Your CAD design lives in a universe of perfect geometry and is very abstract
- [Narrator] Your CAD design lives in the universe, which is abstract, and represents perfect geometry. Every geometric element and ever part feature is stored either as a high-precision numeric value or a perfect mathematical representation of the desired geometry. This abstract allows the sharing and transmission of information without any loss of precision or fidelity. The CAD file allows us to store a set of properties, which define the shapes, sizes, angles, and locations for every feature of a given part.
When you create a shape in most modern CAD programs, that shape is mathematically perfect. For example, if you draw a circle using a CAD program, each and every point on the circle is located exactly one radius away from the center of the circle. If you extend that circle into a cylinder, the cylinder will also represent a stack of circles which is mathematically perfect. Similarly, if you draw a square in your CAD program, your square will be a mathematically perfect square. Each line segment will be the same length, it will be perfectly perpendicular to the adjacent segment, and it will be parallel to the opposite segment.
If we extend that square into the third dimension to create a cube or rectangular solid, those mathematical relationships continue to extend perfectly. The resulting geometry will have opposing faces that are perfectly flat and parallel, with adjacent faces that are perfectly perpendicular. These shapes in our CAD program are based upon mathematical constructs, and the geometric relationships are not affected by changing of the part dimensions. A circle can be big or small without affecting its shape. When you give a part length dimensions in a computer CAD program, those dimensions are perfectly precise and exact.
For example, a one-centimeter cube has sides that are exactly one centimeter long. It's as though the dimension is 1.0000 centimeters, with an infinite number of zeros. Fortunately, we can leave off all the trailing zeros after the dimension in the CAD program. Similar to length dimensions in computer CAD, angular dimensions are perfectly precise and exact. A 90-degree angle dimension represents a perfectly right angle of exactly 90 degrees. A 25.27-degree angle is exactly 25.27 degrees, no more, and no less.
Many features in the CAD programs, such as holes, have a location defined by a particular length dimension. For example, a hole might be placed two centimeters from the right edge of a part and one centimeter from the bottom of that part. As we noted earlier, length dimensions are perfectly precise and exact in computer CAD. As a result, the position of that feature is perfectly precise and exact. Unlike shape, size, angle, and location depend on measurements made from one reference location on the part to other locations on that part.
- Where dimensioning goes wrong
- Basic GD&T rules and symbols
- GD&T from the engineering perspective
- Manufacturing design rules
- Interpreting GD&T in CAD models
- GD&T from the machinist perspective
- Understanding the tolerance limits of your shop
- Communicating your tolerance capabilities to potential customers