3.4.1 A Comparison of Open Die, Impression Die, Rolled Ring and Cold Forging Processes

The choice among the various forging processes is driven by component size, production quantities, and component shape. The following guidelines usually apply.

1. When forgings are very large, when very few are required, or when delivery times are very short, open die forging is the typical choice.
2. As shapes become more complex, and production quantities increase, impression die forging becomes the process of choice provided that the size does not exceed the capability of the impression die equipment.
3. Shaft-like forgings with details on the ends or along the length are candidates for upset forging.
4. Seamless rings may be made by open die forging over a mandrel, impression die forging or ring rolling. Diameters less than one foot may be candidates for impression die forging. Diameters less than one foot up to 30 feet, in low to high quantities, are candidates for ring rolling. When quantities are very low or face heights are too large for ring rolling, open die forging over a mandrel can be the process of choice.
5. Relatively small components that are rotationally symmetrical or axisymmetric, require high strength and high precision, and are produced in larger quantities are candidates for cold forging.

Figure 3-2 The tractor drawbar hitch (top) and truck steering knuckle (bottom) illustrate forging's capability to combine several parts into one.

When impression die forging is chosen, four options are available: blocker type forgings, finished forgings, near-net forgings, and net shape forgings.

Blocker Type Forgings are generally forged in a single impression die, with generous finish allowance. This process is suitable for moderate production quantities. A rough rule of thumb for finish stock is at least 5 mm (0.2 inch) of machining envelope for each 300 mm (12 inches) of dimension for blocker type forgings made from steel. The allowance can be less for aluminum, and should be 25% to 50% more for heat resistant alloys. Draft angles are typically 7^o to 10^o.

Finished Forgings are suitable for high production quantities. They are forged with significantly less finish allowance than are blocker type forgings, and typical FIA guideline tolerances apply. Typical finish allowances are 1.25 to 2.5 mm (0.050 to 0.100 inch) plus draft, which varies from 3^o to 7^o. (See Appendix A Guideline Tolerances For Hot Forged Impression Die Forgings , Appendix B Tolerances for Hot Upset Forgings and Appendix D Specialized Tolerances for Precision Aluminum Forgings.)

Near-Net Forgings are forged with some surfaces requiring little or no machining, and some surfaces may be left as forged. They are similar to finished forgings except they are closer to final configurations. Some forging companies, by virtue of their own special forging equipment, may offer specific improvements over the tolerances and finish allowances considered as "normal" by the FIA Tolerance Guidelines.

Net Shape Forgings are forged to net and near-net shapes with many functional surfaces forged to required tolerances, requiring no machining. For example, tooth forms on net shape forged gears up to 125 mm (5 inch) diameter are being forged to tolerances of ±0.10 mm (±0.004 inch), which is often close enough to eliminate gear cutting operations. However, back faces or shafts are usually machined.

Product features attributable to these four forging processes are illustrated in Figure 3-3.

Figure 3-3 Blocker, finished, near-net and net shape forgings.

Following is a summary of several typical areas where product factors drive the choice.

• Increased forging precision tends to drive up the cost of forging operations somewhat, but it usually reduces the cost of finish machining. As production quantities increase, the reduced cost of machining operations becomes a stronger offsetting factor.
• A very expensive raw material that is difficult to machine will suggest the most chipless process, and small production quantities might best be net or near-net forged. If the material is inexpensive and readily machined, open die forging in small quantities may be the optimum choice.
• Medium size to large rings can be made either by open die forging over a mandrel and finish machining or by a ring rolling process in which finish machining may or may not be required. Production quantities drive the choice. Open die forging is generally more economical in very low quantities; ring rolling becomes more economical as quantities increase.
• Components that have features with rotationally symmetrical or axisymmetric shapes, such as splines, may be candidates for either cold forging or hot impression die forging, depending on complexity and size.