MAE Course 3344 Lecture 8 Sheet Metal Shaping and Forming

MAE Course 3344 Lecture 8 Sheet Metal Shaping and Forming
Professor John J. Mills Mechanical and Aerospace Engineering The University of Texas at Arlington This lecture covers material describing how sheet metal is shaped and formed. It covers chapter 16 of Kalpakjian Professor John J. Mills: Tel (817)

Material Transformation Processes
Assembly SLS Powders Special Pressing Firing/ Sintering Injection Molding Stamping Sheet metal forming Raw Material Continuous Casting/Rolling Rolling Forging/ Press forming Finishing Ingot casting Molten Material Extruding This figure is the overview with the topic highlighted by light blue rectangle with rounded corners Casting Shapes Machining Single crystal pulling Blow molding Current lecture Increasing level of detail Professor John J. Mills: Tel (817)

Overview of Sheet Metal Forming
Shearing to make blanks Fundamentals of forming sheet metal These are the topics covered in this lecture. Items highlighted in red have links the appropriate pages. To activate you need to be in slide show mode or on the browser Professor John J. Mills: Tel (817)

Sheet Metal Forming History
Very old process - back to 5000 BC Original sheet obtained by hammering over a stone anvil Cut to shape with a knife Formed over stone or wooden dies by hammering Now sheet produced by sheet mills Cutting to shape and forming is by machines Sheet metal forming is concerned with the cutting and forming of relatively thin sheet. Large plates with a small value of the ratio of thickness to any other dimension are what is meant by "sheet". Sheet is formed by rolling which is described in lecture 3 Professor John J. Mills: Tel (817)

General Practices Most common commercial material is carbon steel
Most common aircraft and aerospace materials are aluminum and titanium Aluminum increasingly found in automobiles Sheet metal forming consists of three basic processes; Cutting to form a shape (blank) Forming by bending and stretching Finishing Professor John J. Mills: Tel (817)

Sheet Metal Forming Processes
Punching Blanking Fine Blanking Stamping Embossing Shearing Slitting Cutting Sawing Deburring Cleaning Coating Sheet, Plate Blank Bending Roll forming Stretch forming Deep drawing Rubber forming Spinning Peen forming Superplastic forming Explosive forming Magnetic pulse forming This illustrates the overall process. Sheet is cut up by various processes into blanks and then formed by bending stretching etc into shapes. Blanks can also be shapes stamped out by punches and dies and used without further forming. Finishing is an important part of sheet metal forming because the cut edges often have to have their sharp edges removed - a process known as deburring. Making blanks forming finishing Professor John J. Mills: Tel (817)

Sheet Metal Advantages and Disadvantages
light weight, versatile shapes, low cost Disadvantages tooling costs (for high production runs) sheet metal may not be appropriate to design function This lists the advantages and disadvantages of sheet metal for use in products Professor John J. Mills: Tel (817)

Shearing Needed to cut rough blanks from the large sheets
A blank is the term for the rough shape needed to form the final part Rectangular blanks created by shears, saws, rotary cutters These blanks can be further sheared into more complex shapes be further formed (bent, deep drawn, etc) into more complex shapes also be the final product Shearing is the basic process by which all sheet is cut. Shearing can be carried out by a pair of manual shears, linear shearing equipment (which is essentially a giant pair of scissors) or shaped punches and dies. The basic process is the same, the resulting shape differs. The term "shearing," however, is usually employed to describe linear cutting. Cutting of complex shape with shaped punches and dies is called stamping. Professor John J. Mills: Tel (817)

The Basic Shearing Process
Like cutting paper with scissors but using a machine Shearing starts with cracks developed on top and bottom of sheet by exceptionally high shear stresses A fracture process The Punch is typically the moving part The Die is the stationary part. The basic shearing process is essentially one of fracture. For small clearances between the punch and the die, the fracture occurs almost instantaneous with the first impact of the punch on the material. If the gap is larger, but still smaller than the thickness of the sheet, the material is substantially deformed before fracture occurs. If the clearance is only slightly smaller than the thickness `the material may not shear at all but be drawn down with the punch. This is the basis of deep drawing a process discussed later. Shearing always results in two parts (except for slitting and lancing, see later). The part which is wanted is the blank. The other is scrap. Professor John J. Mills: Tel (817)

The Basic Shearing Process Results
Typically creates rough fracture surfaces Smoothing of this surface occurs by rubbing on the shear blades or the die Shears, the machine for cutting metal can operate up to thickness of several inches Professor John J. Mills: Tel (817)

Effect of die clearance on deformation zone
Smaller the clearance, the better the edge This figures shows how with small clearances, fracture occurs almost immediate upon contact of the punch with the material, where as with large clearances, material is pulled into the gap. The pulling of material into the gap is used in the ironing process (a drawing process) discussed later Professor John J. Mills: Tel (817)

Simple Shearing Advantages and Disadvantages
Minimal tooling Stops for dimensions Disadvantages Only simple shapes (rectangles) Professor John J. Mills: Tel (817)

Shearing Operations for more complex shapes
Punching More complex shapes than simple shearing Made by punch and die set Internal part (slug) discarded Blanking Same basic process as Punching but Internal part (slug) retained Fine blanking - a specialized kind of blanking Other operations include Parting Stamping Notching Embossing Lancing Perforating Slitting Nibbling Shaving Steel rules (soft materials only) There are a variety of shearing operations. Punching essentially creates a hole in the required part, whereas blanking creates the required part. The others do various things as illustrated on page 448 of kalpakjian. Professor John J. Mills: Tel (817)

Other shearing processes
Professor John J. Mills: Tel (817)

Punching Circular blanks created by punch and die Punch Workpiece Die
The circle is a view looking down on the punch and die. To create flat blanks, a flat die as illustrated is required. Shaped dies to reduce punching force gives deformed blanks. Workpiece Die Professor John J. Mills: Tel (817)

A Punched Hole This figure shows an enlarge cross-section of the hole in the workpiece. The rollover depth can be almost invisible. And the fracture edge can be burnished or polished by the punch and die set. Professor John J. Mills: Tel (817)

Process variables in shearing with a punch and die and punch force
F = 0.7 T L (UTS) where F force T workpiece thickness L total sheared length (the circumference in this case) UTS Ultimate tensile strength of workpiece material This illustrates a typical cylindrical punch and die and movement of the material under the shearing forces. It also provides and equation for the punch force required. Professor John J. Mills: Tel (817)

Major Processing Factors in Shearing Die Design
Punch shape Bevel Reduces shear forces and noise Double bevel Reduces lateral forces of bevel shear Convex shear All produce at least one part (e.g. the blank) which is bent. If a shaped punch were perfectly flat the impact of the punch on the material being stamped will be large and any misalignment of the punch with the die could be disastrous, resulting in major damage to the punch or the die. Most punches have convex or concave edges to spread the cutting force out and to help with alignment of the punch and die. Note that these shapes all induce bending in one either the blank or the scrap. The amount of bending depends on the size of the part and the bevel angle . The trick is to make sure that the bent part is always scrap (the slug, see later), or expensive operations will be needed to straighten up the blank. To create flat blanks, we must use a flat punch Professor John J. Mills: Tel (817)

The Blank This illustrates the bending that occurs as mentioned earlier. The amount of dishing depends on a number of parameters, including the shape of the punch, the diameter of the punch, the thickness of the material. This figure is for a flat die. Professor John J. Mills: Tel (817)

Major Processing Factors in Shearing
Independent parameters Dependent parameters Punch and Die Shape Material Clearance between punch and die Increased clearance Workpiece ductility and thickness Increased ductility Decreased thickness Dulled tools Speed of punch/shear Decreased speed Increased Lubrication Rougher edge Larger deformation zone Increased burr height Greater ratio of burnished to rough areas Decrease max. punch force This figure attempts to summarize various statements in Kalpakjian p It attempts to show the empirically observed relationships between the various operational parameters and the nature of the cut edge. Professor John J. Mills: Tel (817)

Major Processing Factors in Shearing Die Design
Clearances Depends on Workpiece material Thickness Size of hole Proximity of hole to sheet edge Small holes required larger clearances than large holes Typically range form 2-8% of sheet thickness Can range from 1%(Fine Blanking) to 30% These parameters, spread over the next few slides are factors which influence the nature of the cut, its edges and the forrces invovled Professor John J. Mills: Tel (817)

Fine Blanking A device called a V-shaped Stinger locks the sheet in place Prevents distortion at sheared edges Very tight (<1%) clearances) Therefore tight tolerances possible Professor John J. Mills: Tel (817)

Other Methods of Cutting Sheet Metal
Band saw Very versatile but not very precise Used a lot in job shops Flame cutting Used mostly on thick steel sheet Can cut quite complex shapes but is not precise Leaves a very rough edge and often a heat affected zone Laser-beam cutting Very popular since it can be readily programmed to cut complex shapes Leaves a fine heat affected zone (much smaller than flame cutting) In addition to shearing, sheet can be cut by other methods. Flame cutting is mostly for cutting up thick sheets for further processing. It leaves a nasty edge which must be removed but is often the only way to cut thick sheets. Lasers are a modern development in which the laser simple melts the material and a gas jet blows it out of the cut. Professor John J. Mills: Tel (817)

Other Methods of Cutting Sheet Metal
Friction sawing Cut-off saw Uses abrasive disk Versatile but inaccurate Water jet Uses high pressure jet of water to cut Leaves nice finished edge Limited in materials that can be cut Abrasive water jet Like water jet but with abrasives contained in jet Cuts anything Leaves nice edge and is precise Programmable and can cut almost any shape Water jet is often used in pastry shops to cut dough. Abrasive water jet cutting is frequently used for composites since it leaves a clean edge unlike machining and shearing which leave frayed edges and laser bean which leave a burned edge. Professor John J. Mills: Tel (817)

Equipment Shears A long stationary blade (lower) and a moveable top blade with a table to support the material. Upper blade can be at an angle to reduce forces but this gives a curved blank Professor John J. Mills: Tel (817)

Equipment Saws Punch presses Band Cut-off
A continuous blade that moves at high speeed through a hole in the table which supports the work piece. The material is moved around while the blade is stationary Cut-off Can be band type or a circular rotating blade. The material is clamped to a table and the weight of the blade holder forces the moving blade through the material Punch presses Like forging machines but can provide high repetition rates Professor John J. Mills: Tel (817)

Equipment Presses CNC nibblers Automated punch presses
Used for shaped punches and dies Precision Fast acting Often combine forming operations as well CNC nibblers Can create many shapes using nibbling tools Automated punch presses moves large sheet around to position a specific location over a punch and die which is automatically changed to deliver a variety of shapes and diameters of holes Professor John J. Mills: Tel (817)

Equipment Flame, laser and water jet cutting systems
Typically are robots that have the cutting device on the end of the robot arm (the end effector) The robots are programmed to cut a shape The robot can be a simple as a linear mechanism to move the end effector over a straight line to cut large slabs Professor John J. Mills: Tel (817)

Fundamentals of Sheet Metal Forming
Professor John J. Mills: Tel (817)

Sheet Metal Forming The sheet metal forming process includes bending, stretching, drawing and otherwise deforming sheet with tools and machines to create a product or component. To form sheet metal it must have a yield point and exhibit plastic flow Brittle materials such as ceramics and carbides cannot be formed this by these processes We must understand the mechanical properties of the sheet before deforming it Yield stress, elongation, anisotropy,surface finish, grain size, edge conditions Sheet metal forming differs from rolling forging and extrusion is that the amount of deformation is much more limited. In rolling, forging and extrusion almost all of the material is deformed massively. In sheet metal forming, deformation is often limited to a specific area (e.g. the bend). Deformation in sheet metal is also mostly tensile in nature where as in rolling forging and extrusion it is mostly compressive. Professor John J. Mills: Tel (817)

Basic modes of deformation
Bending Folding the sheet The most common operation The only deformation occurs at the bend Stretching Characterized by uniaxial or biaxial uniform strain Typically the material is grasped by the edges and pulled over a die Drawing Characterized by deforming the sheet into a die with a punch or by other means. Bending can be thought of as a subset of stretching Bending is also mostly carried out in a linear fashion in that bends are mostly made over a straight line. Two dimensional bending also occurs. Drawing is similar to wire or rod drawing in that the material is mostly under tensile stress, but it is often two dimensional (bi-axial strains) rather than one dimensional(uni-axial strains) for wire drawing. Professor John J. Mills: Tel (817)

Sheet Metal Characteristics for Forming
Important properties Elongation Need high uniform elongation True strain at which necking occurs (= strain hardening coeff.) Need large strain hardening exponent This slide lists and describes the important metal characteristics for forming sheet metals. Without a large uniform deformation, the possible shapes that can be created are limited. Non-uniform elongation or yield point elongation gives rise to Lueders bands which is a surface defect. This only occurs in certain materials. Anisotropy arise in the rolling process. Grains are oriented along the rolling direction. This means that a sheet may be bent further in one direction than in the other. Large grain influence mostly the appearance of the product Professor John J. Mills: Tel (817)

Yield point elongation (important for low carbon steels and Al/Mg alloys) Non-uniform elongation Restricts the amount of deformation possible during forming Some parts yield while others do not Leuders bands Professor John J. Mills: Tel (817)

Important properties Anisotropy Produces non-uniform deformation Gives ears during deformation Two kinds Planar Normal Grain size Influences strength of product Influences surface finish Large grains give mottled appearance State of the sheared edges Rough edges cause premature failure during forming State of the sheet surface Professor John J. Mills: Tel (817)

Failure mechanisms include:
Necking As occurs at the ultimate tensile stress Tearing As it sounds Professor John J. Mills: Tel (817)

Formability The term "Formability" integrates the important properties into one word Definition The ability of sheet to undergo the required shape change or deformation without failure Formability is essentially an empirical way of measuring how the material deforms under specific conditions. Professor John J. Mills: Tel (817)

Generic Formability Tests
Tests to measure the formability of the metal Tensile testing Universal test method - stress and strain to failure under uniaxial stress Biaxial tensile testing More generic and representative of forming conditions Very difficult and hence expensive to do properly Cupping A simple generic test for all forms of sheet metal forming Tensile testing gives only a general indication of formability because so many sheet metal forming processes are biaxial. True biaxial testing is very difficult to do properly and small misalignments can lead to misleading data. Therefor the cupping test was developed. None of these is any help in determining bending limits, There for the bend test was developed.. Professor John J. Mills: Tel (817)

Cupping Test and Formability Diagram
The Cupping test Push a round steel punch into firmly held sheet until a crack appears Metric is the amount of deformation when crack appears measures the formability Use Cupping test on various widths to change the strain conditions to provide data on forming limits Narrow widths undergo simple uniaxial tension Large widths undergo equal biaxial stretching The forming limits as a function of major and minor strain is the Forming Limit Diagram The cupping test measures formability by using the measured deformation of circles inscribed on a sheet which has a spherical shape impressed on it. The changes in the circle dimensions along the major and minor axes when failure occurs are plotted as shown in figure 16.14, p 457 to form a forming limit diagram. This diagram can then be used to determine the maximal strains that a specific forming process can have. Professor John J. Mills: Tel (817)

Cupping test Professor John J. Mills: Tel (817)

Forming Limit Diagram This is a very complicated figure which is difficult to explain clearly. Essentially it predicts the limits of failure for a given set of major and minor strains. If the major and minor strain conditions of forming lie above the curve for a particular material, it will fail during forming. Note that a compressive minor strain (I.e a negative minor strain) is beneficial in that greater major strains can be achieved without failure. Many processes take advantage of that phenomenon. Professor John J. Mills: Tel (817)

Forming Limit Diagram Give the limits of major and minor stress for cracking and tearing Carbon steel and brass have higher limits than high strength steel and aluminum alloys and are more formable Increased thickness raises the curves BUT Thicker material difficult to bend around tight radii - see later Note that having a compressive (negative) minor strain is advantageous need special tooling Professor John J. Mills: Tel (817)

Other forming tests Depend on the specific forming method Bending
Stretching Drawing. etc Professor John J. Mills: Tel (817)

Summary Characteristic Importance
Elongation Determines the capability of the sheet metal to stretch without necking and failure; high strain-hardening exponent (n) and strain-rate sensitivity exponent (m) desirable. Yield-point elongation Observed with mild-steel sheets; also called Lueder’s bands and stretcher strains; causes flame like depressions on the sheets surfaces; can be eliminated by temper rolling, but sheet must be formed within a certain time after rolling. Anisotropy (planar) Exhibits different behavior in different planar directions; present in cold-rolled sheets because of preferred orientation or mechanical fibering; causes earing in drawing; can be reduced or eliminated by annealing but at lowered strength. Anisotropy (normal) Determines thinning behavior of sheet metals during stretching; important in deep-drawing operations. Grain size Determines surface roughness on stretched sheet metal; the coarser the grain, the rougher the appearance (orange peel). Professor John J. Mills: Tel (817)

Summary Characteristic Importance
Residual stresses Caused by nonuniform deformation during forming; causes part distortion when sectioned and can lead to stress-corrosion cracking; reduced or eliminated by stress relieving. Springback Caused by elastic recovery of the plastically deformed sheet after unloading; causes distortion of part and loss of dimensional accuracy; can be controlled by techniques such as overbending and bottoming of the punch. Quality of sheared edges Depends on process used; edges can be rough, not square, and contain cracks, residual stresses, and a work-hardened layer, which are all detrimental to the formability of the sheet; quality can be improved by control of clearance, tool and die design, fine blanking, shaving, and lubrication. Surface condition of sheet Depends on rolling practice; important in sheet forming as it can cause tearing and poor surface quality; see also Section 13.3. Professor John J. Mills: Tel (817)

Sheet Metal Forming Processes
Punching Blanking Fine Blanking Stamping Embossing Shearing Slitting Cutting Sawing Deburring Cleaning Coating Sheet, Plate Blank Bending Roll forming Stretch forming Deep drawing Rubber forming Spinning Peen forming Superplastic forming Explosive forming Magnetic pulse forming This is a transition slide to the next topic, forming of the blanks created. Professor John J. Mills: Tel (817)

Summary Shearing of sheet to form flat shapes
The fundamentals of changing that flat shape into a three dimensional one Next lecture discusses different sheet metal forming processes Professor John J. Mills: Tel (817)

MAE Course 3344 Lecture 8 Sheet Metal Shaping and Forming

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MAE Course 3344 Lecture 8 Sheet Metal Shaping and Forming

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