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GD&T (Geometric Dimensioning & Tolerancing) is the modern drafting standard for creating unambiguous documentation of parts. In contrast to traditional tolerancing methods that give acceptable deviation as scalar values attached to measurements, GD&T applies tolerances as deviations from an ideal geometric shape each feature is attempting to achieve. This feature-based tolerancing scheme communicates part intent clearly and improves part acceptance rate through consistent inspection methods.
In a traditional tolerance scheme, all dimensions have a scalar tolerance following the dimension value (±0.01, ±0.02, +0.02/–0.01).
This includes dimensions that define the position of a feature.
The ±0.01 tolerance on the left hole's position compounds with the +0.02/–0.01 tolerance on the right hole's position.
The right hole's position from the left edge of the part may vary as much as –0.02 to +0.03 (total length of 0.05), rather than the +0.02/–0.01 (total length of 0.03) implied by its tolerance.
Image credit to www.mcgill.ca
Geometric Dimensioning & Tolerancing only puts tolerances on FEATURES, using boxes called Feature Control Frames (FCFs).
Dimensions that control the position of features are treated as perfect and do not receive a tolerance.
The 0.25 diameter tolerance on the position of each hole is independent of the behavior of any other holes.
The boxed 12, 26, and 76 positional dimensions define theoretically exact center points, from which are where the tolerances are measured (instead of another feature).
Adapted from ASME Y14.5-2018, p.195
The GD&T method achieves higher precision and clarity by defining each feature relative to a datum reference frame (DRF), which is a theoretical construct that constrains all six degrees of freedom for the part. Defining every feature directly from the DRF removes the potential for compounded tolerances when defining features off of one another, as in traditional tolerancing. The DRF has a hierarchy, wherein primary, secondary, and tertiary datum features each constrain fewer degrees of freedom than each feature before it. This hierarchy communicates which features are most important to the design, and aids in the inspection process by implying an order in which to position the part.
The concept of the Datum Reference Frame (DRF).
The concept of the Datum Reference Frame (DRF).
XY, YZ, and XZ planes define position of features along the X, Y, and Z axes. The U, V, and W rotational angles define orientation of features about the X, Y, and Z axes.
Adapted from ASME Y14.5-2018, p.85
A complete design using GD&T principles requires three main documents:
Design document drawing(s). These define the functional requirements and properties of the product. A good design document will include things like acceptable tolerances and loads, computer models and views of the product, how the product will be used, etc. Good design document drawings improve the performance of the produced item.
A manufacturing process plan. This defines how to produce the product. A thorough plan will include all stock materials and their sourcing vendor/location, a step-by-step list of actions to perform and the machines/tools/processes involved, etc. A good manufacturing plan improves the consistency of the produced item.
Quality dimensional measurement plan. This defines how to verify the produced item meets the functional requirements. A measurement plan should describe the tools needed in measurement (dial gauge, inspection fixture, go/no go gage) and how to calibrate them, how to place the part in the jig, acceptance criteria, etc. A good quality dimensional measurement plan improves the defect rate and confidence in the produced item.
Flowchart of the relationships between design, manufacturing, and inspection.
Credit to www.geotol.com
Example (Position Tolerance of a Hole)
Example (Position Tolerance of a Hole)
The classic example of GD&T's value is positional tolerancing for holes. Traditional XY tolerancing relies on defining the positions of features as linear ranges along the X and Y axes of the drawing plane. This results in square or rectangular tolerance zones in which the feature reference point can sit. However, the nature of a rectangle's diagonals in relation to its sides means the maximum deviation of the feature from the ideal position is greater than the tolerance values.
A square tolerance zone (orange) and a circular tolerance zone (blue) of equal size. As the direction of deviation approaches 45°, the square zone becomes more tolerant of change in the dot's position (red lines), while the circle enforces the same tolerance regardless of angle (yellow lines).
Therefore, the circular tolerance zone is equally strict and understood in all directions of deviation. The square tolerance zone is more lax and unclear the deviation approaches a 45° angle.
Animation made by me in Aseprite!
To address this, GD&T instead uses—as the "G" and "T" imply—Geometric Tolerancing, where the tolerance is clarified further with geometric shapes. If the intention of a designer is to allow a hole to deviate from an ideal position by a distance of T, then a circular tolerance zone is more appropriate than a rectangular one. By definition, a circle is the set of all points in a plane each the same distance from a reference point. The ability to define the tolerance zone as a circle rather than a rectangle means the tolerance value accurately describes the needs of the design, and will allow for more parts to pass inspection.
Comparison of the different tolerancing methods. Drafting notation on the left, and a tolerance zone analysis on the right.
Converting the GD&T tolerance zone to its traditional XY tolerance equivalent.
To achieve the level of precision found in GD&T while using an XY tolerance scheme, the tolerances must be significantly tighter (reduction in side length from overall square at top to white square).
The black regions of the GD&T representation are all portions of the XY tolerance zone which would place the hole out of tolerance. In total, these regions account for 21.5% of the XY zone.
For an example of GD&T in action, check out my full design example where I bring a door hinge bracket concept from idea to a working product!
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