Sheetmetal Working

7/25/2019 Sheetmetal Working

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SHEET METALWORKING

Cutting Operations Bending Operations

Drawing

Other Sheet Metal Forming Operations es an resses or ee e a rocesses

Sheet Metal Operations Not Performed on Presses


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Sheet Metalworking Defined

Cutting and forming operations performed on relativelythin sheets of metal

c ness o s ee me a = . mm n o mm

(1/4 in)

>

Operations usually performed as cold working


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Sheet and Plate Metal Products

Sheet and plate metal parts for consumer andindustrial products such as

. u omo es an ruc s

2. Airplanes

.

4. Farm and construction equipment

5. Small and lar e a liances

6. Office furniture

7. Computers and office equipment


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Advantages of Sheet Metal Parts

High strength Good dimensional accuracy

Good surface finish

Relatively low cost or arge quan es, econom ca mass pro uc on

operations are available


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Sheet Metalworking Terminology

1. Punch-and-die Tooling to perform cutting, bending, and drawing

2. Stamping press

Machine tool that performs most sheet metal

3. Stampings

Sheet metal roducts


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Sheet Metal Processes

1. Cutting Shearing to separate large sheets; or cut part

per me ers or ma e o es n s ee s

2. Bending

3. Drawing

Formin of sheet into convex or concave sha es


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Cutting: Shearing between two sharp cutting edges

(1) just before the

punch contacts work

punc eg ns to pus nto wor ,causing plastic deformation


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3 unch com resses and

penetrates into work causing a

smooth cut surface

opposing cutting edges which

separates the sheet


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Shearing, Blanking, and Punching

Three principal operations in pressworking that cutsheet metal:

ear ng

Blanking


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Shearing - sheet metal cutting operation along a

s ra g ne e ween wo cu ng e ges

subsequent operations

Blanking - sheet metal cutting to separate piece from

surrounding stock

Cut piece is the desired part, called a blank

Punching - sheet metal cutting similar to blanking

except cut piece is scrap, called a slug

Remaining stock is the desired part


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ear ng operat ona) side view of the shearing operation

b) front view of power shears equipped with inclined upper cuttingblade symbol v indicates motion


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(a) Blanking and (b) punching


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Clearance in Sheet Metal Cutting

Typical values range between 4% and 8% of stock

thickness

- If too small, fracture lines pass each other, causing doubleburnishing and larger force

- ,

excessive burr results


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Cutting Forces

Important for determining press size (tonnage)

F = S t L or F= 0.7 t L

where S = shear strength of metal (MPa) = tensile strength (MPa)

t = stock thickness (mm), and


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Deep Drawing

Sheet metal forming to make cup-shaped, box-shaped,or other complex-curved, hollow-shaped parts

Sheet metal blank is positioned over die cavity and

Products: beverage cans, ammunition shells,

automobile body panels


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Stages in Deformation of the Workpiece during Deep Drawing


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-shaped part:

(1) start of operation

contacts work(2) near end of

(b) Correspondingworkpart:

(1) starting blank

2 drawn art


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Clearance in Drawing

Sides of punch and die separated by a clearance cgiven by:

c = .

where t = stock thickness

,

stock thickness


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Drawing Ratio DR

Most easily defined for cylindrical shape:

b

D

DR

= = p

Indicates severity of a given drawing operation

- Upper limit = 2.0


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Reduction r

,

b

pb

Dr

Value of rshould be less than 0.50


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Thickness-to-Diameter Ratio

Thickness of starting blank divided by blank diameterThickness-to-diameter ratio = t/Db

Desirable for t/Db ratio to be greater than 1%

As t/Db decreases, tendency for wrinkling increases


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Blank Size Determination

For final dimensions of drawn shape to be correct,starting blank diameter Db must be right

o ve or b y se ng s ar ng s ee me a an

volume = final product volume

part wall


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Other drawing operations:

Redrawing Reverse Drawing

1 start

(2) finish


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Shapes other than Cylindrical Cups

Square or rectangular boxes (as in sinks), Stepped cups,

Cones,

Cups with spherical rather than flat bases, rregu ar curve orms as n au omo e o y pane s

technical problems in drawing


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Common defects in drawn parts

a wr n ng n ange

b) wrinkling in walls

c) tearing

d) earinge) surface scratches


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Other Sheet Metal Forming on Presses

Other sheet metal forming operations performed onconventional presses

pera ons per orme w me a oo ng

Operations performed with flexible rubber tooling


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Dies for Sheet Metal Processes

Most pressworking operations performed withconventional punch and die tooling

us om es gne or par cu ar par

The term stamping die sometimes used for high


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Components of a punch and die for a blanking operation

Documents

Sheetmetal Working