1. Overview
Plasma cutting technology integrates numerical control technology, power supply technology, and plasma technology.
It is a high-tech option in cutting technology, superior to traditional flame cutting in terms of cutting efficiency, quality, precision, and minimal heat input and distortion.
It is widely applied in various industrial fields, including aerospace and mechanical manufacturing. Hydraulic props, used to control coal mining faces and withstand mine pressure, are primarily made from medium-thickness sheet metal cutting, assembly, and welding.
Plasma cutting, with its myriad advantages, is widely used in hydraulic prop production. The numerical control plasma cutting equipment currently equipped in workshops can handle the cutting of parts of complex shapes.
The technical advantages of plasma cutting become more apparent during batch production.
This article mainly analyzes the common quality issues in plasma cutting production and their causes, and proposes targeted measures to improve product quality and production efficiency.
2. Working Principle of Plasma Cutting
The working principle of plasma cutting mainly utilizes the high temperature and high speed of the plasma flow as the heat source for cutting.
The plasma torch ionizes the compressed gas to generate a high-temperature, high-speed plasma arc that melts the metal.
Concurrently, the high-speed, high-pressure airflow blows the melted metal away, thereby achieving metal cutting, as shown in Figure 1.
3. Influencing Factors and Quality Control Measures for Plasma Cutting
3.1 Arc Starting and Ending Defects and Control Measures
In the process of plasma cutting, defects arising from arc starting and ending are among the common issues.
Arc starting defects are primarily due to uneven sheets and deformations or displacements of the workpiece during cutting, as well as unreasonable arc lines and compensation values.
Because of the unevenness of the sheet, the workpiece falls from the parent material at the moment of cutting completion. At this instant, the workpiece may be burned by the plasma arc, which is the main cause of arc starting defects.
Furthermore, during the non-corner arc starting and ending process, if the compensation value is set higher than the required value, and the arc starting and ending lines are not set to intersect, it can easily lead to arc ending defects.
Control Measures: By controlling the arc extinguishing time, defects related to arc extinguishing can be avoided.
Depending on the actual cutting situation, setting the arc extinguishing time can allow the arc to extinguish as the workpiece falls off, or by using CNC programming techniques, the lead-out line can be omitted to avoid arc-extinguishing defects.
Defects caused by deformation or displacement during the cutting process are relatively minor and can generally be compensated with a circular arc start and arc extinguishing line.
3.2 Difficulty Cutting at Vertex or Sharp Points
For plasma cutting, another common problem is that the vertex or sharp points of the parts are difficult to achieve through plasma cutting, and acute angle sharp points are even more difficult to cut out.
These are mainly due to the characteristics of plasma cutting itself and are difficult to eliminate.
Therefore, if there are no special requirements for the parts, it is acceptable for the workpiece to have small fillets naturally formed during the cutting process.
For workpieces that require vertex cutting, the following two measures can be taken: one is to reduce the effect of trailing on the sharp angle by decelerating at the turning point; the other is to increase the external fillet angle at the sharp corner during the programming process, thereby achieving the cutting of the sharp angle or sharp reduction.
3.3 Problems with Burrs and Slag
Burrs and slag are common phenomena on plasma cut parts. Firstly, when plasma cutting, if the cutting speed is set too fast or too slow, this is the main reason for burrs or slag at the cutting position.
Secondly, due to the damage of plasma equipment itself, such as the burn and aging of the cutting machine nozzle, burrs and slag may occur at the workpiece’s cutting position.
Control Measures:
Determine a reasonable cutting speed through process trials to ensure a reasonable cutting speed and avoid the occurrence of burrs and slag.
Secondly, timely equipment repair and maintenance, and timely replacement of damaged nozzles. The low purity of the protective gas in plasma cutting or the mismatch between the nozzle and the protective gas can cause burrs and slag on the workpiece.
For example, if the slag area below the workpiece is large, it is mainly because the plasma nozzle is blocked by slag, the slag cannot be blown away in time, and it accumulates on the workpiece surface.
3.4 Diagonal Deviation Issues
Diagonal deviation is a common defect in plasma cutting, mainly caused by issues within the plasma cutting machine itself.
This can occur due to the extended operation time of the plasma cutting machine’s travel mechanism, resulting in lateral gear displacement, or due to inadequately spaced installations.
In sum, considering the characteristics and rules of CNC plasma cutting process, appropriate measures can effectively reduce cutting defects and control the quality of cuts.
When there is a discrepancy between the size of the plasma cut part and the blueprint, the kerf compensation can be initially adjusted.
When kerf values are normal, they should equal the actual values of the cutting torch, but when minor changes are needed in the workpiece size, the kerf value can be compensated to achieve the precise contour size required.
Additionally, in slender parts with high internal stresses and sensitivity to thermal deformation stress, the sheet can be separated based on the actual situation to release internal stress and then be cut again.
Alternatively, retaining the machining rod during cutting allows the workpiece and sheet to be integrated, reducing the influence of deformation stress on the workpiece after cutting.
4. Conclusion
Plasma cutting technology is widely utilized in manufacturing, but it still faces challenges such as extensive quality control and large fluctuations in product quality in actual production, directly affecting product creation.
Therefore, it is essential to vigorously promote lean processes, continuously improving the quality of plasma cuts through enhanced management and technical innovation.
