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How-tos for tolerance design

Dimensional tolerance design is crucial to creating components that fit together well and function properly. Overly “loose” tolerances yield too much manufacturing allowance, leading to mismeasured (and therefore often unusable parts — however, needlessly “tight” tolerances demand precision that may drive increases in cost, lead time, and complexity.

Mastering the process of designing for part tolerances can help ensure that:

  • Pieces fit together as planned, resulting in effective and beautiful final products
  • You’re not sinking costs on special, unnecessary labor requirements
  • Inevitable manufacturing errors will not render components useless
  • Natural environmental wear and tear will not impede functionality

There are many elements to consider when designing a component’s dimensional tolerance. Every step of the manufacturing process must be accounted for because everything from the material used to the order of assembly could contribute to part tolerance.

Tips for dimensional tolerance analysis

Dimensional tolerances help engineers understand the margin of manufacturing error that a component can face before becoming unusable. Tolerances are often measured in X, Y, and Z dimensions, but can be applied to any element, such as surface roughness and external temperature. A “fit” is a kind of tolerance that determines how components will fit into one another, such as a shaft fitting into a hole.

You can best determine dimensional tolerances and fits by taking the time to deeply consider each and every element of your component. Consider all the possibilities of a given part and how application-related factors, environmental factors, materials, and more might influence the part’s tolerance. Also, be sure to consider how a certain component’s manufacturing processes could affect the tightness or looseness of its tolerances.

Optimizing your dimensional tolerances will entail striking a desirable balance between performance and cost-effectiveness. Here are some suggestions to keep in mind while designing manufacturing tolerances:

Determine which manufacturing process you will use

Different manufacturing methods require — and be capable of achieving — different tolerances. Always adhere to design for manufacturability (DFM) best practices to ensure that part design is not only optimized for its end-use, but well-suited for the given manufacturing method.

Consider your materials

Different types of materials have differing levels of accuracy, consistency, and shrinkage rates. For example, brass is highly weldable but quite susceptible to stress corrosion cracking. Use a material selection chart and consult your manufacturer in order to select a best-fit material that will meet project requirements and suit all of your tolerances.

Take tolerance stacking into account

Tolerance stacking is the accumulation of tolerance over multiple related tolerances. If not taken into account, the buildup of many smaller tolerances may make it impossible to assemble two mating components.

tolerance design stacking
 The best way to avoid tolerance stacking is to create tolerances using solid points of reference.
Account for environmental conditions

An irregular manufacturing environment will result in more naturally occurring variation. To minimize deviation between components, ensure the manufacturing facility where your parts will be made meets regulatory standards and does not suffer from major fluctuations in temperature or humidity, which could impact manufacturing and part performance. Even more importantly, consider part applications and end-use environmental factors to assess potential degradation, discoloration, deformation, and more.

Be careful when applying tolerances to radius or diameter

A radius’s tolerance will double when measured as a diameter, so it may accidentally be looser or tighter than intended if its measurements are not properly checked. Ensure accuracy by treating dimensional tolerances as if they were continued zeroes, even if they are not.

Common design tolerance errors

Here are three common mistakes designers and engineers make when designing tolerances — and how to avoid them:

Don’t wait to create dimensional tolerances

Considering tolerances at the beginning of the  product development process can prevent time-consuming, costly redesigns. If you adjust for a  part’s specifications early on, you can accelerate production speed and time-to-market — especially if you enlist the help of an expert manufacturer like Fast Radius, who can work alongside you to optimize tolerances from day one.

Don’t over-tighten (or over-loosen) your tolerances

Balancing your tolerances is one of the most difficult aspects of tolerance design. With overly loose tolerances, it’s likely that many components will arrive misaligned and possibly dysfunctional. On the other hand, designing dimensional tolerances too tightly will unnecessarily increase manufacturing, material, and labor costs, which could cost you precious time and money.

Don’t leave out dimensional tolerances

If you leave out dimensional tolerances, your manufacturer will either assign their own tolerances or place a standard ±0.005 in. tolerance on the part. This will likely result in improperly sized pieces, which then have to be remanufactured correctly using proper tolerances.

tolerance design close
Dimensional tolerance design is crucial to creating components that fit together well and function properly.

Regulatory considerations

Following tolerancing regulations and guidelines offer parameters that help ensure you will design proper tolerances every time. These requirements and best practices — some of which are outlined below — can help you find the standard tolerance for a given part, discover formulas to aid the tolerance creation process, and properly account for all materials and production elements of a component.

  1. Geometric dimensioning and tolerancing (GD&T). This system helps standardize the language used to define dimensional tolerances, engineering intent, and other component requirements. GD&T also offers fundamental rules and practices of designing tolerances.
  2. American Society of Mechanical Engineers (ASME). ASME promotes unity among symbols, definitions, and rules for tolerance using GD&T. This helps ensure clear communication and understanding between designers, engineers, and manufacturers.
  3. The International Organization for Standardization (ISO). One of the most universal regulatory bodies, ISO provides standards for manufacturing tolerances. It’s important to note that ISO dimensional tolerance standards differ from ASME’s regarding angle projection, sheet size, and default title blocks and borders.
  4. International Tolerance grades (IT grades). These guides reference industrial processes defined in ISO, and identify how precise a tolerance can be for a dimension given its manufacturing process. IT grades offer a mathematical formula that can help calculate the tolerance for a specific IT grade.

Turn to these resources to ensure your tolerances are appropriate and compliant. Utilizing these tools could help prevent mismeasured tolerances, as well as offer further insight and understanding into the process of forming dimensional tolerances.

Improve tolerances with Fast Radius

Creating dimensional tolerances can be challenging, but employing proper foresight, tools, and tolerancing processes can help you design balanced tolerances that help ensure the long-term performance of your parts. However, defining tolerances requires assessing a range of complex factors — and can prove challenging, even for the most experienced product development teams. To rest assured that every part is well-toleranced and high-performing, consider teaming with a trusted manufacturing partner.

A leading on-demand digital manufacturer, Fast Radius helps you optimize dimensional tolerances in accordance with your chosen manufacturing method, materials, timeline, budget, relevant regulations, and unique project requirements. With a team of seasoned engineers, designers, and advisors, Fast Radius offers expert services ranging from design and prototype optimization to production and fulfillment. Contact us today. Let’s make your vision a reality.

To learn more about optimizing injection molding tolerances and other important engineering concepts like durometer and stress concentration, visit the Fast Radius resource center.

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