17.4 Drawing and Stretching Processes

Chapter 17Chapter 17

Sheet Forming ProcessesSheet Forming Processes(Part 2:(Part 2:Drawing & Stretching) Drawing & Stretching)

(Review)(Review)

EIN 3390 Manufacturing ProcessesEIN 3390 Manufacturing ProcessesSpring, 2011Spring, 2011

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

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Spinning is usually considered for simple shapes that can be directly withdrawn from a one-piece form.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 DrawingDeep DrawingDuring drawing, the material is pulled inward, so its

circumference decrease. Since the volume of material must be the same,

V0 = V f

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.

Deep DrawingDeep Drawing

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.


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

Th

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Defects in Drawing PartsDefects in Drawing Parts


Cup redrawing to further reduce diameter and increase wall height. (Left) forward redraw; (right) reverse redraw.

Two alternatives for converting drawn parts into deeper cups:

Forward redraw – the material undergoes reverse bending as it flows into the die.

Reverse redraw – the stating cup is placed over a tubular die, and the punch acts to turn it inside out.

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

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

Sheet HydroformingSheet Hydroforming

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

Figure 17-51 Two-sheet hydroforming, or pillow

forming.

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 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 = 5.5/3.3 = 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/D p – 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/D p – 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

H)

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.

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

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17.4 Drawing and Stretching Processes