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In forging techniques research is carried out for arriving at a shape called ‘net shape’ so that

negligible machining is needed after forging. The following aspects are considered in the design of forging components that comply with production technique requirements:

1. If the blanks are to be produced by flat-die forging, intersections of two or more cylindrical elements should be avoided (Fig. 6.1). Intersections of cylindrical and flat surfaces should also be avoided (Fig. 6.2). Such combinations present difficulties when they are forged between flat dies; they should be designed as shown in Figs. 6.3 and 6.4, respectively. Ribbed cross-sections should also be avoided, because ribs are unacceptable when flat-die forging is employed. Bosses,

projections, pads, etc. should be avoided on the main surfaces of forgings (e.g. bosses “a” in Fig. 6.5), as well as projections inside the prongs of fork-type parts (e.g. bosses “a” in Fig. 6.6). In the first case (Fig. 6.5), the holes may be spotfaced below the rough surface instead of providing bosses. In the second case (Fig. 6.6), instead of making inner bosses, the height of the outer bosses can be increased.

Fig. 6.1 Undesirable shape for flat die forging.

Fig. 6.4 Desirable shape for flat die forging. Fig. 6.3 Desirable shape for flat die forging.

Fig. 6.5 Component with four bosses.

Fig. 6.6 Component with bosses on inner and outer sides.

2. In a number of cases, it is advisable to replace components having such a complex shape by units consisting of simple welded or assembled elements.

3. The following discussion involves production design aspects of forgings produced in closed dies under hammers or in presses:

The form of a drop forging should ensure easy removal from the die impression. The angle of draft on the side surface of forging should be designed from a plane square to the die parting surface. Draft and dowels should range from 5 to 15° for outer walls and from 7 to 15° for inner walls, when no ejectors are used. When ejectors are applied, it should be between 2 and 10° for outer walls and 3 and 12° for inner walls.

4. Transitions from one rough surface to another must be rounded fillets and corners. Sharp corners are inadmissible on closed-die forgings. To avoid forging rejects and to extend the die life, the fillets of inside corners should have larger radii than outside corners. Depending on the height and height-to- width ratio of the element, radii are taken from 1.5 mm to 12.5 mm for outer corners and 4 mm to 45 mm for inner corners. Adequate radii or chamfers should be provided for the junctions of machined surfaces on the component (Fig. 6.7). In this case, the following condition is to be observed:

where

r_p = corner radius of the machined part r_f = corner radius of the rough-forging z_n = nominal machining allowance

Fig. 6.7 Profile of forging.

If the above condition is not observed, it will be necessary to increase the machining allowances for the adjoining surfaces to ensure that the specified radius is obtained on the finished part. When a chamfer is used instead of a rounded corner, it should be such as can be inscribed by the corner radius.

5. No sharp differences should be allowed in the cross-section areas along the forging because this complicates the forging process and may lead to an increase in rejects due to cramping and

underfilling.

6. Thin walls in forgings reduce die life since the forging cools rapidly and its resistance to metal flow is increased. This also leads to underfilling and increase in the amount of rejects. If thin elements of forgings are adjacent to the parting plane of the dies, there will be a large metal waste and more rejects due to underfilling and breaking-off in the process of cold flash trimming.

7. Symmetrical shapes of forgings relative to die parting planes and symmetrical drafts of projectile walls simplify die-sinking procedures and forging operations, and reduce the number of rejects due to die mismatch. Asymmetrical shapes and non-uniform drafts result in forces tending to shift the upper and lower dies.

8. The sizes of bosses to be drilled and subsequently machined are determined on the basis of the required minimum wall thickness after machining the hole and the possible value of die mismatch. In many cases, oval bosses, with their major axis along the direction of probable shift, will enable minimum wall thickness to be maintained after machining the hole.

9. It is often better to substitute weld parts for one-piece components to have metal and simplify forging operations. For instance, the weldment shown in Fig. 6.8 used in place of a drop forging provides for considerable amount of metal economy and simplifies, to a great extent, the forging process. However, the need for such changes should be checked in each definite case because,

sometimes a one-piece drop forging may be more convenient in production and costs than a weldment (e.g. the lever shown in Fig. 6.9).

Fig. 6.8 Forged and forged cum welded machine part.

Fig. 6.9 Lever design.

10. In designing forgings which are to be produced in horizontal forging machines, the following considerations may be helpful:

(a) Forgings of various forms may be obtained in horizontal forging machines, but the most

suitable for this process are those having the form of solids or revolution of regular geometric shape with flanges, shoulder and through or blind holes. The wall thickness of forging with deep, through or blind holes should not be less than 0.15 of the outside diameter.

(b) Reductions in cross-section along the length of forgings should be avoided because they impede metal flow during the forging process.

(c) Shanks of taper form are also difficult to forge and they should be replaced by cylindrical shanks. (d) The volume of flanges located at the ends or in the middle of a forging must not exceed the volume of a bar having the given diameter d and a length of 10–12 d.

(e) Draft for this type of forging may be very small. For instance, a draft of 0.5° is suitable for the cylindrical section of the forging up-set within the punch cavity and of a length more than one-half of the diameter. A draft of 0.5–1.5° is suitable for shoulders formed in the circular impressions of dies and 0.5–3° on the walls of blind holes with a length of five or more diameters.

(f) Transition from one surface to another must have fillets with radii from 1.5–2 mm.

(g) Carbon and alloy steel fasteners and similar parts having an annealed hardness, BHN = 120–207, are produced by cold heading.

Cold drawn wire or rod is employed in cold heading. The headed elements or components should be, as far as possible, of simple form and with a minimum volume and diameter.

Close tolerance should not be specified, unless required, for the headed parts, as die life will be reduced. Fillet radii of at least 0.2 mm should be provided at all corners.