ISO 8015 – Fundamental Tolerancing Principle (Independency)

Understand ISO 8015 and the independency principle. Learn how tolerances are applied independently in ISO standards and avoid common GD&T mistakes.

Overview of ISO 8015

ISO 8015 defines the fundamental tolerancing principle within the ISO GPS system, known as the independency principle. It states that each specification on a drawing applies independently unless otherwise explicitly stated.

This principle is critical for interpreting ISO standards like ISO 1101 and ISO 5459. However, it is also one of the most misunderstood concepts in engineering, often leading to incorrect assumptions about how size and geometry interact.

Core Principle

Principle

Description

Impact

Independency Principle

Each requirement (size, form, orientation, etc.) is independent unless specified otherwise

Prevents unintended constraints and ensures flexible tolerancing

Core Concepts

Independency Principle

Technical Product Specification (TPS)

Each tolerance requirement is applied separately, without assuming any relationship between size and geometry.

Purpose: Ensures clear, predictable interpretation of specifications.

Each tolerance requirement is applied separately, without assuming any relationship between size and geometry.

Purpose: Ensures clear, predictable interpretation of specifications.

Specification Independence

ISO GPS Integration

Size tolerances do not automatically control geometric form unless explicitly defined.

Purpose: Avoids hidden constraints that can affect manufacturing.

Size tolerances do not automatically control geometric form unless explicitly defined.

Purpose: Avoids hidden constraints that can affect manufacturing.

Modifiers & Exceptions

Model-Based Definition (MBD)

Special symbols or notes can override independency when needed.

Purpose: Allows designers to define relationships when function requires it.

Special symbols or notes can override independency when needed.

Purpose: Allows designers to define relationships when function requires it.

High-Cost Errors

Misunderstanding the independency principle leads to incorrect inspection and rejected parts.

Assuming Size Controls Form

Engineers often assume tighter size tolerances improve geometry, which is not true under ISO rules.

Ignoring Explicit Modifiers

Missing or misinterpreting modifiers can result in incorrect tolerance application and inspection failure.

Failing to apply linked ISO standards results in incomplete or incorrect specifications.

How to Apply ISO 8015

Understand Independency

Recognize that each requirement is separate unless specified otherwise.

Apply ISO Standards Correctly

Use ISO 1101 and related standards with the independency principle in mind.

Avoid Assumptions

Do not assume relationships between size and geometry.

Use Modifiers When Needed

Explicitly define dependencies if function requires it.

Validate Interpretation

Ensure drawings are interpreted consistently across teams and suppliers.

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Comparison & Related Standards

ISO 8015

A principle-based standard defining how tolerances are interpreted independently in ISO systems.

ASME Y14.5

A rule-based system where certain relationships (like envelope requirements) are implied unless modified.

ISO 1101

Defines geometrical tolerancing used alongside limits and fits.

ISO 2768

Provides general tolerances applied alongside ISO principles.