1. Design of sheet metal Bending, forming, and Drawing Dies Chapter Four By: Getachew G/A

2. Introduction  Shaping operation are generally divided in to three groups:  Bending  Forming and  drawing  Bending is defined as shaping metal around a straight axis which extends completely across the material.

3.  Bending frequently is used to increase the rigidity of shaped parts in press working operations.  The simplest bending operation is air bending, so called because the die does not touch the outside of the bend radius. Figure 4.1 air bending

4. similar to bendingexcept that instead of a straight one  Forming is similar to bending, except that the form or bend is along a curved axis instead of a straight one.  A part that is formed generally takes the shape of the die or punch.  Metal flow is not as uniform as in the bending because it may be localized, to some extent, depending upon the shape of the workpiece.

5.  Drawing is the most complicated of the three sheet metal shaping processes.  A drawing operation begins with a flat blank which is transformed into a cup or shell. parent metal  The parent metal is subjected to severe plastic deformation.  Shell forms produced may be cylindrical or rectangular or tapered sides.

6. Bending Dies  When bends are made in two direction, it is desirable to orient the workpiece so that no bends are parallel with the grain direction.  The minimum bend radii are limited by the following factors:  Angle and length of bend  Material property and  Direction of bend in relation to grain

7. equal to  As a rule of thumb, the minimum bend radii for most annealed (soft) metals is equal to the thickness of the metal. Bending Methods  The two bending methods used extensively in presses are:  wiping dies and  “V” bending

8. for a die  V bending is accomplished by using “V” block for a die and a wedged shaped punch to force the metal in to the die. Figure Methosds of bending

9.  The desirable width of the opening in the “V” shaped die is ordinarily: the material thickness 16mm plate  8 times the material thickness up to about 16mm plate. the material thickness heavier thickness  10 or 12 times the material thickness are used for forming heavier thickness of plate. by varying the punch forces the sheet metal  The included angle of the “V” bend can be changed by varying the distance the punch forces the sheet metal into the V die

10.  When the punch does “bottom out”, processes are refereed to as Air Bending. mild steel,  For making right angle bends in the mild steel, both punch and die are generally made with an included angle of 89 0. heat treated and springy material  An included angle of 30 to 70 0 for some heat treated and springy material

11. Springback remaining in the bend area  Elastic stresses remaining in the bend area after bending pressure is released will cause a slight decrease in the bend angle. Metal movement  Metal movement of this type is known as springback.  The magnitude of this spring back will vary according to  material type,  thickness and  hardness.

12. simplest way of combating  Over bending is the simplest way of combating spring back problem.  For low carbon and soft non-ferrous material is from 0 to 2 0  For carbon content of 0.4 to 0.5% carbon steel and half–hard materials spring back may vary from 3 to 5 0.  Spring back may be as high as 10 to 15 0 in hard materials.

13. Bend allowance In bending operationsmaterial near the bend radius is material near the outside of the bend is  In bending operations, the material near the bend radius is under compression while the material near the outside of the bend is under tension. area and area  A neutral plane exists between the area under tension and area under compression. Fig. Bend terms for general angle

14. elastic limit is not exceeded  If the material is uniform in section and its elastic limit is not exceeded, the neutral plane will coincide with the centerline of the material. When elastic limit exceeded the thickness  When the bending forces cause the elastic limit of the material to be exceeded, the neutral plane moves toward the inner surface at a distance of one–third to one–half the thickness of the material.

15.  This is because the material under compression resists bulging much more than the material under tension resists stretching, and therefore, the greatest amount of plastic flow takes place on the tension side of the bend.  The length of the neutral plane does not change as a result of bending.

16. necessary to consider  When a blank or sheet is to be bent, it is necessary to consider the effect of stretching the metal at the outside of the bend. length of the formed part along the neutral plane  Since there is no stretch in the neutral plane, the length of the formed part along the neutral plane will be the correct length.  The curved neutral plane of the bend area is called the Bend Allowance. methods and formulas determining  Several methods and formulas are used for determining the bend allowance.

17. Where: B = Bend Allowance (arc length of neutral axis ), mm A = bend angle, degree IR = inside radius of bend, mm t = metal thickness, mm K = constant of neutral axis location  K is equal to 0.33 when IR is less than 2t. and 0.50 when IR is more than 2t.

18. Example  Calculate the blank length of the part shown in figure below. Solution  Divide the blank into segments and number as shown in figure.  Determine the length of the segment: L 1 = 50-(5+3) = 42mm

20. L 2 = (90/360)(2 π ) (5 + 0.33 x 3) = 9.41mm  Note: Since IR is less than 2T, K = 0.33 L 3 = 100 – (5+3+5+3) = 84mm L 1 = L 5 L 2 = L 4 The blank length = L 1 + L 2 + L 3 + L 4 + L 5 = 42 + 9.41 + 84 + 9.41 + 42 = 186.82mm ----------- Ans. stock material size of the bend anglesize of the bend radius Note: that the formula takes into account the stock material thickness, the size of the bend angle, and the size of the bend radius.

21. Forming Dies  Forming operations may be done to facilitate assembly later in the production line.  The more common classification of forming operations are: Solid form dies Pad type form dies Curling dies Embossing dies Coining dies Bulging dies and Assembly dies

22.  Solid form Dies: Consists of a male punch and female die shaped to the contour of the workpiece.  Allowance for clearance equal to the thickness of the work piece is provided between the male and female die.  Generally used for forming operations in progressive dies.  Screws and dowels are preferably located about 1.5 times their diameter from the edge of the form die block

23. Figure Solid form Dies Figure Pressure pad forming dies

24.  Pressure pad forming dies: may be necessary when the workpiece contains sharp radii or intricate details. applied by  Pressure for the pad may be applied by springs, air or hydraulics.  Springs have the disadvantage of increasing pressure with extended travel.  This may cause excessive stretching and tearing of the metal.

25.  Curling Dies: rolls a raw edge of sheet metal into a roll, curl as shown in the figure.  The purpose is to: strengthen the raw edge provide provide a protective edge and improve improve the appearance of the product.  The curl is often applied over a wiring for increased strength.

26. Figure Embossing diesFigure Curling dies

27.  Embossing dies: Embossing is shallow forming operation in which the workpiece material is stretched over a male die and caused to conform to the male die surface by mating female die surface. on one side on the other  The finished product will have a depressed detail on one side and a raised detail on the other.  An embossed pattern may have more intricate detail than a formed pattern.

28.  Coining dies: Coining is the process of pressing material in a die so that it flows in to the space in the detail of the die face as shown in figure.  Coining operations are generally performed cold.  Coining has two major advantages: 1. Ornate detail can be reproduced with excellent surface finish 2. Very close tolerance can be held

29. Figure Coining Dies Figure Bulging Dies

30. internal forming operation  Bulging dies: Bulging is an internal forming operation used to expand portions of a drawn shell or tube. forming force is applied transmitted  The forming force is applied from inside the workpiece and is transmitted through a medium that will flow and not compress. rubber Urethaneheavy greasewater  The more common media are rubber, Urethane, heavy grease, oil or water.  bulging die must be cut in to section that open, in order to remove the workpieces after bulging

31.  Assembly dies: Two or more parts are assembled, held in position, and then locked in position by:  riveting  staking  crimping or  press fitting

32. Drawing Operations from metal blanks.  Drawing is the name given to the production of cups, shells, box and similar articles from metal blanks. drawing operation along with the final product  A simple drawing operation is shown in figure 4.1, along with the final product. from flat stock  A round blank is first cut from flat stock. punch pushes the blank through the die.  The blank is then placed in the draw die, where the punch pushes the blank through the die.

33. On the return stroke counter bore in the bottom of the die  On the return stroke, the cup is stripped from the punch by the counter bore in the bottom of the die. The top edge of the shell expands  The top edge of the shell expands (spring back) slightly in order to make this possible. no deeper than half its diameter.  A drawing operation is referred to as shallow drawing when the cup is no deeper than half its diameter.  There would be very little thinning of the metal in this case.

34.  Deep drawing is drawing a cup deeper than half it’s diameter. requires considerable skill and experience,  Designing dies for deep drawing requires considerable skill and experience, and only the general principles can be studied. accompanied by an operation called defined as by forcing its through a tight die.  Deep drawing is often accompanied by an operation called ironing, which is defined as the reduction in wall thickness of a shell by forcing its through a tight die.

35.  The thickness of the shell bottom is not changed during an ironing operation, but the walls are both lengthened and made thinner. Fig. 4.1 a simple drawing Operation

36. Metal flow during drawing understand what happens to the metal  To understand what happens to the metal during a drawing operation, the process should be broken in to progressive stage of formation as shown in the figure 4.2. by forcing a punch against the blank  Here a flat circular blank is drawn in to a flat bottomed cup by forcing a punch against the blank, which rests on a die. for easy reference  Note that the blank is divided in to sections for easy reference.

37. Fig. 4.2 (a) Metal flow during drawing

38. Fig. 4.2 (b) Metal flow during drawing

39. During the first stage,  During the first stage, the punch contacts the blanks as shown in figure 4(a). section 1.  The area of punch contact is denoted as section 1. not destroyed by the punch  This section forms the flat bottom of the cup and is not destroyed by the punch. Upon further penetration warped around the punch nose die radius  Upon further penetration of the punch, the metal in section 2 is in bent or warped around the punch nose and die radius as shown in figure 4(b).

40. As the punch penetrates still further  As the punch penetrates still further, the metal that was previously bent over the die radius becomes straightened. Additionally metal is pulled over the die radius  Additionally metal is pulled over the die radius, which in time straightened as the punch progresses downward.  In other words, the outside edge of the blank is being drawn towards the punch.

41. In order to accomplish this successfully  In order to accomplish this successfully, the section of the blank must reduce in circumference. only way to do this  The only way to do this is that the metal should be compressed and become thicker.

42. blank holder enough force  The blank holder should have enough force to be used in preventing formation of wrinkles. force created by the blank holder  The force created by the blank holder also increases frictional forces. Too much blank holder pressure  Too much blank holder pressure may tear the side wall of the drawn cup. side wall transmit area of draw  The side wall transmit the punch force to the area of draw on the die radius.

43. this area becomes thinner due to tension forces.  Tearing will generally occur near the punch radius because this area becomes thinner due to tension forces. the blank edge nears the punch  The amount of compression and thickness increase as the blank edge nears the punch.,thin sheet deeper draws  The metal may tend to wrinkle rather than compress, specially in thin sheet or with deeper draws.

44. Variable that affect metal flow during drawing Radius on Punch size  There is no set rule for the size of the radius on the punch. radius  A sharper radius will require higher forces when the metal is folded around the punch nosed and may result in excessive thinning or tearing at the bottom of the cup.

45. general rule to prevent excessive thinning  A general rule to prevent excessive thinning is to design the punch with a radius of from 4 to 10 times the metal thickness.  If the radius must be less than four times the metal thickness, it may be necessary to:  form it over a larger punch radius and then to develop the specify radius  re strike it to develop the specify radius.

46. when only one draw is necessary cup bottom takes the shape of the punch nose  The radius may be determined by the product design when only one draw is necessary to complete the workpiece because the cup bottom takes the shape of the punch nose. When several redraws are required than that of the preceding shell.  When several redraws are required, the punch radius for each redraw should be proportionately smaller than that of the preceding shell.

47. Draw radius on die should be as large as possible  Theoretically, the radius on the draw die (draw ring) should be as large as possible to permit full freedom of metal flow as it pass over the radius. aids in compressing thickening  The draw ring cause the metal to being flowing plastically and aids in compressing and thickening the outer portion of the blank.

48. However,  However, if the draw radius is too large, the metal will be released by the blank holder too soon and wrinkling will result.  Too sharp radius will:  hinder the normal flow of the metal and  cause uneven thinning of the cup wall, with resultant tearing.

49. general rule the material thickness.  The general rule is to make the draw radius four times the material thickness. metal thickness when drawing shallow cups of heavy gaugeswithout a blank holder.  The draw radius may be increased from six to eight times the metal thickness when drawing shallow cups of heavy gauges metal without a blank holder. relationship of the blank diameter cup diameter.  The Nomograph in fig.4.3 give a more exact method of determining draw die radius, based on the relationship of the blank diameter to the cup diameter.

50. Figure 4.3 Nomograph for determining draw-die radius

51.  Determine the draw die radius from Nomograph, shown in figure 4.3, for the following parameters: Drawn shell diameter(internal), d = 25mm Shell height, h = 20mm Thickness of sheet metal, T = 0.8mm Example

52. Solution  Givens: d = 25mm, h = 20mm, T = 0.8mm = 51.23mm (blank diameter) D - d = (51.23 -25)mm = 26.23mm (to be referred from table) R = 3.66 ± 0.125mm ---------- Ans.

53. Friction workpiece blank draw die surface  The force of static friction between the workpiece blank and draw die surface must be overcome in a drawing operation.  The force of the blank holder adds significantly to the force of static friction. press  A press lubricant is generally applied to reduce friction. Refer press lubricants! The purpose of press lubricant is to provide a film between the workpiece & the punch and the die. ( Refer press lubricants! )

54. Material to be drawn have a great influence on the successes of drawing operation.  The characteristics of the material to be drawn have a great influence on the successes of drawing operation. he most important parameters  The most important parameters considered by the product designer are:  Ductility and  Yield strength. to undergoes a  Ductility is the ability of the metal to undergoes a change of shape occur in a metal.

55. gradequalitylow carbon steel  The following principal factors affected the selection of grade and quality of low carbon steel for deep drawing: 6.Desired finished 7.Grain size 8.Press speed 9.Material Availability 10.Cost 1.Sovereignty of draw (amount of reduction and punch nose radius.) 2.Thickness of sheet 3.Shape of part (round, rectangular, or conical) 4.Ironing requirement 5.Flange requirement

56. Percentage Reduction and depth of draw is generally expressed in terms of  The percent reduction in drawing cylindrical shell is generally expressed in terms of the diameters of the blank D and the drawn shell d, where outside diameter of the blank D = the outside diameter of the blank and internal diameter of the shell d = internal diameter of the shell amount of the work material is to be compressed  This percentage provides an approximate value for the amount of the work material is to be compressed.

57.  The drawability of a metal is often expressed as the percentage reduction from the blank diameter to the cup diameter. calculated from the formula:  Percentage reduction is calculated from the formula: Where: P = percentage reduction d = Internal diameter of drawn shell D = Outside diameter of blank

58. Ratio h/d No of Reductions Reduction % First Draw Second Draw Third Draw Fourth Draw Up to 0.75140 0.75 – 1.524025 1.5 – 3.03402515 3.0 – 4.5440251510 Table: 4.1 Possible number of reductions for a given ratio of shell height to diameter Blank thickness, mm First Draws RedrawsSizing draw Up to 0.381.07t – 1.09t1.08t – 1.1t1.04t – 1.05t 0.4 - 1.271.08t – 1.1t1.09t – 1.12t1.05t – 1.06t 1.3 - 3.181.1t – 1.12t1.12t – 1.14t1.07t – 1.09t 3.5 and up1.12t – 1.14t1.15t – 1.2t1.08t – 1.1t Table: 4.2 draw clearance (t = thickness of original blank)

59. Figure 4.4 Charts for checking percentage reduction in drawing of cups

60. Example  To illustrate the use of the chart shown in the figure 4.4, assume the problem of determining:  whether a cup 190 mm internal diameter can be drawn in three draws of 40, 20, and 15 percent reduction, respectively from 460mm diameter blank.

61. Solutions  To find the diameter of the cup after the first draw, trace the line for the 460mm diameter horizontally to its intersection with the diagonal for 40% reduction. internal diameter of the cup after the first draw  From this intersection, draw a vertical line to the top of the chart and read 276mm which is the internal diameter of the cup after the first draw.

62.  Next, trace a horizontal line from 276mm on the vertical axis until it intersects the diagonal for 20% reduction.  From this point, draw a vertical line to the bottom of the chart and read 221mm which is the internal diameter of the cup after the second draw.  The internal diameter of the cup after the third draw, assumed to be 15% reduction, is found using the same procedure.

63.  i.e by drawing a line horizontally from 221mm to the point of intersection with the diagonal line representing 15% reduction and from there to the bottom of the chart, which gives a reading of 188mm.  Accordingly, it is concluded that a 190mm internal diameter cup can be drawn from 460mm diameter blank in the three assumed reduction.

64. Exercise  Determine the percentage reduction for the workpiece shown in the figure.  Does the depth of the draw indicate that the cup can be formed in one draw?  Compute the clearance.

65. Die Clearance between the punch and the die  Die clearance is the gap left between the punch and the die to allow for the flow of the work material.  The allowance for die clearance may range from 7 to 20% of the metal thickness. When is equal to  When clearance is equal to the metal thickness or less, ironing or burnishing of the metal will occur near the top of the cup.  Table 4.2 shows draw clearance for various blank thickness.

66. Determination of Blank Size  Before designing the die, the designer of draw dies must determine:  the approximate blank size of drawn workpiece.  the number of draws necessary to produce the shell.  Various methods of determining blank size have been developed.  Mathematical  Graphical layout  Surface area  Weight and  Volume

67. shell diameter corner radiusas it affects the blank diameter.  The equation used to calculate the blank size for cylindrical shells of relatively thin metal considers the ratio of the shell diameter to the corner radius d/r as it affects the blank diameter. When d/r is 20 or more When d/r is between 15 and 20 When d/r is between 10 and 15 When d/r is below 10 Where: D = blank diameter d = shell outside diameter h = shell height r= corner radius of punch

68.  The formulas given above are theoretical blank size, which only approximation when applied to actual practice.  Extra metal should be added to the formula of blank diameter to provide for trimming. unevenirregular edge  Trimming is necessary on deeper draws to eliminate the uneven and irregular edge on the rim of the draw cap.  The extra material added to the blank diameter is referred to as trim allowance.

69.  The necessary trim allowance increases as the size of the drawn cup increase. increase the blank diameter by 0.05mm cup diameter up to 10mm  The general rule of trim allowance is to increase the blank diameter by 0.05mm for cup diameter up to 10mm  Therefore add 0.05mm for each of cup diameter. 0.5mm for a cup diameter of 100mm  Example, 0.5mm to the blank diameter for a cup diameter of 100mm.

70. Drawing Force  The amount of force required to shape a symmetrical cup by drawing can be calculated from the following formula: Where: t = thickness of metal UTS = Ultimate Tensile Strength C = constant for friction and bending (0.6 to 0.7 for ductile material) P = drawing pressure d = shell outside diameter D = blank diameter

71. Single and Double–action Draw Dies  Draw dies consisting of only a punch and a die are known as single-action dies.  The simplest type of single action die is shown in the fig. 4.5. Fig. 4.5 Single action draw die  In this die, a pre cut blank is placed on the top of the die.  When the punch pushes the blank through the die on the return stroke, the cup is stripped from the punch

72. Example  Figure below shows a symmetrical cup workpiece with a shell height of 40mm and a shell diameter of 50mm the corner radius is 1.6mm.  The workpiece material is 1020 cold rolled steel 0.8mm thick.  Make the necessary calculation for designing the die for this drawing operation.

73. Fig. Draw shell used in sample problem

74. Solution  Determining the size of the blank:  The ratio d/r = 50/1.6 = 31.25 (> 20)  Then the formula for determining the size of blank will be : = = 102.47mm --------- ans.

75.  For smooth edge, add extra metal for trimming.  According to the general rule of adding 0.05mm to the blank diameter for each 10mm of the cup diameter, = 0.05 x (50/10) = 0.25mm should be added for the trimming. Thus, D would be: D = 102.47mm + 0.25mm = 102.72mm = 105mm --- Ans

76.  Determining percentage reduction :  Percentage reduction is calculated from : P = 100 (1-d/D) = 100 x [1-(50/105)] = 52.38 % reduction  The theoretical maximum percentage reduction for one draw is approximately 50 %.  Therefore, it obvious that the above cup cannot be draw in one operation.

77.  Determining draw ratio:  The depth of draw is expressed as the ratio: = h/d = 40/50 = 0.80  Since the ratio exceeds 0.75, more than 1 reduction is necessary, as was explained. Refereeing to table!  Refereeing to table!

78. End of chapter Four