Application of PLC in the CNC Transformation of Lathes

1 Introduction

PLC is widely used in the control of mechanical manufacturing equipment, but in the CNC transformation of ordinary lathes, using PLC as the core component of the CNC system is still a new topic. With the continuous improvement of PLC technology and functions, this will be a development trend. This article discusses this.

2. Design of control principle of PLC numerical control system for lathe

2.1 Lathe operation requirements

Lathes are generally used to process rotating surfaces, threads, etc. The movements required are generally fast forward, working forward, and fast reverse in the X and Z directions. During the processing, they can be switched between automatic, manual, external circle turning, and thread turning; and they can be operated in a single step.

2.2 Problems that need to be solved by PLC numerical control system

The operation process of a lathe is relatively complex, and PLC is generally only suitable for sequential control of actions. To use PLC to control lathe actions, three problems must be solved:

Figure 1 Schematic diagram of CNC system

1) How to generate the signal to drive the servo mechanism and coordinate the X and Z movements;

2) How to change the feed system speed;

3) How to achieve internal transmission and change of thread lead during thread turning.

These problems can be solved by combining PLC and its control module with corresponding actuators.

2.3 Control Principle of CNC System

The CNC transformation of ordinary lathes is to change the tool holder, X and Z feed to CNC control. According to the transformation characteristics, the servo element adopts a stepper motor, and the implementation of an open-loop control system can meet the requirements. The Z-axis pulse equivalent is 0.01mm, and the X-axis pulse equivalent is 0.005mm. A transistor output type PLC is selected. The pulse signal of the driving stepper motor is generated by programming, and the speed is changed by generating pulses of different frequencies through the program. The X and Z-axis movements can be input manually or automatically controlled by the program. The pulse signal of the thread turning is generated by the spindle pulse generator, connected to the PLC input terminal through the AND gate circuit, and the pulse of the required lead is obtained by the PLC program frequency conversion. The tool holder rotation, the turning tool advance and retreat can be controlled by manual or automatic program. Figure 1 is the schematic diagram of the CNC system.

3. Determine the number of PLC input and output (I/O) points

The designed lathe operations are: total stop at the starting point, fast forward in Z and X directions, work forward and fast backward; forward and reverse rotation of the tool rest; manual, automatic, single step and thread turning conversion. Therefore, 14 input points are required. According to Figure 1, 9 output points are required. The I/O connection diagram is shown in Figure 2 (taking Mitsubishi F1S-30MT as an example).

Figure 2 I/O connection diagram

4. Driver program (ladder diagram) design

4.1 Overall program structure design

The selection of manual, automatic, single-step and threading programs is realized by jump instructions. Figure 3 is the overall program structure diagram. If X12 is closed (X13, X14, X15 are disconnected), its normally closed state is disconnected and the manual program is executed; if X12 is disconnected and X13 is fully closed, the program skips the manual program, the pointer goes to P0, and the automatic program is executed.

4.2 Manual program ladder diagram design

Manual program and automatic program need to be designed according to the specific parts. Here we only take the Z-direction fast forward, working forward and fast reverse actions as an example to illustrate. Its ladder diagram is shown in Figure 4.

Figure 4 Z-axis manual program ladder diagram

In the manual program execution state, press X0, Y1 is turned on, and preparations for starting are made. Press X2, and the auxiliary relay M0 is turned on. Through the combination of T63 timing and Y2 contacts, a pulse signal with a frequency of 103/2i is generated (i is the timing time, set as needed, in ms), driving the Z direction to fast forward. When X3 is pressed (M0 is disconnected), M1 is turned on, and M1 is combined with the timer T32 to make Y2 generate a pulse with a frequency of 103/2j (j>i), which is output by Y2 to achieve working forward. When X4 is pressed, M0 and Y3 are turned on at the same time, and the motor quickly reverses to achieve fast reversal. Due to space limitations, other program ladder diagrams are omitted.

5 Conclusion

The application of CNC lathes in China's machinery manufacturing industry is developing rapidly, but high-precision CNC machine tools are expensive, and in actual production, there are a large number of parts with less complex shapes and general precision requirements, which require CNC lathes with general precision to process.
At the same time, there are a large number of ordinary lathes available in China. CNC transformation of these lathes is an effective way to improve production efficiency and benefits with less investment. In the past, the CNC transformation of lathes used Z80 and 8031 ​​chips as the core components of the CNC system, which is more expensive and the system is more complicated. Using PLC as the CNC system of lathes has the advantages of low cost, simple system, and easy adjustment, and will surely be widely used.