Sheet Metal Processes SHEET

1. Cutting Operations 2. Bending Operations 3. 4. Operations Not Performed on Presses

2

Sheet Metalworking Defined

• Cutting and forming operations performed on • relatively thin sheets of metal • Thickness of sheet metal = 0.4 mm (1/64 in) to 6 mm (1/4 in) • Thickness of plate stock > 6 mm • Operations usually performed as cold working

Three Major Categories of Sheet Metal Processes

• Cutting • to separate large sheets; or cut part perimeters or make holes in sheets • Bending • Straining sheet around a straight axis • Drawing • Forming of sheet into convex or concave shapes Shearing

• A sheet-metal cutting operation along a straight line between two cutting edges Sheet Metalworking Defined

Cutting and forming operations performed on relatively thin sheets of metal • Thickness of sheet metal = 0.4 mm (1/64 in) to 6 mm (1/4 in) • Thickness of plate stock > 6 mm • Operations usually performed as cold working

8 Sheet and Plate Metal Products

• Sheet and plate metal parts for consumer and industrial products such as • Automobiles and trucks • Airplanes • Railway cars and locomotives • Farm and construction equipment • Small and large appliances • Office furniture • Computers and office equipment

9 Advantages of Sheet Metal Parts

• High strength • Good dimensional accuracy • Good surface finish • Relatively low cost • For large quantities, economical mass production operations are available

10 Dies for Sheet Metal Processes

Most pressworking operations performed with conventional punch-and- tooling • The term die sometimes used for high production dies

11 Dies for Sheet Metal Processes (a) Progressive die; (b) associated strip development

12 Sheet Metalworking Terminology

1. “Punch-and-die” • Tooling to perform cutting, bending, and drawing 2. “” • Machine tool that performs most sheet metal operations 3. “Stampings” • Sheet metal products

13 Three Major Categories of Sheet Metal Processes

1. Cutting ◦ Shearing to separate large sheets; or cut part perimeters or make holes in sheets

2. Bending ◦ Straining sheet around a straight axis

3. Drawing ◦ Forming of sheet into convex or concave shapes

14 Power and Drive Systems

• Hydraulic presses - use a large piston and cylinder to drive the ram • Longer ram stroke than mechanical types • Suited to deep drawing • Slower than mechanical drives • Mechanical presses – convert rotation of motor to linear motion of ram • High forces at bottom of stroke • Suited to blanking and

15 I. Cutting

Shearing between two sharp cutting edges

16 Shearing Process

• 1. Punching: shearing process using a die and punch where the interior portion of the sheared sheet is to be discarded. • 2. Blanking: shearing process using a die and punch where the exterior portion of the • shearing operation is to be discarded. • 3. Perforating: punching a number of holes in a sheet • 4. Parting: shearing the sheet into two or more pieces • 5. Notching: removing pieces from the edges • 6. Lancing: leaving a tab without removing any material

Lancing • Lancing is a piercing operation in which a hole is partially punched and one side is bent down

Lancing

Blanking

Piercing

12/25/2019 19 Perforating (Delikleme) • Perforating consists of piercing a large number of closely spaced holes.

12/25/2019 20 Notching (Kertikleme) • Notching is essentially the same as piercing except that the edge of the sheet of metal forms a portion of the periphery of the piece that is punched out.

12/25/2019 21 Nibbling • Nibbling is a variation of notching in which a special machine makes a series of overlapping notches, each further into the sheet of metal.

12/25/2019 22 Nibbling

12/25/2019 23 Shaving • Shaving is a finishing operation in which a very small amount of metal is sheared away around the edge of a blanked part. • Used for greater dimensional accuracy.

12/25/2019 24 Trimming • Trimming is used to remove excess metal that remains after a drawing, , or operation (same as blanking).

12/25/2019 25 Dinking

• Dinking is a modified shearing operation that is used to blank shapes from low- strength materials, primarily rubber, fiber, and cloth.

12/25/2019 26 Shearing

Sheet metal cutting operation along a straight line between two cutting edges • Typically used to cut large sheets into smaller sections for subsequent operations

27 Blanking and Punching

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

(a) Blanking 28and (b) punching https://www.youtube.com/watch?v=YSi655MewC0 Clearance in Sheet Metal Cutting

Distance between the punch and die • Typical values range between 4% and 8% of stock thickness • If too small, fracture lines pass each other, causing double burnishing and larger force • If too large, metal is pinched between cutting edges and excessive burr results

30 II. Bending

Straining sheetmetal around a straight axis to take a permanent bend

(a) Bending of sheet metal (b) both compression and tensile elongation of the metal occur in bending

31 Types of Sheetmetal Bending

• V-bending - performed with a V-shaped die • Edge bending - performed with a wiping die

32 V-Bending • For low production • Performed on a press brake • V-dies are simple and inexpensive

33 Edge Bending

• For high production • Pressure pad required • Dies are more complicated and costly

34 Springback in Bending

Springback = increase in included angle of bent part relative to included angle of forming tool after tool is removed • Reason for springback: • When bending pressure is removed, elastic energy remains in bent part, causing it to recover partially toward its original shape

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

Products: beverage cans, ammunition shells, automobile body panels

36 Maximum Punch Force

퐷푂 퐹푚푎푥 = 휋퐷푝푇(푈푇푆) − 0.7 퐷푝

Deep Drawability

Limiting Drawing Ratio (LDR)

37 Anisotropy r

• Normal anisotropy r is an indication of resistance to the thinning of a material • is defined as the ratio of transverse strain to thickness strain • at the uniform elongation region. • In sheet materials, material properties usually change with directions. • If the r value is equal to 1, the material is considered to be isotropic • Springback • Die Radius • Die load • Punch Radius • Recoil / Warp • Clearance Ratio • Minimum Bend Radius (or • Pad Force maximum curvature)

12/25/2019 41 12/25/2019 42 Redrawing

Additional drawing steps (redrawing) may be required if the shape change of the material is too severe IV. Shapes other than Cylindrical Cups

• or rectangular boxes (as in sinks), • Stepped cups, • Cones, • Cups with spherical rather than flat bases, • Irregular curved forms (as in automobile body panels)

• Each of these shapes presents its own unique technical problems in drawing

44 Ironing  Makes wall thickness of cylindrical cup more uniform  Examples: beverage cans and artillery shells

Ironing to achieve a more uniform wall thickness in a drawn cup: (1) start of process; (2) during process Note thinning and elongation of walls

45 Embossing  Used to create indentations in sheet, such as raised (or indented) lettering or strengthening ribs Embossing: (a) cross-section of punch and die configuration during pressing; (b) finished part with embossed ribs

46 Advantages of Guerin Process Guerin Process  Low tooling cost  Form block can be made of wood, plastic, or other materials that are easy to shape  Rubber pad can be used with different form blocks  Process attractive in small quantity production

47 Several sheet metal parts produced on a turret press, showing variety of hole shapes possible (photo courtesy of Strippet, Inc.)

48 Sheet Metal Operations Not Performed on Presses

1. Stretch forming 2. Roll bending and forming 3. Spinning 4. High-energy-rate .

49 1. Stretch Forming Sheet metal is stretched and simultaneously bent to achieve shape change

Stretch forming: (1) start of process; (2) form die is pressed into the work with force Fdie, causing it to be stretched and bent over the form. F = stretching force

50 2. Roll Bending Large metal sheets and plates are formed into curved sections using rolls

51 2. Roll Forming Continuous bending process in which opposing rolls produce long sections of formed shapes from coil or strip stock

Roll forming of a continuous channel section: (1) straight rolls (2) partial form (3) final form

52 3. Spinning Metal forming process in which an axially symmetric part is gradually shaped over a rotating mandrel using a rounded tool or roller

Conventional spinning: (1) setup at start of process; (2) during spinning; and (3) completion of process

53 4. High-Energy-Rate Forming (HERF)

Processes to form metals using large amounts of energy over a very short time • HERF processes include: • • •

54 Explosive Forming Use of explosive charge to form sheet (or plate) metal into a die cavity • Explosive charge causes a shock wave whose energy is transmitted to force part into cavity • Applications: large parts, typical of aerospace industry

Explosive forming: (1) setup, (2) explosive is detonated, and (3) shock wave forms part and plume escapes water surface 55 Explosive Forming

• video https://www.youtube.com/watch?v=Cmo9MQ3pPF0 Electrohydraulic forming • EHF is based on the ultra-high-speed deformation of metal using shockwaves in water. • The electric arc in water between two electrodes vaporizes the surrounding water, • converting electrical energy into an intense shockwave of mechanical energy. • The shockwave simultaneously transforms the metal workpiece into a visco-plastic state • and accelerates it onto a die, • enabling forming of complex shapes at high speeds in cold conditions. • All of which happens in a matter of milliseconds; • This process is not limited by size • and allows forming of parts up to a few square meters in size. • An array of electrodes can be placed over a large workpiece, enabling pressure distribution according to the product’s topology, • still using a one-sided die to create complex shapes and fine details.

Electromagnetic forming (EMF)

• is a non-contact forming technique. • It is an impulse or high-velocity forming method • using pulsed • by application of Lorentz' forces to the work piece • preferably made of highly electrically conductive material • without having working medium and mechanical contact. Electromagnetic forming (EMF)

• Video

• https://www.youtube.com/watch?v=O02Tny2y7TA Electromagnetic Forming

Sheet metal is deformed by mechanical force of an electromagnetic field induced in workpart by an energized coil • Presently the most widely used HERF process • Applications: tubular parts (1) setup in which coil is inserted into tubular workpart surrounded by die; (2) formed part 61 Electromagnetic Forming

• The ESMF system consists of an R-L-C circuit • and the process starts when a bank is suddenly discharged through an actuator coil. • The transient that is flowing through the actuator coil generates a timevarying magnetic field • this magnetic field induces electric currents in nearby conductor. • These induced currents are called as eddy currents and are created on the workpiece surface. • These eddy currents have different flow directions in contrast to the ones in the coil. • Therefore, a repulsive force is generated between the coil and the workpiece. • If this force is enough to form the metal then it is formed according to the die. Electromagnetic Forming