Ch6 sheetmetw proc

13.04.23 CHAPTER 6 SHEET METAL WORKING PROCESSES

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CHAPTER 6SHEET METAL WORKING PROCESSES

6.1 INTRODUCTION

ME 333 PRODUCTION PROCESSES II

Sheet metalworking includes cutting and forming operations performed on relatively thin sheets of metal (0.4-6 mm).

The tooling used to perform sheet metalwork is called punch and die. Most sheet metal operations are performed on machine tools called presses.

The term stamping press is used to distinguish these presses from forging and extrusion presses. The sheet metal products are called stampings.

The commercial importance of sheet metalworking is significant.

The number of consumer and industrial products that include sheet metal parts: automobile and truck bodies, airplanes, railway cars and locomotives, farm and construction equipment, small and large appliances, office furniture, computers and office equipment, and more. Sheet metal parts are generally characterized by high strength, good dimensional tolerances, good surface finish, and relatively low cost.


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Sheet-metal processing is usually performed at room temperatures (cold working).

The exemptions are when the stock is thick, the metal is brittle, or the deformation is significant.

These are usually cases of warm working rather than hot working.


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The three major categories of sheet-metal processes:(1)cutting (shearing, blanking, piercing) (2)bending (3)drawing. Cutting is used to separate large sheets into smaller pieces, to cut out a part perimeter, or to make holes in a part. Bending and drawing are used to form sheet metal parts into their required shapes.

Fig.6.1 Some cutting operations


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Classification of Sheet Metalworking Processes

Fig.6.2 Basic sheet metalworking operations:(a) bending,(b) drawing, and(c) shearing;

(1) as punch first contacts sheet and

(2) after cutting.

Force and relative motion are indicated by F and v


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Classification of Sheet Metalworking Processes

Fig.6.2 Basic processes involved in forming sheet metal components. (a) Processes involving local deformation.


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6.2. PIERCING AND BLANKING

A commonly used piercing-blanking die set and related terms are shown in the following figure.

Fig.6.3 Components of a punch and die


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Blanking and punching

Blanking and punching are similar sheet metal cutting operations that involve cutting the sheet metal along a closed outline.

If the part that is cut out is the desired product, the operation is called blanking and the product is called blank.

If the remaining stock is the desired part, the operation is called punching. Both operations are illustrated on the example of producing a washer:

Starting stock produced by shearing operation from a big metal sheet

Fig.6.4 Steps in production of washer


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The cutting of metal between die components is a shearing process in which the metal is stressed in shear between two cutting edges to the point of fracture, or beyond its ultimate strength. The metal is subjected to both tensile and compressive stresses; stretching beyond the elastic limit occurs; then plastic deformation, reduction in area, and, finally, fracturing starts and becomes complete.

Fig.6.5 Shearing of sheet metal between punch and die

Blanking punch diameter= Db-2c

Blanking die diameter= Db

Hole punch diameter= Dh

Hole die diameter= Dh+2c


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The cutting of metal between die components is a shearing process in which the metal is stressed in shear between two cutting edges to the point of fracture, or beyond its ultimate strength. The metal is subjected to both tensile and compressive stresses; stretching beyond the elastic limit occurs; then plastic deformation, reduction in area, and, finally, fracturing starts and becomes complete.

Fig.6.5 Shearing of sheet metal between punch and die


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Engineering analysis of metal cutting:

Cutting of sheet metal is accomplished by a shearing action between two sharp edges. The shearing action is illustrated in the figure:

Fig.6.3 Shearing

Fig. 6.6. Shearing of sheet metal between two cutting edges:

(1) just before the punch contacts work;

(2) punch begins to push into work, causing plastic deformation;

(3) punch compresses and penetrates into work, causing a smooth cut surface; and

(4) fracture is initiated at the opposing cutting edges that separate the sheet.

Symbols v and F indicate motion and applied force, respectively.


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At the top of the cut surface is a region called the rollover. This corresponds to the depression made by the punch in the work prior to cutting. It is where initial plastic deformation occured in the work.

Just below the rollover is a relatively small region called the burnish. This results from penetration of the punch into the work before fracture began.

Beneath the burnish is the fractured zone, a relatively rough surface of the cut edge where continued downward movement of the punch caused fracture of the metal.

Finally, at the bottom of the edge is a burr, a sharp corner on the edge caused by elongation of the metal during final seperation of the two pieces.


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Process parameters in sheet metal cutting are clearence between punch and die, stock thickness, type of metal and its strength and length of the cut

Clearance c in a shearing operation is the space between the mating members of a die set (e.g.punch and die).

For optimum finish of cut edge, proper clearance is necessary and is a function of the kind, thickness, and hardness of the work material.

In an ideal cutting operation the punch penetrates the material to a depth equal to about 1/3 of its thickness before fracture occurs, and forces an equal portion of the material into the die opening.

Common die clearances (linear clearance) are 2-5% of the material thickness.

Angular clearance is gradient given to the hole in the die such that cut material will easily be removed. Angular clearance is usually ground from 0.25⁰ to 1.5⁰ per side.


6.2.1. Engineering Analysis_CLEARANCE


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The correct clearance depends on sheet-metal type and thickness t:c = a*t

where a is the allowance (a = 0.075 for steels and 0.060 for aluminum alloys).

If the clearance is not set correctly, either an excessive force or an oversized burr can occur:

Fig.6.7 Effect of clearance: (Left) clearance too small causes less than optimal fracture and excessive forces, and (Right) clearance too large causes oversized burr.


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Figure (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.


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The calculated clearance value must be;-substracted from the die punch diameter for blanking operations or -added to die hole diameter for punching:

Fig.6.8 Die diameter is enlarged with clearance c in punching. In blanking, the punch diameter is decreased to account for clearance. D is the nominal size of the final product.


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An angular clearance must be provided for the die hole to allow parts to drop through it:

Fig.6.9 Angular clearance for the die opening in punching and blanking.


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6.2.2. CUTTING FORCE


The pressure (or stress) required to cut (shear) work material is;

For example to produce a hole of 20mmX20mm in a material 2mm in thickness with 40 kg/mm2 shear strength:

P= 40 kg/mm2x(2x20+2x20)mmx2mm

P= 40x160 kg= 6400 kg force is required.

DtSP P SLt

(for round holes)

(for any contours)

where;S= shear strength of material, kg/mm2

D= hole diameter, mmL= shear length, mmt= material thickness, mm


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6.2.3 TOOLS AND DIES FOR CUTTING OPERATIONS


When the die is designed to perform a single operation (for example, cutting, blanking, or punching) with each stroke of the press, it is referred to as a simple die:

Fig.6.10 The basic components of the simple blanking and punching dies

Simple dies


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More complicated pressworking dies include:• compound die to perform two or more operations at a single position of the metal strip• progressive die to perform two or more operations at two or more positions of

the metal strip

Fig.6.11 Method of making a simple washer in a compound blanking and punching die

Multi-operational dies


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Multi-operational dies

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.


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6.2.4 CENTRE OF PRESSURE


Sheet metal part that to be blanked is of irregular shape the summation of shearing forces on one side of the center of the ram may greatly exceed the forces on the other side. This result in bending and undesirable deflections might happen. Center of pressure is a point, which the summation of shearing forces will be symmetrical. This point is the center of gravity of the line that is the perimeter of the blank. It is not the center of gravity of the area.

r

y2

r

yx2

rSin

α 3

bax

3

hy

Fig.6.14 Center of pressure for some shapes


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Procedure to find center of pressure:

1. Divide cutting edges into line elements, 1,2,3, ...2. Find the lengths l1, l2, l3, ...3. Find the center of gravity of each element as x1, x2, x3, ..., y1, y2, y3, ...4. Calculate the center of pressure from:


l

lx

lll

xlxlxlx

....

....

321

332211

l

ly

lll

ylylyly

....

....

321

332211


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Find the center of pressure and the required cutting force of the following blank (S=40 kg/mm2 and t=2mm).


EXAMPLE

Element l X Y (l)(x) (l)(y)

1 4.00 0.00 6.25 0.00 25.00

2 4.71 1.50 9.20 7.05 43.33

3 3.20 4.00 7.00 12.80 22.40

4 2.50 4.00 5.00 10.00 12.50

5 3.00 1.50 4.25 4.50 12.75

6 1.57 1.00 0.00 1.57 0.00

TOTAL 18.98 35.92 115.98

cm..

.x 891

9818

9235 cm.

.

.y 106

9818

98115

and the cutting force is;P= LtS 189.8mmx2mmx40kg/mm2 =15184 kg


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Since cutting operations are characterized by very high forces exerted over very short periods of time, it is some times desirable to reduce the force and spread it over a longer portion of the ram stroke.Two methods are frequently used to reduce cutting forces and to smooth out the heavy loads.


6.2.5 REDUCING CUTTING FORCES

1. Step the punch lengths; the load may thus be reduced approx. 50%.2. Tapering the punch; grind the face of the punch or die at a small shear angle

with the horizontal. This has the effect of reducing the area in shear at any time, and may reduce cutting force as much as 50%. The angle chosen should provide a change in punch length of about 1.5 times of material thickness. It is usually preferable to a double cut to prevent setup of lateral force components.

Fig.6.15 Different configurations for reducing the cutting force


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Fig.6.– Effect of different clearances when punching hard and soft alloys


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In designing parts to be blanked from strip material, economical strip utilization is of high importance. The goal should be at least 75% utilization.


6.3 SCRAP-STRIP LAYOUT FOR BLANKING

where;t : thickness of the stock,W: width of the stock,B: space between part and edge (1.5t),C: lead of the die (L+B),L&H: dimensions of the work piece.


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Locating the work piece for maximum economy is very important.

100% XTotal

ScrapScrap 100

..% X

Total

UtilUtil


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If two strips (250 mm and 125 mm width) are available for the production of 100 mm blanks, which one have to be preferred for maximum material utilization?

HOMEWORK:


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Bending is defined as the straining of the sheet metal around a straight edge:


6.4 BENDING

Fig.6.15 Bending of sheet metal


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Bending operations involve the processes of V-bending and edge bending:

Fig.6.16 (Left) V-bending, and (Right) edge bending; (1) before and (2) after bending

•V-bending—sheet metal is bent along a straight line between a V-shape punch and die.•Edge bending—bending of the cantilever part of the sheet around the die edge.


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Bending is the process by which a straight length is transformed into a curved length. It is a very common forming process for changing sheet and plate into channel, tanks, etc.For a given bending operation the bend radius can not be made smaller than a certain value, otherwise the metal will crack on the outer tensile surface. Minimum bend radius is usually expressed in multiples of the sheets thickness. It varies considerable between different metal and always increases with cold working. Bend radius is not less than 1 mm and for high strength sheet alloys the minimum bend radius may be 5t or higher.


R - bend radiusBA - bend allowance - bend angleL0 - original lengtht - sheet thickness

Lf L0

Lf=L1+L2+BA Rmin>5t practical


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This is the stretching length that occurs during bending. It must be accounted to determine the length of the blank,


Fig.6.17 Calculation of bend allowance

where A is the bend angle; t is the sheet thickness;R is the bend radius; Kba is a factor to estimate stretching, defined as follows:

Kba = 0.33 for R < 2t Kba = 0.50 for R ≥ 2t

where Lb is the length of the blank, L are the lengths of the straight parts of the blank, BA is the bend allowance,


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The minimum bend radius for a given thickness of sheet can be predicted fairly accurately from the reduction of area measured in tension test, Ar.

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1min rAt

R

2

2min

2

1

rr

r

AA

)A(

t

R

for Ar< 0.2,

for Ar> 0.2, o

for A

AAA

Another common problem is springback. It is the dimensional change of the formed part after pressure of the forming tool has been removed. It results from the change in strain produced by elastic recovery.

2

2

t/R

t/R ratioSpringback

f

o

o

f


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The commonest method of compensating for springback is to bend the part to a smaller radius of curvature than is desired so that after springback the part has the proper radius.

Springback is the elastic recovery leading to the increase of the included angle when the bending pressure is removed.

To compensate for springback two methods are commonly used:1. Overbending—the punch angle and radius are smaller than the final ones.2. Bottoming—squeezing the part at the end of the stroke.

Fig.6.18 Springback in bendingFig.6.19 Compensation of springback by:(a) and (b) overbending; (c) and (d) bottoming


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The force required bending a length L about a radius R may be estimated from;

2tan

)2/(2

2 tR

LtP o

Bending forcesThe maximum bending force is estimated as

where Kbf is the constant that depends on the process, Kbf = 1.33 for V-bending and Kbf = 0.33 for edge bending; w is the width of bending; D is the die opening dimension as shown in the figure:

Fig.6.20 Die opening dimension D, (a) V-bending, (b) edge bending


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Equipment for bending operations

Fig.6.21 Press brake with CNC gauging system Fig.6.22 Dies and stages in the press brake forming of a roll bead


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Deep drawing is the metal working process used for shaping flat sheets into cup-shaped articles such as bathtubs, shell cases, and automobile fenders. Generally a hold down or pressure pad is required to press the blank against the die to prevent wrinkling. Optional pressure pad from the bottom may also be used.


6.5 DEEP DRAWING (Derin Çekme)

Fig.6.23 Drawing of a cup shaped part


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Deep drawing of a cup-shaped part


Fig.6.24 Deep drawing of a cup-shaped part: (Left) start of the operation before punch contacts blank, and (Right) end of stroke


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In the deep drawing of a cup the metal is subjected to three different types of deformations. In the flange part, as it is drawn in, the outer circumference must continuously decrease from that of the

original blank Do to that of the finish

cup Dp. This means that it is subjected to a compressive strain in the hoop (tangential) direction and a tensile strain in the radial direction. As a result of these principal strains, there is a continual increase in the thickness as the metal moves inward. However, as the metal pass over the die radius, it is first bend and then straightened while at the same time being subjected to a tensile stress. This plastic bending under tension results in considerable thinning. Punch region is under very little stress.

Fig.6.23 Types of deformations in different region during deep drawing of a cup shaped part


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ClearanceClearance c is the distance between the punch and die and is about 10% greater than the stock thickness:

c = 1.1t


Holding forceThe improper application of the holding force can cause severe defects in the drawn parts such as (a) flange wrinkling or (b) wall wrinkling if the holding force is too small, and (c) tearing if the folding force is overestimated.

Fig.6.25 Defects in deep drawing of a cup-shaped part


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The force on the punch required to produce a cup is the summation of the ideal force of deformation, the frictional forces, and the force required to produce ironing. Mathematical calculation of the drawing force is very complex. Following approximate equation is developed:

P dt nD

dH

d

De Bo

11 2 2. /

where;

P = total punch load, o= average flow stress,d = punch diameter, D = blank diameter,H = hold drawn force, B = force required to bend,

t = wall thickness, = coefficient of friction,

= efficiency

Drawing force may be calculated for practical purposes by:

P dto when LDR 2 (Limiting Drawing Ratio)


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The drawability of a metal is measured by the ratio of the blank diameter to the diameter of the cup drawn from the blank (usually accepted as punch diameter). For a given material there is a Limiting Drawing Ratio (LDR), representing the largest blank that can be drawn through a die without tearing.

ed

DLDR

Where, is an efficiency term to account for frictional losses. If =1, then LDR=2.7 while =0.7, LDR2 which is used in most practical applications.

Some of the practical considerations which affect drawability:Rd 10tRp should be big enough to prevent tearing.Clearance between punch and die; 20 to 40% greater than “t”.Hold-down pressure; 2% of o and lubricate die walls

The diameter of blank required to draw a given cup may be obtained approximately by equating surface areas.

where; h is height of cup.

D ddh

2 2

4 4 dhdD 42 and


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If the shape change required by the part design is too severe (limiting drawing ratio is too high, or LDR is not sufficient to form a desired cup), complete forming of the part require more than one drawing step. The second drawing step and any further drawing steps if needed, are referred to as redrawing. Throat angle is 10-15. Redrawing is generally done in decreasing ratios as given below:

(D/d)= 1.43, 1.33, 1.25, 1.19, 1.14 and 1.11.If these redrawing steps are not enough to reach required cup diameter, annealing have to be performed and then redrawing can be performed.

6.5.1 REDRAWING

Fig.6.26 Redrawing of a cup


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A 200 mm blank is to be drawn to a 50 mm cup. Estimate the minimum number of draws required using the drawing ratios given below:

6.5.2 EXAMPLE:

56.38>50

Draw 1st 2nd 3rd 4th 5th 6th

Ratio 1.43 1.33 1.25 1.19 1.14 1.10

Solution:LDR=2 D/d = 200/50 = 4 > 2So that redrawing is necessary.

1. 86.13943.1

20043.1 1

1

DD

D

2. 16.10533.1

86.13933.1 2

2

1 DD

D

3. 13.8425.1

16.10525.1 3

3

2 DD

D

4. 69.7019.1

13.8419.1 4

4

3 DD

D

5. 01.6214.1

69.7014.1 5

5

4 DD

D

6. 38.561.1

01.621.1 6

6

5 DD

D


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Therefore annealing should be applied. But it might be better to anneal the blank before 6th draw to reduce number of redraws. We know that LDR=2. So that if annealing is performed after 3rd draw where D3 = 84.13 mm, than ratio to reach required cup diameter is:

68.150

13.84 < 2

Therefore, after 3rd draw, blank is annealed and then redraw with a ratio of 1.68 to obtain required cup diameter. The required number of drawing is then 4.


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The Guerin process involves the use of a thick rubber pad to form sheet metal over a positive form block:


6.6 OTHER SHEET-METAL FORMING OPERATIONS

Fig.6.27 The Guerin process: (Left) start of the operation before rubber pad contacts sheet, and (Right) end of stroke

The Guerin process


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Examples of equipment and products manufactured by the Guerin process:


Fig.6.28 Rubber pad press showing forming tools on the press table

Fig.6.29 A large number of different components can be made simultaneously during one press cycle with rubber pad presses

Advantages: small cost of toolingLimitations: for relatively shallow shapesArea of application: small-quantity production


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It is similar to Guerin process but instead of rubber pad a rubber diaphragm filled with fluid is used:


Fig.6.30 Hydroform process:(1) start-up, no fluid in the cavity;(2) press closed, cavity pressurized with hydraulic fluid;(3) punch pressed into work to form part.

Symbols:v - velocity,F – applied force, andp - hydraulic pressure

Hydroforming

Advantages: small cost of toolingLimitations: simple shapesArea of application: small-quantity production


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In stretch forming the sheet metal is stretched and bent to achieve the desired shape:


Fig.6.Fig.6.3131 Stretch forming: (1) start of the Stretch forming: (1) start of the process; (2) form die is pressed intoprocess; (2) form die is pressed into the work the work causing it to stretched andcausing it to stretched and bent over the form. Symbols: v -bent over the form. Symbols: v - velocity, Fvelocity, Fdiedie - applied force - applied force

Stretch forming

Advantages: small cost of tooling, large partsLimitations: simple shapesArea of application: small-quantity production


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Spinning is a metal forming process in which an axially symmetric part is gradually shaped over a mandrel by means of a rounded tool or roller:


Fig.6.Fig.6.3232 In spinning operation, flat circular blanks are often formed into hollow shapes such as photographic reflectors. In a lathe, tool is forced against a rotating disk, gradually forcing the metal over the chuck to conform to its shape. Chucks and follow blocks are usually made of wood for this operation

Spinning

Advantages: small cost of tooling, large parts (up to 5 m or more)Limitations: only axially symmetric partsArea of application: small-quantity production


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These are metal forming processes in which large amount of energy is applied in a very short time. Some of the most important HREF operations include:


Fig.6.Fig.6.3333 Explosive forming: (1) set-up, (2) explosive is detonated, and (3) shock wave forms part

HIGH-ENERGY-RATE FORMING (HERF)

It involves the use of an explosive charge placed in water to form sheet into the die cavity.

Explosive forming


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Fig.6.Fig.6.3434 Explosively formed elliptical dome 3-m in diameter being removed from the forming die

Advantages: small cost of tooling, large partsLimitations: skilled and experienced laborArea of application: large parts typical of the aerospace industry

Explosively formed elliptical dome 3-m in diameter being removed from the forming die


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This is a HREF process in which a shock wave to deform the work into a die cavity is generated by the discharge of electrical energy between two electrodes submerged in water. Similar to explosive forming, but applied only to small part sizes.


Fig.6.Fig.6.3535 Setup of electrohydraulic forming

Electrohydraulic forming


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The sheet metal is deformed by the mechanical force of an electromagnetic field induced in the workpiece by a coil:


Fig.6.Fig.6.3636 Electromagnetic forming: (1) set-up in which coil is inserted into tubular workpiece surrounded by die, (2) formed part

Electromagnetic forming

Advantages: can produce shapes, which cannot be produced easily by the other processes

Limitations: suitable for magnetic materialsArea of application: most widely used HERF process to

form tubular parts


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If two strips (250 mm and 125 mm width) are available for the production of 100 mm blanks, which one have to be preferred for maximum material utilization?

HOMEWORK:


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THE END



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