First, CNC machine tools have the following outstanding advantages over traditional machine tools, and these advantages come from the power of the computer contained in the CNC system.

1.1 It can process complex parts such as curves and curved surfaces that cannot be processed by traditional machine tools.

Because the computer has superb computing power, it can instantaneously and accurately calculate the amount of motion that each axis should move instantaneously, so it can be combined into complex curves or surfaces.

1.2 can realize the automation of processing, and it is flexible automation, so the efficiency can be increased by 3 to 7 times compared with the traditional machine tool.

Since the computer has the ability to remember and store, the input program can be remembered and stored, and then automatically executed in the order specified by the program to achieve automation. The CNC machine tool can realize the automation of the other workpiece processing by replacing one program, so that the single piece and the small batch production can be automated, so it is called "flexible automation".

1.3 The precision of the machined parts is high, the size dispersion is small, the assembly is easy, and the "repair" is no longer needed.

1.4 can achieve the concentration of multiple processes, reducing the frequent handling of parts between machine tools.

1.5 The benefits derived from the above five. Such as: reducing the labor intensity of workers, saving labor (one person can take care of multiple machine tools), reducing tooling, shortening the trial cycle and production cycle of new products, and responding quickly to market demand.

These superiorities are unimaginable by the predecessors and are an extremely important breakthrough. In addition, numerical control of machine tools is the basis for enterprise information transformation such as FMC (Flexible Manufacturing Unit), FMS (Flexible Manufacturing System) and CIMS (Computer Integrated Manufacturing System). CNC technology has become the core technology and basic technology of manufacturing automation.

Second, the content of CNC transformation

2.1 Restore the original function, diagnose and recover the faulty part of the machine tool and production line;

2.2 CNC, add digital display device on ordinary machine tool, or add CNC system, transform into NC machine tool and CNC machine tool;

2.3 Refurbishment, in order to improve the accuracy, efficiency and automation, renovate the mechanical and electrical parts, reassemble the mechanical parts and restore the original precision; and update the CNC system that does not meet the production requirements with the latest CNC;

2.4 Technology update or technological innovation, in order to improve performance or grade, or in order to use new technology, new technology, on the original basis to greatly improve the level and grade of renovation.

Third, the characteristics of CNC transformation

3.1 Reduce investment and short delivery time

Compared with the purchase of a new machine tool, it can generally save 60% to 80% of the cost, and the transformation cost is low. Especially for large and special machines, it is especially obvious. Generally, large-scale machine tools are rebuilt, only 1/3 of the cost of new machine tools is purchased, and the delivery time is short. However, in some special cases, such as the production and installation of high-speed spindles and pallet automatic exchange devices, it is too laborious and costly. The cost of retrofitting is often increased by 2 to 3 times. Compared with the purchase of new machine tools, it can only save about 50% of investment.

3.2 Mechanical performance is stable and reliable, limited structure

The basic parts such as the bed and column used are heavy and solid casting components, not the welded components. The modified machine tool has high performance and good quality and can be used as a new equipment for many years. However, due to the limitations of the original mechanical structure, it is not advisable to make a breakthrough transformation.

3.3 Familiar with the equipment, easy to operate and repair

When purchasing a new device, it is not known whether the new device can meet its processing requirements. The transformation is not the case, the machining capacity of the machine tool can be accurately calculated; in addition, due to the use of the machine for many years, the operator has already understood the characteristics of the machine tool, and the training time is short and effective in operation and maintenance. Once the modified machine tool is installed, it can be fully loaded.

3.4 can make full use of existing conditions

You can make full use of existing foundations without having to rebuild the foundation as you would when purchasing new equipment.

3.5 can use the latest control technology

According to the development speed of technological innovation, the automation level and efficiency of production equipment can be improved in time, the quality and grade of equipment can be improved, and the old machine tool can be changed into the current level machine tool.

Fourth, the choice of CNC system

There are three main types of CNC systems. When retrofitting, they should be selected according to specific conditions. 4.1. Open-loop system driven by stepping motor (to meet the needs of most machine tools, the price is cheap.)

The servo drive device of the system is mainly a stepping motor, a power stepping motor, an electro-hydraulic pulse motor and the like. The feed command pulse sent by the numerical control system is controlled by the drive circuit and the power is amplified, so that the stepping motor rotates, and the actuator is driven by the gear pair and the ball screw pair. As long as the number, frequency, and energization sequence of the command pulses are controlled, the amount of displacement, speed, and direction of motion of the actuator movement can be controlled. Such a system does not need to feed back the measured actual position and speed to the input end, so it is called an open loop system. The displacement accuracy of the system is mainly determined by the angular displacement accuracy of the stepping motor, the transmission components of the gear screw and the like. Pitch accuracy, so the system's displacement accuracy is low.

The system has simple structure, convenient debugging and maintenance, reliable work, low cost and easy modification.

4.2. Closed-loop CNC system for asynchronous motor or DC motor drag and grating measurement feedback

4.3, AC/DC servo motor drag, encoder feedback semi-closed loop CNC system

At present, there are many manufacturers of CNC systems, such as SIEMENS and FANUC, and famous domestic companies such as Renhe (Economic and Practical) and Shenyang National Engineering Research Center. When selecting the numerical control system, it is mainly based on the various precisions that the machine tool has to achieve after the NC transformation, the power of the drive motor and the requirements of the user.

V. Discussion on modification of main mechanical components in numerical control transformation

A new CNC machine tool must be designed to achieve: high static and dynamic stiffness; small friction coefficient between the motion pairs, no gaps in the transmission; high power; easy to operate and maintain. The above requirements should be met as much as possible when the machine tool is modified. It can't be considered that the numerical control device is connected with the ordinary machine tool to meet the requirements of the numerical control machine tool, and the main components should be modified accordingly to meet certain design requirements, in order to obtain the intended transformation purpose.

5.1, sliding guide pair

For the CNC lathe, in addition to the guiding precision and processability, the guide rail also has good friction and wear resistance, and reduces the dead zone due to frictional resistance. At the same time, there must be sufficient rigidity and reasonable rail protection and lubrication.

5.2, gear pair

The gears of general machine tools are mainly concentrated in the headstock and gearbox. In order to ensure the transmission accuracy, the accuracy of the gears used on CNC machine tools is higher than that of ordinary machine tools.

5.3, sliding screw and ball screw

Screw drive is directly related to the accuracy of the transmission chain. The selection of the lead screw mainly depends on the precision requirements of the workpiece and the drag torque requirement. In general, the sliding screw should be no less than 6 grades. If the nut clearance is too large, replace the nut. The use of a sliding screw is lower than that of a ball screw, but it is difficult to meet the processing of parts with high precision.

The ball screw has low friction loss and high efficiency, and its transmission efficiency can be above 90%; high precision and long service life; the starting torque and the torque during movement are close to each other, which can reduce the starting torque of the motor. Therefore, it can meet the processing requirements of higher precision parts.

5.4, ​​security protection

Efficient must be based on safety. In the transformation of machine tools, it is necessary to take corresponding measures according to the actual situation, and must not be ignored. The ball screw pair is a precision component. When working, it is necessary to prevent dust, especially chips and hard sand from entering the raceway. An integral iron shield can also be added to the longitudinal lead screw. The end faces of the large carriage that are in contact with the sliding guide rails are to be sealed, and it is absolutely prevented that the hard granular foreign matter enters the sliding surface to damage the guide rail.

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