Forging design guidelines

Steel closed-die forgings typically use 5–7° on external walls and 7–10° on internal walls, which shrink onto the die as they cool. Aluminum and precision forgings often run 1–3°, and some precision work goes lower with ejectors. Draft adds metal that is later machined off, so use the least the forger can release reliably.

Mismatch is the sideways offset between the upper and lower halves of a forging, caused by the dies shifting across the parting line. Typical allowances run about 0.015–0.030 in (0.4–0.8 mm) on small and mid-size steel parts under EN 10243-1. Machining stock must cover it on any surface that crosses the parting line.

Plan 0.060–0.125 in (1.5–3.2 mm) per machined surface on typical closed-die parts, and more on large forgings. The stock has to remove scale, decarburized skin, draft, mismatch and die wear so the finished face cleans up everywhere. Too little stock leaves black spots on the machined face.

Webs are usually at least 0.125–0.250 in (3–6 mm) thick, and thicker as the web's plan area grows. Thin webs chill against the die and resist flow, so the tonnage needed to fill them rises steeply. Aluminum and high-alloy steels usually need thicker webs than carbon steel.

A lap forms when metal folds over onto itself instead of flowing smoothly, usually at a sharp fillet or where too much metal enters a cavity too early. The fold leaves a seam that acts as a crack in service. Generous fillets of at least 0.125 in (3 mm), good blocker design and correct billet weight prevent laps.

EN 10243-1, which replaced DIN 7526, is the common reference for closed-die steel forgings. It sets length, thickness, mismatch, residual flash and straightness tolerances by part weight, shape complexity and steel grade, in grade F for standard work and grade E for precision work. Many US forgers quote to their own tables on similar values.