QUALITY · September 10, 2026 · 8 MIN READ

GD&T on Hot Forgings: Which Callouts the Process Can Actually Hold

A hot forging holds general tolerances of roughly ±0.3 to ±1.0 mm as-forged, and geometric callouts tighter than about 0.2 mm on an unmachined surface are not achievable — the die wears, the part contracts unevenly and the parting line moves. If a drawing specifies flatness 0.05 mm or position 0.1 mm on an as-forged face, the requirement is really a machining requirement, and it should be priced and inspected as one.

WHERE THIS APPLIES AT FORGEPRESS CHINA

ForgePress China quotes as-forged tolerance to ISO 8062 class C and machines critical dimensions to ±0.2 mm where the drawing requires it, then holds your GD&T only where the drawing says "after machining". Capacity: HKM3 hot-die forging line 1,000–5,000 t, part weight 0.1–500 kg, maximum envelope 1,200 × 800 mm, as-forged to ISO 8062 class C with critical dimensions machined to ±0.2 mm, process control to ISO 9001, first article 4–8 weeks from die design release.

KEY NUMBERS

As-forged general tolerance: roughly ±0.3 mm on small parts, ±0.5 mm at 100 mm, ±1.0 mm above 300 mm.
Flatness and straightness on an unmachined forging: 0.15–0.30 mm is realistic, 0.05 mm is not.
Draft angle causes the datum problem: a vertical wall has 3–7° of draft, so it cannot serve as a true datum.
Position tolerance on machined holes is normally IT8–IT9, i.e. 0.03–0.09 mm at 50 mm.

The three reasons forged surfaces cannot hold tight GD&T

Die wear. Every impression grows as it wears — 0.05–0.15 mm over a die's life is normal — so a tolerance that was met on the first article may be exceeded at piece 8,000 without anything else changing. A GD&T callout that assumes a stable die will fail late in the run, not early.

Uneven contraction. A forging with a 3:1 section ratio does not cool uniformly, so a nominally flat plate bows. The bow is typically 0.1–0.3 mm over a 200 mm span and is repeatable, which means it can be corrected in the die but not eliminated.

Parting line mismatch. Upper and lower dies are separate pieces; a 0.10 mm mismatch at the parting line is good practice and 0.20 mm is common on large dies. Any callout referenced to the parting line inherits that error.

What is realistic as-forged versus after machining

Characteristic	As-forged (realistic)	After machining	Note
General dimension, 0–100 mm	±0.3 to ±0.5 mm	±0.05 mm (IT9–IT10)	Per DIN 7526 / customer drawing
General dimension, 100–300 mm	±0.5 to ±0.8 mm	±0.08 mm	Grows with part size
Flatness, 200 mm span	0.15–0.30 mm	0.02–0.05 mm	As-forged bow is repeatable
Straightness, 300 mm	0.20–0.40 mm	0.03–0.06 mm	Affected by ejection and cooling
Perpendicularity to a drafted wall	Not meaningful	0.05 mm	Draft makes the wall a poor datum
Position of a forged hole	±0.5 mm	±0.05 mm	Forged holes are usually cored and drilled
Concentricity, two forged diameters	0.3–0.6 mm	0.03–0.05 mm	Die mismatch dominates
Surface profile	0.5–1.0 mm	0.05–0.10 mm	Use profile instead of ± tolerances

Ranges reflect closed-die hot forging of carbon and low-alloy steel parts in the 1–10 kg class. They are planning values, not acceptance criteria — the drawing and the agreed inspection plan govern.

The datum problem nobody mentions

GD&T assumes datums are stable features. In hot forging, the natural datum candidates are drafted walls, a parting line and a curved as-forged surface — all of which are either non-vertical or not flat. A perpendicularity callout to a 5° drafted wall is technically defined and practically meaningless, because the wall's own form error swamps the tolerance.

The correct approach is to establish three small machined datum pads on the first operation, then reference all geometric callouts to those pads. This costs one extra fixturing step and removes almost all inspection disputes. If a drawing already has datum targets, check that they can actually be machined.

How to specify GD&T on a forging drawing

Use profile of a surface rather than coordinate ± tolerances for as-forged contours — it controls the whole surface and matches how the die actually works.
Apply tight geometric callouts only to machined surfaces, and say so in a note: "geometric tolerances apply after machining".
Give the as-forged condition its own note with a general tolerance class, e.g. DIN 7526 grade F, rather than leaving it to the title block.
Define datums on machined pads, not on drafted walls or the parting line.
State the inspection stage for each callout: as-forged, after machining, or both.

Frequently asked questions

What tolerance can hot forging hold as-forged?

Typically ±0.3 to ±0.5 mm on dimensions up to 100 mm, ±0.5 to ±0.8 mm from 100 to 300 mm, and ±1.0 mm and above on large parts. Geometric tolerances of 0.15–0.30 mm are realistic for flatness or straightness; below 0.1 mm requires machining.

Why can I not use the side wall as a datum?

Because it has draft — 3 to 7° by design so the part can be ejected. A drafted wall is not perpendicular to the parting plane, and its own form error is larger than the tolerance you are trying to control. Machine a small datum pad instead.

Should I use position or profile tolerance?

For as-forged contours, profile of a surface. It controls form, orientation and location in one callout and matches the way a die produces a surface. Position tolerance is the right choice for machined holes and for patterns of features.