Deep Drawing. Sheet Metal Forming Processes. Deep Drawing. Deep Drawing. Deep Drawing. Deep Drawing Parameters and Practices.

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1 Sheet Metal Forming Processes Deep Drawing Deep Drawing Like shearing but with clearance > material thickness The Process Place round blank over die Clamp in place Punch travels downward into die metal deforms by stretching into the die Deep Drawing Deep Drawing Advantages Very high production rates (e.g. cans) Can make parts no other process can make Containers of all shapes and sizes Disadvantages Limited shapes Material quality critical to obtaining good parts Dies are expensive c = Clearance D b = blank diameter D p = Punch diameter R d = die corner radius R p = Punch corner radius F = drawing force F h = holding force Deep Drawing Initial step bending of edge straightening of side wall Deep Drawing Parameters and Practices Blankholder force Too high - workpiece tears Too low - - cup gets wrinkles Blankholder force is 0.7 to 1 % of the total of the yield and ultimate tensile strength of the workpiece Radius/diameter of punch Diameter of blank Thickness of blank Thinning and drawing final cup shape

2 Deep Drawing Parameters and Practices Clearance between punch and die Clearances usually 1.07 to 1.14 thickness Too low - blank pierced Friction/lubrication Radius of entry into die Radius on end of punch too small - cracking at corners Too large - wrinkling Punch force Deep Drawing Forces and Stresses Stress states are complex, force difficult to compute Under blankholder Compressive through thickness Tensile along radius Compressive (poisson's effect along circumference In die Mostly tensile in punch direction Also tensile at right angles to punch direction Stresses cause material to thin in punch direction and shrink around die in circumferential direction Deep Drawability Normal Anisotropy Deep drawability is a complex phenomenon An important empirical metric is Limiting Drawing Ratio (LDR) given by LDR = Maximum blank diameter Punch diameter LDR is related to fundamental properties of the material being drawn through a mechanical property called: Normal or Plastic Anisotropy Normal Anisotropy, R = width strain/thickness strain LDR is proportional to the average normal anisotropy It is measured by measuring the deformations in length on width of a normal tensile specimen LDR and Anisotropy Deep draw cup with ears

3 Metal Flow Between die and punch flow is similar to that in drawing Under the blankholder much smaller metal movement and is uniform In non-circular products flow becomes nonuniform To control flow in non--circular products use Drawbeads The bending and straightening of the metal as it flows provides greater resistance to metal movement Design and location important to good products Design blank appropriately Cut corners of square blanks at 45 o Metal Flow Drawing Defects Sheet Metal Forming Processes a) Wrinkling in flange - small holding force b) Wrinkling in the wall - insufficient holding force, wrinkling initially occurring on the flange. c) Tearing - high stress, sharp die radius d) Earing - anisotropy of the material e) Surface scratches - Die or punch not having a smooth surface, insufficient lubrication Ironing & Redrawing Ironing Reduces thickness variation from top to bottom of cup Like drawing a sheet where the die is formed by the clearance between the punch and its die Redrawing Used for complex shapes Multistep dies Reverse redrawing Allows very deep cups to be formed (Beverage cans) Redrawing Reverse Redrawing

4 Deep Drawing of Cans Deep Drawing of Cans 7 Sheet metal operations 2 - Deep Drawing 1 - Blanking 3 - Redrawing Deep Drawing of Cans Deep Drawing of Cans 4 - Ironing 6 - Necking 5 - Doming 7 - Seaming Sheet Metal Forming Processes Stretch forming Stretch Forming Used predominantly to make aircraft parts Like stretch bending for tubes, but uses flat sheet Material is pulled across a tool while being subjected to tensile stress

5 Stretch Forming Stretch forming Advantages Suitable for low volume production Versatile Economical, inexpensive dies Disadvantages Cannot form parts with sharp corners or depressions Other Related Processes Rubber Forming Rubber forming Large flexible rubber base acts as a die The rubber acts like a semi fluid medium pushing the sheet against the die shape Advantages Only one side of the die needs to be made Disadvantages Shapes are limited Other Related Processes Hydroforming or fluid forming Uses a fluid medium separated from the workpiece by a rubber membrane Fluid kept under high pressure (100 MPA, 15,000 psi) during the forming cycle Other Related Processes Spinning Similar to forming pots with clay on a potters wheel Only for axisymmetric parts Usually involves multiple passes May require elevated temperatures for thick parts or low ductility materials

6 Types of Spinning Conventional thickness retained, outside diameter typically reduced Shear spinning/power spinning/flow turning/ hydrospinning/spin forging Thickness reduced, outside diameter retained Conventional Spinning Shear spinning Also known as power spinning, flow turning, spin forging Related to Ironing Types of Spinning Tube Spinning Wall thickness reduced Can be executed internally or externally forward or backward Used to make Pressure vessels Car and truck wheels Rocket and missile parts Other Related Processes Shock Wave Forming Shock created by Explosives (Explosive Forming) Electric spark ( Electrohydraulic forming) Gas Uses a hydrostatic shock wave to push material into a die. Still need to clamp the workpiece to the die Other Related Processes Magnetic pulse forming Pulsed magnetic forces move the metal against a die Often used to seal a tube against a cylindrical part

7 Forming Equipment Like forging presses Important parameters for forming equipment Stiffness Rigidity Several types Mechanical Hydraulic Pneumatic Pneumatic/hydraulic Forming Equipment Selection parameters include: Type of forming Size and shape of the workpiece Length of stroke of the slides Number of strokes per minute Speed and shut height Number of slides Maximum force Type of controls Die change mechanism Forming Equipment Shapes A Large Stamping Press C-Frame most common Easy access O or box- type stiffer Can have several presses linked by automated transfer equipment Break Press

8 Die set components Types of die sets Strippers Knockouts Shedders Compound dies Progressive dies Summary Introduction Die sets are tools to hold and aid in the alignment of.. cutting forming bending etc. Introduction Introduction Guide bushings and guide pins align the punch and die holders Shank fits into the press ram Flange to fix the die holder to the press Introduction Stamping tools are mounted between the punch and die holders. Accurate slip fit is maintained between the guide pins and the guide bushings. Precision die sets ball bearing bushing are used Selection feeding method rigidity required viewing access alignment accuracy Die Sets

9 Die Sets Die Sets Strippers Applies pressure to the stock strip sticking occurs Punches Designed to.. not to deflect withstand stripping forces not to rotate with cutting action life extended by heat treatment strength wear resistance Punches Punch deflection avoided by making the body diameter of the punch larger than the cutting edge. Punches

10 Piercing Punches Smaller and longer than blanking punches Designed to withstand shock, deflection, and buckling Higher likelihood of damage easy removal/ replacement Smaller diameter punches need support for most of their length Punches Thin short punches - a quill provides support Punches Thin punches - a quill provides support Punches Punches To prevent the punch from turning in the punch plates. Die Blocks

11 Die Blocks Sectioned blocks large die blocks difficult machining intricate shapes Combine one or more operations in one station during one stroke of the press. Compound dies Compound dies Advantages Pierced holes are held to close tolerances and concentricity with edges. Larger parts can be blanked in a small press Compound dies are economical one press one operator Compound dies Advantages Besides cost savings relocation errors can be minimized. Elimination of inaccuracy Compound dies Disadvantages Certain parts or blank shapes and sizes, compound dies are not suitable. Too much die detail crowded into one area, Example, Punched hole too close to the edge of the blank => failure Compound Dies Multiple operations within same stroke

12 Made with two or more stations in a row. As the press ram descends each station performs an operation on the workpiece idler station Stock strip advances series of stations one or more die operation The part remains attached to the scrap skeleton until the last station. Cut-off Each press ram stroke complete workpiece Scrap skeleton cut-up by a scrap cutter : Operations Several stamping operations are combined and performed in successive stations. Other names Cut & carry die Follow die Gang die Operations which may be included in a progressive die: Piercing, blanking, forming, drawing, and cutoff : Pilots Parts must be fed and located accurately in a progressive die. Stock positioning Direct pilots holes punched in the part Indirect pilots holes pierced in scrap-strip. : Pilots Advantages Not affected by workpiece change Size and location not limited (as with direct pilots) Disadvantages Material width and lead may increase Drawing affects scrap-strip

13 : Stations Distance from one station to the next must be the same. Station-to-station distance = The distance the strip moves to relocate at each station : Costs Die cost high Production runs high volume Press cost high : automatic press with cut-off, feed, straightener etc. Operator requirement reduced : main task is to replace the coil of stock. Scrap high : nesting is limited

14 Simple Die : Trimming