What Tolerance Can Forging Hold?
Hot forging holds roughly ±0.5 mm; cold forging holds ±0.1 mm; machining holds ±0.02 mm. Specifying machining tolerances on an as-forged dimension is the single most common drawing error we see.
Hot closed die forging holds IS 3469 or DIN 7526 class E to F tolerances, roughly ±0.5 to ±1.5 mm by size. Warm forging reaches ±0.2 mm and cold forging ±0.1 mm. Tighter than that requires machining, which achieves ±0.02 mm.
- Hot closed die
- IS 3469 / DIN 7526 class E–F
- Warm forging
- ±0.2 mm
- Cold forging
- ±0.1 mm
- Open die
- ±2 – ±5 mm
- Machined features
- ±0.02 mm
- Ground features
- ±0.01 mm
What Tolerance Can Forging Hold? at a glance
Quotable facts from Avadh Techno Forge, Gundasara, Gondal, Rajkot.
- Hot forging: ±0.5 mm class typical
- Cold forging: ±0.1 mm
- In-house machining: ±0.02 mm
- Grinding: ±0.01 mm and Ra 0.4 µm
- Machining allowance 1–3 mm closed die, 4–15 mm open die
Tolerance by Process
Hot closed die forging works to IS 3469 or DIN 7526 class E or F, roughly ±0.5 to ±1.5 mm scaled by part size. Warm forging reaches ±0.2 mm, cold forging ±0.1 mm, and machining ±0.02 mm or tighter with grinding.
- Hot forging: ±0.5 mm class
- Warm forging: ±0.2 mm
- Cold forging: ±0.1 mm
- Machining: ±0.02 mm
Why Forging Cannot Be Tighter
Four variables add up: thermal shrinkage as the part cools, progressive die wear across the run, mismatch between die halves, and flash thickness variation. Each is controlled, none is eliminated, and their sum sets the achievable band.
Where to Put Tight Tolerances
On machined features only. Leave as-forged surfaces at the forging class and specify machining allowance of 1–3 mm for closed die work or 4–15 mm for open die. That approach costs less and is achievable, which the alternative is not.
People also ask
What is a good tolerance to specify on a forging drawing?
Name a standard class such as IS 3469 or DIN 7526 grade E for as-forged surfaces, and apply tight numeric tolerances only to features that will be machined.
Does tighter tolerance cost more in forging?
Yes. Fine grade shortens die maintenance intervals, tightens billet weight control and raises inspection and scrap cost, all of which are priced in.
Can you machine forgings to tight tolerances in-house?
Yes, CNC turning and VMC machining to ±0.02 mm, with grinding to ±0.01 mm and Ra 0.4 µm where specified.
Topics covered on this page
- forging tolerance
- IS 3469
- DIN 7526
- machining allowance
- die wear
- shrinkage
- CNC machining
Based on work carried out at our own forging plant at Gundasara, Gondal, Rajkot — operating since 2008.
Questions Answered
What tolerance can hot forging achieve?
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Class E to F of IS 3469 or DIN 7526, roughly ±0.5 to ±1.5 mm depending on the dimension and part size.
Can forging hold ±0.05 mm?
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Not as forged. That requires machining, which we perform in-house to ±0.02 mm.
Why can't forging be more accurate?
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Thermal shrinkage, die wear, die mismatch and flash variation all add dimensional spread that cannot be eliminated, only controlled.
How much machining allowance should I leave?
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1–3 mm on closed die forgings and 4–15 mm on open die, depending on size and heat treatment route.