Die Maintenance Handbook Chapter 13

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Forming Flanges and Hems

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Flanges on metal parts fall into three basic types. The simplest isthe straight flange, which is a straight bend. Concave flanges aretermed stretch flanges because the metal formed into the flangestretches. Convex flanges are called shrink flanges because themetal is compressed or shrunk. There are also combinations of these types of flanges that occur in a single operation.

FLANGING OPERATIONS

There are factors that place limits on the flanging process, in-cluding:

• the severity of deformation accomplished before fracture occurs;• the amount of wrinkling or puckering permissible on a com-

pression flange; and• press energy or force available for flanging large areas with

conventional wipe flanges high in the press stroke.

FLANGE TYPES

Figure 13-1 illustrates a number of different flanges. Thestraight flange is the most common type. The problems associ-ated with straight flanges are springback and scoring.

Close tolerance bends should not be specified unless necessaryto enhance the appearance or function of the part. A variation in


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elastic recovery or springback is a problem. This results from varia-tions in material properties.

Forming stretch flanges involves stretching the metal during thebending operation. The greatest amount of stretch occurs at the edgeof the flange. The stretch amount is essentially zero at the bendradius.

Edge splitting can be a problem when stretch flanging (see Fig-ure 13-2). The likelihood of splitting depends on the material prop-erties and the edge condition resulting from shearing or trimming.Tensile stress can be reduced by shorter flange lengths or by pro-

viding notches or scallops. Notches will reduce the flange strength.The metal in a shrink flange compresses and causes it to shortenin length. The amount of shrinkage is greatest at the edge of theflange and diminishes to zero at the bend radius.

Irregular or curved flanges tend to have less springback prob-lems than straight flanges. A reverse flange is a combination of a stretch and shrink flange. Another combination flange is the

joggled flange. While the majority of the joggled flange is straight,the corners are stretch and shrink flanges, respectively.

A flanged hole is a type of stretch flange. Some flanged holeapplications are locating bosses, holes for tapped threads, openingsfor heat transfer tubes, and non-chafing passages for wires. Flang-ing an opening in a stamping can greatly increase the rigidity of thepart.

Figure 13-2. (a) Edge splitting can be a problem when stretch flanging. (b)Tensile stress can be reduced by providing notches or scallops.


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MINIMUM BEND RADII

The minimum bend radii vary depending on the type of metal.Most annealed metals can be bent to a radius equal to the thick-ness, although some softer metals can be bent to an inside ra-dius one-half metal thickness. Short bend-radius lengths reduce

the minimum bend radius. This is not a practical considerationif the minimum bend-radius length is eight or more times themetal thickness.

The rolling direction in sheet or strip metal limits the mini-

mum bend radius. An angle of 90° between the bend axis and thedirection of rolling allows most metals to bend to the smallestpossible radii.

Bend allowances are discussed in Chapter 12.

Strain

A simple general equation expresses the strain at the edge of

the stretch flange, where most failures begin:

2

1

1 x R

e R

= −(13-1)

where:

e x = strain at flange edge R

1= flange-edge radius before forming (in.[mm])

R2 = flange-edge radius after forming (in.[mm])

Circle grid analysis (CGA) is an excellent method for determin-ing the actual amount of strain at a flanged edge. Expanding a drilled, deburred hole with a lubricated conical punch to deter-mine the forming limit is one type of comparative test performedon materials to be flanged.

Edge conditions such as burrs and rough fractured edges re-

duce stretch-flange formability. Such edge conditions result in ex-cessive cold working of the metal.


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STOCK THICKNESS VARIATION

Producing close-tolerance stampings requires stock thicknessvariations to be held to a minimum. Figure 13-3 illustrates sectionsthrough a typical bending or forming die. If the stock thickness iscorrect, the part will be properly formed as shown in Figure 13-4.

If the stock is too thin, angles will be under-bent as illustratedin Figures 13-5 and 13-6. Severe die damage can result from at-tempting to form stock that is too thick for the die clearances.Large lateral forces are developed that can greatly exceed the

applied press tonnage. The force is multiplied by wedge-like ac-tion as shown in Figure 13-7. This type of damage often occursdue to operator inattention or an automation malfunction re-sulting in two blanks being formed in the die at one hit, as shownin Figure 13-8.

Figure 13-3. Section through a typical bending or forming die illustratingforming of the correct stock thickness.


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Figure 13-4. A correctly formed part produced in a die. Die clearances, diegeometry, and stock specifications must be correct to produce properlyformed stampings.

Figure 13-5. If the stock is too thin the angles will be under-bent.


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Figure 13-6. An incorrectly formed part produced from stock that is too thinresults in the bend or form angles being less than required.

Figure 13-7. Section through a bending or forming die shows that if the

stock is too thick, malformed parts and severe damage can occur.


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HEMMING OPERATIONS

Hems are primarily used to provide a smooth rounded edge andfor the attachment of one sheet metal part to another. They are a very effective way to eliminate a dangerous sheared edge. Hems

find extensive use in automobiles to join closure panels. A sharply bent flattened hem requires materials with high duc-

tility. Materials that do not have the ductility required to form a flattened hem may require a teardrop or rounded edge hem. Thesame minimum bend radii considerations based on material form-ability apply to hems used for attachment to other sheet metalparts (see Figure 13-9).

Tooling

Flanges and hems made in one operation require complex tool-ing. However, to simplify tooling, hemming a part that has been

Figure 13-8. Section through a bending or forming die shows that severedamage can occur if more than one thickness of stock is in the die.


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flanged in a previous operation may reduce overall cost. Figure13-9 illustrates four different types of hems (Smith 1990).

Using Hems for Assembly of Stampings

The sequence of a typical hemming stamping assembly opera-tion after bending a 90° flange in the outer panel is to place theinner panel in the flanged outer panel and bend the outer panelan additional 45°. The partially assembled panels are next trans-ferred to the hem die, where the final assembly takes place byflattening the pre-bent hem.

PRESSURES

Typical hemming pressures, including seaming pressures, aregenerally seven times the forming pressure required for 90° bends,and may be as high as a ratio of 40:1. Variables are: stock thickness,

Figure 13-9. Four different types of hems are shown; the bottom two show

hem types used to assemble stampings.


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tensile strength, size of area to be flattened or hemmed, and tight-ness of the hem.

The amount of pressure required depends upon the thicknessof the stock, the length of the bend, the width of the die, whethera lubricant is used, and the amount of wiping, ironing, or coining present.

The use of Equation 13-2 for deriving bending pressures is validfor V-shaped dies only. For channel forming and U-forming, multi-ply the result by 2. If forming a channel with a flat bottom, a blank holder is necessary. Multiply the blank holder area in square

inches by 0.15 to derive the approximate required tonnage, thenadd to the bending force derived from the equation. Equation 13-2is used to determine V-bending forces for air bending in a die suchas that used for simple press-brake tooling.

2 KLSt F

W =

(13-2)

where:

F = bending force (lbf [N]) K = die-opening factor: varies from 1.20 for a die opening of

16 times metal thickness to 1.33 for a die opening of 8times metal thickness

L = length of bent part (in. [mm])S = ultimate tensile strength (psi [Pa])t = metal thickness (in. [mm])W = width of V-channel or U-forming lower die (in. [mm])

The approximate force required to bend or flange a sheet (Fig-ure 13-10) is given by:

2

1 2

0.167 (theoretical)SLt

F r r C

=

+ +

(13-3)

2

1 2

0.333 (for wiping dies)SLt

F r r C

=

+ +

where:

F = bending force (lbf [N])S = ultimate tensile strength (psi [Pa])


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L = length of bend (in. [mm])t = sheet metal thickness (in. [mm])r1 = punch radius (in. [mm])r 2 = die radius (in. [mm])C = die clearance (in. [mm])

The flanging pad must grip the part firmly to insure the partwill remain in tight contact with the male die half during flanging.To determine if pad force is sufficient, use the following equation:

3 p

SLt F = (13-4)

where:

F p= pad force (lbf [N])S = ultimate strength of the material (psi [Pa]) L = flange length (in. [mm])t = material thickness (in. [mm])

Figure 13-10. Factors entered to determine flanging force requirements.


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REFERENCE

Smith, D. 1990. Die Design Handbook. Section 6, Bending of Metals. Dearborn, MI: Society of Manufacturing Engineers.

Die Maintenance Handbook Chapter 13

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