Director of Process Room, AVIC Beijing Changkong Machinery Co., Ltd. Zhang Guangming
Machining deformation of slender shafts in shaft parts is a processing difficulty often encountered. Take a model of precipitation hardened stainless steel slender shaft parts as an example (see Figure 1). The difficulty of machining this part is that the length-to-diameter ratio of the parts is 30, and the hardness of the base is 37-42HRC (usually the length-to-diameter ratio is greater than 16 and belongs to the elongated shaft parts). , the rigidity of the parts in the process is poor). After the part is solid solution, the surface is hardened during the roughing process, and the deformation is severe, and the deformation amount is between 0.2 and 1 mm.
figure 1
1. Analysis of the status quo of traditional process processing
(1) Conventional route rough car shape→ rough car shape→solid solution→car shape→car shape→study center hole→fine car outer circle→car shape→grinding circle→toothing→deburring→rough cutting→aging → Grinding center hole → grinding outer circle → fine cutting → grinding outer circle → deburring → fluorescence penetration test → ultrasonic cleaning → inspection → ultrasonic cleaning → oil seal into the warehouse.
(2) Problems after the parts are solid solution, because the hardness of the base of the parts is low, the strength and hardness of the surface of the parts are improved during the semi-finishing process, and the plasticity is reduced by the work, and the hardness is as high as 38-44HRC. The deformation is large, and the subsequent finishing cannot be performed. The original process plan could not meet the requirements of the design drawings. Since the part deformation rejection rate is 80%, the part qualification rate is low, which cannot meet the normal production needs.
(3) Analysis of the cause of the problem Workpiece material problem: Precipitation hardening stainless steel has high strength, high toughness, good corrosion resistance and oxidation resistance, and its processing characteristics are: 1 The problem of work hardening is serious. The degree of hardening can be more than twice the hardness of the matrix material, and the depth of the hardened layer can reach a few tenths of a millimeter, which has adverse effects on the cutting process. 2The cutting force is large and the cutting temperature is high. The strength and hardness of the surface of the hardened part further accelerate the wear of the tool and quickly improve the cutting resistance. 3 The tool wears a lot. The tool is prone to bond, diffusion, abrasive, boundary and groove wear during the cutting process, which makes the tool lose its ability to cut. 4 The material has poor rigidity during cutting. According to the GJB2294-95 standard precipitation hardening stainless steel solution treatment, the hardness of the part base is ≤ 363HB, and the lower limit of the hardness value is not required, which causes most unstable factors in the processing.
Machining Retention Problem: The length-to-diameter ratio of the part is 30, which belongs to the slender shaft type. Generally, a clip and a top clip are used in the process. The part has a large plastic deformation during the cutting process. If the part is left in the roughing process, This will result in a less rigid part of the part during finishing, resulting in increased cutting forces during machining. At the same time, the work hardening of stainless steel is serious, and the work hardening phenomenon often occurs simultaneously with the surface residual stress and micro cracks, which makes the performance of the surface layer of the part less difficult to cut and directly affects the deformation of the part.
The influence on the top force: the original process uses a CNC machine tool with a clip and a top clamp. The machine tailstock is pneumatic. Due to the large machining allowance, the top tightening force is also increased accordingly, and the clamping force of the slender shaft parts is for the part. The length of the difference is not good enough to adjust the top force.
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