CNC Knowledge: After working in machines for so many years, I finally understand the difference between CN and CNC!

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(Numerical control, called CNC) refers to the use of discrete digital information to control the operation of machines and other devices, which can only be programmed by the operator himself.

CNC

Application of CNC technology

The development of CNC technology is quite rapid, which has greatly improved the productivity of mold processing. Among them, the processor with faster calculation speed is the core of the development of CNC technology. CPU improvement 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 CNC technology has undergone great changes in recent years that it is worthy of our consideration of the current application of CNC technology in the mold making industry.

Processing time of program blocks and others As processor processing speed increases and CNC manufacturers apply high-speed processors to highly integrated CNC systems, CNC performance has improved significantly. A more responsive and responsive system achieves much more than faster program processing speeds. In fact, a system capable of processing part programs at a relatively high speed may also function as a slow processing system, because even a fully functional CNC system has potential issues that can become speed bottlenecks. treatment.

Today, most mold factories realize that high-speed machining requires more than just the processing time of a short machining program. In many ways, the situation is similar to driving a race car. Does the fastest car always win the race? Even a casual spectator of a car race knows that there are many factors other than speed that influence the outcome of a race.

First of all, the driver’s knowledge of the circuit is important: he must know where the sharp turns are in order to slow down appropriately and negotiate them safely and efficiently. When processing molds at high feed speeds, the trajectory monitoring technology to be processed in the CNC can obtain information about the occurrence of sharp curves in advance, and this function plays the same role.

Likewise, the responsiveness of a driver to other driver movements and uncertainties is similar to the amount of servo feedback in a CNC. Servo feedback in CNC mainly includes position feedback, speed feedback and current feedback.

When a driver rides on the track, the consistency of his movements and his ability to skillfully brake and accelerate have a very significant impact on the driver’s on-the-spot performance. Likewise, the bell-shaped acceleration/deceleration and path monitoring functions to process of the CNC system use slow acceleration/deceleration instead of abrupt speed changes to ensure smooth acceleration of the machine tool.

Additionally, there are other similarities between racing cars and CNC systems. The power of the racing motor is similar to that of the drive device and the CNC motor. The weight of the racing car is comparable to the weight of the moving components of the machine tool. The stiffness and strength of the racing car are similar. rigidity of the machine tool. The CNC’s ability to correct trajectory-specific errors is very similar to a driver’s ability to keep a car in its lane.

Another situation similar to today’s CNC is that racing cars that are not the fastest often require drivers with all-round skills. In the past, only high-end CNCs could guarantee high machining precision while cutting at high speed. Today, mid- and low-end CNCs have the capabilities to get the job done satisfactorily. Although the high-end CNC offers the best performance currently available, it is also possible that the low-end CNC you are using has the same processing characteristics as the high-end CNC of similar products. In the past, the factor that limited the maximum feed rate for mold processing was CNC, but today it is the mechanical structure of the machine tool. When the machine tool is already at its performance limit, a better CNC will not improve performance further.

Intrinsic characteristics of CNC systems

Here are some basic CNC features in today’s mold processing process:

1. Non-uniform rational B-spline (NURBS) interpolation of curved surfaces

This technology uses interpolation along a curve, rather than using a series of short straight lines to fit the curve. The application of this technology has become quite common. Many CAM software programs currently used in the mold industry offer 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 improve the quality of surface finishes, improve smoother motor operation, increase cutting speeds, and enable smaller part programs.

2. Smaller instruction unit

Most CNC systems transmit movement and positioning instructions to the machine tool spindle in units of at least 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 (0.000001 mm). After the control unit is reduced by 1,000 times, higher processing precision can be achieved and the motor can run more smoothly. Smooth motor operation allows some machine tools to operate at higher accelerations without increasing bed vibration.

3. Acceleration/deceleration of the bell curve

Also called S-curve acceleration/deceleration or crawl control. Compared with the linear acceleration method, this method can achieve a better acceleration effect of the machine tool. Compared to other acceleration methods, including linear and exponential methods, the bell curve method can generate lower positioning errors.

4. Monitoring of leads to be processed

This technology is widely used and has many performance differences that differentiate its operation in low-end control systems from that in high-end control systems. Generally speaking, the CNC implements program preprocessing via monitoring of the machining path to ensure better control of acceleration/deceleration. Depending on the performance of the different CNCs, the number of program blocks required to monitor the trajectory to be processed varies from two to several hundred, which mainly depends on the minimum processing time of the part program and the acceleration/acceleration time constant. deceleration. Generally speaking, to meet the processing requirements, at least fifteen trajectory tracking program blocks are required to be processed.

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 faster, and the CNC’s control of the machine tool also becomes more precise.

The functions of the digital servo system are as follows:

1) Current loop sampling speed will be increased, coupled with improved current loop control, thereby reducing motor heating. 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 precision of the screw. Additionally, increasing the sampling speed can also increase the speed loop gain, which helps improve the overall performance of the machine tool.

2) Since many new CNCs use high-speed sequences to connect to servo loops, the CNC can obtain more working information about the motor and drive device through the communication link. This improves the maintenance performance of the machine tool.

3) Continuous position feedback enables high precision machining at high speeds. The acceleration of CNC operating speed makes the position feedback rate a bottleneck limiting the operating 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 limited by the signal type. Thanks to the serial return, this problem will be solved. Accurate feedback accuracy is achieved even when the machine tool is operating at very high speeds.

6. Linear motor

In recent years, the performance and popularity of linear motors have improved significantly, 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 machine tool’s linear motor to have a maximum output force of 15,500 N and a maximum acceleration of 30 g. The application of other advanced technologies has reduced the size and weight of machine tools and significantly improved cooling efficiency. All of these technological advances give linear motors greater advantages over rotary motors: higher acceleration/deceleration rates; more precise positioning control, greater rigidity of the internal dynamic braking movement;

External additional features: open CNC system

Machine tools using open CNC systems are developing very quickly. The communication speeds of currently available communication systems are relatively high, leading to 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 advantage is that even if machine tool hardware becomes obsolete, open CNC still allows its performance to evolve based on existing technology and processing requirements. Other functions can be added to Open CNC using other software. These properties may be closely related to mold processing, or they may have little to do with mold processing. Generally, the open CNC system used in the molding workshop has the following common function options:

Inexpensive online communications;

Ethernet;

Adaptive control function;

Interfaces for barcode readers, tool serial number readers and/or pallet serial number systems;

Ability to save and modify a large number of part programs;

Collection of stored program control information;

File processing function;

Integration of CAD/CAM technology and workshop planning;

Universal control interface.

This last point is extremely important. Because there is an increasing demand for simple-to-use CNCs in mold processing. In this concept, the most important thing is that different CNCs have the same control 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 allow 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 know the C language. In addition, the open system controller allows setting different machine operation modes according to needs individual. This allows operators, programmers and maintenance personnel to configure settings according to their own needs. When in use, only the specific information they need appears on the screen. Adopting this method can reduce the display of unnecessary pages 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. Through five-axis machining, the number of tools and/or 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 processing time machining will also be reduced. CNCs are becoming more and more capable, allowing CNC manufacturers to offer more five-axis functionality.

Functions that were previously only available in high-end CNCs are now also used in mid-range products. For manufacturers who have never used five-axis machining technology, applying 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;

Allows you to define tool offsets once the part program is completed;

Support universal program design so that post-processed programs can be used interchangeably between different machine tools;

Improve the quality of finish;

It can be used for machine tools with different structures, so it is not necessary to indicate in the program whether the spindle or workpiece rotates around the central point. Because this will be solved by the CNC settings.

We can use the example of ball milling cutter compensation to illustrate why the five-axis is particularly suitable for mold processing. In order to accurately compensate for the offset of the ball cutter as the workpiece and tool rotate around the central pivot axis, the CNC must be able to dynamically adjust the amount of tool compensation in the X directions, Y and Z. Ensuring continuity of tool cutting contact points is beneficial to improve finishing quality.

Additionally, five-axis CNC uses include 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 modify the tool vector.

When the tool center axis 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 offset of the tool diameter 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 perform feed movements in the direction of the axis of rotation, all these Offsets must be updated dynamically to account for the constantly changing orientation of the tool.

Another CNC feature called “tool center point programming” allows programmers to set the path and speed of the tool center point. The CNC ensures that the tool moves according to the program through commands in the direction of the rotation axis and the linear axis. This feature prevents the center point of the tool from changing with tool change. This also means that in five-axis machining, the tool offset can be directly input like in three-axis machining, and it can also be explained via. another post-program. Changing the tool length. This feature of rotating the spindle to achieve axis movement simplifies tool programming post-processing.

Using the same function, the machine tool can also achieve rotational motion by rotating the workpiece around a central pivot axis. 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 advance slowly in the direction of the tool vector, and also allows the direction of the tool tip vector to be changed without changing the position of the tool tip (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.