Chapter 36 Quality Engineering (Review) EIN 3390 Manufacturing Processes Summer A, 2011.
EIN 3390 Chap 17 Sheet-Forming Processes Part 1 Spring 2012
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Transcript of 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