Sheet-Metal Forming Processes and Equipment

FIGURE 1 Examples of sheet-metal parts.

(a) Stamped parts.
(b) Parts produced by spinning.

Examples of sheet-metal parts

TABLE 1 General Characteristics of Sheet-metal Forming Processes (in alphabetic order)

Forming ProcessCharacteristics
DrawingShallow or deep parts with relatively simple shapes, high production rates, high toolling and equipment costs
ExplosiveLarge sheets with relatively simple shapes, low tooling cost but high labor cost, low-quantity production, long cycle times
IncrementalSimple to moderately complex shapes with good surface finish;low production rates, but no dedicated tooling required; limited materials
Magnetic-pulseShallow forming, bulging, and embossing operations on relatively low strength sheets, requires special tooling
PeenShallow contours on large sheets, flexibility of operation, generally high equipment costs, process also used for straightening formed parts
RollLong parts with constant simple or complex cross sections, good surface finish, high production rates, high tooling costs
RubberDrawing and embossing of simple or relatively complex shapes, sheet surface protected by rubber membranes, flexibility of operation, low tooling costs
SpinningSmall or large axisymmetric parts; good surface finish; low tooling costs, but labor costs can be high unless operations are automated
StampingIncludes a wide variety of operations, such as punching, blanking, embossing, bending, flanging, and coining; simple or complex shapes formed at high production rates; tooling and equipment costs can be high, but labor cost is low
StretchLarge parts with shallow contours, low-quantity production, high labor costs, tooling and equipment costs increase with part size
SuperplasticComplex shapes, fine detail and close dimensional tolerances, long forming times (hence production rates are low), parts not suitable for high-temperature use

 

 

FIGURE 2

(a) Schematic illustration of shearing with a punch and die, indicating some of the process variables.
Characteristic features of
(b) a punched hole and
(c) the slug.
(Note that the scales of (b) and (c) are different.)

Schematic illustration of shearing with a punch and die

 

FIGURE 3

(a) Effect of the clearance, c, between punch and die on the deformation zone in shearing. As the clearance increases, the material tends to be pulled into the die rather than be sheared. In practice, clearances usually range between 2 and 10% of the thickness of the sheet.
(b) Microhardness (HV) contours for a 6.4-mm (0.25-in.) thick AISI 1020 hot-rolled steel in the sheared region.

clearance

 

FIGURE 4

(a) Punching (piercing) and blanking.

(b) Examples of various die-cutting operations on sheet metal.

Lancing involves slitting the sheet to form a tab.

Punching (piercing) and blanking

 

FIGURE 5

(a) Comparison of sheared edges produced by conventional (left) and by fineblanking (right) techniques.

(b) Schematic illustration of one setup for fine blanking.

sheared edges

 

FIGURE 6 Slitting with rotary knives.

This process is similar to opening cans.

Slitting with rotary knives

 

FIGURE 7 An example of Taylor-welded blanks

Production of an outer side panel of a car body by laser butt welding and stamping.

Taylor-welded blanks

 

FIGURE 8 Examples of laser butt-welded and stamped automotive-body components.

laser butt-welded and stamped automotive-body components

 

FIGURE 9 

Schematic illustrations of the shaving process.

(a) Shaving a sheared edge.

(b) Shearing and shaving combined in one stroke.

Schematic illustrations of the shaving process

 

FIGURE 10 Examples of the use of shear angles on punches and dies.

the use of shear angles on punches and dies

 

FIGURE 11  Schematic illustrations

(a) before and (b) after blanking a common washer in a compound die.

Note the separate movements of the die (for blanking) and the punch (for punching the hole in the washer).

(c) Schematic illustration of making a washer in a progressive die.

(d) Forming of the top piece of an aerosol spray can in a progressive die.

Note that the part is attached to the strip until the last operation is completed.

blanking a common washer in a compound die

 

TABLE 2  Important Metal Characteristics for Sheet-forming Operations

CharacteristicImportance
ElongationDetermines the capability of the sheet metal to stretch without necking and failure; high strain-hardening exponnet (n) and strain-rate sensitivity exponent (m) are desirable
Yield-point elongationTypically observed with mild-steel sheets (also called Luder’s bands or stretcher strains); results in depressions on the sheet surface; can be eliminated by temper rolling, but sheet must be formed within a certain time after rolling
Anisotropy (planar)Exhibits different behavior in different planar directions, present in cold-rolled sheets because of preferred orientation or mechanical fibering, causes earing in deep drawing, can be reduced or eliminated by annealing but at lowered strength
Anisotropy (normal)Determines thinning behavior of sheet metals during stretching, important in deep drawing
Grain sizeDetermines surface roughness on stretched sheet metal; the coarse the grain, the rougher is the apperance (like an orange peel); also affects material strength and ductility
Residual stressesTypically caused by nonuniform deformation during forming, results in part distortion when sectioned, can lead to stress-corrosion cracking, reduced or eliminated by stress relieving
SpringbackDue to elastic recovery of the plastically deformed sheet after unloading, causes distortion of part and loss of dimensional accuracy, can be controlled by techniques such as overbending and bottoming of the punch
WrnklingCaused by compressive stresses in the plane of the sheet; can be objectionable; depending on its extent, can be useful in imparting stiffness to parts by increasing their section modulus; can be controlled by proper tool and die design
Quality of sheared edgesDepends on process used; edges can be rough, not square, and contain cracks, residual stresses, and a work-hardened layer, which are all detrimental to the formability of the sheet; edge quality can be improved by fine blanking, reducing the clearance, shaving, and improvements in tool and die design and lubrication
Surface condition of sheetDepends on sheet-rolling practice; important in sheet forming, as it can cause tearing and poor surface quality

 

FIGURE 12

(a) Yield-point elongation in a sheet-metal specimen.

(b) Lüder’s bands in a low-carbon steel sheet.

(c) Stretcher strains at the bottom of a steel can for household products.

Yield-point elongation in a sheet-metal specimen

 

FIGURE 13 

(a) A cupping test (the Erichsen test) to determine the formability of sheet metals.

(b) Bulge-test results on steel sheets of various widths. The specimen farthest left is subjected to, basically, simple tension. The specimen that is farthest right is subjected to equal biaxial stretching.

determine the formability of sheet metals

 

FIGURE 14 

(a) Strains in deformed circular grid patterns.

(b) Forming-limit diagrams (FLD) for various sheet metals. Although the major strain is always positive (stretching), the minor strain may be either positive or negative. R is the normal anisotropy of the sheet, as described in Section 4.

Strains in deformed circular grid patterns

 

FIGURE 15 

The deformation of the grid pattern and the tearing of sheet metal during forming. The major and minor axes of the circles are used to determine the coordinates on the forming-limit diagram in Fig. 14b.

The deformation of the grid pattern and the tearing of sheet metal during forming

 

 

FIGURE 16 

Bending terminology. Note that the bend radius is measured to the inner surface of the bent part.

Bending terminology

 

FIGURE 17 

(a) and (b) The effect of elongated inclusions (stringers) on cracking as a function of the direction of bending with respect to the original rolling direction of the sheet.

(c) Cracks on the outer surface of an aluminum strip bent to an angle of 90°. Note also the narrowing of the top surface in the bend area (due to the Poisson effect).

The effect of elongated inclusions (stringers) on cracking

 

TABLE 3  Minimum Bend Radius for Various Metals at Room Temperature

MaterialCondition
SoftHard
Aluminum alloys06T
Beryllium copper04T
Brass (low-leaded)02T
Magnesium5T13T
Austenitic stainless steel0.5T6T
Low-carbon, low-alloy, and HSLA0.5T4T
Titanium0.7T3T
Titanium alloys2.6T4T

 

 

FIGURE 18 

Relationship between R/T and tensile reduction of area for sheet metals. Note that sheet metal with a 50% tensile reduction of area can be bent over itself in a process like the folding of a piece of paper without cracking.

Relationship between RT and tensile reduction of area for sheet metals

 

 

FIGURE 19 

Springback in bending. The part tends to recover elastically after bending, and its bend radius becomes larger. Under certain conditions, it is possible for the final bend angle to be smaller than the original angle (negative springback).

Springback in bending

 

 

FIGURE 20  Methods of reducing or eliminating springback in bending operations.

Methods of reducing or eliminating springback in bending operations

 

FIGURE 21 

Common die-bending operations showing the die- opening dimension, W, used in calculating bending forces.

Common die-bending operations

 

 

FIGURE 22  Examples of various bending operations.

Examples of various bending operations

 

 

FIGURE 23  (a) through (e) Schematic illustrations of various bending operations in a press brake. (f) Schematic illustration of a press brake.

Schematic illustrations of various bending operations in a press brake

 

FIGURE 24   (a) Bead forming with a single die. (b) through (d) Bead forming with two dies in a press brake.

Bead forming with a single die

 

 

FIGURE 25  Various flanging operations.

(a) Flanges on flat sheet.

(b) Dimpling.

(c) The piercing of sheet metal to form a flange. In this operation, a hole does not have to be pre-punched before the punch descends. Note, however, the rough edges along the circumference of the flange.

(d) The flanging of a tube.

Note the thinning of the edges of the flange.

Various flanging operations

 

 

FIGURE 26 

(a) Schematic illustration of the roll-forming process.

(b) Examples of roll-formed cross sections.

Schematic illustration of the roll-forming process

 

 

FIGURE 27  Methods of bending tubes.

Internal mandrels or filling of tubes with particulate materials such as sand are often necessary to prevent collapse of the tubes during bending.

Tubes also can be bent by a technique in which a stiff, helical tension spring is slipped over the tube. The clearance between the outer diameter of the tube and the inner diameter of the spring is small; thus, the tube cannot kink and the bend is uniform.

Methods of bending tubes

 

 

FIGURE 28 

(a) The bulging of a tubular part with a flexible plug. Water pitchers can be made by this method.

(b) Production of fittings for plumbing by expanding tubular blanks under internal pressure. The bottom of the piece is then punched out to produce a “T.”

The bulging of a tubular part with a flexible plug

 

FIGURE 29  Schematic illustration of a stretch-forming process. Aluminum skins for aircraft can be made by this method.

Schematic illustration of a stretch-forming process

 

FIGURE 30  The metal-forming processes involved in manufacturing a two-piece aluminum beverage can.

metal-forming processes

 

 

FIGURE 31 

(a) Schematic illustration of the deep-drawing process on a circular sheetmetal blank. The stripper ring facilitates the removal of the formed cup from the punch.

(b) Process variables in deep drawing. Except for the punch force, F, all the parameters indicated in the figure are independent variables.

Schematic illustration of the deep-drawing process

 

 

FIGURE 32 

Strains on a tensile-test specimen removed from a piece of sheet metal. These strains are used in determining the normal and planar anisotropy of the sheet metal.

Strains on a tensile-test specimen removed from a piece of sheet metal

 

 

TABLE 4  Typical Ranges of Average Normal Anisotropy, Ravg for Various Sheet Metals

Zinc alloys0.4-0.6
Hot-rolled steel0.8-1.0
Cold-rolled, rimmed steel1.0-1.4
Cold-rolled, aluminum-killed steel1.4-1.8
Aluminum alloys0.6-0.8
Copper and brass0.6-0.9
Titanium alloys (α)3.0-5.0
Stainless steels0.9-1.2
High-strength, low-alloy steels0.9-1.2

 

 

FIGURE 33 

The relationship between average normal anisotropy and the limiting drawing ratio for various sheet metals.

The relationship between average normal anisotropy and the limiting drawing ratio for various sheet metals

 

 

FIGURE 34 

Earing in a drawn steel cup, caused by the planar anisotropy of the sheet metal.

Earing in a drawn steel cup

 

 

FIGURE 35 

(a) Schematic illustration of a draw bead.

(b) Metal flow during the drawing of a box-shaped part while using beads to control the movement of the material.

(c) Deformation of circular grids in the flange in deep drawing.

Schematic illustration of a draw bead

 

 

FIGURE 36 

An embossing operation with two dies. Letters, numbers, and designs on sheetmetal parts can be produced by this process.

An embossing operation with two dies

 

FIGURE 37 

(a) Aluminum beverage cans. Note the excellent surface finish.

(b) Detail of the can lid, showing the integral rivet and scored edges for the pop-top.

Aluminum beverage cans

 

 

FIGURE 38 

Examples of the bending and embossing of sheet metal with a metal punch and with a flexible pad serving as the female die.

Examples of the bending and embossing of sheet metal

 

 

FIGURE 39 

The hydroform (or fluid-forming) process. Note that, in contrast to the ordinary deep-drawing process, the pressure in the dome forces the cup walls against the punch. The cup travels with the punch; in this way, deep drawability is improved.

The hydroform (or fluid-forming) process

 

FIGURE 40 

(a) Schematic illustration of the tube-hydroforming process.

(b) Example of tube-hydroformed parts. Automotive-exhaust and structural components, bicycle frames, and hydraulic and pneumatic fittings are produced through tube hydroforming.

Schematic illustration of the tube-hydroforming process

 

FIGURE 41  

Hydroformed automotive radiator closure.

Hydroformed automotive radiator closure

 

 

FIGURE 42 

Sequence of operations in producing a tubehydroformed component:

(1) tube as cut to length;

(2) after bending;

(3) after hydroforming.

Sequence of operations in producing a tubehydroformed component

 

 

FIGURE 43 

Schematic illustration of expansion of a tube to a desired cross section through (a) conventional hydroforming and (b) pressure sequence hydroforming.

Schematic illustration of expansion of a tube to a desired cross section

 

 

FIGURE 44  

View of the tube-hydroforming press, with bent tube in place in the forming die.

View of the tube-hydroforming press

 

 

FIGURE 45 

(a) Schematic illustration of the conventional spinning process.

(b) Types of parts conventionally spun. All parts are axisymmetric.

Schematic illustration of the conventional spinning process

 

 

FIGURE 46 

(a) Schematic illustration of the shear-spinning process for making conical parts. The mandrel can be shaped so that curvilinear parts can be spun. (b) and (c) Schematic illustrations of the tube-spinning process.

Schematic illustration of the shear-spinning process for making conical parts

 

 

FIGURE 47 

(a) Illustration of an incremental-forming operation. Note that no mandrel is used and that the final part shape depends on the path of the rotating tool.

(b) An automotive headlight reflector produced through CNC incremental forming. Note that the part does not have to be axisymmetric.

Illustration of an incremental-forming operation

 

FIGURE 48 

Types of structures made by superplastic forming and diffusion bonding of sheet metals. Such structures have a high stiffness-to-weight ratio.

Types of structures made by superplastic forming and diffusion bonding of sheet metals

 

 

FIGURE 49 

(a) Schematic illustration of the explosive-forming process.

(b) Illustration of the confined method of the explosive bulging of tubes.

 Schematic illustration of the explosive-forming process

 

 

FIGURE 50 

(a) Schematic illustration of the magnetic-pulse- forming process used to form a tube over a plug.

(b) Aluminum tube collapsed over a hexagonal plug by the magnetic pulse- forming process.

Schematic illustration of the magnetic-pulse- forming process

 

 

FIGURE 51 

(a) A selection of common cymbals.

(b) Detailed view of different surface textures and finishes of cymbals.

A selection of common cymbals

 

 

FIGURE 52 

Manufacturing sequence for the production of cymbals.

Manufacturing sequence for the production of cymbals

 

 

FIGURE 53 

Hammering of cymbals.

(a) Automated hammering on a peening machine;

(b) hand hammering of cymbals.

Hammering of cymbals

 

FIGURE 54 

Methods of manufacturing honeycomb structures:

(a) expansion process;

(b) corrugation process;

(c) assembling a honeycomb structure into a laminate.

Methods of manufacturing honeycomb structures

 

FIGURE 55 

Efficient nesting of parts for optimum material utilization in blanking.

Efficient nesting of parts for optimum material utilization in blanking

 

 

FIGURE 56 

Control of tearing and buckling of a flange in a right angle bend.

Control of tearing and buckling of a flange in a right angle bend

 

 

FIGURE 57 

Application of notches to avoid tearing and wrinkling in right-angle bending operations.

Application of notches to avoid tearing and wrinkling in right-angle bending operations

 

 

FIGURE 58 

Stress concentrations near bends.

(a) Use of a crescent or ear for a hole near a bend.

(b) Reduction of severity of tab in flange.

Stress concentrations near bends

 

 

FIGURE 59 

Application of (a) scoring or (b) embossing to obtain a sharp inner radius in bending. Unless properly designed, these features can lead to fracture.

Application of scoring or embossing to obtain a sharp inner radius in bending

 

 

FIGURE 60 

(a) through (f) Schematic illustrations of types of press frames for sheet forming operations. Each type has its own characteristics of stiffness, capacity, and accessibility.

(g) A large stamping press.

Schematic illustrations of types of press frames for sheet forming operations

 

 

FIGURE 61 

Cost comparison for manufacturing a round sheet- metal container either by conventional spinning or by deep drawing.

Note that for small quantities, spinning is more economical.

Cost comparison for manufacturing a round sheet- metal container

 

2017-12-07T11:23:58+00:00 Forming|

2 Comments

  1. Drakier December 12, 2017 at 10:12 am - Reply

    Looks like some good data ty

  2. Dave April 12, 2018 at 7:23 pm - Reply

    Great post – very information filled and useful! Keep up the good work 🙂

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