The Stamping Springback Compensation Technology Study for An

Auto B Pillar

Guanglin Shi

, Jinhai Cheng

, Shaowen Lv

and Xuezhao Zhou

Institute of Mechanical Engineering, Guangxi University of Science and Technology,

Chengzhong District, Liuzhou City, China

Keywords: Spring back Compensation; CAE simulation; Reverse Engineering .

Abstract: The stamping spring back problem of a sheet metal is always a difficult point to solve in production, and it

also becomes a hot issue in academic circle. This paper introduced a method to reduce the springback

deformation of an auto B pillar. A Handy Scanner was used to acquire the points cloud datum of a drawn B

pillar; afterwards, all surfaces of this part were reconstructed in the Geomagic Studio software, taking

advantage of this datum. Consequently, the dimension difference between the reconstruction and primitive

numerical model was obtained by using the Geomagic Qualify Probe software, and it was the springback

value of this auto B pillar after drawn. To reduce the springback value of this part, according to the positions

presenting springback maximum, some offset dimensions on stamping die surfaces were compensated.

Finally, DYNAFORM software was used to simulate the forming process with the improved die, to analyse

the improved springback results of this part. The result shows this method is effective to reduce springback.

1 INTRODUCTION

The forming precision is an important indicator to

the qualification of a stamping part, and it is badly

influenced by the spring back problem. In realistic

stamping production, the spring back of a sheet

metal is always difficult to predict and solve. To

reduce the spring back of a sheet metal, many

researchers have been doing a great deal of work.

Luc Papeleux, Jean- Philippe Ponthot[1] detailedly

studied several parameters’ influence on spring back

with FEA method, including BHF(blank holder

force), friction, spatial integration, time integration

scheme and constitutive laws, using a 2D U-draw

bending case. Michael Krinniger, Daniel Opritescu[2]

et al. studied the influence to the springback

behaviour with different bending parameters, punch

velocity and materials by some experimental

investigation, and they developed an extendable

metal model, then they advised that we should

consider these factors’ influence to spring back in

the design process, so that we obtained a sheet metal

with enough dimensional accuracy. L. Komgrit, H.

Hamasaki[3] et al. introduced a new technology to

eliminate spring back, they pushed up the bottom of

the sheet metal with a counter punch, and they

testified their method by experiment and simulation;

H. Naceur, Y.Q. Guo[4] et al. introduced a method

in order to reduce the springback effects by

optimizing the geometry of tools in sheet metal

forming, they made optimized design to tools’ radii,

thickness distribution and material parameters

according to a new response surface method and

their experiment. Finally, they validated their results

by using STAMPACK and ABQUS software.

With the progression of science and technology,

nowadays we can use Reverse Engineering to

acquire exact difference between the end product

and primitive numerical model, and we also can use

CAE technology to simulate stamping process, and

analyse the springback of an end product. These new

techniques break a new path to solve this problem.

The B pillar is a main part supporting a car’s

body structure, meanwhile, it bearing the pressure

from the front door and rear door, so it must has

sufficient strength and stiffness. The B pillar has a

complicated surface construction, it’s manufacturing

processes including drawing, triming, reshaping and

punching, among which the drawing process has an

important influence on its’ final forming precision.

Aiming at a whole car having more light weight, this

B pillar was made of high strength steel, which

allowed much less thickness to ensure sufficient

mehcanical properties, and in this case the thickness

of part was 1mm. The parameters of the material of

this B pillar was illustrated as follow table:

Table 1: Mechanical Properties of Material.

2 SPRINGBACK DETECTION

2.1 Part Drawn Numerical Modelling

After the B pillar was manufactured in drawing

process, a stamping springback would occur in some

locals of this part. Springback is an inevitable

phenomenon in forming process, and it directly

influences dimensional precision and final shape of a

sheet metal [5]. Springback causes difference in

dimension between the final product and the

primitive numerical model designed by engineer. To

acquire this difference value, we can use Reverse

Engineering Technology. Handy Scanner is a device

which is in common use in optical scanning field. In

this study case, we used a handy scanner whose type

was Handy Scan 700, its resolution ratio running up

to 0.05mm, and maximum precision was 0.030mm.

After some adjusting and pre-processing work was

done, we used this instrument to scan the drawn part.

Our site work was shown as follow:

Figure 1: Using Handy Scan 700 to scan a drawn part.

Then we used this points cloud datum acquired

from Handy Scan 700 to reconstruct a numerical

geometric model of the drawn B pillar in the reverse

modelling software, Geomagic Studio. The

following drawing was the reconstructed model.

Figure 2: The reconstructed model of a drawn part.

2.2 Springback Detection

In this section, we used the Geomagic Qualify Probe

software, to detect the difference between the drawn

model and the primitive numerical model. Inputting

the both numerical models into this software, the

primitive model as reference model and the

reconstructed model as test model, aligning these

two models, clicking 3D contrasting menu, we can

obtain the difference value in dimension as the

following figure.

From this figure, we can observe the maximum

differences was +3.64mm and minimum -5.21mm,

representing the springback values were 3.64mm

and 5.21mm on the local positions, and Geomagic

Qulify Probe already marked the positions by two

highlight points. Additionally, we also can see most

of the difference values be from -1.91mm to

1.91mm, the average dimensional difference of these

two models was ±0.66mm, and the standard

deviation was 0.87mm, so these deviations were

acceptable in general. The next work was to reduce

the oversize deviations by some means.

Figure 3: Springback value contour.

3 SPRINGBACK

COMPENSATION

3.1 Springback Value Quantization

Using Geomagic Qualify Probe to make some

sections crossing the local positions in which the

maximum springback and oversize springback were

located, we got some section boundary lines from

the reference model, and some points around these

lines from the test model. Along each of these

section boundary lines we selected some reference

points from the test model, then projecting these

points to the section line along the line’s normal

direction, we can obtain the crossover points on the

section boundary lines; dimensioning the distance

from each point to the opposite crossover point, we

can obtain the springback value of each point on the

primitive geometry model designed by engineer. The

springback plus maximum and minus maximum

sections were shown by the following graph:

Figure 4: Springback plus maximum section.

Figure 5: Springback minus maximum section.

3.2 Die Profile Compensation

From the above several chapters, we already

obtained the actual precise difference values in

dimension between the primitive geometry model

and the drawn model, to compensate these

difference values, in this paper we adopted one

method which changed the molded surfaces of the

die and punch. According to the springback value of

each point, we changed these points position to the

reverse side of the section boundary, in other words,

we giving two times of difference value along the

normal direction of the section line to compensate

springback value. Based the original model and

these changed points, we can generate a new model

whose surfaces were renewed with springback

compensated points, and this model constituted the

new die. By using the CATIA software, inputting

the original model and carrying out the same

operations, assigning the original model as the entity

to be changed, selecting the generated line from

those compensated points as target entities, on

condition that all curves were continuous by its’

tangents, taking advantage of the function of CATIA

which reconstructs surfaces from fitting NURBS

curves, subsequently, we got the new die whose

surfaces can offset the springback.

The section boundary lines cut from primitive

model and compensated model were compared as

following graph:

Figure 6: Die profile reconstructed with Springback

compensation by CATIA software.

4 COMPENSATION EFFCT TEST

4.1 Simulation Model Establishment

We had obtained the compensated die model in

CATIA, and the next work was to test whether our

improvement to the die was effective, and whether

the springback of the B pillar drawn from a drawing

die was reduced to meet our expectation. All these

work would done in the DYNAFORM software,

which took a simulation to the forming process.

In CATIA, we saved the compensated die

surface as *.igs format, inputting this *.igs file to the

software DYNAFORM, off seting this shell model

with a distance equal to the thickness of a blank, so

we got the punch shell model; meanwhile we

extracted surfaces from the blank and blank holder

in their primitive solid structures to save as *.igs

format files, also inputting these files to

DYNAFORM as the blank and blank holder shell

models, meshing and positioning these shell models

including die, punch, blank, blank holder, the

established models in DYNAFORM as following :

Figure 7: Dynaform simulation models.

4.2 Forming Simulation

4.2.1 Simulation Process Parameters

According to the practical condition, we set some

process parameters in drawing simulation process as

following table:

Table 2: Simulation Process Parameters.

4.2.2 Simulation Results

After simulation completed, we used the

postprocessor module of DYNAFORM to open

*.d3plot file generated to observe forming results.

The following figure was the FLD (Forming Limit

Diagram) of the B pillar, and from this FLD we can

see the forming properties be good in general, there

being no crack and serve wrinkle happened on the B

pillar surface, most of its surface being in sufficient

drawing state, a small quantity of crack which

happened on the border of this B pillar would be

trimmed in later process, having no influence to the

whole forming quality. In brief, the whole forming

was well distributed, and the drawing process was

qualified.

Figure 8: FLD of the B pillar drawing simulation.

4.3 Springback Analysis Based on Die

Dimension Compensated

The purpose of all work above was to decrease the

springback of the B pillar by changing die molded

surface, and whether our purpose would achieve can

be confirmed though using springback analysis

module in DYNAFORM software. Dynaform

simulates forming process with dynamic explicit

algorithm and calculates springback process with

static implicit algorithm, ensuring calculating

efficiency in simulation and calculating precision in

springback analysis [6]. Inputting the *.dynain file

generated with drawing simulation above to

DYNAFORM, after setting all parameters related

with analysis, submitting mission to solver to

calculate. After DYNAFORM solver finished this

mission, we can open *.d3plot file to see analysis

results in DYNAFORM postprocessor. The results

contained two frames *.d3plot files, one was the

starting model and the other was the model with

springback. Exporting each frame result file, we

obtained numerical model of each frame. The

springback was the difference between the starting

model and the springback model. We also used

Geomagic Qualify Probe to compare the different

value between the two models. The springback

results which located the springback maximum and

minimum positions were displayed with highlighted

points as following figure. The difference values

were displayed in the meanwhile.

Figure 9: Springback of a B pillar stamped by an

improved die.

From this figure above, we can recognize the

maximum springback value was +2.09mm and the

minimum was -2.82mm, which were largely reduced,

compared with the original springback values which

were +3.64mm and -5.21mm. Additionally, we also

can see most of the difference values locate in the

interval from -1.05mm to 1.05mm, the average

dimensional difference of these two models was

changed to be ±0.53mm, and the standard deviation

was 0.60mm; That was to say, all deviations were

reduced in some extent, so the analysis results

supported our improving work to be effective.

5 CONCLUSIONS

Taking a comprehensive survey on our whole work

in this paper, we introduced a systematic method to

reduce springback of a car B pillar, and we used

optical scanner, HandyScan 700, to obtain points

datum of a drawn B pillar, contrasting difference

between the points datum and primitive model in

Geomagic Qualify Probe to obtain springback values;

According to these springback values, we changed

die surface with a compensation value in CATIA;

Afterwards, we used DYNAFORM to simulate the

drawing process with the reconstructed die model,

and analysed the springback of the B pillar which

stamped with new die and punch whose surfaces

were compensated. The results confirmed that our

improving work to die was effective. This method

solves the problem which is difficult to predict and

measure the springback of a final sheet metal, so it

can be valuable in some engineering projects.

Because of the springback phenomenon, the

dimensional precision of a car B pillar is very

difficult to ensure; To obtain a qualified B pillar, the

traditional method to debug press tools mainly

depends on the experience of a bench worker. In that

case, the workload of the worker is very heavy;

moreover, it takes the worker enormous amount of

time to find where the press tools must be modified,

so that the traditional method lengthens the cycle of

press tools’ development and manufacturing. By

using our method introduced in this paper, we can

accurately find where the press tools should be

modified with a high efficiency, so it can shorten

press tools debugging cycle; Besides this, as the case

mentioned in this paper, we decreased the average

deviation value of the B pillar from 0.66mm to

0.53mm, so the results also satisfied us, thus it can

enhance the forming qualification and precision of a

sheet metal.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the financial

support of Guangxi Education Department Project

for Science and Technology, under project The

Precision Control Method Study for Whole Life

Cycle of a Die with reference KY2015ZL125.

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