# Metal Forming: The Ultimate Guide

Intrigued by the marvels of metal stamping? In this blog post, we dive into the fascinating world of hole flanging, necking, and bulging. Our expert mechanical engineer will guide you through the intricacies of these processes, explaining key concepts and sharing insider insights. Get ready to expand your knowledge and appreciate the art behind shaping metal like never before!

## Hole Flanging

Hole flanging is a punching method that uses a mold to turn the edge of the hole of the part to an upright or a straight edge at a certain angle.

Hole Flanging Type:

Depending on the shape of the blank and the edge of the hole being flanged, there are flanging holes on the flat plate, and also on the curved surface, such as the flanging hole on the tube blank; the flanged holes can be rounded or non-rounded.

### 1.1  Round hole flanging

1. Deformation characteristics of round hole flanging

Deformation characteristics of round hole flanging:

• Deformation is local and mainly occurs in the annular part (d1-d0) at the bottom of the punch. This area is the deformation area of the round holeflanging.
• The material in the deformation zone is stretched in the tangential and radial directions, resulting in deformation that is elongated in both the tangential and radial directions and has a reduced thickness.
• The deformation area is not uniform, the radial elongation is not obvious, the tangential deformation is large, and the more the mouth is extended, the more the mouth is thinner.

1. Forming limit of round hole flanging

The forming limit is expressed by the hole-flanging factor K:

Limit hole-flanging factor Kmin.

Factors affecting the limit hole flanging coefficient:

1. Process design of round hole-flanging

(1) Technology of round hole

• The radius of the fillet between the vertical edge after flangingand the flange should meet: material thickness t <2mm, r = (2 ~ 4) t; material thickness t> 2mm, r = (1 ~ 2) t;
• If the above requirements cannot be met, a reshaping process needs to be added after turning the holes in order to set the required fillet radius.
• Afterflanging, the thickness of the vertical edge mouth is most severely reduced, and the thickness at the thinnest part is:

(2) Process arrangement for round hole

Usually, before flanging the hole, it is necessary to pre-punch the hole for the hole-flanging, and then determine whether it can be turned at one time according to the height of the hole and the coefficient of the hole-flanging, and then determine the forming method of the hole-flanging parts.

(3) Process calculation of plate hole-flanging

1) Determine the diameter of the pre-punched hole

2) Calculate the height of the hole to determine whether the hole can be successfully turned once.

3) Determine the number of turning holes

When the hole-flanging height H <Hmax, it can be flanged at one time.

(4) Process calculation of drawing the bottom hole first and then flanging the hole

1) Calculate the hole-flanging height h that can be reached after the pre-drawing is:

2) Calculate pre-punching diameter and drawing height before hole-flanging:

3) Deep drawing process calculation

(5) Calculation of the hole flanging force

When using a cylindrical flat bottom punch to flange a hole, it can be calculated as follows:

The force for flanging holes with a tapered or spherical punch is slightly less than the value calculated by the above formula.

1. Round hole mold

(1) Mold structure of round hole-flanging

Formal hole-flanging die

Inverted hole-flanging mold

Blanking, deep drawing, punching, and hole-flanging compound dies

(2) Structure and size design of the working part of the hole-flanging die

1) Structure and size of round hole punch

2) Clearance C between convex and concave die

### Flanging

Flanging refers to a stamping method that uses a mold to turn the edges of the product into an upright or straight edge at a certain angle.

According to the shape of the flanged outer edge:

• Inner curved flange on the outer edge
• Outercurved flange on the outer edge

### 2.1   Inner curved flange on the outer edge

The deformation is similar to a round hole-flanging, which belongs to elongation.

The deformation area is mainly tangentially stretched, and the deformation at the edges is the largest, which is easy to crack.

The degree of deformation is:

### 2.2 Outer curved flange on the outer edge

The outer-curved flanging deformation of the outer edge is similar to shallow drawing, and belongs to compression type deformation.

The deformation zone of the billet mainly generates compressive deformation under the action of tangential compressive stress, which is easy to lose stability and wrinkle.

The degree of deformation can be expressed as:

### 2.3  Outer edge flanging method

• Calculation method of blank size
• Mold structure: steel mold or soft mold
• Need to control the rebound
• For vertical edges with different directions, the method of segmented flanging should be adopted

Hole-flanging, Flanging and Shaping

Thinning hole-flanging or flanging refers to a deformation process that uses a smaller die gap to force the thickness of the vertical edge to become thinner and increase in height.

### Necking

Necking is a stamping method that uses a mold to reduce the radial dimension of the end of a hollow or tubular part.

### 3.1  Necking deformation characteristics

1. Necking deformation characteristics

• Zone A-the non-deformed zone that has undergone plastic deformation
• Area C-non-deformed area waiting for deformation
• Zone B-the deformation zone that is being deformed
• Preventing instability and wrinkling is the main problem to be solved in the necking process
1. Neck forming limit

The degree of necking deformation is expressed by the ratio of the neck diameter after necking to the diameter of the blank before necking.

Shrinking coefficient: m = d / D

The minimum value of the necking coefficient obtained under the premise of ensuring the stability of the necking member is called the limit necking coefficient [m].

[m] is related to the plasticity of the material and the supporting structure of the mold.

Necking die for different supporting methods

### 3.2   Necking process design

1. Determination of blank size

See Table 6-4 for the determination of the size of the blank of the necking piece.

1. Confirmation of necking times

When the actual necking coefficient m is smaller than the limit necking coefficient [m], the necking cannot be performed at one time.

The number of necking can be calculated by:

1. Calculation of necking force

When there is no support necking, the necking force is:

### 3.3  Necking die structure

Necking die without support

Necking die with external support

Necking and flaring compound dies

### Bulging

Bulging is a stamping method that uses a mold to plastically deform the interior of a hollow part under the action of two-way tensile stress to obtain a convex part.

### 4.1  Bulging deformation characteristics (two cases)

The deformation area is almost the entire blank or the open end, and the open end of the blank is contracted and deformed.

Therefore, the deformation in the deformation area is a deformation state in which the circumference is elongated, axially compressed, and the thickness is reduced.

The deformation zone is limited to the part to be swollen in the middle of the blank.

The deformation zone mainly produces elongation deformation in the circumferential direction and thinning in the thickness direction.

Bulging is an elongation-forming process.

Preventing bursting is the key problem to be solved in the bulging process.

### 4.2  Bulging forming limit

The degree of bulging deformation is expressed by the ratio of the maximum diameter of the convex bulging obtained after bulging and the diameter of the blank before bulging, that is, the bulging coefficient:

The larger the bulging coefficient value, the greater the degree of bulging deformation.

### 4.3  Bulging process design

1. Determination of bulging blank

When bulging, the length of the blank when axially allowed to deform freely:

1. Calculation of bulging force

σZ – The true stress in the bulging deformation area, take σZb in approximate estimation.

### 4.4  Bulging method and bulging mold structure

• Steel molds or soft molds can be used. Soft molds are widely used.
• Soft mold medium can be rubber, paraffin, PVC plastic, high pressure liquid and high pressure gas.

Rubber bulging mold

High-pressure liquid punch bulging

Hydraulic bulging of tee joint

### 5.1  Beading, convex hull pressing

Beading and convex hull pressing are embossing methods that use a mold to produce convex hulls or ribs (reinforcing ribs) on the part.

Features of beading and convex hulls forming

• Deformation zone is local
• The deformation zone is stretched in both directions and the thickness is reduced.It is an elongation type, and the main failure form is pull-break
• The quality of the bulge is good

The forming limit of the beading can be expressed by the amount of change in the length of the deformation zone before and after the beading

1. Compress convex hull

The forming limit of the convex hull can be expressed by the height h of the convex hull

## 4 Types of Metal Stamping Process

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Author

#### Shane

##### Founder of MachineMFG

As the founder of MachineMFG, I have dedicated over a decade of my career to the metalworking industry. My extensive experience has allowed me to become an expert in the fields of sheet metal fabrication, machining, mechanical engineering, and machine tools for metals. I am constantly thinking, reading, and writing about these subjects, constantly striving to stay at the forefront of my field. Let my knowledge and expertise be an asset to your business.

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