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Design Considerations

For forged pieces that are produced in two-part impression dies, the designer should take into account the following: the parting line, the draft, the presence of ribs, bosses, webs, and recesses, and the machining allowance.

Parting Line

The mating surfaces of the two halves of the die define a parting line around the edges of the forging as they come together. Determining the position of the parting line is a critical step in the forging design. The parting line should be indicated on the forging drawing.

It is preferable that the parting line lie in one plane perpendicular to the axis of the die motion. It can be located such that one die half will be completely flat, and the line will surround the largest projected area of the piece.

If this is not possible, it is desirable to preserve symmetry to prevent high side-thrust forces on the dies and the press.

The inclination of any portion of the parting line from the principal parting plane should not exceed 75^o, in general, much shallower angles are preferred.

It is essential to select the parting line not to entail any undercuts in either die impression, since the forging must be ejected out of the die after it is made.

The parting line location can be changed to control the grain flow given the above recommendations are fulfilled.

Draft

The die impressions are tapered to facilitate removing the forging from the dies. Generally, all forged surfaces that lie parallel to the die motion are tapered. This taper also helps the flow into deeper die cavities. It is called the draft and is specified by the angle with respect to the die-motion axis.

It is conventional to specify a standard draft angle for all affected surfaces on a forging. Also, it is conventional to call for matching draft on both die halves to make surfaces of unequal depth meet at the parting line.

Low-draft and no-draft forging can be produced in some metals, such as aluminum and brass. This is done in cases when reduction or elimination of draft yields significant benefits.

Ribs, Bosses, Webs, and Recesses

It is easier to manage metal flow when the ribs and bosses are not too high and narrow, and the web is relatively thick and uniform in thickness. Deep recesses are easier to forge if they have spherical bottoms.

Machining Allowance

The machining allowance should allow for the worst-case buildup of draft and for all tolerances. Extra metal is sometimes provided to keep critical machined surfaces away from the grain-flow pattern that occurs in the flash region near the parting line.

Source: Bralla, Design for manufacturability Handbook