What are some tips and best practices for adding tolerances and fits to your part drawing?

1 Know the standards

Depending on your industry, location, and customer, you may need to follow different standards for tolerances and fits. Some of the most common ones are ISO, ANSI, ASME, and DIN. These standards define the symbols, notation, terminology, and methods for specifying tolerances and fits. You should familiarize yourself with the relevant standards and use them consistently throughout your drawing.

2 Choose the right fit

Fits are classified into three categories: clearance, interference, and transition. Clearance fits allow some clearance between the mating parts, interference fits require some force or heat to assemble the parts, and transition fits can have either clearance or interference depending on the actual dimensions. The choice of fit depends on the function, assembly, and performance of your part. For example, clearance fits are suitable for parts that need to rotate or slide, interference fits are suitable for parts that need to transmit torque or resist separation, and transition fits are suitable for parts that need to align or locate.

3 Use the minimum tolerance

Tolerances indicate the acceptable range of variation in the dimensions of your part. The smaller the tolerance, the more precise and accurate your part is. However, smaller tolerances also mean higher manufacturing costs, more inspection time, and more potential for errors and rejects. Therefore, you should use the minimum tolerance that meets the functional requirements of your part. You can use tolerance tables, charts, or calculators to find the appropriate tolerance for your fit and dimension.

4 Apply tolerances logically

Tolerances can be applied in different ways, such as limit dimensions, plus/minus dimensions, geometric tolerances, or statistical tolerances. You should choose the method that best conveys your design intent and minimizes ambiguity and confusion. You should also apply tolerances logically and consistently, avoiding unnecessary or contradictory tolerances. For example, you should not use both limit dimensions and plus/minus dimensions for the same feature, or use a tolerance that is larger than the fit allowance.

5 Use datum references

Datum references are points, lines, or planes that serve as the basis for measuring and locating other features. They help to establish the orientation, alignment, and position of your part in relation to the mating part or the coordinate system. You should use datum references to define geometric tolerances, such as flatness, parallelism, perpendicularity, or concentricity. You should also use datum references to control the cumulative effect of tolerances, such as runout, total runout, or position.

6 Check your drawing

Before finalizing your drawing, you should check it for accuracy, completeness, and clarity. You should verify that your tolerances and fits are correct, consistent, and compatible with the standards, the function, and the manufacturing process of your part. You should also check that your drawing is easy to read, understand, and interpret by others, such as machinists, inspectors, or customers. You should avoid errors, omissions, or discrepancies that could lead to miscommunication or mistakes.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?