How do you interpret and apply tolerances and deviations on a drawing?

1 Types of tolerances and deviations

Tolerances and deviations can be classified into different types, depending on their purpose, location, and expression. Geometric tolerances define the shape, orientation, and location of the features, for instance parallelism, perpendicularity, concentricity, or symmetry. Dimensional tolerances refer to the size or length of the features, such as width, height, depth, or diameter. Material tolerances are associated with physical or chemical properties of the materials, such as strength, hardness, or composition. Lastly, surface finish tolerances indicate the quality or texture of the surfaces, including roughness, flatness, or waviness. These are usually indicated by symbols and datums on the drawing, plus or minus values or a range of values.

2 How to interpret tolerances and deviations

To interpret tolerances and deviations on a drawing, you must first identify the type of tolerance or deviation and its location on the drawing. Look for symbols, datums, values, ranges, standards, or specifications that indicate the tolerance or deviation. Then, understand the meaning and implication of the tolerance or deviation by referring to relevant codes, norms, or manuals. After that, compare the tolerance or deviation with the nominal dimension, shape, or position of the element. Calculate the upper and lower limits of the tolerance or deviation to check if they are within the acceptable range. For example, if you see a dimensional tolerance of ±0.5 mm for a width of 100 mm, it means that the actual width can vary from 99.5 mm to 100.5 mm.

3 How to apply tolerances and deviations

To apply tolerances and deviations on a drawing, you need to use appropriate tools and methods to measure the actual dimensions, shapes, or positions of the elements - such as a caliper, micrometer, level, or laser scanner. Then, compare the measured values with the tolerance or deviation limits and determine if they comply with the drawing - you can use a gauge, template, chart, or software for this. Finally, record and report the results of the measurement and comparison - you can use a form, log, report, or database to document your data and communicate the findings and recommendations.

4 Best practices for tolerances and deviations

To ensure that tolerances and deviations are interpreted and applied correctly and consistently on a drawing, you should use clear and unambiguous symbols, values, and annotations to indicate the tolerances and deviations. Make sure the expression of the tolerances and deviations avoids confusion, contradiction, or redundancy. Additionally, use realistic and reasonable tolerances and deviations that reflect the design requirements, manufacturing capabilities, and quality standards. It’s also important to use common and standardized tolerances and deviations that conform to industry norms and regulations. Avoid arbitrary or custom tolerances and deviations that may cause confusion or inconsistency.

5 Benefits of tolerances and deviations

Tolerances and deviations are essential for construction drawings, as they ensure the design intent is met and that the elements perform as expected. These provide a margin of error that allows for variations in the fabrication and installation processes. Additionally, they maintain the quality and reliability of the elements, while also controlling costs and optimizing resources. Tolerances and deviations also specify an acceptable level of deviation that guarantees functionality and durability of the elements, while balancing accuracy with efficiency of the production and assembly processes.

6 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?