Forging Design Considerations

July 7, 2017 | Author: Chinmay Das | Category: Forging, Engineering Tolerance, Metalworking, Industries, Industrial Processes
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Module-II of PDPT

2.1 FORGING OPERATION Forging is the operation where the metal is heated and then a force is applied to manipulate the metal in such a way that the required final shape is obtained.

Figure 2.1.1: Forging machine

Figure 2.1.2: Grain structure in forging

Two types of operations are used in forging in order to arrive at the final shape. They are as follows. Drawing out: This is the operation in which the metal gets elongated with a reduction in the cross sectional area. For this purpose, the force is to be applied in a direction, perpendicular to the length axis. Upsetting: This is applied to increase the cross sectional area of the stock at the expense of its length. To achieve this, force is applied in a direction parallel to the length axis. Because of the manipulative ability of the forging process, it is possible to closely control the grain flow in the specific direction, such that the best mechanical properties can be obtained based on the specific application. Some examples of desirable grain flow directions are given in the adjacent figure. In the crane hook it is possible to get the two types of grain patterns by proper arrangement of operations. The grain pattern obtained without bending is shown in fig.2.1.3 (a), where as the one in (b) is obtained by bending after drawing out. As a result, the grain flow is also bent along the hook and thus provides the necessary strength for lifting loads. The gear blank shown in (d) is obtained by upsetting the blank and then finish forging, whereas the one in (c) is obtained without upsetting the blank. This provides radial grain flow which is essential for good strength in gear teeth for several applications. Figure 2.1.3: Grain flow directions in forging

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Module-II of PDPT Forging Design Considerations Parting Plane: A parting plane is the plane at which the two die halves of the forging meet. It could be a simple plane or irregularly bent, depending on the shape of the forging. • The parting plane should be the largest cross sectional area of the forging, since it is easier to spread the metal than to force into deep pockets. • A flat parting plane is more economical. • It should be chosen in such a way that equal amount of material is located in each of the two die halves. • It may be required to put more metal into the top die half since metal would flow more easily in top half. • If the punching of hole, which is perpendicular to the parting plane, is required then it may be necessary to choose a parting plane which distributes the hole evenly and provides sufficient strength to the punch. The below mentioned figure shows one possible parting plane which simplifies the lower part of the die. But the punch in the upper die half becomes excessively long and may buckle. By changing the parting line as shown in the next figure, it is possible to punch from both sides, thus reducing the machining. This also provides a smaller height to diameter for the punch increasing its rigidity.

(a) (b) Figure 2.1.4: Parting line to reduce the depth of a punched hole Draft: It is the taper put on all the forging sides arranged parallel to the travel of the press slide or hammer ram. This makes it easier for the metal to fill up the working volume of the die impressions and facilitates the removal of the forging. Standard practice indicates the use of 2 to 12 0 draft angles depending on the type of die, rib height, and the material to be processed. Internal surfaces require more draft than external surfaces. The forgings of non-ferrous alloys need smaller drafts than the steel ones. In upset forgings, the draft problem is minimized because the part is held securely by the gripper die during the punch withdrawal and the gripper itself gets opened to release the component. So for upset forgings smaller value of draft angle is considered.

Draft position Outside Inside

Height or Depth (mm) Up to 25 Above 25 Up to 25 Above 25

Drop forgings Normal Close ( degree) (degree) 5 to 7 3 to 7 5 to 10 3 to 7 7 to 10 5 to 8 8 to 12 5 to 9 Table I: Recommended draft angles

Upset forgings Normal Close ( degree) (degree) 3 to 5

2 to 4

5 to 7

4 to 6

Fillet and Corner Radii: Since forging involves flow of metal in orderly manner, therefore it is necessary to provide a streamlined path for the flow of metal so that defects’ free forging is produced. When two or more surfaces meet, a corner is formed which restricts the flow of metal. These corners are rounded off to improve the flow of metal. Fillets are for rounding off the internal angles, whereas corner is that of the external angle. Let us consider the flow of metal over a corner as shown in the figure 2.1.5(a). Because of large corner radius provided, metal is allowed to flow smoothly into the pocket. But when corner radius is small or not provided as in figure 2.1.5(b), the metal flow is first hindered and when it finally enters the

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Module-II of PDPT cavity, the metal would fold back against itself forming a defect called lap or cold shut. Nominal fillets and corner radii are taken from the tables to suit the weight and required accuracy of the forgings. Sharp fillets and radii increase the tendency towards forging defects and accelerate the die wear. To avoid this fillets are taken to be larger than corner radii.

Figure 2.1.5: Effect of corner radius on the flow of metal Depth or Fillet, mm Corner radius, mm Height, mm 15 5 2.5 25 8 4.0 40 12 4.5 50 15 5.0 65 18 5.5 75 20 6.0 Table II: Recommended fillet and corner radii for drop forgings Upset diameter Fillet, mm Corner radius, mm Stock diameter Up to 1.25 6.5 6.5 1.25 to 3.00 3.5 3.5 Over 3.00 3.0 3.0 Table III: Recommended fillet and corner radii for upset forgings Shrinkage Allowance: The forgings are generally made at a high temperature of 1150 to 13000 C. At this temperature, the material gets expanded and when it is cooled to the atmospheric temperature, its dimensions would be reduced. Hence a shrinkage allowance is added on all the linear dimensions. Length or width, mm Up to 25 26 to 50 51 to 75 76 to 100 101 to 125 126 to 150 Each additional 25

Commercial, mm 0.08 0.15 0.23 0.30 0.38 0.45 Add 0.075 Table IV: Recommended shrinkage allowance

Close, mm 0.05 0.08 0.13 0.15 0.20 0.23 0.038

Die Wear Allowance: This allowance is considered to account for the gradual wear of the die which takes place with the use of the die.

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Module-II of PDPT Net mass of forging, Kg Up to 0.45 0.46 to 1.35 1.36 to 2.25 2.26 to 3.20 3.21 to 4.10 4.11 to 5.00 Each additional 1 add

Commercial, mm 0.80 0.88 0.95 1.03 1.11 1.18 0.083 Table V: Recommended die wear allowance

Close, mm 0.40 0.45 0.48 0.53 0.55 0.60 0.041

Finish or Machining Allowance: It is provided on the various surfaces which need to be further machined. The amount of allowance to be provided should consider accuracy & surface finish required on the forged products, and also depth of decarburized layer, scale pits etc formed during the forging operation. Greatest dimension, mm Minimum allowance per surface, mm Up to 200 1.5 201 to 400 2.5 401 to 600 3.0 601 to 900 4.0 Above 900 5.0 Table VI: Recommended finish allowance for drop forgings Greatest diameter, mm Minimum allowance per surface, mm Up to 50 1.5 51 to 200 2.5 Above 200 3.0 Table VII: Recommended finish allowance for upset forgings Cavities: The cavities and ribs including holes can be produced up to a certain depth only in drop forging because the punch needs to have the necessary strength to withstand the forging load. Thin long punches are likely to wear out quickly and need reconditioning of the die.

Figure 2.1.6: Cavity configuration in drop forging Materials Aluminium, Magnesium Steel , Titanium

Ratio of h : W L=W 1.0 1.0

L ≥ 2W 2.0 1.5

Table VIII: Maximum limits of depth In addition to these allowances, the various tolerances that are applicable to the forgings such as mismatch tolerance, weight tolerance, residual flash tolerance, thickness tolerance, burr tolerance, etc. are also considered while arriving at the final dimensions of the die.

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Module-II of PDPT The forging tolerance is the value of permissible deviation from the nominal forging size indicated in the drawing. Overlap is the stock left on the work where forging is inconvenient, this stock projects beyond the outline of the ready article. Overlaps are resorted to for the sake of simpler shape of the forging to facilitate its manufacture. The overlaps are removed by machining. A sample component after providing the necessary allowances and tolerances is shown.

Figure 2.1.7: Allowances shown on forged component

Figure 2.18: Forging component as affected by allowances and tolerances

Reference: 1. 2. 3.

Manufacturing Technology by P.N.Rao, TMH , page 254 -264 Dies, Moulds and Jigs by V. Vladimirov, MIR Publishers, page 301-304 Manufacturing Engineering and Technology by Kalpakjian and Schmid, Pearson Education, page 353-355

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Module-II of PDPT Review Questions 1. 2. 3. 4. 5. 6. 7. 8.

Explain the features of a typical forging die. How parting plane selection in case of forging is different from that of casting? Why corner radius is less than fillet radius in forging die? On which section of forged products machining allowance is provided? Draft angles are less in upset forging compared to drop forging? Name various positive and negative allowances considered in forging. Why larger values for draft angle are considered for internal surfaces? Take two solid cylindrical specimens of equal diameter but different heights and compress them (frictionless) to the same percent reduction in height. Show whether final diameters will be different or same. 9. Why tolerances are considered in die design? 10. Why ratio of rib height to web height is important in die design?

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