Analysis on Numerical Control Transformation of Ordinary Machine Tools

CNC machine tools are increasingly used in the machining industry. The development of CNC machine tools is full-featured and high-performance on the one hand; on the other hand, it is a simple and practical economical CNC machine tool with basic functions of automatic machining, which is convenient for operation and maintenance. The purpose of the transformation of CNC machine tools is to require the machine to be stable and reliable, and to operate at the lowest possible failure rate. The microcomputer control system will be used to comprehensively analyze the C620-1 lathe transformation process.

First, the choice of computer control system

At present, there are many types of numerical control systems on the market. Before choosing, we should first have a full understanding of the functions of the CNC machine tools to be modified. The CNC system can be selected based on the principle of reasonable price, advanced technology and convenient service. This is very important to achieve a higher performance price ratio. The functions of the CNC system should be matched with the functions of the CNC machine tool that is ready to be modified, and the excess CNC function should be minimized. Therefore, the overall configuration plan is as follows:

— The PC uses an industrial PC to meet the control requirements of the control system.

- The motion control card uses a PCL motion control card manufactured by ADVANTECH. The card is a high-speed three-axis stepper motor motion control card with 16-bit digital input and output ports for three-axis linkage.

- The optocoupler circuit is designed by itself. Its function is to isolate the signal interference of the external interference signal to the motion control card and improve the stability of the system.

—The machine body is modified from the C620-1 lathe. The original screw, slide box, gearbox, etc. are removed, and the original three-jaw chuck is retained.

—The stepper motor and its driver are based on Nanjing Stone. The input voltage of the driver is 45V. Considering the step angle of the stepping motor and the pitch of the screw, the pulse equivalent of the X axis of the system is 0.00125, and the pulse equivalent of the Z axis is 0.0025. It is fully capable of meeting the processing accuracy requirements of 0.005mm.

- All kinds of limit switches, deceleration switches and zero return switches are installed on the machine body. The limit switches act as hardware limit. When the lathe machining workpiece exceeds the machining range, the lathe automatically stops machining. The function of the deceleration switch is to turn the deceleration switch on when the lathe tool holder returns to zero and walks to the zero point of the lathe, and informs the lathe to decelerate to the zero position.

Second, the transformation of the mechanical part

In order to give full play to the technical performance of the numerical control system, it is ensured that the modified lathe is repeatedly positioned under the system control, and the microcomputer has the following changes to the mechanical parts without creeping, long service life and beautiful appearance.

1. Spindle system transformation

The original spindle gearbox was removed, using the inverter + belt + spindle + encoder. The spindle speed of the system is also stepped speed regulation, a total of 7 stages, the speed can be from 40r/min to 1200r/min, but the speed of each gear can be realized by setting the inverter as needed. With the five terminals of the inverter, the motor can be used to control the forward, reverse, stop and speed of the motor. This inverter is a Panasonic product, in which the seven-speed frequency is set in A20~A26, and the port should be controlled by the port controller (A1). This system does not use manual mechanical speed control, and all speeds are controlled by the S command in the G code. The software can read the G code and automatically control the spindle motor speed.

2. Feed system design

The feed system should minimize the intermediate drive. The X and Z two-axis feed system of this lathe removes the intermediate transmission link of the original feed system, and directly adopts the transmission scheme of stepping motor + rigid coupling + ball screw. The stepper motor and the ball screw nut can be installed by removing the original screw and adding a small number of mechanical accessories. According to calculations, the torque of the stepper motor is required to be 5N.m. The stepping motor we selected is 86BYG250C-SAFRBC- 0302 of Nanjing Stone Co., with a step angle of 0.9/1.8 and a torque of 7.5N.m. The drive model is SH20806 and the input voltage is 45V.

3. Bed modification

In order to make the modified machine tool have higher starting rate and precision retention, the wear resistance of machine parts and components should be fully considered, especially the wear resistance of machine tool guides.

The old bed was used in the transformation, and the guide rail was made of GCR15 bearing steel to HRC56-62, and fixed on the cast iron bed with screws and adhesive. The working surface of the guide rail before bonding is ground and has a surface roughness of Ra0.8mm to improve the bonding strength.

4. Automatic tool changer

In order to meet the multi-step machining and application of the CNC system T function on one machine, the WZD4 automatic tool holder was finally determined after many investigations and trials. The tool holder is small in size and has high repeatability. It is suitable for powerful turning and is safe and reliable.

Third, efficiency and effectiveness

After the lathe has been modified, it not only maintains the wide range of shifting range of the original machine tool spindle transmission part, but also can be programmed and controlled by the programming language in the machine motion control. Practice has proved that the machine tool is easy to operate, reduces the labor intensity of the operator, and improves the product quality, and the cost of the transformation is only half of the price of purchasing a simple CNC lathe.