Dimensional Specifications
Dimensional specifications are often expressed in standardized terminology and conventions. Geometric Dimensioning and Tolerancing (GD&T) is one way to communicate engineering specifications.
Geometric Dimensioning is the description of the physical measures of an object using standardized terminology and symbology. Most people understand the concept of measuring a dimension of a physical object, so we won’t elaborate here.
Tolerancing is a derivative of tolerance. Tolerance is the acceptable deviation around a target value. Generally, there will be an upper limit (at or above the target) and a lower limit (at or below the target) indicated in a tolerance specification.
Tolerance
The importance of tolerance is best illustrated looking at its impact on two interfacing components.
Every standardized part will have target dimensions. However, there will always be a deviation from the target values. Some degree of deviation is to be expected and the allowed deviation is typically stated in a specification to ensure compatibility and interchangeability with other parts.
When you have two or more interfacing components, the actual dimensions of each part can affect how well those parts mate. The dimensions of interfacing parts will consider the dimensions of one another. The tolerance of the interfacing parts should consider the impact on one another. When you combine dimensional deviation of two parts, the deviation of each part can either complement or compound with the deviation of the other; a concept we will call tolerance stacking.
If we use an example of a pipe and a slip fitting, we can see how the tolerance/deviation of each component is important.
If the pipe is undersized and the fitting is oversized, the pipe will fit loosely in the fitting. Depending on the amount of deviation from the target (tolerance) for each part, the connection may leak.
If the pipe and fitting were exactly at the target dimensions (or were equally undersized or oversized), the connection would be tighter. It would be less likely to leak.
If the pipe was oversized and the fitting was undersized, you might have a hard time fitting the two parts together, but it probably wouldn’t leak (assuming you can get them together).
The tolerances of interfacing parts must be carefully coordinated to ensure a suitable fitment.
- Not Too Loose: The gaps between the interface must not be too significant, as this would result in a loose fit, leaking, and/or rattling, as applicable.
- Not Too Tight: If there is too much overlap of tolerances, there is a higher likelihood that the parts will not fit together.
Diagram Symbology
Engineering drawings are typically presented as a Geometric Dimensioning and Tolerance (GD&T) diagram. A GD&T diagram will generally have a wireframe drawing of the component, with callouts of specifications to particular features. Features with dimensional requirements may have requirements presented in feature control frame. This object uses the following convention:
Many symbols may be used in a GD&T diagram. All of them are important for a fabricator or manufacturer. However, many of the corresponding specifications require highly specialized equipment to test. As such, they are not practical for the average Joe to check. Most of the symbols used in a feature control frame are beyond the scope of evaluation for us, as consumers. For completeness, we have included a link to the following table on Scribd to help navigate the symbols and conventions that you may see in a GD&T diagram:
We don’t see many of these conventions used in the TDP. Of those that are used, we are only interested in some of them (as consumers looking to verify dimensions).
Let’s use an example diagram from the TDP to highlight the parts of a typical diagram that are of interest to us.
A: This is a callout to another part of the diagram. Typically, this will be used to show additional detail of a section, especially if there are several subtle parameters to define.
B: This shows the diameter (Ø .380) in the diagram units with non-uniform tolerance (notice the + .005 and – .000). We read this as this dimension should be 0.380″, may not be smaller than 0.380″, and may be up to 0.385″ in diameter.
C: This shows the depth (.100) in the diagram units with uniform tolerance (± .003). Length will also be reflected in the same way. We read this as this dimension should be 0.100″, may not be smaller than 0.097″, and may be up to 0.103″ in length.
D: This shows diameter (Ø .157) in the diagram units with uniform tolerance (± .001). We read this as this dimension should be 0.157″, may not be smaller than 0.156″, and may be up to 0.158″ in diameter.
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