Understanding Forging Dimensional Tolerance Standards — DIN EN ISO 13920, Standard Forging Tolerance Classes & Machining Allowance
A technical guide to forging dimensional tolerance standards — how standard forging tolerance classes (per DIN EN ISO 13920 and equivalent standards) work, the difference between as-forged tolerance and finish-machined tolerance, and how to correctly specify machining allowance so your forged component drawing matches achievable forging process capability. Shivam Forge, Rajkot, India. Call +91-9265772827.
Forging dimensional tolerance is governed by fundamentally different physics than machining tolerance, and understanding this difference is essential to specifying a forged component drawing that's both achievable and cost-effective. As-forged dimensions — the dimensions produced directly by the forging process itself, before any subsequent machining — carry inherently wider tolerance than machined dimensions, because hot metal forming involves thermal contraction variation, die wear over a production run, and material flow behavior that machining's cold, tool-controlled material removal simply doesn't share. Standard forging tolerance classes (as codified in DIN EN ISO 13920 and comparable standards) exist to formalize this achievable as-forged tolerance range, and the critical specification principle they enable is straightforward: any dimension requiring tighter tolerance than standard as-forged capability allows should be specified as a machined dimension with adequate machining allowance (stock) left on the forging, not as an as-forged dimension held to an unrealistic tolerance the forging process cannot reliably achieve.
Standard classification of achievable as-forged dimensional tolerance based on component size and forging process type, providing a recognized reference for specifying realistic as-forged dimensions rather than arbitrary or unrealistically tight tolerance values.
Standard forging tolerance classes distinguish between linear dimensional tolerance (length, width, height) and geometric tolerance (straightness, flatness, symmetry), since these are governed by somewhat different aspects of the forging process and die condition.
Guidance on how die wear across an extended production run can affect achievable as-forged tolerance over time, and how tolerance class selection should account for this rather than assuming first-article tolerance capability holds indefinitely.
Explanation of why closed-die forging generally achieves tighter as-forged tolerance than open-die forging, given the fundamentally different die constraint each process applies to material flow.
Guidance on specifying adequate machining stock allowance on any feature requiring tighter final tolerance than as-forged capability provides, ensuring sufficient material remains for machining to reach final tolerance without risking material shortage.
Guidance on identifying which component features can reasonably remain as-forged (accepting standard as-forged tolerance) versus which require machining to final tolerance, avoiding unnecessary machining cost on features where as-forged tolerance is functionally adequate.
Engineering drawing review identifying any as-forged tolerance callouts that exceed realistic forging process capability, flagging these for correction (either loosening the as-forged tolerance or converting the feature to a machined dimension) before tooling commitment.
Support for either applying standard published tolerance class values or agreeing custom project-specific tolerance requirements where a customer's application justifies tolerance tighter or looser than standard classification, documented in the purchase agreement.
A recurring source of friction between component designers and forging suppliers is a drawing specifying tight dimensional tolerance directly on an as-forged feature — a tolerance value that might be entirely reasonable for a machined dimension, but that the forging process itself simply cannot reliably achieve, given the fundamentally different physics governing hot metal forming compared to cold, tool-controlled machining. Understanding why this mismatch occurs, and how to correctly specify forged component drawings to avoid it, starts with recognizing that as-forged tolerance and machined tolerance are not the same category of precision and shouldn't be specified as if they were interchangeable.
As-forged dimensional variation arises from several sources inherent to the forging process itself: thermal contraction as the hot-formed component cools from forging temperature to room temperature, which varies somewhat with section thickness and cooling rate across the part; die wear accumulating gradually across an extended production run, which can shift achievable dimension slightly as tooling ages; and the material flow behavior of hot metal filling a die cavity, which behaves differently — and with somewhat more inherent variation — than the precisely controlled material removal machining represents. Standard tolerance classification systems like DIN EN ISO 13920 exist to formalize the achievable tolerance range these process characteristics actually produce, providing a recognized, standards-based reference rather than leaving tolerance specification to guesswork or unrealistic assumption.
The practical specification principle this understanding enables is genuinely simple, even though it's frequently overlooked: any dimension requiring tighter final tolerance than standard as-forged capability provides should be specified as a machined dimension, with adequate machining stock (allowance) left on the as-forged part to support reaching that tighter final tolerance through subsequent machining — rather than specifying the tight tolerance directly on the as-forged dimension itself, which sets up an unachievable requirement regardless of forging process quality or supplier capability. Conversely, features where standard as-forged tolerance is functionally adequate shouldn't be needlessly specified as machined dimensions either, since this adds unnecessary machining cost without functional benefit.
For component designers and purchasers seeking guidance on correctly specifying forged component dimensional tolerance — distinguishing as-forged from machined features, applying appropriate machining allowance, and selecting appropriate tolerance classification — Shivam Forge's engineering team provides drawing review and tolerance feasibility guidance. Contact us at +91-9265772827 or sales@shivamforge.com with your drawing for a manufacturability and tolerance review.
As-forged tolerance is the dimensional precision achievable directly from the forging process itself, without subsequent machining — inherently wider than machined tolerance due to thermal contraction variation, die wear, and material flow behavior in hot metal forming. Machined tolerance, produced through cold, tool-controlled material removal, can achieve significantly tighter precision, which is why tight-tolerance features are specified as machined dimensions rather than as-forged.
DIN EN ISO 13920 provides a standard classification system for achievable as-forged dimensional tolerance based on component size and forging process type, giving engineers a recognized reference for specifying realistic as-forged tolerance rather than arbitrary values that may not match actual forging process capability.
This depends on the specific feature's final tolerance requirement, the as-forged tolerance class applicable to that dimension, and the component's material and geometry — we recommend engineering review of your specific drawing to confirm adequate machining allowance is specified, since insufficient allowance risks a machined feature not cleaning up (removing all as-forged surface) after machining.
Specifying tighter tolerance than a dimension functionally requires typically increases cost — either forcing unnecessary machining on features that could remain as-forged, or requiring tighter die tolerance control that adds forging process cost — without delivering functional benefit. Matching tolerance specification to actual functional requirement, rather than defaulting to tight tolerance everywhere, is more cost-effective.
Yes — die wear across an extended production run can gradually affect achievable as-forged tolerance, which is why tolerance class selection and ongoing dimensional monitoring should account for this rather than assuming first-article tolerance capability holds indefinitely across the full production run.
Why Choose Shivam Forge
Shivam Forge delivers precision hot-forged components from our integrated Shapar, Rajkot facility — covering forging, CNC machining, heat treatment, and quality inspection under one roof.
