Editing Forging (section)

===== Design of impression-die forgings and tooling =====
Forging dies are usually made of [[alloy steel|high-alloy]] or [[tool steel]]. Dies must be impact- and wear-resistant, maintain strength at high temperatures, and have the ability to withstand cycles of rapid heating and cooling. In order to produce a better, more economical die the following standards are maintained:<ref name="Degarmo393"/>
*The dies part along a single, flat plane whenever possible. If not, the parting plane follows the contour of the part.
*The parting surface is a plane through the center of the forging and not near an upper or lower edge.
*Adequate [[Draft (engineering)|draft]] is provided; usually at least 3° for aluminium and 5° to 7° for steel.
*Generous fillets and radii are used.
*Ribs are low and wide.
*The various sections are balanced to avoid extreme difference in metal flow.
*Full advantage is taken of fiber flow lines.
*Dimensional tolerances are not closer than necessary.

Barrelling occurs when, due to [[friction]] between the work piece and the [[die (manufacturing)|die]] or [[punch (tool)|punch]], the work piece bulges at its centre in such a way as to resemble a [[barrel]]. This leads to the central part of the work piece to come in contact with the sides of the [[die (manufacturing)|die]] sooner than if there were no friction present, creating a much greater increase in the pressure required for the punch to finish the forging.

The dimensional tolerances of a steel part produced using the impression-die forging method are outlined in the table below. The dimensions across the parting plane are affected by the closure of the dies, and are therefore dependent on die wear and the thickness of the final flash. Dimensions that are completely contained within a single die segment or half can be maintained at a significantly greater level of accuracy.<ref name="Degarmo394">Degarmo, p. 394</ref>
{| class="wikitable"
|+ Dimensional tolerances for impression-die forgings<ref name="Degarmo394"/>
|-
! Mass [kg (lb)]
! Minus tolerance [mm (in)]
! Plus tolerance [mm (in)]
|-
| 0.45 (1)
| 0.15 (0.006)
| 0.46 (0.018)
|-
| 0.91 (2)
| 0.20 (0.008)
| 0.61 (0.024)
|-
| 2.27 (5)
| 0.25 (0.010)
| 0.76 (0.030)
|-
| 4.54 (10)
| 0.28 (0.011)
| 0.84 (0.033)
|-
| 9.07 (20)
| 0.33 (0.013)
| 0.99 (0.039)
|-
| 22.68 (50)
| 0.48 (0.019)
| 1.45 (0.057)
|-
| 45.36 (100)
| 0.74 (0.029)
| 2.21 (0.087)
|}
A lubricant is used when forging to reduce friction and wear. It is also used as a [[thermal barrier]] to restrict heat transfer from the workpiece to the die. Finally, the lubricant acts as a parting compound to prevent the part from sticking in the dies.<ref name="Degarmo394"/>