Understanding Dimensioning Techniques in AutoCAD

Welcome back to the AutoCAD Essentials for Mechanical Engineers series! In this blog, we’ll explore dimensioning techniques in AutoCAD, focusing on how to effectively use the tools for various mechanical engineering drawings. Proper dimensioning ensures that your design is communicated clearly, so manufacturers and other engineers can follow the specifications with precision. 

Whether you're a beginner using AutoCAD 2D drawing or advancing into AutoCAD 3D, understanding how to dimension accurately is critical for creating professional and accurate mechanical drawings. 

Selecting Objects for Dimensioning in AutoCAD 

One of the most commonly used tools in AutoCAD is Linear Dimensioning. You can invoke this by selecting an object or by specifying origin points. 

Two Modes of Linear Dimensioning: 

• Selecting the Object Directly
• After invoking the Linear Dimension command, right-click or press Enter to bring up the object selection tool. Click on the object to be dimensioned. In this mode, the first and second extension lines are automatically based on the type of object selected, such as a line or an arc. If you are working with a circle, the endpoints of the diameter are used as the origins for the extension lines. When selecting the circle, if the point is close to the north or south, the dimension is drawn horizontally. If it's closer to the east or west, the dimension is vertical. This approach is particularly useful in AutoCAD for mechanical engineers, where precise measurements are key. 
• Using Offset from Origin: The offset distance between the endpoints and the extension line can be specified manually. This helps maintain clarity in your drawing, especially when using CAD software to detail complex parts. For instance, when dimensioning arcs, this approach ensures the extension lines don’t clutter the design. 

This method can also be applied to polylines or exploded objects, allowing you to dimension individual elements in a design. This feature is handy when working on complex shapes in mechanical AutoCAD. 

Dimensioning Circular Elements: Radius and Diameter 

When working with circular elements, understanding how to apply radius and diameter dimensions is crucial in AutoCAD mechanical drawings. To dimension a circle: 

1. Radius: 

Use the Dimension Radius command to specify the radius of a circle. Select the circle, and the center point will be automatically placed. You can adjust the position of the dimension line based on where you want the measurement to appear. For example, if the radius exceeds 90°, the text alignment changes to improve readability. This allows you to fit dimensions neatly inside or outside the circle, which is essential for AutoCAD 2D and AutoCAD 3D projects. 

2. Diameter: 

For diameter measurements, use the Dimension Diameter command. The DI symbol is prefixed to the dimension to indicate diameter. Like with radius, you can position the dimension text inside or outside the circle for clarity. This tool is frequently used in CAD programs when designing mechanical parts like shafts or holes. 

Highlighting the Center Point of Circular Elements 

If you want to highlight the center point of a circle for additional referencing, use the Dimension Center command. This is especially useful when referencing datums or tolerances in mechanical designs. Center marks are automatically placed, allowing for easy referencing in further tolerance analysis or assembly drawings. 

Angular Dimensioning 

In AutoCAD for mechanical engineers, angular dimensions are often necessary to measure angles between two lines or points on an arc. The Dimension Angular command allows you to dimension: 

• Arcs and Circles: Select the arc or circle, and AutoCAD will calculate the angle between the endpoints of the arc. You can also invoke options such as MText or Text to edit or add information to the dimension text, which is useful when annotating parts like gears or brackets. 
• Quadrant Option : If the angle measurement needs to be placed outside the extension line limits, you can use the quadrant feature. This allows you to shift the dimension text to different quadrants of the design, enhancing the clarity of your drawings. For instance, if you're working on an AutoCAD 2D drawing of an angular part, positioning the dimension text in a different quadrant can make the drawing cleaner and more professional. 

Using Baseline Dimensions 

The Baseline Dimension tool is another essential feature for AutoCAD mechanical engineers. It allows you to measure multiple dimensions from the same baseline, improving accuracy and consistency in your drawings. 

Here’s how to use it: 

• Setting a Baseline: First, you need to establish the baseline using the Dimension Linear or Aligned commands. Once this is set, you can create additional dimensions from this baseline. This is helpful when designing steps or features that need to be referenced from a common starting point. 
• Continuing Dimensions: After the initial baseline is set, additional points can be added, and AutoCAD automatically refers to the previous dimension for consistency. This is particularly useful in AutoCAD mechanical crash courses, where consistency is vital for large-scale projects. The dimensions are measured end-to-end from the baseline, maintaining clarity and precision. 

Real-World Example: Measuring Along an Arc 

Let's consider an example where you need to measure dimensions along an arc. This is a common requirement in mechanical CAD design. Suppose you have an arc, and you need to place evenly spaced holes along its length. After drawing the arc, place a circle at one point on the arc and use the Array tool to replicate the circle along the arc. 

Now, to dimension the angle between these holes: 

1. Invoke the Angular Dimension Command: 

Select the center point of the arc and then the first and last points of the circles. This will calculate the angle between them. 

2. Using Baseline Dimensions: 

You can also use baseline dimensions to measure each hole relative to the center point of the arc. This method ensures accuracy and is particularly useful in CAD programs for AutoCAD mechanical drawings. 

Conclusion 

Mastering dimensioning techniques in AutoCAD is critical for creating precise and professional mechanical engineering drawings. Whether you're working on AutoCAD 2D or AutoCAD 3D, understanding how to dimension accurately is essential for communicating your design effectively. 

By learning the ins and outs of baseline dimensions, angular dimensions, and radius/diameter dimensions, you'll be better equipped to produce quality mechanical CAD designs. 

For further learning, consider enrolling in an AutoCAD crash course or an AutoCAD free certification course. These courses can help enhance your skills and knowledge, making you a more proficient user of this powerful CAD software. 

This blog is part of our ongoing AutoCAD Design Series. If you missed the previous posts, check them out here.  

Would you like to have a more interactive experience going through the AutoCAD essentials? 

Skill-Lync has released a FREE comprehensive course covering AutoCAD Essentials for Mechanical Engineers in detail! Check it out here.

If you’re looking to go deeper into AutoCAD Essentials check out Skill-Lync’s AutoCAD certification course.   

Check out our hands-on course today and add AutoCAD to your list of skills!  

Let’s get #IndustryReady together, one skill at a time! 

Start Course Now