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Definition of a Feature
In GD&T, a feature is a surface or a group of surfaces. In most cases, a surface is something you can see or touch on an actual part. Surfaces are the boundaries between the part and empty space.
Understanding Features
Parts consist of features (surfaces). In the figure below, the part on the left has been decomposed or deconstructed into its constituent features. On most parts, like this one, features are the boundaries between the part and empty space around it. On one side of the surface is the material the part is made of (e.g. aluminum, ABS plastic, stainless steel), and on the other side is the environment the part exists in, such as our atmosphere, the vacuum of space for parts used in extraterrestrial environments, seawater for parts used under the ocean, etc.
Part Decomposed into its Constituent Features (Surfaces)
Feature in this context is part of the language of GD&T. It is jargon, technical language. Feature is a commonly used term in many environments. We use it in everyday discussions at home, and it is used in many technical disciplines. Thus, feature may mean something different to different people. In the context of product design and manufacturing, feature may mean something different to different people depending on what their job is. For example, a designer, a machinist, and an inspector may have different ideas about what a feature is. That is why we have a formal definition of feature in ASME Y14.5 standard. However, ASME Y14.5 doesn’t own the term feature. We don’t own the right to say we have the only definition. For these reasons, I sometimes use the term tolerancing features instead of feature when talking about GD&T in hopes it provides clarity.
Where is the Feature?
Note that for parts like a vessel that holds fluids, such as a gas tank on a car, there are features (surfaces) on the inside and the outside. In such a case, these may be both sides of formed sheet metal. On other parts, there may be features that are truly inside the part, that have material on both sides of a surface. For example, consider a bimetallic strip used in a thermostat, which consists of copper and steel strips bonded to one another. The interface between the strips represents distinct surfaces (features) within the part, but the surfaces have material on both sides, copper on one side and steel on the other. Another example is overmolded parts, which is common on many consumer products, such as hand tools. A common technique is overmolding, in which a part is embedded in and/or coated by another material. Screwdrivers and pliers are common examples. Another more recent example is in additive manufacturing where surfaces of different materials, structures, or densities are printed adjacent to one another. Thus, while most features exist on the boundaries of a part, some features are internal.
Overmolded Tools with Internal Features
Features: From Simple to Complex
There are many types of features in GD&T because there are many geometric shapes that are used in products. For example, a surface may be a simple flat surface, a cylinder, two parallel planes, a sphere, a cone, a wedge, a torus, a linear extruded shape like hex socket or Allen wrench, or a complex surface like an automobile fender, an airplane wing, or the ergonomic surface of a computer mouse. See the examples below.
Types of Features – From Simple to Complex (from top to bottom)
In a general sense, we can separate features into two broad categories: features of size and other types of features. Features of size are important because the envelope principle only applies to features of size, which means that if a feature is a feature of size, then the feature must conform to an additional requirement, the envelope principle, that does not apply to other features.
Planar Surface The simplest feature is a planar surface.
Features of size Features of size are the next simplest feature. A feature of size consists of a surface that contains opposed points, such as a cylinder, a sphere, or opposed parallel planes. In the ASME Y14.5-2009 we changed the name of features of size to regular features of size. This is because we identified a new class of features that are similar to features of size but do not meet the criteria to be a regular feature of size.
Two criteria must be met to be a regular feature of size. To be a regular feature of size:
The feature must have one of three shapes: a cylinder, a sphere, or opposed parallel planes (e.g. key or keyway)
AND
The size of the feature must be dimensioned a certain way: the size of the cylinder, sphere, or opposed parallel planes must be defined by a directly-toleranced dimension (e.g. limit dimension or dimension with ± size tolerances).
Features of size are very common on parts. Cylindrical holes and pins, keys and keyways are common examples, and they are very important in machinery. Features of size are so common and so important that they are treated differently than other types of features. Features of size are the only type of feature that can be rigorously defined by a directly-toleranced dimension. The rigor comes from the envelope principle.
Features of Size (Regular Features of Size) – Inch
Features of Size (Regular Features of Size) – Metric
Opposed Points A pair of points that have opposed surface normal vectors. A surface normal vector points away from a surface into space, and the vector is perpendicular (normal) to the surface at that point. Points are opposed if their surface normal vectors are collinear (e.g. they lie on the same line) and point in opposite directions.
Opposed points for shaft and block in the figure above are shown in the following figure. Opposed points are shown in black and points that are not opposed are shown in red. Note that the upper right corner of the block is rounded. Thus, all points along the 1.50 ±.02 width and 1.50 ±.02 height of the block do not include opposed points. In the following figure, the surface normal vectors are clearly not collinear along lines that intersect the radius and the opposite surface. This is okay. It is very common for features of size to include areas in which there are no opposed points. The radiused area is a good example of this. Consider if there was a keyway in the shaft. The points along the cylinder opposite the keyway would not be opposed. As long as there are enough opposed points on the feature to function, and if it meets the criteria above, the feature may be considered as a regular feature of size.
Opposed Points and Points That Are Not Opposed
Examples: Features of size
Features of Size (Regular Features of Size) – Inch
The features in the figure above have the correct nominal shape, cylinder, sphere, or opposed parallel planes AND their size is defined by a directly-toleranced dimension. Thus, these are regular features of size, and the envelope principle (Rule #1) applies to these features.
Features of Size (Regular Features of Size) – Metric
The features in the figure above have the correct nominal shape, cylinder, sphere, or opposed parallel planes AND their size is defined by a directly-toleranced dimension. Thus, these are regular features of size, and the envelope principle (Rule #1) applies to these features.
Examples: Features That Are Not Regular Features of Size
Note that the following figures include the same geometry as the examples that define regular features of size. Thus, the features have the correct shape, e.g. the features are cylindrical, spherical, or opposed parallel planes. However, the features are not defined by a directly-toleranced dimension, or their size is controlled by another source, or the “size” of the feature is not defined by a single size dimension.
Each of the features that were defined as a feature of size in the previous examples are number 1 – 6 in the following figurse. See the explanations after the figure for the reason the dimensioned feature is not a feature of size. Note, the only reason we care if a feature is a regular feature of size is that the envelope principle (Rule #1) only applies to regular features of size.
Features That Are Not Regular Features of Size – Inch
Features That Are Not Regular Features of Size – Metric
The following explanations apply to the Features That Are Not Regular Features of Size for inch and metric above.
Shaft diameter: Cylindrical feature. The shaft diameter is defined as a stock dimension. This means that the size and the form of the feature are controlled by the stock supplier or manufacturer. For example, it is common to purchase ¾” round rod or shaft from a supplier, such as American Steel or Ryerson. The manufacturer, a steel mill in this case, provides the tolerances for the stock material. Often, the stock material complies with a standard specification, such as ISO or ASTM. Thus, the dimension on the drawing is similar to a reference dimension. It clarifies that the feature’s diameter is controlled by another source, not by dimensions and tolerances on the drawing.
Shaft length: Opposed parallel planes. The ends of the shaft are defined by a basic dimension. A flatness tolerance applies to the flat surface on one end of the shaft, and a profile of a surface tolerance applies to the flat surface on the other end of the shaft. The ends of the shaft must conform to the specified geometric tolerances, but the length of the shaft is not defined as a regular feature of size.
Block thickness: Opposed parallel planes. The thickness of the block is defined by a basic dimension. A profile of a surface tolerance, specified with “2X”, applies to the flat surfaces on both sides of the block. The sides of the block must conform to the specified geometric tolerances, but the thickness of the block is not defined as a regular feature of size.
Block height: The height of the block is defined by a reference dimension. That means that the dimension does not control the height of the block, as the height of the block is defined somewhere else. Even if the reference dimension included a tolerance within the parentheses, it still would not control the height of the block. Reference dimensions are FYI only, they are not contractually binding.
Block width: Opposed parallel planes. The width of the block is defined by a chain of two directly-toleranced dimensions. Thus, the width of the block is not defined as a size – it is defined as the sum of two distances. Note that while we show examples of chained dimensions in ASME Y14.5, we do not explain what the chained tolerances mean. Thus, the deviation allowed between the edges of the block and hole and the overall deviation allowed across the width of the block are unclear and ambiguous.
Hole diameter: Cylindrical feature. The hole diameter is defined using a radius dimension instead of a diametral dimension (a dimension value preceded by a diameter symbol). In ASME Y14.5 we only define radii as two-dimensional arcs – we do not explain what a cylindrical hole defined by a radial dimension and tolerance means. That is, we do not explain which requirements are defined by such a specification and thus the conformance criteria for the specification are unclear.
Radii (R, CR, SR)
Fillets and Rounds
Chamfers
Holes
Hole sets
Keys and Keyways
etc.