For manufacturing engineers, purchasing the official ISO 20457:2018 PDF from your national standards body is an essential investment. The standard belongs on every plastics engineer’s reference shelf alongside ISO 2768 (for machined components) and ISO 22081 (for surface profile tolerancing).
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The heart of the ISO 20457 tolerance system is its classification into . The standard defines nine tolerance grades, ranging from TG1 (very precise) to TG9 (very coarse). The appropriate tolerance grade depends on the part’s functional requirements, material properties, and manufacturing process capabilities. iso 20457 tolerance table pdf
An important distinction exists between and ISO 22081 . While both standards address general geometrical specifications, they serve different purposes. ISO 22081 specifies general geometrical tolerances using surface profile as a geometric feature—either without datums or with a datum system that blocks all non-redundant degrees of freedom. By contrast, ISO 20457 provides a distance-dependent tolerance zone width approach specifically designed for plastics, recognizing that shrinkage effects cause dimensional variation proportional to the distance between features. However, as noted by tolerance experts, ISO 20457 does not fully implement the distance-dependent approach consistently throughout the standard.
: The industry standard for most technical plastic parts with moderate fit requirements. Class C (Low Precision) Share public link The heart of the ISO
The ISO 20457 tolerance table, found within the standard’s PDF, is a cornerstone of modern geometric product specification. It translates the abstract concept of “general tolerance” into specific, actionable numbers for linear dimensions, angles, flatness, straightness, perpendicularity, and runout. By organizing these values into four clear classes (f, m, c, v), the standard balances manufacturing economy with functional assurance. For any design engineer or manufacturing professional, understanding how to read and apply the tables in ISO 20457 is not a niche skill—it is a fundamental practice that turns a drawing into a reliable, producible product.
Word spread the way things do in small towns: through a customer with a recommendation, an apprentice who saw the coupling run smoother than anything they'd seen in months. Orders returned—one-off repairs at first, then short runs for local manufacturers who needed a nimble supplier. People liked that Ana could promise tolerances and deliver parts that met them. They liked that she paid attention to the subtle language of standards. The ISO 20457 table, once just a relic, became the backbone of her shop's revival. An important distinction exists between and ISO 22081
While the full PDF document outlines extensive data across different nominal dimensions, the table below provides a simplified overview of standard dimensional tolerances (± in mm) for common size ranges across typical Tolerance Groups. Nominal Dimension (mm) TG3 (Precision) TG4 (Fine) TG5 (Medium) TG6 (Standard) TG7 (Coarse) 1 to 3 3 to 6 6 to 10 10 to 18 18 to 30 30 to 50 50 to 80 80 to 120 120 to 180 180 to 250
But success brought scrutiny. A larger supplier, threatened by the renewed local capacity, tried to undercut them with cheaper components made overseas. For a while, price pressure cut deep. Ana remembered the reasons she nearly closed years ago—lost contracts, unpaid bills, the slow math of survival. She considered compromise. Could they loosen tolerances to chase lower cost? The tolerance table was still on her wall, a quiet reminder of why standards mattered. She called the team together.
: Engineering drawings often include the note: General tolerances ISO 20457 – TG6 . ISO 20457 Tolerance Table Overview