Abstract: This paper analyzes and studies the working principle of a plasma cutting machine and optimizes the cutting process. Improving the control precision of the control system to enhance processing accuracy achieves a low-cost, high-return effect. Optical isolation was implemented between the control system and peripheral circuits to reduce external interference and increase system security. A suitable motor drive was selected based on calculations. The optimized design of the control system improved the cutting accuracy and anti-interference performance of the cutting machine. Experiments demonstrate that the cutting accuracy can reach approximately ±0.5mm.
1. Introduction
CNC plasma cutting refers to CNC cutting using CNC flame, plasma, laser, and waterjet cutting machines. It involves fully automated, efficient, high-quality, and high-utilization CNC cutting based on optimized nesting programs provided by CNC cutting software. CNC cutting represents a modern high-tech production method, a product of the combination of advanced optimized nesting calculation technology and computer numerical control (CNC) technology with cutting machinery. Currently, manual and semi-automatic cutting are still relatively common in my country's manufacturing industry. With the continuous progress of my country's modernization, the amount of steel used in the machinery manufacturing industry is constantly increasing, and the requirements for cutting precision and efficiency are also constantly rising. Highly intelligent and highly integrated CNC cutting equipment greatly reduces production costs and resource waste, becoming an inevitable trend in the development of modern manufacturing.
For materials that are difficult to cut using traditional methods, CNC plasma cutting machines can be used. Furthermore, in terms of cutting speed, when cutting thin ordinary carbon steel plates of small thickness, CNC plasma cutting is several times faster than traditional oxygen cutting, while maintaining a smooth cut surface and exhibiting good thermal deformation. The quality of parts cut by CNC plasma cutting machines has a crucial impact on product quality, and scientifically optimizing the cutting path is of great value. Analysis of the causes of thermal deformation in cut parts: While the probability of thermal deformation during CNC plasma cutting is relatively small, residual internal stress is unavoidable during the rolling and cooling of metal sheets. This is because, during the cutting process, due to the influence of localized high-temperature heat sources, the metal sheet expands along the cutting direction. The expanding metal sheet is then constrained by the surrounding mother plate, resulting in significant stress at the cut edge.
CNC plasma cutting machines are characterized by multi-functionality, high efficiency, high speed, high precision, low labor intensity, and high automation. They are suitable for processing a wide variety of small-batch, complex-shaped, and frequently changeable parts, and are widely used in many industries such as automotive, locomotive, pressure vessel, chemical machinery, nuclear industry, general machinery, construction machinery, and steel structure. CNC plasma cutting can cut various metal materials that are difficult to cut with oxygen (and some non-metallic materials can also be cut using the plasma arc). Its main advantages are that when cutting thin ordinary carbon steel plates with a relatively small thickness, the cutting speed can reach 5-6 times that of oxygen cutting, the cut surface is smooth, the thermal deformation is small, and the kerf width and cut angle are large. When cutting thin plates, a near-perpendicular cut surface can be obtained by using special cutting torches or processes. The processing quality of the parts cut by CNC plasma cutting machines plays a crucial role in the overall product, so effectively preventing deformation of the processed parts is particularly important.
2. Cutting process optimization
To minimize cutting deformation, the metal sheet must be accurately and securely positioned before cutting. Where possible, a multi-point contact electromagnetic platform should be used for appropriate leveling to eliminate uneven residual internal stress and improve flatness. CNC plasma cutting machines combine high-speed, high-temperature, and high-energy plasma cutting with computer control to cut metal sheets. Their operation is automatically controlled according to a pre-programmed sequence; they recognize the processing program. Therefore, selecting a suitable cutting process before processing—including the starting point, direction, sequence, and speed—plays a decisive role in the quality of the cut.
2.1 Selection of the starting point of the arc
Generally, the ideal starting point for cutting is at the edge of the metal sheet or in the middle of the kerf in a previously cut workpiece. If the distance is too large, the arc may not form or may break, resulting in incomplete cutting and product waste or scrap. If the distance is too small, a short circuit between the nozzle and the workpiece can burn out the nozzle, disrupting the cutting process. Practice has shown that a nozzle height of 6-8 mm from the workpiece is generally suitable, although this can be slightly less than 6-8 mm for air plasma cutting and water recompressed plasma arc cutting.
2.2 Selection of Cutting Direction
The correct cutting direction should ensure that the last cut edge is mostly detached from the mother plate. If it detaches too early, the surrounding corner frames will not be able to withstand the thermal deformation stress that occurs during the cutting process, causing the cut part to shift and resulting in dimensional deviations.
2.3 Cutting Speed Selection
Cutting speed is the relative speed between the torch and the workpiece during the cutting process. A suitable cutting speed is crucial for a straight cut surface. Cutting speed is determined by factors such as material thickness, cutting current, gas type and flow rate, nozzle structure, and appropriate backswing. At the same power, increasing the cutting speed will result in a slanted cut. During cutting, the torch should be perpendicular to the workpiece surface, but to facilitate slag removal, a slight backswing angle is acceptable (generally no greater than 3°). Therefore, to improve productivity, the highest possible cutting speed should be selected while ensuring complete penetration.
2.4 The effect of cutting order
The cutting sequence refers to the order in which nested parts of various sizes are cut on a steel plate. Generally, the principle of "inside before outside, small before large" should be followed: that is, cut the inner contour of the workpiece (or the parts nested within it) first, then cut the outer contour; cut smaller parts first, then larger parts. Otherwise, deformation may occur when cutting the inner contour or other small parts on a metal sheet, resulting in the scrapping of the workpiece.
Selection of 3-step motor drive
The stepper motor model selected for the CNC plasma cutting machine designed in this article is:
(1) The model of the horizontal drive motor of the worktable is MT86STH60-6004A.
(2) The longitudinal drive motor of the cutting gun frame is MT42STH33-1334A.
A stepper motor, as the actuator of a CNC plasma cutting machine, is a mechanism that converts electrical pulse signals into angular displacement. The entire cutting machine is controlled by a driver and controller, as shown in Figure 1. The driver in this system performs circular distribution and power amplification of the control pulses, energizing the stepper motor windings in a specific sequence to control the motor's rotation. When a pulse signal and a positive direction signal are given to the driver, the sequence of energizing the motor windings after passing through the circular distributor and power amplification is as follows: the four states change cyclically, causing the motor to rotate clockwise. If the direction signal becomes negative, the energizing sequence changes to counter-clockwise rotation.
To improve stepper motor performance, microstepping drivers are used to change the motor's operating rotation angle. The working principle of a microstepping driver is to change the magnitude of the current in phases A and B, thereby altering the angle between the resulting magnetic fields. One step angle is subdivided into multiple steps. Stepper motors all have a fixed resonant region; the resonant region for two-phase and four-phase stepper motors is generally between 180-250Hz (step angle 1.8 degrees). Higher motor drive voltage, larger motor current, lighter load, and smaller motor size cause the resonant region to shift upwards, and vice versa. To maximize motor output torque, prevent step loss, and reduce overall system noise, the operating point should generally be significantly offset from the resonant region. To reduce vibration during low-speed operation, a driver with microstepping is used in the design. When using microstepping, a higher microstepping value results in smoother current flow and more stable motor rotation.
Drivers generally have a microstepping function. After microstepping, the stepper motor step angle is calculated using the following method:
Step angle = Motor's inherent step angle / Microstepping
Select the MT-2HB05HM driver model based on the calculated microstepping ratio and the stepper motor model.
4 Control System I/O
I/O interfaces are crucial in control systems, serving as the relay point for receiving feedback signals and issuing control signals. Most systems require isolation and protection for the control system. This isolation and protection mechanism can convert I/O control levels; it can isolate the controller from the outside world, preventing large currents from the main circuit from flowing into the controller and causing it to burn out.
4.1 Optimization of Optocoupler Input Module
Generally, limit switches, start switches, and pause switches use mechanical switches. To prevent interference, normally closed contacts of the mechanical switches are usually used and connected as shown in Figure 2.
The system requires consistent logic for emergency stop, start, pause, and limit switches, all connected to normally closed contacts. The system automatically detects the status of the start position upon power-on, using this information as the basis for control. Therefore, if no start switch is connected, the corresponding start position should be connected to 24V ground.
4.2 Output Module Optimization
The system output has two connection methods: one utilizes the relays provided at the back of the system to directly connect to inductive loads such as electromagnetic relays; the second is a passive connection with only two paths. Control signal = 0: Switch/relay on (+24V forms a loop, active low, signal issued); Control signal = 1: Switch/relay off (+24V does not form a loop, signal withdrawn). The optocoupler output circuit connection is shown in Figure 3.
5 Typical Wiring of Control System
This optimized design utilizes the SH2012AH-QC CNC system from Beijing Star Microstep Co., Ltd., a dedicated CNC system for cutting machines. This system features resistance to plasma interference, lightning strike protection, and surge protection. When applied to plasma processing, it automatically controls corner speed, performs power-off recovery, automatic power-off recovery, retraction, segment selection, and hole point selection. It is suitable for the external piercing of thick plates and allows for selective hole point selection, greatly simplifying user operation. The system also features a small line segment processing function, making it widely applicable to metal blanking, advertising, and wrought iron work. It includes a parts library with 24 graphic types (expandable and customizable), containing commonly used basic machining parts.
The CNC system for the cutting machine is suitable for flame/plasma cutting, and its input/output interfaces have functions for both flame and plasma cutting. Figure 4 shows a typical wiring diagram of this system applied to plasma cutting.
In actual control wiring, the stepper motor driver uses a common anode connection. The height control interfaces (7, 8, 19, 20) and their common terminals in the DB25 interface of the CNC system are left floating. When not using limit switches, the limit switch interfaces are terminated with their common terminals.
The prototype was tested before and after the modification. A 1.5cm thick steel plate was used for the test. 1cm of the steel plate was cut off each time, and the error was measured. The average error of 20 sets was obtained. The difference before and after the modification can be clearly seen, as shown in Figure 5.
The experiment shows that the error after the improvement is significantly lower than that before the improvement. The error after the improvement is controlled at about 0.5mm. Therefore, the modified control system has stronger anti-interference ability and higher cutting accuracy.
6. Conclusion
This paper selects a suitable driver based on the stepper motor microstepping factor and pulse equivalent calculation to achieve the required cutting accuracy. Optocoupler technology was incorporated into the input and output modules of the control circuit of the control system, significantly reducing interference from external circuits and thus improving the cutting accuracy of the equipment. Improvements were made to the wiring path of the control system, resulting in better and smoother operation and enhanced equipment stability. Through these improved control system designs, the equipment accuracy was greatly improved, reaching approximately 0.5mm, significantly increasing the utilization rate of the sheet metal.
