3.5.4.5 Design Rules for Parts Made by Cold and Warm Forging

1. Commercially made cold forgings typically weigh less than 23 kg (50 lb), although larger forgings have been cold forged.
2. Net shape cold forgings should be considered for products made in high volumes with surfaces that are difficult or expensive to machine due to geometric configurations.
3. Shapes that can be made by upsetting and bending, such as bicycle pedal cranks, are good candidates for cold forging.
4. Net or near-net shapes, such as tripot inner races or universal joint crosses, can be manufactured using cold or warm lateral extrusion. (See Figure 3-22.)
5. Consider replacing heat treatment with cold forging to work harden the product to yield strengths exceeding 550 MPa (80,000 psi).
6. Specify the material with the lowest possible amount of carbon and lowest alloying level.
7. Cold forgings do not require draft angles to release them from the tooling.
8. Solid or tubular shaped products with either through or blind holes, with net formed splines or other axial features, can be made by cold forging.
9. When specifying blind holes, keep in mind:
       • Holes that are deep in proportion to their diameter are difficult to forge.
   
   

   Figure 3-22 Net or near-net shapes, such as tripot inner races or universal joint crosses, can be manufactured using cold or warm lateral extrusions.

   
       • Maintain uniform side wall thicknesses.
       • The wall at the bottom of the blind hole should be at least as thick as the side walls.        (See Figure 3-23.)
   
   

   Figure 3-23 The hole on the left can be forged; the one on the right will require a drilling operation.

   
   
10. Consult with the forger to determine the net shape capability, and design net shape surfaces within those capabilities.
11. When designing solid shapes, minimize the difference between the largest and smallest diameters of the part (See Figure 3-24.)
12. Avoid undercut diameters in products to be cold forged. They can be forged in some cases if the undercut is wide as illustrated in Figure 3-25. Consult with the forger.
13. Avoid extremely thin or thick wall sections when designing tubular parts.
    
    

    Figure 3-24 Minimize diameter ratios.

    Figure 3-25 Undercuts can be forged in some cases if they are wide compared with the diameter of the feature.

    
    
14. Avoid sharp corners: use fillets and radii.