【本著作除另有註明外，採取創用CC「姓名標示－非商業性－相同方式分享」台灣3.0版授權釋出】 授課教師：楊宏智教授 Manufacturing Engineering Technology Sheet-Metal Forming Processes and Equipment 【本著作除另有註明外，採取創用CC「姓名標示－非商業性－相同方式分享」台灣3.0版授權釋出】 【本著作除另有註明外，採取創用CC「姓名標示－非商業性－相同方式分享」台灣3.0版授權釋出】

Manufacturing Engineering Technology 機械製造 楊宏智（台大機械系教授）

Chapter Outline Introduction Shearing Sheet-metal Characteristics and Formability Formability Tests for Sheet Metals Bending Sheets, Plates, and Tubes Miscellaneous Bending and Related Operations Deep Drawing Rubber Forming and Hydroforming Spinning Superplastic Forming Specialized Forming Processes Manufacturing of Metal Honeycomb Structures Design Considerations in Sheet-metal Forming Equipment for Sheetmetal Forming Economics of Sheetforming Operations

Introduction Products made of sheet metals are common Pressworking or press forming is used for general sheet-forming operations, as they are performed on presses using a set of dies

Introduction A sheet-metal part produced in presses is called a stamping Low-carbon steel has low cost and good strength and formability characteristics Manufacturing processes involving sheet metal are performed at room temperature

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

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

Advantages of Sheet Metal Parts High strength Good dimensional accuracy Good surface finish Relatively low cost Economical mass production for large quantities

Sheet Metalworking Terminology Punch-and-Die - tooling to perform cutting, bending, and drawing Stamping press - machine tool that performs most sheet metal operations Stampings - sheet metal products

Three Basic Types of Sheet Metal Processes 1.Cutting (Shearing) Shearing to separate large sheets Blanking to cut part perimeters out of sheet metal Punching to make holes in sheet metal 2.Bending Straining sheet around a straight axis 3.Drawing Forming of sheet into convex or concave shapes

Sheet Metal Cutting (Shearing) (1) Just before punch contacts work; (2) punch pushes into work, causing plastic deformation; (3) punch penetrates into work causing a smooth cut surface; and (4) fracture is initiated at opposing cutting edges to separate the sheet

Shearing Before a sheet-metal part is made, a blank is removed from a large sheet by shearing The edges are not smooth and perpendicular to the plane of the sheet

Punch and Die Sizes Die size determines blank size Db Punch size determines hole size Dh c = clearance

Shearing Processing parameters in shearing are The shape of the punch and die The speed of punching Lubrication The clearance, c, between the punch and the die When clearance increases, the zone of deformation becomes larger and the sheared edge becomes rougher Extent of the deformation zone depends on the punch speed Height, shape, and size of the burr affect forming operations

Clearance in Sheet Metal Cutting Distance between punch cutting edge and die cutting edge 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

Clearance in Sheet Metal Cutting Recommended clearance is calculated by: c = at where c = clearance; a = allowance; and t = stock thickness Allowance a is determined according to type of metal

Sheet Metal Groups Allowances Metal group a 1100S and 5052S aluminum alloys, all tempers 0.045 2024ST and 6061ST aluminum alloys; brass, 0.060 soft cold rolled steel, soft stainless steel Cold rolled steel, half hard; stainless steel, 0.075 half hard and full hard

Shearing Clearance Clearance control determine quality of its sheared edges which influence formability of the sheared part Appropriate clearance depends on: 1. Type of material and temper 2. Thickness and size of the blank 3. Proximity to the edges of other sheared edges When sheared edge is rough it can be subjected to a process called shaving

Shearing: Characteristics and Type of Shearing Dies Punch and Die Shape Punch force increases rapidly during shearing Location of sheared regions can be controlled by beveling the punch and die surfaces

Shearing Punch Force Maximum punch force, F, can be estimated from Friction between the punch and the workpiece can increase punch force T = sheet thickness L = total length sheared UTS = ultimate tensile strength of the material

Shearing EXAMPLE 16.1 Calculation of Punch Force Estimate the force required for punching a 25-mm diameter hole through a 3.2-mm thick annealed titanium- alloy Ti-6Al-4V sheet at room temperature. Solution UTS for this alloy is 1000 MPa, thus

Blanking and Punching Blanking - sheet metal cutting to separate piece (called a blank) from surrounding stock Punching - similar to blanking except cut piece is scrap, called a slug

Shearing: Shearing Operations Punching is where the sheared slug is scrap Blanking is where the slug is the part to be used and the rest is scrap Die Cutting Shearing operation consists of: Perforating: punching holes in a sheet Parting: shearing sheet into pieces Notching: removing pieces from the edges Lancing: leaving a tab without removing any material

Shearing: Fine Blanking smaller clearance used pressure pad w/ v-shaped stringer locks the sheet metal movement of punch, pressure pad, cushion are controlled by triple-action hydraulic presses

Shearing Operations: Slitting Shearing operations are through a pair of circular blades, follow either a straight line, a circular path, or a curved path

Shearing Operations: Nibbling A contour is cut by a series of overlapping slits or notches Simple tools can be used to produce complex shape

Shearing: Tailor-welded Blanks Laser-beam butt welding involves two or more pieces of sheet metal with different shapes and thicknesses The strips are welded to obtain a locally thicker sheet and then coiled The welded assembly is then formed into a final shape. Resulting in: Reduction in scrap Elimination of the need for subsequent spot welding Better control of dimensions Improved productivity

Shearing: Tailor-welded Blanks EXAMPLE 16.2 Tailor-welded Sheet Metal for Automotive Applications Production of an outer side panel of a car body is by laser butt welding and stamping

Shearing: Tailor-welded Blanks EXAMPLE 16.2 Tailor-welded Sheet Metal for Automotive Applications Some of the examples of laser butt-welded and stamped automotive-body components.

Characteristics and Type of Shearing Dies Transfer Dies Sheet metal undergoes different operations arranged along a straight line or a circular path Tool and Die Materials Tool and die materials for shearing are tool steels and carbides Lubrication is needed for reducing tool and die wear, and improving edge quality

Characteristics and Type of Shearing Dies Progressive Dies a different operation is performed at the same station with each stroke of a series of punches

Characteristics and Type of Shearing Dies For high product production rates The part shown below is the small round piece that supports the plastic tip in spray cans

Characteristics and Type of Shearing Dies Compound Dies Operations on the same sheet may be performed in one stroke with a compound die Limited to simple shapes due to: Process is slow Complex dies is more expensive

Shearing: Miscellaneous Methods of Cutting Sheet Metal Other methods of cutting sheets Laser-beam cutting Water-jet cutting Cutting with a band saw Friction sawing Flame cutting

Sheet-metal Characteristics and Formability Yield-point elongation having both upper and lower yield points. This behaviour results in Luder’s bands Typically observed with mild-steel sheets Results in depressions on the sheet surface Can be eliminated by temple rolling (but sheet must be formed within a certain time after rolling)

Sheet-metal Characteristics and Formability Grain Size Affects mechanical properties and surface appearance Smaller the grain size, stronger is the metal Dent Resistance of Sheet Metals Dents caused by dynamic forces from moving objects that hit the sheet metal Dynamic yield stress, instead of static yield stress, should be the significant strength parameter

Formability Tests for Sheet Metals Sheet-metal formability is the ability of the sheet metal to undergo the desired shape change without failure Sheet metals may undergo 2 basic modes of deformation: (1) stretching and (2) drawing Cupping Tests In the Erichsen test, the sheet specimen is clamped and round punch is forced into the sheet until a crack appears The punch depth is a measure of formability of the sheet Easy to perform, but does not simulate exact conditions of actual forming, and not reliable for complex parts.

Formability Tests for Sheet Metals Forming-limit Diagrams To develop a forming-limit diagram, the major and minor engineering strains are obtained Major axis of the ellipse represents the major direction and magnitude of stretching Major strain is the engineering strain and is always positive Minor strain can be positive or negative Curves represent the boundaries between failure and safe zones

Formability Tests for Sheet Metals Forming-limit Diagrams Forming-limit diagrams is to determine the formability of sheet metals

Forming-Limit Diagram (FLD) Blank stretched over a punch, deformation observed and measured in the region where failure has occurred The curves represent the boundaries between failure and safe zones Different mat'ls have different FLDs, and the higher the curve, the better is the formability Compressive minor strain is associated with a higher major strain than a tensile minor strain of the same magnitude The effect of sheet-metal thickness is to raise the curves Friction, lubrication at punch/sheet-metal interface, and surface scratches, are important factors

Bending Sheets , Plates, and Tubes Bending is a common industrial forming operation Bending imparts stiffness to the part by increasing its moment of inertia Outer fibers are in tension, while the inner in compression Poisson effect cause the width to be smaller in the outer region and larger in the inner region

Bending Sheets, Plates, and Tubes Approximate bend allowance is For ideal case, k = 0.5, Minimum Bend Radius Engineering strain during bending is Minimum bend radius, R, is

Bending Sheets, Plates, and Tubes Minimum Bend Radius Increase the bendability by increase their tensile reduction of area Bendability also depends on the edge condition of the sheet Improve resistance to edge cracking by removing the cold-worked regions Cold rolling results in anisotropy by preferred orientation or mechanical fibering

Bending Sheets, Plates, and Tubes Springback Plastic deformation is followed by elastic recovery when the load is removed, called springback Springback can be calculated by

Bending Sheets, Plates, and Tubes Compensation for Springback Springback is compensated for by overbending the part One method is stretch bending where the part is subjected to tension while being bent Bending Force Excluding friction, the maximum bending force, P, is For a V-die, it is modified to

Miscellaneous Bending Operations Examples of various bending operations Roll Bending Plates are bent using a set of rolls. Curvatures can be obtained by adjusting the distance between the three rolls

V-Bending and Edge Bending Low production Performed on a press brake V-dies are simple and inexpensive Edge-Bending: High production Pressure pad required Dies are more complicated and costly

Miscellaneous Bending and Related Operations Beading Periphery of the sheet metal is bent into the cavity of a die The bead imparts stiffness to the part by increasing the moment of inertia of that section

Miscellaneous Bending and Related Operations Flanging In shrink flanging, the flange is subjected to compressive hoop stresses and cause the flange periphery to wrinkle

Bending Operations most common forming operation paper clip, file cabinet etc

Press Brakes – Bending Equipment Sheet metal or plate can be bent easily with simple fixtures using a press complex bends with long narrow bed and short adjustable strokes metal bent between interchangeable dies

Roll Forming Also called contour-roll forming or cold-roll forming Used for forming continuous lengths of sheet metal and for large production runs Dimensional tolerances, springback, tearing and buckling of the strip have to be considered

Bending of Tube Stock Stretch bending of tube: (1) start of process and (2) during bending

Tube Bending Large dia. thin wall, with small bend radius tend to cause wrinkle at inner side of the tube Oldest method of bending a tube is to first pack its inside with loose particles and then bend it into a suitable fixture Thick tube can be formed to a large bend radius without the use of fillers or plugs

Tube Forming tubular part placed in a split-female die and expanded with a polyurethane or rubber plug punch retracted; plug returns to its original shape and removed by knockout rod finished part removed by opening the split die (water pitcher) production of fitting for plumbing

Manufacturing of Bellows (a) Bulged tube : Tube bulged at several equidistant locations (b) Compressed tube : Bulged tube compressed axially to collapse bulged regions, thus forming bellows

Stretch Forming Sheet metal is gripped by two sets of jaws The jaws stretch the metal sheet and wrap it around a form block (die) Most of deformation is induced by tensile stretching, and forces on the form block are far less Very little springback results Form block often made of wood or low-melting metal Produce large parts in low or limited quantity

Stretch Forming Sheet metal is clamped along its edges and then stretched over a male die Die moves upward, downward, or sideways Used to make aircraft wing-skin panels, fuselages, and boat hulls

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 then punch pushes metal into opening Products: beverage cans, ammunition shells, automobile body panels Also known as deep drawing (to distinguish it from wire and bar drawing)

Deep Drawing - a punch forces a flat sheet metal into a die cavity - depth greater than diameter

Deep Drawing Material beneath punch remains unaffected and becomes cup bottom Cup wall is formed by pulling the remainder of disk inward the radius of die - Hoop stress tends to cause buckling or wrinkling  - Pressure ring is used to suppress wrinkle

Deep Drawing Wrinkling can be reduced if a blankholder is loaded by maximum punch force The force increases with increasing blank diameter, thickness, strength and the ratio

Redrawing Cup becomes longer as it is redrawn to smaller diameters since volume of the metal is constant If the shape change is too severe, more than one drawing step is required. The second drawing step, and any further drawing step is referred as redrawing

Ironing If the clearance between the punch and the die is large, the drawn cup will have thicker walls Thickness of the cup wall can be controlled by ironing, where drawn cup is pushed through one or more ironing rings

Deep Drawing: Deep Drawability Earing In deep drawing, the edges of cups may become wavy and the phenomenon is called earing Earing is caused by the planar anisotropy Planar anisotropy of the sheet is indicated by

Deep-drawing Practice Earing Too high a blankholder force increases the punch force and causes the cup wall to tear Draw beads are needed to control the flow of the blank into the die cavity and reduce the blankholder forces

Deep-drawing Practice CASE STUDY 16.1 Manufacturing of Food and Beverage Cans Aluminum beverage cans has excellent surface finish Detail of the can lid is shown

Rubber Forming Dies are made of solid materials, such as steels and carbides The dies in rubber forming is made of a flexible material (polyurethane membrane) In the bending and embossing of sheet metal, the female die is replaced with a rubber pad

Hydroforming In the hydroform, or fluid-forming process, the pressure over the rubber membrane is controlled throughout the forming cycle Control of frictional conditions in rubber forming is a factor in making parts successfully

Rubber Forming and Hydroforming In tube hydroforming metal tubing is formed in a die and pressurized internally by a fluid, usually water Rubber-forming and hydroforming processes have the advantages of: Capability to form complex shapes Flexibility and ease of operation Low tooling cost

Tube Hydroforming CASE STUDY 16.2 Tube Hydroforming of an Automotive Radiator Closure Figure shows a hydroformed automotive radiator closure Sequence of operations: (1) tube as cut to length; (2) afterbending; (3) after hydroforming

Spinning Spinning is a process that involves the forming of axisymmetric parts over a mandrel A circular blank of flat sheet metal is held against a mandrel (form block of desired shape) and rotated while a rigid tool deforms and shapes the material over the mandrel; Disk of sheet metal progressively shaped by localized pressure with small roller Suitable for conical and curvilinear shapes

Shear Spinning (Forming) A simplified version of the spinning process in which each element of the blank maintains its distance from the axis of rotation. Metal flow is entirely in shear and no radial stretch has to take place to compensate for the circumferencial shrinkage

Shear Spinning Also known as power spinning, flow turning, hydrospinning, and spin forging Use to produce an axisymmetric conical or curvilinear shape while reducing the sheet’s thickness and maintaining its maximum (blank) diameter

Tube Spinning Tube Spinning The thickness of hollow, cylindrical blanks is reduced by spinning them on a solid, round mandrel using rollers Can be carried out externally or internally Various external and internal profiles can be produced from cylindrical blanks with constant wall thickness

Spinning Incremental Forming Simplest version is incremental stretch expanding A rotating blank is deformed by a steel rod with a smooth hemispherical tip to produce axisymmetric parts CNC incremental forming uses a CNC machine tool to follow contours at different depths across the sheet-metal surface Advantages are low tooling costs and high flexibility in the product shapes

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 (1) Setup, (2) explosive is detonated, and (3) shock wave forms part and plume escapes water surface

Explosive Forming The peak pressure, p, is given by The mechanical properties of parts similar to those made by conventional forming methods The dies may be made of aluminum alloys, steel, ductile iron or zinc alloys p = pressure, psi K = constant that depends on the type of explosive

Explosively Formed Part Explosive used as a source of energy Rapid conversion of explosive charge into gas generates a shock wave Pressure of shock wave is sufficient to form sheet metal no limit to the size of the workpiece (suitable for low quantity of large parts, ie aerospace application)

Electromagnetic Forming coil current rapidly discharged from capacitor bank eddy current generated in the tube (workpiece) repelling force between the coil and the tube forces generated collapse the tube Higher the electrical conductivity of the workpiece, the higher the magnetic forces

Peen Forming Used to produce curvatures on thin sheet metals by shot peening one surface of the sheet Surface of the sheet is subjected to compressive stresses The process also induces compressive surface residual stresses, which improve the fatigue strength of the sheet

Laser Forming Involves the application of laser beams as a heat source in specific regions of the sheet metal Process produce thermal stresses, which can cause localized plastic deformation of the sheet In laser-assisted forming, the laser acts as a localized heat source, thus reducing the strength of the sheet metal at specific locations Improve formability and increasing process flexibility

Superplastic Forming The behavior of superplastic are where tensile elongations were obtained within certain temperature ranges Superplastic alloys can be formed into complex shapes by superplastic forming Have high ductility but low strength Advantages: Complex shapes can be formed Weight and material savings Little residual stresses Tooling costs are lower

Superplastic Forming Limitations of superplastic forming: Part will undergo shape changes Must be formed at sufficiently low strain rates Diffusion Bonding/Superplastic Forming Fabricating of complex sheet-metal structures by combining diffusion bonding with superplastic forming (SPF/DB) Application for aerospace industry Improves productivity and produces parts with good dimensional accuracy and low residual stresses

Specialized Forming Processes CASE STUDY 16.3 Cymbal Manufacture

Manufacturing of Metal Honeycomb Structures A honeycomb structure has light weight and high resistance to bending forces, used for aircraft and aerospace components 2 methods of manufacturing honeycomb materials: Expansion process Corrugation process

Manufacturing of Metal Honeycomb Structures A honeycomb structure consists of a core of honeycomb bonded to two thin outer skins Has a high stiffness-to-weight ratio and is used in packaging for shipping consumer and industrial goods

Design Considerations in Sheet-metal Forming Blank Design Poorly designed parts will not nest properly Blanks should be designed to reduce scrap to a minimum

Equipment for Sheet-metal Forming Press selection for sheet-metal forming operations depends on: Type of forming operation Size and shape of workpieces Number of slides Maximum force required Type of mechanical, hydraulic, and computer controls Features for changing dies Safety features

CNC Turret Press Parts Sheet metal parts produced on a turret press, showing variety of hole shapes possible (photo courtesy of Strippet Inc.)

Economics of Sheet-forming Operations Sheet-forming operations are versatile and can produce the same part The costs involved depend on die and equipment costs and labor For small and simple sheet-metal parts, die costs and lead times to make the dies are low Deep drawing requires expensive dies and tooling Equipment costs depend on the complexity of the forming operation