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.

CNC

CNC technology application

The development of CNC technology is quite rapid, which greatly improves the productivity of mold processing. Among them, 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 operation speed, but the speed itself also involves the improvement of CNC technology in other aspects. Because of the great changes in CNC technology in recent years, it is worth a review of the current application of CNC technology in the mold manufacturing industry.

Block processing time and others Due to the increase in CPU processing speed, and CNC manufacturers applying high-speed CPUs to highly integrated CNC systems, the performance of CNC has been significantly improved. Faster and more sensitive systems achieve more than just higher program processing speed. In fact, a system that can process part machining programs at a fairly 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 The bottleneck of processing speed.

At present, most mold factories realize that high-speed machining requires more than short processing time. In many ways, this situation is very similar to driving a car. Will the fastest car win the race? Even a spectator who watches the car race occasionally knows that in addition to speed, there are many factors that affect the outcome of the race.

First of all, the driver's knowledge of the track is important: he must know where there are sharp turns so that he can slow down appropriately and pass the curve safely and efficiently. In the process of processing molds with high feed speed, the to-be-processed trajectory monitoring technology in CNC can obtain the information of sharp curves in advance. This function plays the same role.

Similarly, 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 a driver is driving around the track, the consistency of the movement, whether he can brake and accelerate skillfully, has a very important impact on the driver's performance on the spot. Similarly, the bell-shaped acceleration/deceleration and to-be-processed track monitoring functions of the CNC system use slow acceleration/deceleration instead of abrupt speed changes to ensure smooth acceleration of the machine tool.

In addition, there are other similarities between the car and the CNC system. The power of the racing engine is similar to the drive 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. The rigidity and strength of the racing car are similar to the strength and rigidity of the machine tool. The CNC's ability to correct certain path errors is very similar to the driver's ability to control the car in the lane.

Another situation that is similar to the current CNC is that those cars that are not the fastest often require technically skilled drivers. In the past, only high-end CNCs could ensure high machining accuracy while cutting at high speed. Nowadays, the functions of the middle and low-end CNC are also likely to complete the work satisfactorily. Although high-end CNC has the best performance that can be obtained at present, there is also the possibility that the low-end CNC you use has the same processing characteristics as the high-end CNC of similar products. In the past, the factor limiting the maximum feed rate for mold processing was CNC, and today it is the mechanical structure of the machine tool. In the case where the machine tool is already at the performance limit, better CNC will not improve the performance.

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 curved surfaces

This technique uses interpolation along the curve instead of using a series of short straight lines to fit the curve. The application of this technology has been quite common. Many CAM software currently used in the mold industry provides an option to generate part programs in NURBS interpolation format. At the same time, the powerful CNC also provides five-axis interpolation and related features. These properties improve the quality of surface finishing, improve the smoothness of the motor operation, increase the cutting speed, and make the part processing program smaller.

2. Smaller command unit

Most CNC systems transmit motion and positioning commands to the machine tool spindle in units of not less than 1 micron. After making full use of the CPU processing power to improve this advantage, the minimum command 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 and the motor can run more smoothly. The smooth operation of the motor allows some machine tools to run at a higher acceleration without increasing the vibration of the bed.

3. Bell curve acceleration/deceleration

Also called S-curve acceleration/deceleration, or crawl control. Compared with the linear acceleration method, this method can make the machine tool get better acceleration effect. Compared with other acceleration methods, it also includes straight-line method and exponential method, and the bell-shaped curve method can obtain smaller positioning error.

4. Track to be processed

This technology has been widely used, and this technology has many performance differences, so that its working mode in low-end control systems can be distinguished from that in high-end control systems. In general, CNC is to preprocess the program through the monitoring of the machining trajectory to ensure that it can obtain more excellent acceleration/deceleration control. According to the performance of different CNCs, the number of program blocks required for the monitoring of the machining path varies 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, in order to meet the processing requirements, at least fifteen to-be-processed track monitoring program blocks are required.

5. Digital servo control

The development of digital servo system is so rapid that most machine tool manufacturers choose this system as the machine tool servo control system. After using this system, CNC can control the servo system in a more timely manner, and CNC control of the machine tool also becomes more precise.

The role of the digital servo system is as follows:

1) The sampling speed of the current loop will be increased, plus the improvement of the current loop control, thereby reducing the motor temperature rise. 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 acceleration of the sampling speed can also increase the gain of the speed loop, which all help to improve the overall performance of the machine tool.

2) Because 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 machining with high-speed feed. The acceleration of CNC operation speed makes the position feedback rate become the bottleneck restricting the operation speed of the machine tool. In the traditional feedback method, as the sampling speed of the external encoder of CNC and electronic equipment changes, the feedback speed is restricted by the type of signal. Using serial feedback, this problem will be well resolved. Even if the machine tool runs at a very high speed, it can achieve precise feedback accuracy.

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. To date, Fanuc has installed at least 1,000 linear motors. Some advanced technologies of GE Fanuc make the linear motor on the machine tool 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 the weight, and greatly improved the cooling efficiency. All these technological advances have made linear motors more powerful when compared to rotating motors: higher acceleration/deceleration rates; more accurate positioning control, higher rigidity; higher reliability; internal dynamic control move.

Additional external features: open CNC system

Machine tools with open CNC systems are developing very quickly. At present, the communication speeds of the available communication systems are relatively high, so various types of open CNC structures appear. Most open systems combine the openness of a standard PC with the functions 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. With the help of other software, other functions can also be added to the open CNC. These properties may be closely related to mold processing, or may have little to do with mold processing. Generally, the open CNC system used in the mold shop has the following commonly used function options:

Low-cost network communication;

Ethernet

Adaptive control function;

Interface for connecting barcode reader, tool serial number reader and/or pallet serial number system;

The function of saving and editing a large number of parts programs;

Collection of stored program control information;

File processing function;

CAD/CAM technology integration and workshop planning;

Universal operation interface.

This last point is extremely important. Because of the increasing demand for CNC machining with simple tooling. In this concept, the most important thing is that different CNCs have the same user interface. In general, operators of different machine tools must be trained separately, because different types of machine tools and machine tools produced by different manufacturers use different CNC interfaces. The open CNC system creates an opportunity for the entire workshop to use the same CNC control interface.

Now, even if the owner of the machine does not understand C language, he can design his own interface for CNC operation. In addition, the controller of the open system allows different machine operation modes to be set according to individual needs. In this way, operators, programmers and maintainers can set 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 operations.

Five-axis machining

In the process of manufacturing complex molds, the application of five-axis machining has become more and more extensive. The use of five-axis machining can reduce the number of tooling or/and machine tools required to process a part. The number of equipment required for the machining process will be minimized, while also reducing the total machining time. The function of CNC is getting stronger and stronger, which allows CNC manufacturers to provide more five-axis features.

What used to be only available in high-end CNCs is now 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. Using the current CNC technology for five-axis machining, the five-axis machining has the following advantages:

Reduce the need for special tools;

Allow the tool offset to be set after completing the part program;

Support 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 for machine tools with different structures, so there is no need to indicate in the program whether the spindle or the workpiece is rotating around the center point. Because this will be solved by CNC parameters.

We can use the example of spherical milling cutter compensation to explain why the five axes are particularly suitable for mold processing. In order to accurately compensate the offset of the spherical milling cutter when the parts and tools rotate around the central axis, the CNC must be able to dynamically adjust the tool compensation in the three directions of X, Y, and Z. To ensure the continuity of the cutting contact of the tool is conducive to improving the quality of finishing.

In addition, the use of five-axis CNC is also manifested 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 center 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 components in the X, Y, and Z directions. In addition, the original tool diameter offsets in the X and Y axis directions are also divided into X, Y, and Z axis components. Since in cutting engineering, the tool can make a feed motion 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 uses rotary and linear axis commands to ensure that the tool moves according to the program. 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 tool offset can be directly input like the three-axis machining, and it can also be explained by the post-program again. Change in tool length. This realization of the motion characteristics of the rotary shaft by rotating the spindle simplifies the post-processing of the tool programming.

Using the same function, the workpiece can be rotated around the central axis, and the machine tool can also obtain rotary motion. The newly developed CNC can dynamically adjust the fixed offset and rotate the coordinate axis to match the movement of the part. The CNC system also plays an important role when the operator uses manual methods to achieve the slow feed of the machine tool. The newly developed CNC system also allows the axis to feed slowly in the direction of the tool vector, and allows the direction of the tool tip vector to be changed without changing the position of the tool tip (see the 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 machining tools. However, with the gradual development of the new five-axis machining function and the acceptance of this function, the true five-axis mold machining machine may be more common.