Basic Dimensions
Spot's master wants to build a fence to keep Spot in his own yard. The fence comes in ten foot lengths, so Spot's master wants to dig his post holes exactly 10 feet apart.
In the figure below, we see that the basic dimensions denote the exact theoretical locations where Spot's master wants the post holes to be. Basic dimensions are dimensions in rectangles as seen below.
We also see that almost none of the post holes actually end up in line with the basic dimensions. Most are close, some are quite far off. We will need geometric tolerances to tell us exactly how far away from the exact theoretical positions the actual post holes can be.
The most difficult thing for most people to understand about basic dimensions is that they are not dependent on the locations of the features that they are dimensioning. So for instance below, the fact that one hole or another is not exactly where it's supposed to be does not change anything about where we theoretically want them all to be. Or in other words, if one hole gets dug in the wrong place, that does not affect the rest of the holes.
In the figure below, we see that the basic dimensions denote the exact theoretical locations where Spot's master wants the post holes to be. Basic dimensions are dimensions in rectangles as seen below.
We also see that almost none of the post holes actually end up in line with the basic dimensions. Most are close, some are quite far off. We will need geometric tolerances to tell us exactly how far away from the exact theoretical positions the actual post holes can be.
The most difficult thing for most people to understand about basic dimensions is that they are not dependent on the locations of the features that they are dimensioning. So for instance below, the fact that one hole or another is not exactly where it's supposed to be does not change anything about where we theoretically want them all to be. Or in other words, if one hole gets dug in the wrong place, that does not affect the rest of the holes.
There are several ways to indicate basic dimensions. Enclosing them in rectangles is the most common. But sometimes, if all of the dimensions on the drawing are basic, the rectangles are omitted and a general note is added indicating that all dimensions are basic.
Sometimes dimensions are derived from a 3D CAD model and considered basic. In that case, there should be a note on the drawing as shown below.
Sometimes dimensions are derived from a 3D CAD model and considered basic. In that case, there should be a note on the drawing as shown below.
