Parametric Dimensioning in AutoCAD
Some designers think that AutoCAD® is simple “Stone Age” software, which makes drawings based upon user inputs with no relation between corresponding features of geometry. To some extent that is true, but AutoCAD can make intelligent geometries that can retain design intent and some veteran users avoid using these parametric tools.
I surely want to encourage users to use these parametric tools and in this article I will try to introduce you to simple yet powerful features of AutoCAD parametric tools.
Why Constrain Drawings At All?
When making assembly drawings it is often required that one feature of a drawing remains related to other features with certain parameters, hence making the complete drawing a single unit.
In a simple, non-parametric drawing, if you change dimension or orientation of any feature it will not affect other related geometries, which can result in serious errors for these kinds of assembly drawings. Parametric dimensioning is a smart alternative in these situations.
Autodesk considers parametric tools so important that it dedicated an entire ribbon tab to it. A dedicated status toggle is also provided with the sole purpose of creating geometric constraints automatically. Figures 1 and 2 are snapshots of the Parametric tab and Infer Constraints Status toggle, respectively, from AutoCAD 2016.
Figure 1: The Parametric tab in AutoCAD 2016
Figure 2: Constraints Status toggle
Types of Constraints
Broadly, constraints have been divided in two types—geometric and dimensional.
Geometric constraints are designed to specifically restrict degrees of freedom of geometry according to some defined set of restrictions. For example, a perpendicular constraint will restrict a line to remain at an angle of 90 degrees with respect to another line. In this scenario, the length of both lines does not matter—only their orientation in plane is restricted.
Dimensional constraint is used to restrict dimensional values of geometries such as length, radius, and angle. In this case, orientation of an object is not at all controlled.
Parametric Dimensioning in Action
Let’s use an example of this very simple geometry (see Figure 3).
Figure 3: Sample geometry
Let’s assume that we want to apply the following constraints on this geometry
Length of the outer rectangle should always remain twice its width.
Radius of the green circle should be one fourth the length of the rectangle.
The circle should always remain in the geometrical center of the rectangle.
Let’s first start with geometric constraints. From the Geometric panel of Parametric tab, select Equal Constraint and click on both longer lines of the rectangle. Now exit this command and repeat this process again on the shorter lines of rectangle.
Go to the Dimensional panel of Parametric tab and select linear. Now click close to the left top left vertex of this rectangle and then close to the top right vertex of this rectangle. A dimension d1 with length of rectangle will appear. Click on the drawing area to place this dimension.
Again select linear from the Dimensions panel and click close to the end points of any shorter lines. A dimension d2 will appear. Double-click dimension d2 thus formed and change its value to d1/2. This will ensure that width of the rectangle always remains half its length.
Figure 4: Linear command
Now click on Radius on the Dimensions panel of Parametric tab and click on the green circle. Place the dimension in the drawing area and change rad1 value thus formed to d1/4. This will ensure the radius of the circle always remains one fourth the length of the rectangle.
Click on Linear on the Dimensions panel and click at a point close to the center of the vertical line on the left of the rectangle, and then click on the circle. A dimension d3 will be formed. Change the value of d3 to d1/2. Again click on Linear and click on the center of the longer line of the rectangle and click on the circle. Now change the value of dimension d4 to d2/2. These settings will ensure that the circle remains at the geometrical center of the rectangle (see Figure 5).
Figure 5: Circle at the geometric center of the rectangle
Testing the Geometry
Now let’s check whether our geometry conforms to the criteria laid down by us earlier. Double-click on the d1 dimension and change its value to 40 units. Instantly, all other related dimensions will get updated to conform to our criteria. Check this with other values of d1 as well and every time it will conform to our criteria.
Using Parameters Manager
I have used division parameters throughout this example for defining one dimension with respect to others. I have used parameter such as d2=d1/2 and rad1=d1/4. AutoCAD permits many more parameters including multiplication, addition, subtraction, logarithmic, and exponential values. You have plenty of parameters to select for your geometry and with these parameters you can design a fairly complex parametric geometry with ease.
Conclusion
Constraints are a great tool if you are working with drawings where you need to maintain design intent. It can ease the design process and increase accuracy by reducing manual calculations for geometrical features.
