【Abstract】This paper introduces theoretical research, technology development and application of high-hardness material chipping after quenching in CNC machining, as well as important achievements, mainly including research and development of cutting parameters, machining parts modeling and other technologies. Practical application results show that improved parameters can greatly improve processing quality, improve quality of machined surface, and reduce rectification cost of chipping problem.

In recent years, CNC machining technology has been widely used in aerospace, precision instruments and other industries. After years of development in my country's manufacturing industry, level of machining technology has been continuously improved, among which CNC machine tools have contributed. But at the same time, current market has higher and higher requirements for mechanical products, especially in high-precision fields such as aerospace and automobile manufacturing, processing quality of parts must be guaranteed. In order to effectively control machining error of mechanical parts and make their quality reach specified range, people also put forward higher requirements for machining accuracy of CNC machine tools. This paper takes edge chipping problem that is easy to occur in the process of CNC machining of molds as an example, analyzes influencing factors, discusses solutions and countermeasures to solve problem, and hopes to be helpful to related work.
In the process of mold manufacturing, quenching is one of main ways to improve hardness of base material. Cr12MoV is a commonly used mold material for high-strength plate cutting processes. After quenching, hardness of Cr12MoV can reach 58~62HRC, which is a high-hardness mold material. As we all know, the higher hardness of material, the more brittle it is, which is also known as "it is too rigid and easy to break". Since machine tool rotates at high speed during CNC machining of this material, hardness of material is high, resulting resistance is greater, resulting in the problem of chipping during machining process at the edge of substrate, which seriously affects quality of mold and cannot meet customer's "zero defect" requirement.

Hardness of Cr12MoV workpiece (2EC, HC, SA3F, etc.) of a certain model mold material after quenching reaches 61~66HRC, chipping phenomenon occurs in finishing stage after quenching, as shown in Figure 1 and Figure 2.

 

Figure 1 Severe state of edge chipping

 

Figure 2 State of micro zigzag chipping

Table 1 shows status investigation of collapsed parts.

Table 1 Status of collapsed parts

Cutting edge type and chipping phenomenon are shown in Table 2.

Table 2 Cutting edge type and chipping phenomenon
Table 2 Summary: ①Due to flat surface of straight trimming profile, tool is basically cutting with tip of tool, cutting speed is almost 0, processing state is extruded, and cutting edge is seriously chipped; ②When machining steep contours and side trimming contours, tool is cut by side edge of tool, and no chipping phenomenon is found.
According to analysis, main reason for extrusion at the edge of edge is large cutting resistance, so verification is carried out by changing processing parameters and reducing cutting resistance.
By changing angle of tool axis, cutting speed is increased, that is, swing angle processing, replacement of sharper inserts, reduction of processing step, and reduction of feed speed to reduce tool resistance.

Verification scheme is shown in Table 3.

Table 3 Verification scheme
Problem of chipping of high-hardness workpieces can be solved by using scheme 3 processing, that is, using finishing tools, step distance is changed from 1.5mm to 1mm, speed is changed from 4,500 rpm to 3,000 rpm, and feed is changed from 4,500mm/min to 2,000mm/min.

Implementation measures are shown in Table 4.

Table 4 Implementation Countermeasures

Through optimization of process parameters such as selection of CNC machining tools, adjustment of step distance, change of speed and feed, following conclusions are drawn:
(1) Finishing tools are used in the semi-finishing process.
(2) Step distance is controlled within 1mm, speed is reduced to S3,000 rpm, and feed is adjusted to 2,000mm/min. After adjusting according to above conditions, problem of high hardness edge chipping can be alleviated, as shown in Figure 3.
 
Figure 3 Processing edge effect after adjustment