A question we are often asked here at GD&T Basics is, “what is a basic dimension?” – These dimensions are often fairly confusing as they look very similar to other dimensions you may be familiar with.
There are three different types of dimensions on prints: Reference Dimensions, Tolerance Dimensions, and Basic Dimensions. We’ll review brief descriptions of each and then dive deeper into Basic Dimension.
A Reference Dimension is a dimension given to any features that are just for reference – or visualization purposes. These are called out on a drawing using parenthesis (i.e., 5.125). It also could be a dimension that is described on another drawing or elsewhere. Either way, the dimension does not get inspected. Think of a Reference Dimension like a foot-long sandwich – sure it’s called a “foot long” but, that’s just a close-enough referenced size, so you know how much food you are getting.
A Tolerance Dimension is a dimension used if a feature’s size needs to be called out or to specify the location or something without applying Geometric Dimensioning and Tolerancing principles to it. They are typically given a plus/minus “±” tolerance but can have asymmetric limits as well. Most commonly, the nominal value and the tolerance to that dimension are shown right next to each other like this: 5.125 ± .125. This means the dimension can go from 5.000 to 5.250 – and could even be written like this:
Unlike the other two types of dimensions, a Basic Dimension is a theoretically exact dimension defining the ideal exact point in space where something needs to be. These dimensions are placed in boxes and never have a tolerance following them. As you could guess, though, making something hit an exact location or dimension would be impossible for anything that we make.
A basic dimension of .375:
This is why we need something else to give the dimension a tolerance – and this is where our GD&T symbols come in. Instead of having a dimension listed like 5.125 ± .125, you can think of the basic dimension like the nominal value (5.125), and the Geometric Symbol sets the tolerance (±.125) for the dimension.
Another way to think about basic dimensions is that they are the dimensions of your perfect CAD model – the ideal state of your part. It is only when your CAD model is put on a drawing that you then give it the manufacturing tolerances.
Basic Dimensions are typically used within the GD&T framework to control the location or geometry of features. The best example of when basic dimensions are used is when specifying True Position. Take a look at this drawing below:
The basic dimensions are those dimensions in the boxes – the 30 and the 15. They do have tolerances, though, as these are set by the Ø0.05 position tolerance in the feature control frame for the 4 holes. These basic dimensions are giving the exact location that the ideal perfect axes of the holes; essentially, this is the CAD model dimension. So, the CAD location of the upper left hole would be exactly 15 from Datum C and 45 from datum B. (You can add the 30 and 15 together since they are exact to get 45.) However, it doesn’t matter how good your manufacturing process is; you will never hit that exact location consistently. The Position tolerance of Ø0.05 is giving the tolerance for how far the manufactured hole could be from its perfect/ideal location.
Here are a few final things to keep in mind with Basic Dimensions:
- Basic Dimensions are essentially the perfect target you are trying to hit without the tolerance on the dimension itself. The tolerance for how far off this perfect target you can be is in the feature control frame for the given feature.
- They do not have tolerances by themselves – therefore, they must be accompanied by a geometric tolerance if they are specifying the size, location orientation, or form of a feature.
- They can also be used to specify the exact measurement points for securing a part for inspection. In this case, the designer is specifying the exact measurement locations but leaving the tolerance of these up to the gage tolerances (which usually have very tight tolerances anyway).
- For locating the True Position of a feature, you must use basic dimensions – you cannot use ± with Position as this would be doubling up the tolerance.
- Basic dimensions are also very commonly used with GD&T Profile to specify the exact size and shape of the surface along with its location and orientation.
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