Basics of tolerancing and geometric dimensioning

Tolerance limits before GD&T was introduced

Before the introduction of GD&T, production characteristics were defined by X-Y areas. For example, when drilling a mounting hole, the hole had to be in a specific X-Y area. The exact tolerance definition, however, specified the position of the hole in relation to the intended position, with the accepted area being a circle. The X-Y tolerance left a zone where inspection could give a false negative because although the hole was not in the X-Y rectangle, it would be within the circumscribed circle. The new system became the military standard in the 1950s. Currently, the GD&T standard is defined by the American Society of Mechanical Engineers (ASME Y14.5-2018) for the USA and ISO 1101-2017 for the rest of the world. It mainly deals with the overall geometry of the product, while other standards describe specific features such as surface roughness, texture and screw threads.

Why is it worth implementing GD&T processes?

For functional assemblies, multi-part products, or parts with complex functionality, it is crucial that all components work well together. All relevant fits and features must be specified in a way that minimizes the impact on the manufacturing process and associated investments while ensuring functionality. Increasing tolerance by a factor of two can increase costs by twice or even more due to higher rejection rates and tool changes. GD&T is a system that allows designers and inspectors to optimize functionality without increasing costs.

The most important benefit of GD&T is that the system describes the design intent, not the resulting geometry. Like a vector or pattern, it is not an actual object, but a representation of it. For example, a feature standing at a 90-degree angle to the base surface may be tolerated to be perpendicular to that surface. This will define two planes separated from each other in which the central plane of the feature must fit. Or when drilling a hole - the most important thing is to tolerate it in terms of alignment with other features.

Describing a product's geometry in relation to its intended functionality and manufacturing approach is ultimately simpler than having to describe everything in linear dimensions. It also provides a communication tool with suppliers, customers and quality controllers.

When GD&T is done correctly, it even allows for statistical control of processes, reducing product rejection rates, assembly failures and quality control effort, saving companies significant resources. As a result, different departments are able to work more in parallel because they have a common vision and language of what they want to achieve.

How does GD&T work?

Engineering drawings must show the dimensions of all features of the part. Next to the dimensions, a tolerance value must be specified with a minimum and maximum allowable limit. Tolerance is the difference between the minimum and maximum limits. For example, if we have a table that we accept with a height of 750 mm to 780 mm, the tolerance will be 30 mm.

However, the table tolerance implies that we will accept a table that is 750 mm high on one side and 780 mm on the other, or has a corrugated surface with a variation of 30 mm. To properly tolerate the product, we need a symbol that communicates the design intent of the flat top surface. Therefore, we must include an additional flatness tolerance in addition to the overall height tolerance.

Similarly, a cylinder of a tolerated diameter will not necessarily fit in its bore if the cylinder is slightly bent during the manufacturing process. Therefore, it also needs straightness control, which would be difficult to communicate using traditional plus-minus tolerances. Another example would be a pipe that must fit perfectly into the complex surface to which it is welded - requiring control of the surface profile.

Tolerancing is the determination of appropriate deviations for all specific design features to maximize the product acceptance rate within the boundaries of the manufacturing processes, depending on the visual and functional purpose of the part.

In the metric system, there are international tolerance classes (IT) that can also be used to specify tolerances using symbols. For example, the symbol 40H11 indicates a 40 mm diameter hole with a loose fit. The manufacturer then only needs to check the hole feature master table to obtain the exact tolerance value.

In addition to individual tolerances, engineers must consider system-level effects. For example, when a part comes out at the maximum allowable dimensions, does it still meet overall requirements such as product weight and wall thickness? This is called the maximum material condition, while its counterpart is the minimum material condition.

Tolerances also add up. If we create a chain link where each hole has a tolerance of plus 0.1 mm and each pin has a tolerance of minus 0.1 mm, this means we will still accept a difference in length of 20 mm with 100 links. When installing repeating features, such as a perforated hole pattern, first set the pattern and then determine the mutual distances, rather than relating to a fixed edge or plane of the part.

The standards apply not only to designers and engineers, but also to quality inspectors, telling them how to measure dimensions and tolerances. The use of specific tools such as digital micrometers and calipers, height gauges, surface plates, dial indicators, and coordinate measuring machines is important in the practice of tolerancing.

When measuring and defining parts, geometry exists in a conceptual space called a coordinate system reference. This is comparable to the coordinate system in 3D printing software. A coordinate system reference is a point, line, or plane that is used as the starting point for measurements. Make sure you define reference features appropriate to the functionality of your part. Unless you are matching the characteristics of one part to the characteristics of other parts in the assembly, you can often use a single reference. Always make sure your main reference has a reliable location for outputting other measurements, for example where the final part will have little unpredictable variability.

Guidelines for Tolerating GD&T

Engineering drawing must accurately show the product without adding unnecessary complexity or limitations. The following guidelines are helpful to consider:

• Clarity of the drawing is most important, even more than its accuracy and completeness. To improve clarity, place dimensions and tolerances outside part boundaries and apply them to visible lines in real profiles, use a unidirectional reading direction, convey part function, group and/or space dimensions, and use white space.
• Always design to the tightest tolerance possible to reduce costs.
• Use the overall tolerance defined at the bottom of the drawing for all part dimensions. The specific tighter or looser tolerances specified in the drawing will then override the general tolerance.
• Tolerate the functional features and their interrelationships first, then move on to the rest of the parts.
• If possible, leave the GD&T work to manufacturing experts and do not describe manufacturing processes in a technical drawing.
• Do not specify an angle of 90 degrees as this is assumed automatically.
• Dimensions and tolerances are valid at 20°C / 101.3 kPa unless otherwise stated.

GD&T symbols

Each feature of GD&T is determined by different elements. GD&T symbols are divided into the following groups:

Shape elements define the shape of features, including:

• Straightness is divided into line element straightness and axis straightness.
• Flatness means straightness in many dimensions, measured between the highest and lowest points on a surface.
• Roundness or circularity can be described as straightness curved in a circle.
• Cylindricality is basically flatness curved into the shape of a cylinder. It includes straightness, circularity and conicity.

Profile elements describe the three-dimensional tolerance zone around the surface:

• The line profile compares a two-dimensional cross-section to an ideal shape. The tolerance zone is defined by two offset curves unless otherwise specified.
• The surface profile creates two offset surfaces between which the feature surface must fit.

Orientation elements refer to dimensions that vary by angle, including:

• Angularity is the flatness at an angle to a reference and is determined by two reference planes separated by a tolerance value.
• Perpendicular means flat at 90 degrees to the reference. Specifies two ideal planes between which the feature plane must fall.
• Parallel means straightness with respect to distance. Parallelism for an axis can be determined by defining the tolerance zone as cylindrical, placing a diameter symbol in front of the tolerance value.

Location elements define feature locations using linear dimensions:

• Position is the location of features in relation to each other or to references and is the most commonly used element.
• Concentricity compares the location of the feature axis to the reference axis.
• Symmetry ensures that non-cylindrical parts are similar on both sides of the reference plane.

Rapid prototyping and production of parts using 3D printing

In this article, we have discussed Geometric Tolerancing and Dimensioning (GD&T), which is of great benefit to designers and engineers working on complex products where dimensions must be tightly controlled.

GD&T conveys not only linear dimensions but also the design intent, which helps to clearly communicate the engineering design to project stakeholders. GD&T encourages designers to optimally tolerate their parts for their chosen manufacturing process, as different manufacturing techniques bring different characteristic variations.

Companies from industries such as aviation, automotive, medicine and many others are implementing digital production tools. 3D printing is a catalyst for efficiency, giving workers - from production engineers to tool operators - the tools to streamline supply chains, improve production and get products onto the market faster market – saving significant amounts and weeks or months of time.