Specifying tolerance for a dimension

What is the rationale when defining a tolerance range for a dimension on the engineering drawing when manufacturing the component

Specifying tolerance for a dimension is a crucial aspect of engineering and manufacturing processes. Tolerance refers to the allowable deviation or variation in a specified dimension, ensuring that the final product meets the desired quality standards. This is particularly important in industries where precision is paramount, such as aerospace, automotive, and electronics.

When specifying tolerance, engineers define both upper and lower limits within which the dimension must fall to guarantee functionality and interchangeability of parts. Tolerance values are often denoted using geometric symbols, such as plus-minus (+/-), and are specified in units of measurement relevant to the dimension being controlled.

Tolerance not only accounts for inevitable variations in the manufacturing process but also helps optimize production costs by allowing for a certain level of imperfection without compromising the overall performance of the product. Effective tolerance management is essential for achieving the desired balance between precision and cost efficiency in the design and production of components.

Deciding the tolerance for a dimension is not a one off activity

It may be revisited during the design process especially for critical applications.

  1. First arrive at a tolerance range depending on the manufacturing process and known generic precision of the process. Machining , forging, casting or sheet metal forming
  1. Apply a generic tolerance from International tolerance grade table ISO 286
  2. If it’s a dimension related to a mating part then deciding the fit is an important step . Tolerances would be defined based on that fit
  3. If there are multiple parts in assembly . Tolerance stackup analysis would be necessary to ascertain if tolerances considered are Ok .
  4. Verify decided tolerance with process capability studies during actual production

Tolerance depends on

  • Manufacturing process
  • Functional requirements
  • Assembly - Assigning fits for mating parts.
  • Aesthetics - Fit and Finish 
  • Precision

Tolerance levels :

The international tolerance grade divides tolerance ranges in levels from IT1 to IT 18

IT1 being very closed tolerance and IT 18 being very open tolerance.

IT0 tolerance grades are generally used for Gauges design for measuring equipment.

Here is a table of IT grades

Common manufacturing processes mapped to Tolerance grades.

For a design engineer, designing a component and trying to arrive at a preliminary value of tolerance

The first step is deciding the manufacturing process . For example , whether the design geometry will be machined, forged or will it be manufacturing through sheet metal operations.

For a machined part the tolerance can be closed but sheet metal parts not so much .

Next step is to look up the Tolerance grade table (ISO 286 ) and select a tentative tolerance grade.

As the design matures and more information is available , the tolerance value maybe revised based on design intent or as an input from tolerance analyses which would be detrimental to function.

Then once initial pilot production starts and real time manufacturing data is available .

Based on the capability of the manufacturing process, the tolerance may further be changed

Specifying tolerances for dimensions is a critical aspect of engineering design, ensuring that manufactured parts meet functional requirements while accounting for variations in the manufacturing process. Several factors influence the selection of tolerance values for a given dimension:

Function and Fit Requirements:

The intended function of the part and the required fit with mating components play a crucial role in determining tolerance. Tighter tolerances may be necessary for precision components, while looser tolerances may suffice for non-critical features.

Assembly Requirements:

Tolerance specifications should consider the ease of assembly. If parts with tight tolerances are challenging to assemble or risk interference fits, it may be necessary to relax tolerances for practical assembly.

Manufacturing Processes:

The choice of manufacturing processes influences tolerance selection. Different processes, such as machining, casting, or injection molding, have inherent capabilities and limitations. Some processes may naturally achieve tighter tolerances than others.

Material Properties:

Material properties, including thermal expansion, can affect the dimensional stability of a part. Tolerance values need to account for potential material variations and the impact of environmental factors.

Cost Considerations:

Tighter tolerances often require more precise manufacturing processes and can increase production costs. Balancing the desired precision with cost considerations is essential to achieve an economical yet functional design.

Statistical Process Control:

Statistical methods, such as Six Sigma, may be employed to determine appropriate tolerance values based on process capability and the desired level of quality. Statistical process control helps ensure that most manufactured parts fall within the specified tolerances.

Critical Features:

Some features of a part may be more critical to its function than others. Critical dimensions may warrant tighter tolerances to ensure the overall performance and reliability of the product.

Industry Standards and Regulations:

Industry-specific standards and regulations often provide guidelines for tolerance selection. Adhering to these standards ensures compliance with industry norms and facilitates interoperability with components from different manufacturers.

Intended Use and Environment:

The operating environment and conditions under which the part will be used can influence tolerance specifications. For example, parts subject to extreme temperatures or harsh conditions may require specific tolerances to maintain functionality.

Design Intent:

The designer's intent and the purpose of the dimension within the overall design should guide tolerance selection. Understanding the criticality of each dimension helps prioritize where tighter or looser tolerances are appropriate.

Communication with Suppliers:

Collaboration with suppliers and manufacturers is crucial. Communicating design intent, performance requirements, and expectations regarding tolerances helps align the design with the capabilities of the manufacturing process.

Previous Experience and Lessons Learned:

Drawing on past experiences and lessons learned from similar projects or components can inform tolerance decisions. Knowledge gained from previous designs and manufacturing processes contributes to more informed decision-making.

In summary, specifying tolerances for dimensions involves a thoughtful consideration of multiple factors, including functional requirements, manufacturing processes, cost considerations, and industry standards. The goal is to strike a balance that ensures the part's functionality, manufacturability, and cost-effectiveness while meeting the overall objectives of the design.

To learn more about GD&T have a look at this course 

Geometric Dimensioning and Tolerancing: Basics

    Categories: GD & Tolerancing