Chapter 17 Sheet Forming Processes (Part 2) Drawing & Stretching, Alternative Methods, Pipe Welding,...

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Chapter 17Chapter 17Sheet Forming ProcessesSheet Forming Processes

(Part 2)(Part 2)

Drawing & Stretching, Alternative Drawing & Stretching, Alternative Methods, Pipe Welding, and PressesMethods, Pipe Welding, and Presses

EIN 3390 Manufacturing ProcessesEIN 3390 Manufacturing ProcessesSpring 2012Spring 2012

17.4 Drawing and Stretching 17.4 Drawing and Stretching ProcessesProcesses

Drawing refers to the family of operations where plastic flow occurs over a curved axis and the flat sheet is formed into a three-dimensional part with a depth more than several times the thickness of the metal

Application: a wide range of shapes, from cups to large automobile and aerospace panels.

17.4 Drawing and Stretching 17.4 Drawing and Stretching ProcessesProcessesTypes of Drawing and Stretching

• Spinning• Shear forming or flow turning• Stretch forming• Deep drawing and shallow drawing• Rubber-tool forming• Sheet hydroforming• Tube hydroforming• Hot drawing• High-energy-rate forming• Ironing• Embossing• Superplastic sheet forming

17.4 Spinning17.4 Spinning

Spinning is a cold forming operation◦Sheet metal is rotated and progressively shaped over a male form, or mandrel

◦Produces rotationally symmetrical shapes Cones, spheres, hemispheres, cylinders, bells, and parabolas

SpinningSpinning

Figure 17-34 (Above) Progressive stages in the spinning of a sheet metal product.

SpinningSpinning

Figure 17-35 (Left) Two stages in the spinning of a metal reflector. (Courtesy of Spincraft, Inc. New Berlin, WI.)

SpinningSpinning

Tooling cost can be extremely low. The form block can often be made of hardwood or even plastic because of localized compression from metal.

With automation, spinning can also be used to mass-produce high-volume items such as lamp reflectors, cooking utensils, bowls, and bells.

Spinning is usually considered for simple shapes that can be directly withdrawn from a one-piece form. More complex shapes, such as those with reentrant angles, can be spun over multipiece or offset forms.

Shear FormingShear FormingShear forming is a version of spinningA modification of the spinning process in which

each element of the blank maintains its distance from the axis of rotation.

No circumferential shrinkageWall thickness of product, tc will vary with the

angle of the particular region:tc = tb(sin

where tb is the thickness of the starting blank.• Reductions in wall thickness as high as 8:1

are possible, but the limit is usually set at about 5:1, or 80%

Shearing FormingShearing Forming

Direct Shear FormingDirect Shear Forming

Figure 17-36 Schematic representation of the basic shear-forming process.

Material being formed moves in the same direction as the roller

Reverse Shear FormingReverse Shear Forming• Material being formed

moves in the opposite direction as the roller

• By controlling the position and feed of the forming roller, the reverse process can be used to shape con- cave, convex, or conical parts without a matching form block.

Stretch FormingStretch Forming

Figure 17-39 Schematic of a stretch-forming operation.

An attractive means of producing large sheet metal parts in low or limited quantities.

A sheet of metal is gripped by two or more sets of jaws with stretching or wrapping around a single form block.

Stretch FormingStretch FormingMost deformation is induced by the tensile stretching, so the forces on the form block are far less than those encouraged in bending or forming.

Very little springback and the workpiece conforms very closed to the shape of the tool.

Wrinkles are pulled out before they occur since stretching accompanies bending or wrapping

Stretch FormingStretch FormingForm blocks can be made of wood, low-melting-point

metal, or even plastic because forces on form block are low.

Quite popular in the aircraft industry to form aluminum, stainless steel into cowling, wing tip, scoop, and other large panels.

Low-carbon steel can be stretched to produce large panels for automotive and truck industry.

If mating male and female dies are used to shape the metal while it is being stretched, the process is known as stretch-draw forming.

Deep Drawing and Shallow Deep Drawing and Shallow DrawingDrawing Drawing is typically used

to form solid-bottom cylindrical or rectangular containers from sheet metal.

When depth of the product is greater than its diameter, it is known “Deep drawing”.

When depth of the product is less than its diameter, it is known “shallow drawing”. Figure 17-40 Schematic of the deep-

drawing process.

Deep Drawing and Shallow Deep Drawing and Shallow DrawingDrawing Key variables:

◦ Blank and punch diameter

◦ Punch and die radius

◦ Clearance◦ Thickness of the

blank◦ Lubrication◦ Hold-down

pressureFigure 17-4 Flow of material during deep drawing. Note the circumferential compression as the radius is pulled inward

Deep Drawing and Shallow DrawingDeep Drawing and Shallow Drawing

During drawing, the material is pulled inward, so its circumference decrease. Since the volume of material must be the same,

V0 = Vf

the decrease in circumferential dimension must be compensated by a increase in another dimension, such as thickness or radial length.

Since the material is thin, an alternative is to relieve the circumferential compression by bulking or wrinkling.

The wrinkling formation can be suppressed by compressing the sheet between die and blankholder service.

Figure 17-42 Drawing on a double-action press, where blankholder uses the second press action

The hold-down force is independent of the punch position.

The restraining force can be varied during the drawing operation.

Multi-action presses are usually specified for the drawing of more complex parts.

Once a drawing process has been designed and the tooling manufactured, the primary variable for process adjustment is hold-down pressure or blankhoder force.

If the force is too low, wrinkling may occur at the start of the stroke. If it is too high, there is too much restrain, and the descending punch will tear the disk or some portion of the already-formed cup wall.

Deep DrawingDeep Drawing

As cup depth increases or material is thin, there is an increased tendency for forming the defects.

Limitations of Deep DrawingLimitations of Deep DrawingWrinkling and tearing are typical limits

to drawing operationsTrimming may be used to reach final

dimensions

Figure 17-45 Pierced blanked, and drawn part before and after trimming

Defects in Drawing PartsDefects in Drawing Parts

Forming with Rubber Tooling or Forming with Rubber Tooling or Fluid PressureFluid PressureBlanking and drawing operations

usually require mating male and female die sets

Processes have been developed that seek to◦Reduce tooling cost◦Decrease setup time and expense◦Extend the amount of deformation for a single set of tools

Alternative Forming OperationsAlternative Forming OperationsSeveral forming

operations replace one of the dies with rubber or fluid pressure ◦Guerin process

Other forming operations use fluid or rubber to transmit the pressure required to expand a metal blank ◦Bulging

Figure 17-47 Method of blanking sheet metal using the Guerin process.

Figure 17-48 Method of bulging tubes with rubber tooling.

Guerin Process (Rubber-die forming)Guerin Process (Rubber-die forming)Guerin process was developed by aircraft industry for

small number of duplicate parts. The sheet materials can be aluminum up to (1/8”) thick and stainless steel up to 1/16”. Magnesium sheet can also be formed if it is heated and shaped over heated form block.

Sheet HydroformingSheet HydroformingSheet hydroforming is a family of processes in

which a rubber bladder backed by fluid pressure replaces either the solid punch or female die set

Advantages◦Reduced cost of tooling◦Deeper parts can be formed without fracture

◦Excellent surface finish◦Accurate part dimensions

Sheet HydroformingSheet Hydroforming

Figure 17-50 (Above) One form of sheet hydroforming.

Figure 17-51 Two-sheet hydroforming, or pillow

forming.

Tube HydroformingTube Hydroforming Process for manufacturing strong, lightweight, tubular components Frequently used process for automotive industry Advantages

◦ Lightweight, high-strength materials◦ Designs with varying thickness or varying cross section can be made◦ Welded assemblies can be replaced by one-piece components

Disadvantages◦ Long cycle time◦ Relatively high tooling cost and process setup

Figure 17-52 Tube hydroforming. (a) Process schematic.

Additional Drawing OperationsAdditional Drawing OperationsHot-drawing

◦Sheet metal has a large surface area and small thickness, so it cools rapidly

◦Most sheet forming is done at mildly elevated temperatures

High-Energy Rate Forming (HERF)◦Large amounts of energy in a very short time◦Underwater explosions, underwater spark discharge,

pneumatic-mechanical means, internal combustion of gaseous mixtures, rapidly formed magnetic fields

Ironing◦Process that thins the walls of a drawn cylinder by

passing it between a punch and a die

Hot-Drawing Processes Hot-Drawing Processes

Figure 17-5 Methods of hot drawing a cup-shaped part. (Up left) First draw. (Up right) Redraw operation. (Lower) Multi-die draw. (Courtesy of United States Steel Corp., Pittsburgh, PA)

Explosive Forming ProcessesExplosive Forming Processes

Ironing ProcessesIroning Processes

Additional Drawing OperationsAdditional Drawing OperationsEmbossing

◦Pressworking process in which raised lettering or other designs are impressed in sheet material

Superplastic sheet forming◦Materials that can elongate in the range of 2000 to 3000% can be used to form large, complex-shaped parts with ultra-fine grain size and performing the deformation at low strain rates and elevated temperature.

◦Superplastic forming techniques are similar to that of thermoplastics

Embossing ProcessEmbossing Process

Properties of Sheet MaterialProperties of Sheet MaterialTensile strength of the material is important in

determining which forming operations are appropriate.

Sheet metal is often anisotropic- properties vary with direction or orientation. A metal with low-yield, high-tensile, and high-uniform elongation has a good mechanical property for sheet-forming operations.

Majority of failures during forming occur due to thinning or fracture

Strain analysis can be used to determine the best orientation for forming

Engineering Analysis of DrawingEngineering Analysis of Drawing

Engineering Analysis of DrawingEngineering Analysis of DrawingIt is important to assess the limitation of the

amount of drawing that can be accomplished.

Measures of Drawing:

1) Drawing ratio (cylinder) DR = Db/Dp

Where Db – blank diameter, Dp – punch diameterThe greater the ratio, the more severe is the

drawing. An approximate upper limit on the drawing ratio is a value of 2.0. The actual limiting value for a given drawing depends on punch and die corner radii (Dp and Dd), friction conditions, depth of draw, and characteristics of the sheet metal (ductility, degree of directinality of strength in the metal).

Engineering Analysis of DrawingEngineering Analysis of Drawing2) Reduction r (another way to characterize a

given drawing)r = (Db - Dp )/Db

It is very closely related to drawing ratio. Consistent with Dr <= 2.0, the value of r should be less than 0.5.

3) Thickness-to-diameter ratio: t/Db

Where t – thickness of the starting blank, Db – blank diameter. The ratio t/Db is greater than 1%. As t/Db decreases, tendency for wrinkling increases. If DR , r, t/Db are exceeded by the design, blank must be draw in two or more steps, sometimes with annealing between steps.

Engineering Analysis of DrawingEngineering Analysis of DrawingExample: Cup DrawingFor a cylindrical cup with inside diameter = 3.0” and height

= 2.0”, its starting blank size Db = 5.5”, and its thickness t = 3/32”, please indicate its manufacturing feasibility.

Solution:DR = Db/Dp = 5.5/3.0 = 1.833 <2.0r = (Db - Dp )/Db = (5.5 – 3.0)/5.5 = 45.45% < 50%t/Db = (3/32)/5.5 = 0.017 > 1%

So the drawing operation is feasible.

Engineering Analysis of DrawingEngineering Analysis of DrawingDrawing Force

F = Dpt(TS)(Db/Dp – 0.7)

Where F – drawing force, lb(N); t – thickness of blank, in. (mm); TS - tensile strength, ib/in2 (Mpa); Db and D p – starting blank diameter and punch diameter, in. (mm). 0.7 – a correction factor for friction. The equation is the estimation of the maximum force in the drawing.

The drawing force varies throughout the downward movement of the punch, usually reaching its maximum value at about one-third the length of the punch stroke.

Clearance c: about 10% than the stock thickness (t) c = 1.1 t

Engineering Analysis of DrawingEngineering Analysis of DrawingHolding Force

Fh = 0.015Y[Db2 – (Dp + 2.2t + 2Rd)2]

Where Fh – holding force in drawing, ib (N); Y – yield strength of the sheet metal, lb/in2 (Mpa); t – starting stock thickness, in. (mm); Rd – die conner radius, in. (mm). The holding force is usually about one-third the drawing force [1].

[1]: Wick, C., et al., “Tool and Manufacturing Engineers, 4th ed. Vol. II.

Engineering Analysis of DrawingEngineering Analysis of DrawingExample Forces in Drawing

Determine the (a) drawing force, and (2) holding force for the case in previous example for feasibility, where tensile strength of the metal = 70,000 lb/in 2 and yield strength = 40,000 lb/in 2 , the die corner radius = 0.25”.

Solution:(a) F = Dpt(TS)(Db/Dp – 0.7)

=(3.0)(3/32)(70,000)(5.5/3.0 – 0.7) =70,097 lb

(b) Fh = 0.015Y[Db2 – (Dp + 2.2t + 2Rd)2]

= 0.015(40,000){5.52 – [3.0 + 2.2(3/32) + 2(0.25)]2}= 1,885 (30.25 – 13.74) = 31,121 lb

Engineering Analysis of DrawingEngineering Analysis of DrawingBlank Size Determination

Assume that the volume of the final product is the same as the that of the starting sheet-metal blank and the thinning of the part wall is negligible.

For a cup with its height H and the same diameters Dp in the bottom and top:

Db2/4 = Dp

2/4 + Dp H, and

Db = SQRT(Dp2 + Dp

Design Aids for Sheet Metal Design Aids for Sheet Metal FormingForming

Figure 17-57 (Left) Typical pattern for sheet metal deformation analysis; (right) forming limit diagram used to determine whether a metal can be shaped without risk of fracture. Fracture is expected when strains fall above the lines.

A pattern is placed on the surface of a sheet.Circles have diameters between 2.4 and 5 mm (0.1

– 0.2”).During deformation, the circles convert into

ellipses. Regions where the enclosed area has expanded

are locations of sheet thinning and possible failure.

Regions where the area has contracted have undergone sheet thickening and may be sites of buckling or wrinkles.

Using the ellipses on the deformed pattern, the major strains (strain in the direction of the largest radius) and the associated minor strain (strain 900 from the major) can be determined for a variety of locations.

If both major and minor strains are positive, the deformation are stretching, and the sheet metal will decrease in thickness.

If the minor strain is negative, this contraction may partially or whole compensate any positive stretching in the major direction. The combination of tension and compression is known as drawing, and the thickness may decrease, increase, or stay the same, depending on relative magnitude of the two strains.

17.5 Alternative Methods of 17.5 Alternative Methods of Producing Sheet-Type ProductsProducing Sheet-Type ProductsElectroforming

◦Directly deposits metal onto preshaped forms or mandrels

◦Nickel, iron, copper, or silver can used ◦A wide variety of sizes and shapes can be

made by electroformingSpray forming

◦Spray deposition◦Uses powdered material in a plasma torch◦Molten metal may also be sprayed

17.6 Pipe Welding17.6 Pipe Welding

Lap-welded pipe◦Skelp has beveled edges and the rolls form the

weld by forcing the lapped edges down against a supporting mandrel.

◦The process is used primarily for large sizes of pipe, with diameters from about 50 mm (2”) to 400 mm (14”). Product length is limited to about 6 to 7 m (20 to 25 ft).

17.7 Presses17.7 PressesFactor for selection of presses:

type of power (mechanical, hydraulic), number of slides or drives, type of drive, stroke length for each drive, type of frame or construction, and the speed of operation.

17.7 Presses17.7 Presses

Figure 17-58 Schematic representation of the various types of press drive

mechanisms.

Types of Press FrameTypes of Press Frame

Figure 17-60 Inclinable gap-frame press with sliding bolster to accommodate two die sets for rapid change of tooling. (Courtesy of Niagara Machine & Tool Works, Buffalo, NY.)

Types of Press FrameTypes of Press Frame

Figure 17-61

A 200-ton (1800-kN) straight-sided press. (Courtesy of Rousselle Corporation, West Chicago, IL.)

Special Types of PressesSpecial Types of PressesPresses have been designed to perform

specific types of operationsTransfer presses have a long moving

slide that enables multiple operations to be performed simultaneously in a single machine

Four-slide or multislide machines are used to produce small, intricately shaped parts from continuously fed wire or coil strip

Figure 17-62 Schematic showing the arrangement of dies and the transfer mechanism used in transfer presses. (Courtesy of Verson Allsteel Press Company, Chicago, IL.)

Figure 17-63 Various operations can be performed during the production of stamped and drawn parts on a transfer press. (Courtesy of U.S. Baird Corporation, Stratford, CT.)

Figure 17-65 Schematic of the operating mechanism of a multislide machine. The material enters on the right and progresses toward the left as operations are performed. (Courtesy of U.S. Baird Corporation, Stratford, CT.)

SummarySummarySheet forming processes can be grouped in

several broad categories◦Shearing◦Bending◦Drawing◦Forming

Basic sheet forming operations involve a press, punch, or ram and a set of dies

Material properties, geometry of the starting material, and the geometry of the desired final product play important roles in determining the best process

Chapter 17 Sheet Forming Processes (Part 2) Drawing & Stretching, Alternative Methods, Pipe Welding,...

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