EIN 3390 Chap 17 Sheet-Forming Processes Part 1 Spring 2012

8/10/2019 EIN 3390 Chap 17 Sheet-Forming Processes Part 1 Spring 2012

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Homework for Chapter 16


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Chapter 17

Sheet Forming Processes

(Part 1)Shearing & Bending

EIN 3390 Manufacturing ProcessesSpring, 2012


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17.1 Introduction

Sheet metal processes involve planestress loadingsand lower forces thanbulk forming

Almost all sheet metal forming is

considered to be secondary processing The main categories of sheet metal

forming are: Shearing

Bending Drawing


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17.2 Shearing Operations

Shearing- mechanical cutting of materialwithout the formation of chips or theuse of burning or melting Both cutting blades are straight

Curved blades may be used to producedifferent shapes Blanking Piercing Notching Trimming


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Shearing Operations

Fracture and tearing begin at theweakest pointand proceedprogressively or intermittently to thenext-weakest location

Results in a rough and ragged edge Punch and die must have proper

alignment and clearance

Sheared edges can be produced that

require no further finishing


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Figure 17-1Simple blanking with a punch and

die.

Shearing Operations


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Shearing Operations


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Classification of Metal formingOperations


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Types of Shearing

Simple shearing-sheets of metal aresheared along astraight line

Slitting- lengthwiseshearing processthat is used to cutcoils of sheet

metal into severalrollsof narrowerwidth

Figure 17-5Method of smooth shearing a rod by

putting it into compression during shearing.

Figure 17-6A 3-m (10ft) power shear for 6.5 mm

(1/4-in.) steel. (Courtesy of Cincinnati

Incorporated, Cincinnati, OH.)


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Figure 17-2(Left)

(Top) Conventionally

sheared surface

showing the distinct

regions of deformation

and fracture and

(bottom) magnified

view of the sheared

edge. (Courtesy of

Feintool Equipment

Corp., Cincinnati, OH.)

Shearing Operations


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Piercing and Blanking Piercing and blanking are shearing operations where a

part is removed from sheet material by forcing ashaped punch through the sheet and into a shapeddie

Blanking- the piece being punched out becomesthe workpiece

Piercing- the punchout is the scrap and the

remaining strip is the workpiece

Figure 17-8(Above) (Left to Right) Piercing,

lancing, and blanking precede the forming of the

final ashtray. The small round holes assist

positioning and alignment.

Figure 17-7Schematic showing the

difference between piercing and

blanking.


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Fine Blanking OperationsFine Blanking - the piece being punched out

becomes the workpiece and pressure pads are usedto smooth edges in shearing

Figure 17-3(Top) Method of obtaining a smooth edge in

shearing by using a shaped pressure plate to put the

metal into localized compression and a punch and

opposing punch descending in unison.


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Figure 17-4

Fineblanked

surface of the

same

component as

shown in

Figure 17-2.

(Courtesy of

Feintool

Equipment

Corp.,

Cincinnati,

OH.)

Shearing Operations


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Types of Piercing and Blanking

Lancing- piercing operation that formseither a line cut or hole

Perforating- piercing a large numberof closely spaced holes

Notching- removes segments fromalong the edge of an existing product

Nibbling- a contour is progressively cut byproducing a series of overlapping slits ornotches


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Sheet-metal Cutting Operations


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Types of Piercing and Blanking

Shaving- finishing operation in which a smallamount of metal is sheared away from theedge of an already blanked part

Cutoff- a punch and a die are used to separate

a stamping or other product from a strip ofstock Dinking- used to blank shapes from low-

strength materials such as rubber, fiber, or cloth


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Sheet-metal Shaving Operations

Figure 17-10The dinking process.

Figure The shaving process.


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Sheet-metal Cutting Operations


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Tools and Dies for Piercing andBlanking

Basic components of apiercing and blanking dieset are: punch, die, andstripper plate

Punches and dies should

be properly aligned so thata uniform clearance ismaintained around theentire border

Punches are normally

made from low-distortionor air-hardenable tool steel

Figure 17-11The basic components of

piercing and blanking dies.


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Blanking Operations

Figure 17-12Blanking with a

square-faced

punch (left) and

one containing

angular shear

(right). Note the

difference inmaximum force

and contact

stroke. The total

work (the are

under the curve)

is the same forboth processes.


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Progressive Die Sets

Progressive die sets-two or more sets ofpunches and diesmounted in tandem

Transfer dies moveindividual parts fromoperation to operationwithin a single press

Compound diescombine processessequentially during asingle stroke of theram

Figure 17-16Progressive piercing and blanking

die for making a square washer. Note that the

punches are of different length.


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Design for Piercing and Blanking

Design rules Diameters of pierced holes should not beless than the thickness of the metal with aminimum 0f 0.3 mm (0.025)

Minimum distance between holes or the edge

of the stock should be at least equal to themetal thickness

The width of any projection or slot should be atleast 1 times the metal thickness and neverless than 2.5 mm (3/32)

Keep tolerances as large as possible Arrange the pattern of parts on the strip tominimize scrap


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Design Clearance


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Clearance CalculationThe recommended clearance is:

C = atWhere c clearance, in (mm); a allowance; and t =

stock thickness, in (mm).

Allowance ais determined according to type of metal.

From Mikell P. Groover Fundamentals of Modern Manufacturing.


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Design Die and Punch Sizes

For a round blank of diameterDbis determined as:

Blank punch diameter = Db - 2c

Blank die diameter = Db

For a round hole (piercing) ofdiameter Dhis determined as:

Hole punch diameter = Dh

Hole die diameter = Db + 2c


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Cutting Forces

Cutting forces are used to determine size of thepress needed.

F = StLWhere S shear strength of the sheet metal, lb/in2(Mpa); t

sheet thickness in. (mm); and L length of the cut edge,in. (mm).

In blanking, punching, slotting, and similar operations, L isthe perimeter length of blank or hole being cut.

Note: the equation assumes that the entire cut alongsheared edge length is made at the same time. In thiscase, the cutting force is a maximum.


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Angular Clearance

for slug or blank to drop through the die, the die opening

must have an angular clearance of 0.25 to 1.50on eachside.


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Example for Calculating Clearanceand Force

Round disk of 3.0 dia. is to be blanked from a half-hardcold-rolled sheet of 1/8 with shear strength = 45,000lb/in2. Determine (a) punch and die diameters, and (b)blanking force.

(a).

From table , a = 0.075,

so clearance c = 0.075(0.125) = 0.0094.Die opening diameter = 3.0

Punch diameter = 3 2(0.0094) = 2.9812 in(b)

Assume the entire perimeter of the part is blanked at one

time.

L = p Db= 3.14(3) = 9.426

F = 45,000(9.426)(0.125) = 53,021 lb


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Design Example

Figure 17-18Method for making a simple washer in a compound piercing and blanking die.

Part is blanked (a) and subsequently pierced (b) in the same stroke. The blanking punch

contains the die for piercing.


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17.3 Bending Bendingis the plastic

deformationof metalsabout a linear axis withlittle or no change in thesurface area

Forming- multiple bendsare made with a single die

Drawing and stretching-axes of deformation arenot linearor are notindependent

Springbackis the

unbending that occursafter a metal has beendeformed

Figure 17-19(Top) Nature of a bend in sheet metalshowing tension on the outside and compression on

the inside. (Bottom) The upper portion of the bend

region, viewed from the side, shows how the center

portion will thin more than the edges.


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Angle Bending (Bar Folder andPress Brake) Bar folders make angle bends up to 150

degrees in sheet metal

Press brakes make bends in heavier sheets

or more complex bends in thin material

Figure 17-22Press brake dies can form a variety of angles and contours. (Courtesy of

Cincinnati Incorporated, Cincinnati, OH.)


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Bar Folder

Figure 17-20Phantom section of a bar folder, showing position and operation of

internal components. (Courtesy of Niagara Machine and Tool Works, Buffalo, N.Y.)


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Press Brake

Figure 17-21(Left) Press brake with CNC gauging system. (Courtesy of DiAcro Division,

Acrotech Inc., Lake City, MN.)(Right) Close-up view of press brake dies forming corrugations.

(Courtesy of Cincinnati Incorporated, Cincinnati, OH.)


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Design for Bending

Several factors are important in specifying a bending

operation Determine the smallest bend radius that can be formed

without cracking the metal

Metal ductility

Thickness of material

Figure 17-24Relationship between the

minimum bend radius (relative to

thickness) and the ductility of the metal

being bent (as measured by the reduction

in area in a uniaxial tensile test).


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Considerationsfor Bending

If the punch

radius is largeand the bendangle is shallow,large amounts ofspringback are

oftenencountered

The sharper thebend, the morelikely the surfaces

will be stressedbeyond the yieldpoint

Figure 17-25Bends should be made with the bend

axis perpendicular to the rolling direction. When

intersecting bends are made, both should be at an

angle to the rolling direction, as shown.


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Design Considerations Determine the dimensions of a flat blank that will

produce a bent part of the desired precision Metal tends to thin when it is bent

Figure 17-26One method of determining the starting blank size (L) for several

bending operations. Due to thinning, the product will lengthen during forming. l1, l2,

and l3are the desired product dimensions. See table to determine Dbased on size

of radius Rwhere t is the stock thickness.


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Air-Bend, Bottoming, andCoining Dies Bottoming dies contact and

compress the full areawithin the tooling

Angle of the bend is setby the geometry of the

tooling Air bend dies produce the

desired geometry bysimple three-point bending

If bottoming dies go

beyond the full-contactposition, the operation issimilar to coining

Figure 17-27Comparison of air-bend (left) and

bottoming (right) press brake dies. With the air-

bend die, the amount of bend is controlled by

the bottoming position of the upper die.


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Roll Bending

Roll bending is a continuous form of three-point bending Plates, sheets, beams, pipes

Figure 17-28(Left)

Schematic of the roll-

bending process;

(right) the roll bending

of an I-beam section.

Note how the material

is continuously

subjected to three-point bending.

(Courtesy of Buffalo

Forge Company,

Buffalo, NY.)


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Tube Bending Key parameters: outer diameter of the

tube, wall thickness, and radiusof thebend

Figure 17-30(a) Schematicrepresentation of the cold roll-

forming process being used to

convert sheet or plate into tube.

(b) Some typical shapes

produced by roll forming.


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Roll Forming

Roll forming is a process by which a metal strip isprogressively bent as it passes through a series offorming rolls

Only bending takes place during this process, and allbends are parallel to one another

A wide variety of shapes can be produced, but

changeover, setup, and adjustment may take severalhours

Figure 17-31Eight-roll sequence for the roll forming of a box channel. (Courtesy of the

Aluminum Association, Washington, DC.)


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Seaming and Flanging

Seaming is a bending operation that can be used to

join the ends of sheet metal in some form ofmechanical interlock

Common products include cans, pails, drums, andcontainers

Flanges can be rolled on sheet metal in a similarmanner as seams

Figure 17-31Various types of seams used on sheet metal.


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Straightening

Straightening or flattening is the opposite of bending Done before subsequent forming to ensure the use of

flat or straight material Various methods to straighten material

Roll straightening (Roller levering)

Stretcher leveling- material is mechanically gripped andstretch until it reaches the desired flatness

Figure 17-33Method of straightening rod or sheet by passing it through a set of

straightening rolls. For rods, another set of rolls is used to provide straightening in the

transverse direction.


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Engineering Analysis of Bending Bending radius R is normally specified on the inside of the

part, rather than at the neutral axis. The bending radius isdetermined by the radius on the tooling used for bending.

Bending Allowance: If the bend radius is small relative tosheet thickness, the metal tends to stretch during bending.

BA = 2A(R + Kbat)/360 Where BA bend allowance, in. (mm); A - bend angle, degrees; R

bend radius, in. (mm); t sheet thickness; and Kba- factor to estimatestretching. According to [1], if R < 2t, Kba = 0.33; and if R>=2t, Kba=0.5. [1]: Hoffman, E.G., Fundamentals of Tool Design, 2nded.


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Engineering Analysis of Bending

Spring back: When the bending pressure is removed atthe end of deformation, elastic energy remains in the bendpart, causing it to recover partially toward its originalshape.

SB = (A Ab)/Ab

Where SB springback; A included angle of sheet-metalpart; and Ab included angle of bending tool, degrees.

From Mikell P. Groover Fundamentals of Modern Manufacturing.


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Bending Force: The force required to perform bendingdepends on the geometry of the punch and die and the strength,thickness, and width of the sheet metal. The maximum bendingforce can be estimated by means of the following equation basedon bending of a simple beam:

F = (KbfTSwt2)/D

Where F bending force, lb (N),; TStensile strength of the sheet

metal, lb/in2. (Mpa); t sheet thickness, in. (mm); and D dieopening dimension. Kbfa constant that counts for differences inan actual bending processes. For V-bending Kbf=1.33, and foredge bending Kbf=0.33

Engineering Analysis of Bending


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Metal to be bent with a modulus of elasticity E = 30x106lb/in2., yield

strength Y = 40,000lb/in2, and tensile strength TS= 65,000 lb/in2.

Determine (a) starting blank size, and (b) bending force if V-diewill be used with a die opening dimension D = 1.0in.

(a)

W = 1.75, and the length of the part is: 1.5 +1.00 + BA.

R/t = 0.187/0.125 = 1.5 < 2.0, so Kba =0.33

For an included angle A = 1200

, then A = 600

BA = 2pA(R + Kbat)/360 =2p60(0.187 + 0.33 x 0.125)/360 = 0.239

Length of the bank is 1.5+1+0.239 = 2.739

(b) Force:

F = (KbfTSwt2)/D

= 1.33 (65,000)(1.75)(0.125)2/1.0

= 2,364 lb

Example for Sheet-metal Bending


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Homework

Review questions (page 457 - 458):

6, 10, 20, 26

Problems (page 458):1: a, b

EIN 3390 Chap 17 Sheet-Forming Processes Part 1 Spring 2012

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