The punching process of motor iron core is an important part of the motor manufacturing process. The prominent issue with punching is that the inner diameter of the stator exceeds the tolerance. According to the punching theory, the size of the holes punched on the sheet metal is equal to the size of the convex mold, and the size of the fallen material is equal to the size of the concave mold. According to this principle, when designing a mold, the outer diameter of the convex mold is taken as the aperture size of the punched part, and the inner diameter of the feeding concave mold is taken as the outer diameter size of the feeding part. To ensure that the dimensions of the workpiece meet the requirements of the drawing, the punch of the punched part is processed according to the middle value of the hole, and the inner diameter of the blanking die is processed according to the middle size of the blanking part.
The inner diameter of the stator punch of the separation mold is also processed according to the above principle, that is, the inner diameter of the stator punch is equal to the inner diameter of the punch stator. In theory, the inner diameter of the separated stator laminations should be equal to the size of the inner diameter punch. But the actual dimensions measured are all larger than the inner diameter of the convex mold, as shown in Table 1.
Table 1 Analysis of Actual Dimensions of Various Models of Components
2 Reasons Analysis
The reason for the occurrence of out of tolerance phenomenon that violates this punching principle is because the elastic rebound phenomenon of the workpiece during material dropping is ignored. It is believed that as long as a reasonable mold gap value is adopted, the rebound amount will be very small and will not affect the dimensional accuracy of the workpiece. Therefore, this factor was not considered in the design of the separation mold. This approach is correct for punched pieces with good rigidity (such as stator slots not processed during separation). However, when separating, the stator slots are often already processed, and for this type of slot punch with extremely poor rigidity, the impact of elastic rebound on size cannot be ignored. Elastic rebound includes positive rebound and negative rebound. For punching, positive rebound refers to the hole size of the punched part being smaller than the punch size; Negative rebound refers to the situation where the aperture of the punched part is larger than the size of the punch. The larger inner diameter of the stator punch than the punch size is caused by negative elastic rebound. There are many factors that can cause negative rebound, among which the most important are the special contour of the workpiece, the punching process of the workpiece, and the structure of the mold.
2.1 Punching process for punching sheets
The punching process adopts the composite die punching method, and the entire punching process is completed through three steps, as shown in Figure 1.
Cutting billet (square) - rotor re punching - stator re punching - separation of stator and rotor.
Process 1: Rotor slots, shaft holes, ventilation holes.
Process 2: Position the shaft hole, punch the stator slot, and shape the stator.
Process 3: Position and separate with the shaft hole.
This process method involves punching slots first and then separating the stator laminations. So, during separation, the inner diameter profile of the stator is composed of a discontinuous network of arc-shaped tooth ends, which has very poor strength. Therefore, the inner diameter of the stator is prone to elastic deformation during the separation process.
2.2 Deformation and force analysis during separation process
Due to the special structure of the punching plate, the strength of the teeth is poor. The teeth of the stator punching plate bear radial tension during the separation process, resulting in significant tensile deformation. After the inner diameter of the broken stator is separated in this tensile state, it returns to its original state under the action of tension, resulting in negative rebound phenomenon of stator inner diameter expansion. This is the main reason for the stator inner diameter deviation.
2.3 Structure of Separation Mold
The current working process of the separation mold is as follows: the shaft holes of process 2 (shaft holes, ventilation holes, rotor slots, stator slots, and stator outer circles have been processed) are positioned on the fixed axis of the separation mold, and the separated rotor punch is punched out of the concave mold by a punching plate. The stripping plate pushes the stator punch out of the convex mold. From the perspective of mold structure, this separation mold cannot effectively prevent the elastic deformation of the punch.
The punching plate in the concave mold cannot press the material during punching, so the material under the punching plate is not constrained and can deform freely (in fact, the punching plate cannot press the material, because considering the convenience of taking the rotor punching plate, it is received by the punching plate when cutting the material).
3 Improvement Measures
The above reasons indicate that there are two reasons for the stator inner diameter exceeding the tolerance. One is that the process method of punching the groove first and then separating it causes a difference in the rigidity of the punching plate. Secondly, during punching, the board cannot be pressed. The above reasons cannot be easily improved because the shape of the punching plate is determined by the motor structure. Punching the groove first and then separating it is a traditional process that our factory has been implementing for decades. Stamping equipment and fixtures are arranged according to this process. So to solve the problem of inner diameter deviation, we can only find other ways on the mold.
Since the negative rebound phenomenon of the punch cannot be avoided, can we design the punch to be smaller than the theoretical size based on the rule between the actual size of the stator inner diameter after separation and the actual size of the punch.
The actual size of the YJ105A stator punching plate, which is φ 540.07mm, is 0.07mm larger than the intermediate value of the required inner diameter of the stator, which is φ 540+0.05mm. Therefore, the design size of the punch, which is φ 540+O.Olmm, has been changed to φ 539.93 ± O.Olmm. Similarly, YJ85A has changed the design size of the punch from φ 680+0.012mm to φ 679.01+0.012mm
Through the above improvements, the inner diameter size of the stator not only meets the requirements of the drawing, but also approaches the middle value of the tolerance required by the drawing (see Table 2). After practical testing, the quality of the components remains stable, and the above experience can be used as a reference for similar component manufacturers.