Looking to learn more about metal stamping and die design forming? Look no further than this comprehensive guide from MachineMfg.
Whether you’re a seasoned professional or just starting out in the industry, this article has everything you need to know about hole flanging, non-round hole flanging, necking, and bulging.
From the deformation characteristics to the forming limits and process designs, this guide covers it all.
So if you’re looking to expand your knowledge and improve your skills in metal stamping and die design forming, be sure to check out this must-read article.
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
- 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.
- 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:
- Material properties
- Pre-punching quality
- Punch shape
- Relative thickness of material
- 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.
- 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
1.2 Non-round hole flanging
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
Further reading: Thinning flanging
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
- 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
- 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
- Determination of blank size
See Table 6-4 for the determination of the size of the blank of the necking piece.
- 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:
- 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
- Determination of bulging blank
When bulging, the length of the blank when axially allowed to deform freely:
- Calculation of bulging force
σZ – The true stress in the bulging deformation area, take σZ=σb 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
Beading, convex hull pressing and embossing
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
- Beading
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
- Compress convex hull
The forming limit of the convex hull can be expressed by the height h of the convex hull
