Discuss the influencing factors and improvement of the accuracy of automobile covered outer plate parts

In the increasingly fierce competition in the automobile market, improving product refinement is the most effective means to improve product competitiveness. The quality of the exterior cover parts of the automobile body is determined by the shape, dimensional accuracy and appearance quality of the parts. The size of the outer cover of the car directly affects the assembly of the whole vehicle and needs to be effectively controlled.

Factors that affect the accuracy of cover outer plate parts

The accuracy of automotive covered outer plate parts is mainly affected by two stages, namely the process design stage and the mold manufacturing debugging stage. Process design stage: The rebound compensation is mainly based on CAE analysis data, or refers to the experience value of the rebound amount of past models for compensation.

With the improvement of CAE software computing power, the springback compensation in the process design stage, especially the springback compensation of the pulling and post-process processes, has gradually narrowed the difference between CAE analysis and actual debugging of parts, but it cannot completely eliminate the springback. Part springback is the most difficult part of stamping production to control, making it particularly important to improve dimensional accuracy in later mold debugging.

There are many factors affecting the dimensional accuracy of the mold after delivery, mainly in the following four aspects: (1) equipment: mold, inspection tool machining equipment accuracy, mold research and distribution, debugging and production machine tool accuracy, production between press process transmission (end tool, manipulator); (2) Mold: mold reference hole accuracy, profile processing accuracy and mold matching symbol type (3) Inspection tool: gauge positioning method, measurement method and detection block accuracy. (4) Materials, formula: stamping process parameters, material specifications (material grade, material thickness).

Press parallelism measurement

Parallelism data logging

Press verticality measurement

Measurement view

Research on improving the dimensional accuracy of parts in the manufacturing stage of mold inspection tools

Equipment accuracy inspection and management.

Due to the difference in mold processing, debugging and production equipment, the dimensional accuracy of mold manufacturing parts will be biased, so it is necessary to control the press to improve the accuracy of the mold benchmark. From a certain point of view, under the influence of temperature difference, when CNC equipment is applied in different time periods in automobile manufacturing, the accuracy of the molds produced by its manufacturing is different.

In order to reduce the influence of machine tools on the accuracy of mold manufacturing, it is best to manufacture the outer cover parts mold of the same model in a unified equipment, debugging or production. However, due to the differences between mold factories and main engine factories, it is difficult to guarantee, so it is necessary to manage the accuracy of the equipment.

The parallelism, perpendicularity of the press, the parallelism of the top crown and the height of the ejector rod directly affect the accuracy of the stamping parts, especially the drawing and flanging shaping processes. The parallelism measurement method of the press is mainly the slider to the lower dead center state, that is, the parallelism of the slider to the 180° state. According to the reference formula 0.02+0.1L/1000, the left and right references: 0.02+0.*4800/1000=0.5mm.

Measurement data

Lower mold

Upper mold

The measurement of verticality is that the press is in an unloaded state, and the air pressure of the balancer needs to ensure that the weight of the slide has been balanced, so that the press as a whole is measured under the premise of complete static load. Usually measure fluctuation data in a fixed interval, generally using the following dead point as a reference, and measure the length interval of a fixed interval upward.

Reference formula: 0.08+0.32S/100 (S is the measurement interval). For stamped parts with deep drawing and a large forming area, the hydraulic equipment is more stable when selected. Different press production or development will cause the size of the drawn formed parts, and the key control parts are best fixed for the same production line production or research and distribution, so as to avoid the size of stamping parts affected by equipment replacement. In addition to the press, the factors that affect the size of the part are also affected by the transmission between processes, such as manipulators, end grippers, parts shelves and other factors.

Mold manufacturing impact and improvement methods

The dimensional accuracy of automotive outer cover parts is the surface size of the part obtained after the mold is formed, and it is especially important to improve the accuracy of the automobile cover mold in the manufacturing stage of automobile outer cover parts. Mold manufacturing is based on the design and manufacturing size of the part as the basic basis and criterion, and it is necessary to consider the possible impact of springback and forced shaping of the part size to cause the deformation of the part.

When calculating the dimensions of the formed parts, it is necessary to consider the calculation by referring to the previous CAE analysis and corresponding empirical data. In addition to the pre-springback analysis to control the accuracy, it is also necessary to improve the manufacturing accuracy of the mold reference hole. The reference hole is the most basic criterion for CNC machining of a single part of the outer cover mold during the manufacture of automobile parts, and the connection of the upper and lower mold bases can be effectively realized through the reasonable analysis of the positioning of the reference hole parts.

During the test, it is necessary to use the method of reference hole measurement on the whole vehicle to strengthen the clarity of the accuracy of the outer cover parts of the car, and timely accurately process the unreasonable mold to fundamentally reduce the occurrence of position shift phenomenon when the upper and lower mold groups are molded. In addition, attention should also be paid to improving the machining accuracy of the processing standards of each part of the mold. The gap and beads of the drawing sequence mold can control the inflow of materials, so the impact on the accuracy of the part is very large, and the guarantee of mold accuracy is particularly important.

Positioning method of the outer plate of the hair cap

Positioning design

Fender positioning method

Inspection block: identification of measuring points (product extensions)

Inspection block: designed as a module (product handpiece)

In the mold manufacturing stage, it is also particularly important to improve the accuracy of mold profile manufacturing. The mold profile will leave a knife pattern during processing, and the roughness of the surface of the mold drawn sequence face will be reduced, which can also improve the dimensional accuracy of the formed part. Improving the manufacturing accuracy of the mold profile can also reduce the workload of the later fitter’s research and rectification.

At the same time, it can also be used to improve the level of fitter technology to improve the roughness of the surface of the mold profile, so as to improve the accuracy of parts. Inspection tool design and manufacturing to improve the accuracy of parts The inspection tool design of stamping parts is based on GD&T in the stamping field, which needs to be combined with the RPS point setting of the whole vehicle.

In the design and manufacturing process of the inspection tool, the inspection tool can be better set according to the RPS point setting of the whole vehicle. For example, the outer plate of the hair cover inspection fixture, most of the parts of the outer plate of the hair cover are non-porous, and can only be positioned as a contour. Profile positioning is data-instable and less repeatable during measurement. The fixture positioning point design can be optimized, and the positioning point can be set on the straighter boundary of the flange surface, which can be set on the windshield side.

When positioning, pay attention to the use of R angle as the positioning surface, the rebound of the flange surface is large, which is easy to cause inaccurate positioning, so the positioning is set on the R angle surface contact length of 2~3mm, and the lower flange surface is forced to air to ensure the stability of accuracy.

Side positioning method

The influence of different materials on the inflow of parts

The difference in inflow between material A and material B usage

When measuring, you need to use the CH hole for positioning, measure the size of the reference position, and adjust the size of the reference position in place first. Then use the positioning reference positioning on the gauge, and the measurement method of the outer plate without holes is as follows: Positioning with CH hole: observe the reference situation of the gauge when the CH hole is positioned, and modify this reference value to the theoretical state. Observe the amount of free springback to confirm whether the part is abnormal and whether the part interferes with the gauge. Whether the part of the reference part is close to zero.

Confirm the clamping method: confirm whether the reference is to be clamped, whether the installation order is correct, and confirm the contour clearance with the gap ruler. The detection block confirms the difference in surface. Measuring parts: Refer to the requirements of the part accuracy table to measure the dimensions in all directions. For example, for fenders parts, the flange edge hole is preferred as the main and auxiliary positioning in the Z direction when the gauge design is positioned, and the contour positioning is selected in the X direction, and the straight side position is also preferred for contour positioning, rather than the arc position shown.

The measurement of the fenders should first confirm whether the positioning hole on the flange face is fitted to the reference surface, then align the prisms, and then lock the positioning pin of the hole on the flange face. The contour surface that fits with the front door is positioned in the X-direction. The initial measurement also needs to be measured at full scale by the detection of the CH hole, including the measurement of the size at the reference point. The measurement is only taken when the size of the contour positioning reference is changed to standard.

In the design of the gauge, in addition to the influence of positioning, it may also be affected by the design of the detection block, if the measurement is at the crescent bay in the fender, the measurement error will be large because it is an arc. In order to reduce the error caused by the measurement, the detection block can be designed as a product extension surface design inspection block, and the detection block is marked with an arc line to facilitate the repeatability of each measurement. Mating surfaces such as mirrors and A-pillars are best designed as modules to evaluate the impact on the appearance of the vehicle.

For large outer covers such as side walls, the gauge design recommends using three reference holes as positioning, and generally designing a total of three datum positioning in the A-pillar, B/C pillar and rear retention position. The main positioning adopts a cylindrical pin, and the rest of the secondary positioning adopts a diamond-shaped pin. When measuring, you need to first locate the pin of the main positioning A-column, followed by the auxiliary pin of the rear position, and finally the auxiliary pin of the B/C column, before the full-size measurement, you can confirm whether the part has an offset before and after by looking at the side wall of the AB column, and by looking at the threshold sidewall and the side wall of the fixed side beam, you can see whether the part is offset up and down.

If there is an offset, it is necessary to check whether there are abnormalities in the mold pattern, positioning, and transmission between processes. The main accuracy problems of the side circumference are in the rear retention position of the tail, the four-corner window position, and the rear door mating position, which is generally caused by the large or small expansion coefficient in the early stage, insufficient rebound amount, and the accuracy of mold processing in the later stage and the symbol of mold research.

Accuracy improvement can first from the rear door with the position priority can ensure the accuracy of the gap gap of the whole door seam, the improvement of the position accuracy of the four-corner window is recommended to ensure that the hole is guaranteed when the symbol type is OK. The post-guarantee position accuracy improves the accuracy of the design surface and hole and has the requirements of local reference, it is recommended that machining treatment is preferred during rectification, and the machine tool data can be used to find the reference machining modification after the tool data to improve the accuracy of the side perimeter post-protection.

The management of the gauge can also improve the accuracy of the gauge, and the common management is to regularly use the three coordinates for accuracy calibration. The management of the gauge should be included in the routine of testing, and it should be frequently confirmed whether the status of the gauge is abnormal and whether the detection block is loose. If there is a change in the inspection, or the product is changed, it is necessary to rectify the inspection tool and recalibrate it before using it to avoid the influence of the inspection tool on the accuracy of the part.

Materials, formulas: stamping process parameters, material specifications (material grade, material thickness)

The forming process of sheet stamping is divided into deep drawing and expansion. Deep drawing refers to the pressing force applied by the mold to the edge part of the material, in order to prevent the part from warping, and the plate can flow freely in the die of the mold, and the deformation occurs in the edge part of the sheet. Expansion refers to the pressing of the sheet metal through the upper mold and the lower mold or the stiffeners on the mold during the stamping process, so that the sheet metal cannot flow freely and the deformation occurs. Therefore, the process parameter setting of stamping molding will have a direct impact on the forming of the part.

The forming process of the material in the mold is plastic deformation, so the performance of the material is directly related to the formability of the part, and the material specification will affect the elongation, wrinkling trend, cracking and springback of the material, resulting in the difference in the accuracy of the part. Material A and material B are two different materials that will cause differences in the inflow of drawing sequence material, resulting in different springback amounts of parts, which ultimately affects the accuracy of the part.


Part size control is mainly divided into springback compensation in the early process analysis stage, and the influence of various accuracy of the design and manufacture of mold inspection tools (including debugging and production) in the later stage. The early rebound compensation is mainly based on CAE analysis and empirical data, and there are many influencing factors in the design and manufacture of mold inspection tools in the later stage, which need to pay attention to and control the influencing factors such as equipment, molds and inspection tools.

Covering mold design is a time-consuming, labor-intensive and cost-intensive process, which requires the manufacture of parts that meet the size of the product, and the mold needs repeated mold testing and mold modification, which prolongs the training cycle of new vehicles. The management of body cover springback control, equipment accuracy and mold inspection tool design and manufacturing can achieve good conditions, which can reduce the workload of later mold revision, shorten the quality cultivation cycle, and enhance the competitiveness of products.

Source: Sheet Metal Forming House

Lower mold

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