Tolerances for Machined Forgings: How to Split ISO 2768, EN 586-3 and GD&T on One Drawing

IWhy do tolerances for machined forgings need three layers?

ISO 2768-1 applies to dimensions created by cutting or by forming sheet metal. An as-forged surface varies with die wear, mismatch, die-closure thickness and thermal shrinkage, which is a different order of variation from machining. Those surfaces should reference a forging tolerance standard or tolerances agreed on the forging drawing.

In practice, give each part two drawings. The forging drawing defines the blank: parting line, draft, mismatch and flash limits. The machining drawing defines machined surfaces, the datum reference frame and finished tolerances. Both drawings should share one datum scheme, and the gap between them is the machining allowance. See how to size machining allowance on an aluminum forging for the calculation.

IIHow do ISO 2768-1 general tolerances work, and when should you choose fine over medium?

ISO 2768-1:1989 remains the current edition; ISO last reviewed and confirmed it in 2022. The drawing note is simply “ISO 2768-m” in or near the title block. The table lists permissible deviations for linear dimensions.

Source: ISO 2768-1:1989, general tolerances for linear dimensions. Ranges are over the lower value up to and including the upper value; values in mm.

Class f is not a quality upgrade. Once the title block says f, every untoleranced machined dimension must be cut and inspected to the fine class, which adds cycle time and inspection cost. A better practice is to keep class m in the title block and call out the dimensions that actually affect fit and function (see section VI).

Some online calculators label class v as suitable for forgings. That is misleading: class v is still an ISO 2768-1 class for machined and sheet-metal dimensions. As-forged surfaces belong under a forging tolerance standard.

IIIISO 2768-2 has been replaced by ISO 22081. Is “ISO 2768-mK” still valid?

ISO 22081:2021 was developed by ISO/TC 213 as a geometrical product specification (GPS) standard. It provides tools for general geometrical specifications and general size specifications, clarifies the rules for applying them, and is written differently from the old H, K and L classes. ISO confirmed the standard after its review in June 2026.

For buyers and SQEs, the goal is not chasing the newest edition. It is making sure both parties read the drawing the same way. A practical approach by drawing status:

• Legacy production drawings: if the title block says ISO 2768-mK, agree with the supplier before first article how untoleranced flatness, perpendicularity and similar requirements will be judged.
• New or revised drawings: keep ISO 2768-1 for general size tolerances, and either adopt ISO 22081 for general geometry or call out important form and location requirements individually.
• No general geometric tolerances on as-forged surfaces: flatness or straightness of forged surfaces should follow the forging tolerance standard or values agreed on the forging drawing.

IVWhich standard covers tolerances on as-forged surfaces?

EN 586-3 specifies dimensional and form tolerances for hot-formed aluminum and aluminum alloy forgings. Its scope covers die forgings that are not machined all over up to 2,000 mm shape dimension, 600 mm die-closure dimension and 4,000 cm² projected area, and die forgings machined all over up to 6,000 mm and 22,000 cm². Larger or complex forgings use tolerances agreed between supplier and purchaser.

A forging drawing needs more than length, width and height. Draft, mismatch, residual flash, ejector marks, fillet radii and die-closure dimensions all affect how the part is located and how much stock is left for machining. Most U.S. drawings do not cite EN 586-3; ASM International’s chapter on forging design dimensions and tolerances and the Forging Industry Association’s Product Design Guide for Forging are common references, with agreed values written directly on the forging drawing.

Forging geometry and parting-line design also decide which surfaces can stay as forged. For design guidance from the forging side, see AFT’s aluminum forging DFM guide.

VWhat tolerance applies to a dimension between a forged surface and a machined surface?

Take a boss height measured from a machined mounting face to an unmachined forged top. The dimension carries both forging and machining variation. If the drawing only says ISO 2768-m, the machine shop may assume class m applies, while the forged surface alone can vary by more than that. Three habits prevent the argument:

• Dimension from machined datums: locate unmachined features from the datum face and datum holes created in the first operation, not by chaining from another forged surface. See datum and fixture strategy for machining aluminum forgings.
• State the range: where a forged dimension affects assembly, give it an individual tolerance on the machining drawing instead of relying on the larger-of-two rule.
• Specify minimum wall: where a machined feature approaches a forged surface, a minimum wall or minimum remaining thickness protects strength better than a nominal dimension.

VIHow should bores, sealing faces, threads and hole positions be toleranced?

Sources: ISO 2768-1, ISO 22081, EN 586-3, ASME Y14.5; AFT machining practice. Actual tolerances depend on the drawing and functional requirements.

For anodized or hardcoated parts, also state whether dimensions apply before or after finishing. Coating buildup and thread treatment are covered in designing machined aluminum forgings for anodizing and hardcoat.

VIIWhat changes on ASME Y14.5 drawings, and how does AFT review them?

There is a second difference that is easy to miss. ASME Y14.5 applies the envelope principle by default (Rule #1), so a size tolerance also limits form. ISO drawings follow the independency principle of ISO 8015 by default, judging size and form separately unless the envelope requirement is indicated. The same bore can pass under one system and fail under the other.

When AFT receives an RFQ, the engineering team reviews the forging and machining drawings together: which surfaces are machined and which stay as forged, whether both drawings share one datum scheme, whether the title-block general tolerances suit a forging, and whether every functional feature has its own tolerance. Questions are raised before quoting, not discovered at first article.

On the inspection side, AFT measures machined dimensions on four Zeiss CMMs aligned to the machining datums, checks forged contours with a handheld 3D scanner, and uses a contour tracer and surface roughness tester for profiles and sealing faces. Records are kept under IATF 16949:2016; see manufacturing and quality control for the full flow.