Sheet metal forming embraces a wide array of manufacturing processes Typically work with thin sheets; usually steel

Transcription

1 Sheet metal forming embraces a wide array of manufacturing processes Typically work with thin sheets; usually steel 1

2 A sheet metal part produced in presses is called a stamping [(a) above] Stamp to force downward to pound Low carbon steel is the most common sheet metal material because of its low cost and generally good strength and formability characteristics Aluminum is the most common s/m material where corrosion resistance is a concern; beverage cans, kitchen utensils Most s/m processes are performed at room temperature Stamped parts generally formed in one compression operation Spun parts ( spinning ) generally formed individually, by shaping a blank over a rotating mandrel; expensive unless performed automatically 2

3 Summary of characteristics of many of the sheet metal forming processes These processes considered throughout the chapter 3

4 One process involves punching This is actually a shearing operation Sometimes the product is the part that is punched out and sometimes it is the part remaining Note that the profile of the punched edge is not uniform, (b) 4

5 Clearance is a major factor in determining shape and quality of sheared edge As clearance increases, the zone of deformation becomes larger, and the sheared edge becomes rougher -Removal of resultant burrs incurs more work and more cost Increase speed to increase edge quality With increasing speed, the heat generated by plastic deformation is confined to a smaller and smaller zone 5

6 Punching (piercing) : the sheared slug is scrap Blanking: keep (use) the slug & scrap the rest The green is kept (in above illustrations) Die cutting: A shearing operation that consists of elements in (c): Perforating: Punching a number of holes in a sheet Parting: Shearing the sheet into two or more pieces Notching: Removing pieces or various shapes from the edges Lancing: Leaving a tab without removing any material 6

7 Fine Blanking (a): Compare traditional and fine blanking Can produce very smooth and square edges by fine blanking Note V shaped stinger to hold part in position Fine blanking employs clearances approx. 1% of the thickness of the sheet - Compare normal clearances 2% to 10% 7

8 Slitting A shearing operation performed by a pair of circular blades Blades may follow straight or curved line Still have a burr Can result in distortions of the sheared edges 8

9 An example of Taylor-welded blanks (keep the green) First:: Five different pieces created by blanking These pieces may be different materials and thicknesses Second: Five pieces joined by laser butt (edge to edge) welding Third: Stamp to final shape It is important that the pieces be properly aligned prior to welding because of the thin sections involved 9

10 Examples of parts for automobiles using the Taylor-welded technology Note different thicknesses in laser welded components Note capability to produce a variety of relatively thin but structurally strong shapes 10

11 Shaving: An operation to remove burrs and create smoother sheared edges 11

12 If surfaces of both surfaces of the punch and die are both flat then may require large shear force to simultaneously cut all the edges. Several strategies to reduce this force: Beveling either punch or die face (note (c) and (d)) Desirable to have symmetry to have equal distribution of the forces (b) Is an example of non symmetrical and this causes lateral forces which must be contained 12

13 Compound Dies Used to perform several operations on the same sheet in one operation This example: (a) & (b): Outside perimeter of washer is cut first; followed by cutting the inside diameter, all in one stroke (c) Illustrates a different variation: the washer is cut in two stages, i.e. each washer requires two strokes 13

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15 Stress-strain diagrams show different patterns for various metals & alloys Some metals have a zone of yield point elongation This occurs in a zone within which the metal is stretched unevenly between upper and lower yield points and exhibits stretch marks. This behaviour results in Luder s bands. 15

16 Cupping Test The earliest tests developed to predict s/m formability The round punch or steel ball is pressed onto specimen until crack forms; then measure punch depth, d. In (b), note the distortion at edges of narrower test strips (a) Erichsen Test (b) Bulge Test 16

17 Flat specimens are cut to various widths, then tested (previous illustration) The surfaces of the flat specimens are marked in circles, prior to testing After testing, the resultant shape may be still circular, but more likely, elliptical. The deformation along major & minor axes are measured. Major axis strain is always positive (larger diameter than original); minor axis strain may be positive or negative, The FLD, Forming Limit Diagram, is constructed showing the boundaries between failure (higher) & safe (lower) regions Example: Aluminum A: Major +20%; Minor -10% = to lower left ( below ) curve, therefore there would be no tear in that location of the specimen B: Major +80%; Minor -40% = to above right of curve; therefore there would be tear in that location of the specimen 17

18 Illustrates the deformation discussed on previous slide 18

19 Bending is one of the most common industrial forming operations Bending imparts strength to flat (or curved) sheet. Edges, ribbed, rolled contours This diagram shows the terminology in bending 19

20 Look carefully at (c) to see the reduction in bend length at the bend. 20

21 Minimum Bend Radius The radius at which a crack first appears at the outer fibres of a sheet being bent. The bend radius is usually expressed as a reciprocal of the thickness. Example: 3T means the smallest radius to which the sheet can be bent without cracking is three times the thickness of a sheet 21

22 R = T(50/r -1) Where R = bend radius T = thickness r = tensile reduction of area of the sheet metal (expressed as a %) 22

23 Compensate for springback by overbending the part 23

24 (a) Overbending (b) Overbending (c) Bottoming the punch by coining to produce local compressive forces (d) Bottoming 24

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28 Beading The periphery of the s/m is bent into the cavity of the die 28

29 Flanging Bending edges of s/m, usually 90 o Shrink Flange Subject flange to compressive hoop forces Stretch Flange Tensile forces Reverse Flange Alternate shrink & stretch Dimpling The hole is first punched, then expanded into a flange Piercing Can create flanges with a shaped punch 29

30 Can be used to create a wide variety of cross sectional shapes as continuous lengths, later cut to length 30

31 Bending tubes Often requires special tooling because of the tendency for buckling & folding 31

32 Bulging Place a tubular, conical, or curvilinear part into a split-female die and then expanding the part, usually with a polyurethane plug Punch is then retracted; plug returns to its original shape, and the formed part is removed by opening the split dies 32

33 Stretch Forming The s/m is clamped along its edges and then stretched over a male die The die moves upward, downward, or sideways In most cases the blank is a rectangular sheet clamped along its narrower edges and stretched lengthwise, thus allowing the material to shrink in width. Controlling the amount of stretching is important to prevent tearing 33

34 Deep Drawing The name refers to the capability to produce deep parts, although many shallow depth parts are produced Deep drawing is important due to widespread use. 34

35 Deep Drawing Important variables are: -Properties of s/m -Ratio of Blank diameter, D o, to Punch diameter, D p, D o / D p, -Clearance between punch & die, c -Punch radius, R p -Die-corner radius, R d -Blankholder force -Friction -Lubrication -These are all independent variables (except F) 35

36 Anisotropy Cold rolling of a sheet results in anisotropy by preferred orientation or by mechanical fibering due to the alignment of any impurities, inclusions, and voids that may be present For a tensile test, length, width, and thickness strains occur and the strains in the width and thickness directions need not be equal. r = strain w /strain t Important in deep drawing because of the high amount of stretching undergone by the metal 36

37 The Normal anisotropy, R avg, can be calculated for a sheet using R 45 and R 90 Normal anisotropy r 0 = r 45 = r

38 LDR = Maximum blank diameter = D o Punch diameter Note: both scales are logarithmic D p 38

39 The metal can stretch more in direction of rolling 39

40 Draw beads are often necessary to control the flow of the blank into the die cavity Beads restrict the free flow of the s/m by bending and unbending it during the drawing cycle, thereby increasing the force required to pull the sheet into the die cavity Draw beads are useful in drawing box-shaped and nonsymmetric parts. Various regions undergo different types of deformation during drawing 40

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42 Case Study 42

43 In rubber forming, one of the dies (the female die) is made of a flexible material, typically a polyurethane membrane. Polyurethanes are selected because of their abrasion resistance, fatigue life, resistance to cutting & tearing. 43

44 In the hydroform process, the pressure over the rubber membrane is controlled throughout the forming cycle. This procedure allows close control of the part during forming and prevents wrinkling or tearing. Deeper draws are obtained than during regular deep drawing because the pressure around the rubber membrane forces the cup against the punch. This increases the friction which reduces the longitudinal tensile stresses in the cup and thus delays fracture 44

45 In tube hydroforming, metal tubing is formed in a die and pressurized internally by a fluid, usually water This process can produce intricate hollow shapes 45

46 Case Study Note varying cross sections weight reduction Most components made by tube hydroforming, holes perforated, then components are assembled through robotic gas-metal arc welding 46

47 Case Study Sequence in formation of one component by tube hydroforming 47

48 Case Study Used in the components in case study 48

49 Case Study The machine 49

50 Spinning A process that involves the forming of axisymmetric parts over a mandrel by the use of various tools and rollers 50

51 Shear spinning This operation produces an axisymmetric conical or curvilinear shape Prefer two rollers applied oppositely, to balance the forces acting on the mandrel 51

52 Incremental forming A class of processes that are related to conventional metal spinning (a) The rotating blank is deformed by a steel rod with a smooth hemispherical tip to produce axisymmetric parts (b) A part that has been produced by CNC incremental forming. The part does not have to be axisymmetric 52

53 Examples of products made by combining diffusion bonding with superplastic forming. (SPF/DB). 53

54 Explosive Forming The s/m blank is clamped over a die and the entire assembly is lowered into a tank filled with water. The air in the die cavity is evacuated, an explosive charge is placed at a certain height and the charge is detonated. The explosion generates a shock wave with a pressure that is sufficient to form sheet metals 54

55 In Electromagnetically Assisted Forming the energy stored in a capacitor bank is discharged rapidly through a magnetic coil. In a typical example, a ring shaped coil is placed over a tubular workpiece. The tube is then collapsed by magnetic force over a solid piece, thus making the assembly an integral part. The mechanics of the process is based on the fact that a magnetic field produced by the coil crosses the metal tube and generates eddy currents in the tube. In turn, these eddy currents produce their own magnetic firld. The forces produced by the two magnetic fields oppose each other. The repelling force generated between the coil and the tube then collapses the tube over the inner piece 55

56 Case Study Cymbals can be made with a variety of sizes, weights, hammerings and surface finishes 56

57 Case Study First cast, then cooled Rolled successively (up to 14 times) Roll the bronze at a different angle with each pass to minimize anisotropy Heat and then stretch formed into the bell shape that determines cymbal s tones Follow with centre drilling of hole for hanging and trimmed on a rotary shear to approximate final diameters. Another stretch forming step that determines the characteristic form that controls the pitch. 57

58 Case Study Peening & hammering Work hardening Stress relief Tone Appearance Automatic or manual 58

59 Manufacturing metal honeycomb structures Common in nonmetallic materials Metal honeycombs involve operations that are best classified as s/m forming processes Two basic methods of manufacturing honeycomb materials: -Expansion process is the more common of the two -The stacked sheets with bands of adhesive are heated (cured) to create strong bonds at the adhesive joints; then sliced & pulled apart -In corrugation process: adhesive is applied at node lines, corrugated sheets are stacked in block, then cured 59

60 Material scrap is the primary concern in blanking operations Note careful design of the part (with full knowledge of process of manufacture) to minimize scrap in these three examples 60

61 In bending operations, the main concerns are: -Material fracture -Wrinkling -Inability to form the bend A s/m part with a flange that is to be bent will force the flange to undergo compression -The compression can cause buckling Note improvement by cutting a relief notch (middle), or by redesign of part (at right) 61

62 Right angle bends are susceptible to tearing and wrinkling Three illustrations of utilizing a notch at the bend to minimize tearing 62

63 The bend radius is a highly stressed area. Therefore, desirable to remove all stress concentrations from this area Holes near bends: undesirable -First: can holes/bends be redesigned to be further apart? -If not, then may use a crescent or an ear as shown in these illustrations (a) Bending flanges -Avoid tabs & notches -When tabs are necessary, use large radii to reduce stress concentrations 63

64 Bending & notches - Important to orient notches properly wrt grain direction Fig : Bends s/b perpendicular to rolling direction (or at least at an angle to it) to avoid cracking Fig : Bending sharp radii can be accomplished through scoring or embossing; but this can result in fracturing Ensure burrs are not present in a bend allowance (since they are brittle because of strain hardening) and can fracture 64

65 Sheet Metal Forming Equipment - The basic equipment consists of mechanical, hydraulic, pneumatic or hydraulicpneumatic presses with variety of features & computer controls -Traditional C frame is widely used for ease of tool & workpiece accessibility -C Frame is not as stiff as pillar or double column -Accessibility is a less important consideration for automation -Variations on basic C Frame for wider pieces and ease of manual use & accessibility Many s/m presses are very much smaller than the large machine illustrated here 65

66 In general Often a particular part or shape can be made in several alternate ways. For example: A cup-shaped part can be formed by deep drawing, spinning, rubber forming, explosive forming, impact extrusion, casting, fabrication Appropriate process is often a function of many variables, including volumes 66

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