The reason and solution of dimension out of tolerance in the blanking process of motor die

1 The introduction

The blanking of motor core is an important link in the process of motor manufacturing.The prominent problem of blanking is that the inner diameter of stator is out of tolerance.According to the blanking theor, the size of the hole punched out on the plate is equal to the size of the punch, and the size of the material dropped is equal to the size of the punch.According to this principle, when designing the mould, the outside diameter size of the punch is the aperture size of the punching part, and the inside diameter size of the blanking die is the outside diameter size of the blanking part.In order to ensure that the size of the parts meets the requirements of the drawing, the punch of the punching parts 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 parts.
The size of the stator punch inner diameter punch of the separation die is processed according to the above principle, that is, the size of the stator punch inner diameter is equal to the size of the stator punch inner diameter.Theoretically, the inner diameter of stator punch after separation should be equal to the size of inner diameter punch.However, the measured actual size is all larger than the size of the inside punch, as shown in table 1.

2 Cause analysis
The reason why there will be a violation of the punching principle of the overshoot phenomenon, because the neglect of blanking parts of the elastic springback phenomenon, that as long as the reasonable mold gap value, springback will be very small will not affect the size of the parts of the precision, so in the separation of die design did not consider this factor.This approach is correct for plates with good rigidity (e.g., stator slots are not machined during separation).However, the stator slots are often machined when the stator slots are separated. For the slots with poor rigidity, the effect of elastic springback on the dimension cannot be ignored.Elastic springback has positive springback and negative springback.For punching, positive springback means that the hole size after punching is smaller than the punch size;Negative rebound means that the aperture of the parts after punching is larger than the size of the punch.The reason why the inner diameter of stator punch is larger than the punch size is due to the negative elastic spring.There are many factors causing negative rebound, the most important of which are the special outline of the parts, the parts blanking process and the die structure.

2.1 blanking process of blanking
The blanking process adopts the technological method of composite die blanking, and all blanking processes are completed after three working procedures, as shown in figure 1.
Cut blank (square) -- rotor repulse -- stator repulse -- stator repulse -- stator separation.
Step 1: rotor slot, shaft hole, ventilation hole.
Step 2: position the stator slots and stator shapes by axis holes.
Step 3: axis hole positioning and separation.
This technique is used for stator blanking by first punching slots and then separating.Therefore, the stator inner diameter profile during the separation is a discontinuous network composed of curved tooth ends, and its strength is very poor, so the stator inner diameter is easy to produce elastic deformation in the separation process.
2.2 deformation and force analysis in the separation process
Due to the special structure of the laminations, less intensity of teeth, the teeth of the stator laminations department under radial tension in the separation process, so the larger tensile deformation, in this state of tensile fracture of the stator inner diameter size after the separation under the pull back to its original state, so the stator inner diameter size swell the negative springback phenomenon.This is the main reason of stator inner diameter overshoot.
2.3 structure of separation module
Currently, the working process of the separation die is as follows: position the axle holes of the process 2 blanking (axle holes, ventilation holes, rotor slots, stator slots and stator outer circles have been processed) on the centering shaft of the separation die. After separation, the rotor blanking is knocked out from the concave die by the beating plate, and the stripper pushes the stator blanking from the punch die.Based on the structure of the die, the separation die can not effectively prevent the elastic deformation of the die.
The punching plate in the die in blanking can not press material, so the material under the punching plate is not subject to any constraints and free deformation (in fact, can not let the punching plate press material, because of the consideration of the rotor punching plate is convenient, is by beating the plate to lay the material).
3. Improvement measures
According to the above reasons, there are two reasons for stator inner diameter overshoot. One is the poor rigidity of the blanking sheet caused by the technological method of punching slot first and then separating.Second, the blanking board can not press.The above reasons cannot be easily improved, because the shape of the blanking sheet is determined by the structure of the motor. It is a traditional process that our factory has been carrying out for decades, and the stamping equipment and tooling are arranged according to this process.So to solve the problem of inner diameter overshoot, we have to find other ways on the mold.
Since it is impossible to avoid the negative rebound phenomenon of the punch, can the size of the punch be smaller than the theoretical size in the design according to the rule between the actual size of stator inner diameter and the actual size of the punch after separation.
Namely, the actual size of the YJ105A stator punching serving, namely, 540.07mm, serving with the median value of 540+0.05mm greater than the required diameter of the stator, namely, 540.07mm, and serving with 539.93±O.O LMM, namely, YJ85A, and serving with 679.01+0.012mm, namely, the design size of the punch, namely, 540+O.O LMM, serving with 539.93±O.O LMM.
Through the above improvements, the stator inner diameter size not only meets the requirements of the drawing, but also is close to the middle tolerance of the drawing (see table 2).Through the practice inspection, the parts quality maintains the stability, the above experience may provide the similar parts manufacturer the reference.