which can make the motor run more smoothly. The smooth running 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 creep control. Compared with using linear acceleration method, this method can make the machine tool get better acceleration effect. Compared with other acceleration methods, including linear and exponential methods, the bell curve method can obtain smaller positioning errors.
4. Track monitoring to be processed
This technique is widely used and has numerous performance differences that differentiate how it works in low-level control systems from how it works in high-level control systems. Generally speaking, CNC realizes the preprocessing of the program by monitoring the processing trajectory, so as to ensure better acceleration/deceleration control. According to the performance of different CNCs, the number of program blocks required for monitoring the trajectory to be processed varies from two to hundreds, which mainly depends on the shortest processing time of the part program and the time constant of acceleration/deceleration. Generally speaking, in order to meet the processing requirements, at least fifteen track monitoring program blocks to be processed are needed.
5. Digital servo control
The development of digital servo system is so rapid that most machine tool manufacturers choose this system as the servo control system of 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 has become more precise.
The role of the digital servo system is as follows:
1) The sampling speed of the current loop will be increased, which together with the improvement of the 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, faster sampling speeds can also increase the gain of the velocity loop, which can help improve the overall performance of the machine tool.
2) Since many new CNCs use high-speed sequences to connect to the servo loop, through the communication link, the CNC can obtain more working information of the motor and drive. This improves the maintainability of the machine tool.
3) Continuous position feedback allows high-precision machining at high feed rates. The acceleration of CNC operation speed makes the speed of position feedback become the 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 resolved. Sophisticated feedback accuracy is achieved even when the machine tool is running at high speeds.
6. Linear motor
In recent years, the working performance and popularity of linear motors have been significantly improved, so many machining centers have adopted this device. So far, Fanuc has installed at least 1,000 linear motors. Some advanced technologies of GE Fanuc 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 of the machine tool, reduced its weight, and greatly improved its cooling efficiency. All these advances in technology give linear motors more advantages than rotary motors: higher acceleration/deceleration rates; more accurate positioning control, higher stiffness; higher reliability; internal dynamic braking.
External additional features: open CNC system
Machine tools using open CNC systems are developing very rapidly. At present, the communication speed of the alternative communication system is relatively high, so there are many types of open CNC structures. Most 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 hardware of the machine tool is outdated, the open CNC still allows its performance to change with existing technology and processing requirements. Additional functionality can also be added to Open CNC with the help of other software. These properties can be closely related to mold processing, or have little to do with mold processing. Typically, an open CNC system used in a mold shop has the following common functional options:
Inexpensive network communication;
Ethernet;
Adaptive control function;
Interfaces available for barcode readers, tool serial number readers and/or pallet serial number systems;
The ability to save and edit a large number of part programs;
Stored program controls the collection of information;
file processing functions;
Integration of CAD/CAM technology and workshop planning;
Common operation interface.
This last point is extremely important. Because the demand for easy-to-operate CNC is increasing in mold processing. In this concept, the most important thing is that different CNCs have the same operator interface. As a general rule, operators of different machine tools must be trained separately because different types of machine tools, as well as machines from different manufacturers, use different CNC interfaces. Open CNC systems create the opportunity to use the same CNC control interface for the entire shop floor.
Now, machine tool owners can design their own interface for CNC operation even if they do not know the C language. In addition, the open system controller allows setting different machine operation modes according to individual needs. In this way, operators, programmers and maintenance personnel can set up according to their own requirements. When in use, only the specific information they need appears on the screen. Using this method can reduce unnecessary page display and help simplify CNC operation.
Five-axis machining
In the process of making 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 reduced to a minimum, and the total machining time is also reduced. CNCs are becoming more powerful, allowing CNC manufacturers to offer more five-axis features.
The functions that only high-end CNCs had in the past are now also used in mid-range products. For those manufacturers who have never used 5-axis machining technology, the application of these features makes 5-axis machining easier. Using the current CNC technology for five-axis machining enables five-axis machining to have the following advantages:
Reduce the need for specialized tools;
It is allowed to set the offset of the tool after completing the part program;
Support the design of general 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 of different structures, so that it is not necessary to specify in the program whether the spindle or the workpiece is rotating around the center point. Because it will be resolved by CNC parameters.
We can use the example of compensation of a spherical milling cutter to illustrate why five-axis is particularly suitable for mold processing. When the parts and tools rotate around the pivot axis, in order to accurately compensate the offset of the spherical milling cutter, the CNC must be able to dynamically adjust the compensation amount of the tool in the three directions of X, Y, and Z. Ensuring the continuity of the cutting point of the tool is conducive to improving the quality of finishing.
Additionally, five-axis CNCs are useful for features associated with rotating a tool about a spindle, features associated with rotating a part about a spindle, and features that allow the operator to manually change the tool vector.
When the central axis of the tool is used as the axis of rotation, the original tool length offset in the Z-axis direction will be divided into three 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 be fed along the axis of rotation, all these offsets must be updated dynamically 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. CNC ensures that the tool moves according to the program through commands in the direction of the rotary axis and linear axis. This feature makes the center point of the tool no longer change with the change of the tool, which also means that in the five-axis machining, the offset of the tool can be directly input like the three-axis machining, and the change of the tool length can also be explained by post-programming again. This kinematics of the rotary axis by rotating the spindle simplifies the post-processing of tool programming.
Using the same function, the machine tool can also obtain rotary motion by rotating the workpiece around the central axis. The newly developed CNC is able to match the movement of the part by dynamically adjusting fixed offsets and rotating coordinate axes. The CNC system also plays an important role when the operator manually implements the slow feed of the machine tool. The newly developed CNC system also allows the axes to be traversed slowly along the direction of the tool vector, and also to change the direction of the tool nose vector without changing the position of the tool nose (see illustration above).
These features enable operators to easily use the 3+2 programming method widely used in the mold industry when using five-axis machining tools. However, as new five-axis machining capabilities are gradually developed and accepted, true five-axis mold making machines may become more common.
