How to Unfold Special-Shaped Holes on Irregular Surfaces?

In this post, the crack propagation simulation model of welded joint with holes composed of weld and base metal is established;

The effects of hole shape, size and position on the stress intensity factor at the crack tip in 6061 aluminum alloy welded joints for aviation were studied.

Flanging of special-shaped hole is a difficult problem in sheet metal unfolding. The mainstream methods include neutral layer unfolding, 3D software sheet metal unfolding and so on.

Both are commonly used in typically formed parts. Irregular surfaces are cumbersome, need post-optimization, and may not even be realized.

There is also an unconventional one-step forming expansion method, which belongs to the rough type.

Although it is fast and widely applicable, it has a large dimensional deviation, which is generally only used as a reference for hole shape expansion.

Limitations of conventional deployment

Limitations of neutral layer deployment

The special-shaped hole is composed of convex arc, concave arc and straight line with different radius, and the stress state and deformation properties of each part are different.

  • The straight part can be regarded as bending deformation;
  • The convex arc part can be regarded as hole turning deformation;
  • The concave arc part can be regarded as drawing deformation.

The top of the drawing cavity of 8228 mesh cover is circular, the bottom is rounded rectangle, and the flanging of the oil tank hole is 5° inclined to the vertical direction, which is mainly composed of straight lines on both sides and concave arcs at both ends.

According to the conventional expansion, it is necessary to find the corresponding formula according to the hole turning form corresponding to each arc or straight line, and then obtain the expansion size of each section.

The joints between each section of the unfolding line calculated by the theory are generally not smooth, so the unfolding line with good quality must be obtained after post-transition treatment.

The whole process is relatively complex and takes a relatively long time. Moreover, as shown in Fig. 1, the 8228 mesh cover has 52 such holes in total.

3D drawing of 8228 mesh cover product

Fig. 1 3D drawing of 8228 mesh cover product

According to the consistency of hole shape, 26 holes need to be expanded, which requires 26 calculations. It can be seen that the calculation workload of the whole oil tank hole is quite large.

In addition, in theory, the concave arc needs to be calculated according to the flanging of the round hole, but there are only three cases when looking up the corresponding formula, namely (a) the flanging of the flat part, (b) the flanging of the deep drawing part, and (c) the flanging after deep drawing.

In this example, the flanging of the oil groove hole on the quadrangular surface can only be relatively approximate, and there is no specific formula to calculate it accurately.

Therefore, the neutral layer expansion has certain limitations.

For the development of special-shaped holes on irregular surfaces, this scheme will not only take a long time, but also the flanging concave arc section of four corner oil tank holes is not applicable, or needs to be improved and adjusted in the later stage.

Limitations of general 3D sheet metal unfolding

Taking UG 3D software as an example, 3D sheet metal unfolding can be roughly divided into two categories: parametric sheet metal unfolding and nonparametric sheet metal unfolding.

Whether the former or the latter, the personnel dealing with flanging and unfolding are required to have certain professional sheet metal knowledge and 3D modeling foundation.

Parametric sheet metal unfolding requires that some operations such as bending and edge bending must be completed under “sheet metal module” in the process of 3D modeling.

The requirements for the comprehensive quality of modelers are relatively high, which is mainly reflected in the setting of some specific parameters involved.

If there is no accumulation of relevant experience, there may be large deviation in the size of the expanded product parametric sheet metal modeling.

Parameterless sheet metal unfolding has less stringent requirements for existing three-dimensional products.

It does not need parameters and does not have to be generated under the “sheet metal module”. However, recognizable bending edges are also required to recognize and expand automatically.

Generally speaking, 3D sheet metal unfolding is more practical in single-stage or multi-stage bending parts whose main surface is plane.

It is difficult to unfold a special-shaped hole on an irregular surface simply by using a three-dimensional “sheet metal module” or the corresponding nonparametric unfolding function.

Limitations of one-step forming deployment

One step forming and unfolding generally belongs to the rough estimation of the approximate unfolding shape of special-shaped holes.

In the real operation process, its accuracy and operation time are directly related to the size of surface virtual meshing.

The finer the mesh, the higher the precision of the expansion line and the corresponding operation time will increase; On the contrary, the larger the mesh, the lower the accuracy, the faster the speed, and each has its own disadvantages.

A new method of small slice subdivision expansion

The oil tank hole flanging of the 8228 mesh cover can not be well realized by the above expansion methods, so it is urgent to study a new method.

So, can this oil tank hole flanging really be simplified into ordinary bending?

There is no doubt that the straight edge segment can be realized, but how to deal with the arc at both ends needs further thinking.

Inspired by one-step forming, the oil groove hole surface can be subdivided into 5 parts, 10 parts or even more small enough pieces.

In this way, it can be infinitely approximated as a straight edge flanging with a very small width, and the more accurate expansion size of the straight edge flanging can be obtained by the neutral layer expansion method.

Expansion of lines on both sides

It is known that although the flanging of the oil tank hole is irregular, the two straight lines can be simplified by flanging approximately.

According to the previous experience of sheet metal unfolding of mesh cover, it can be simplified to unfold according to the inner material and add compensation, without looking up the table and applying a lot of formulas.

In terms of angle, because the hole direction of the hole turning die needs to be optimized to be vertical upward and the included angle of the hole is 10 °, the two side surfaces composed of straight lines are actually variable angle surfaces. The specific deployment process is as follows:

(1) Rotate the outer edge of flanging along the bending line (orange surface in Fig. 2) and offset it by 0.12mm for compensation.

Simplified expanded partial view of lines on both sides

Fig. 2 Simplified expanded partial view of lines on both sides

  • Intersect with the drawing surface of the substrate to obtain an intersecting curve, that is, straight edge expansion.

(3) In the later stage, the intersecting curve and two end arcs need to be simplified into straight line segments and arcs.

The unfolding of the straight-line part is generally conventional, and those with general bending unfolding experience can think of this treatment.

In fact, it can be completed in 2D environment with AutoCAD, but the operation in 3D should be more convenient, fast, intuitive and efficient.

Expansion of concave arc at both ends

The concave arc at both ends is actually a conic, not an arc, so in a strict sense, the neutral layer expansion method previously introduced is not applicable.

If the neutral layer is to be used for expansion, the concave arc needs to be segmented and approximated into a small concave arc, and then look up the table and apply the formula for calculation.

The results obtained can roughly ensure the accuracy, but the operation is too complex and the operability is not strong.

Here, a new method of “small slice subdivision expansion” is used to divide the concave arc surfaces at both ends into infinite pieces, and then 7 pieces are extracted equidistant.

Because the sheet is small enough, it can be understood that the width of these 7 sheets is 0, that is, they are 7 straight lines.

As shown in Fig. 2, use the three-dimensional bisection command to divide the concave arc bending lines at both ends into 7 reference points, and measure and record the line length from these points to the flanging contour line in turn.

Enter the UG sketch, and the datum plane is perpendicular to the blanking direction.

The length of the line segment just measured corresponds to the radius value of the diameter marked by the dotted circle in Fig. 3.

Here, according to the process requirements, negative compensation is made for the arc flanging height at both ends, and the green thin solid line is the compensated position.

Then, the spline curve is used for line tangent approximation with each arc to form the orange line in Figure 3, which is the expansion line of the arc segment.

The other concave arc is also treated with this scheme to obtain the final complete expansion line.

Simplified expanded view of concave arc at both ends

Fig. 3 Simplified expanded view of concave arc at both ends

Post optimization of expansion line

Through the above processing, we get the expansion lines of lines and arcs, but these are splines. It is not reasonable for post-processing.

In the past, other development lines were projected to the two-dimensional environment according to the blanking direction, and then smoothly transformed into straight lines and arcs one by one, which took a lot of time.

In contrast, for the obtained spline expansion line, in UG 3D software, you can simply use a “simplify curve” command to simplify all splines into lines and arcs, and it is also a smooth transition.


At present, this new method of “small piece subdivision” has been officially adopted in some recent new products of our company.

The oil groove holes of 9100, 8325 and other mesh covers are irregular curved surface special-shaped hole flanging.

The unfolding obtained by this method and the sheet metal parts obtained by trial production can meet the design requirements.

The whole process from product modeling to sheet metal unfolding can be completed in the same three-dimensional environment.

The mutual conversion between software is omitted, and there is no huge amount of calculation. The whole process is clear.

The most important thing is that the simplified line obtained by this method is already the expansion line in space.

In the AutoCAD environment, the expansion can only be processed on the plane, and the real expansion line can be obtained only by post projection conversion.

This new expansion method effectively avoids some unnecessary errors in the conversion process, and it is easier to ensure the quality of the expansion line.

It is believed that in the increasingly modern and high-end sheet metal industry, this new method of “small piece subdivision” will inevitably form a new trend and lead to the development of sheet metal parts in the future.

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