ENGINEERING DRAWING - DIMENSIONING

Audio: Engineering drawing – dimensioning

In Introduction to the Engineering drawings, we showed you the building blocks of the engineering drawing. All of these building blocks are equally important for defining the drawing clearly and unambiguously. In this article, we are going to talk about dimensions. Dimensions are necessary to define the size and shape of an engineering object. We will dig deeper into the main elements of dimensioning and methods that we can use to define the size and shape of an object.

Introduction

Imagine that you are buying a new cabinet online. You are looking at the pictures of the cabinet, and you are satisfied with its form and aesthetics, and you would like to buy it. As you are limited with the space, you are looking for the dimension of the cabinet to see if it will fit in your space. At least, what you will look for is width, depth, and the height of the cabinet (usually noted in WxDxH format). The next logical step is to measure your free space and compare it with the defined cabinet size. If the cabinet does not fit in your free space, you will not purchase this cabinet.

The similar thing we are doing with the components that we are manufacturing. When we create a drawing of the 3D object, we create all the necessary views (orthographic views and/or pictorial views) for its unambiguous representations, and we add dimensions to it. Then, we send this drawing to our supplier to manufacture it. After components have been manufactured, we would (or our supplier) inspect the component’s size to see if they are created according to our drawing specification. In case some of the dimensions are outside of the specified ones, the supplier will have to rework the component or create a new one.

Properly defined dimensions ensure that everyone involved understands the size, shape, and tolerances of the object being depicted. Furthermore, they guide machinists and fabricators in determining the appropriate tools, techniques, and tolerances required to produce the part accurately. Accurately defined dimensions help convey the design intent. It provides information about critical features, relationships between different parts, and important geometric characteristics.

Dimensioning for those reasons is the most important part of the drawing creation, and it can get very complex as the complexity of our product gets higher. You could have ten (or more) different components as a part of one complex assembly. These components could be fixed to each other, while others could move in respect to each other.

Defining proper dimensions to be inspected and defining the proper tolerances is one of the hardest parts of creating engineering drawings. These concepts will be explained in another article because they are so important that they will need a few articles to be completely clarified. But before we go to these complex topics, first, we must learn the basics.

Elements of dimensions

As we previously mentioned, we use dimensions to define the size and shape of an engineering component. In other words, with the use of dimensioning technics on the engineering drawings, we define the physical boundaries of the component, like the size and location of the different features.

Let us now investigate a simple example. You can see the simple steel plate drawing with its corresponding dimensions in the picture below.

This steel plate is 70mm long, 35 mm wide, and 7 mm high. You probably noticed these thin lines with the arrows on the sides and the number in the middle (or on the side). This is how we represent dimensions on the engineering drawings. The rules for dimensioning are defined by ISO 129-1:2018.

When I was a student, our professors insisted on manual drawing creation. What I mean by manual is creating components and assemblies by hand with pen on paper. When we got to the point where we understood all drawing rules, then we moved to create drawings using CAD software. Fast forward to today, as a mechanical design engineer, I never created a drawing with pen and paper. I created a lot of freehand sketches, but I never created an official mechanical drawing by hand.

Let us think about one more thing. In the real world, most engineering companies use CAD software for designing products and for creating engineering drawings. Most of CAD software already contains different standard rules for creating drawings. With that being said, we will not dissect every rule to its core. At this point, I think that it is good enough for you to learn the basic terminology and then move to perfect your craft of dimensioning drawings unambiguously.

Extension line, dimension line, nominal value, and terminator

Let us now look at the most important elements of dimensions:

Extension lines – are drawn perpendicular to the feature that is being dimensioned.
Dimension line – is perpendicular to the extension line, and above this line, the nominal value is shown.

If you are interested in the line types and thickness used to show these lines check: Basic elements of engineering drawings.

3. Nominal value – represents the nominal value of the dimension, i.e., it represents a physical value of the feature in specific units. The nominal value is always above the dimension line, and it is on a natural scale (1:1). The tolerances are always added to the nominal value. Read more about the tolerances: Engineering tolerances – Introduction.

4. Terminators – the first Terminator was filmed in 1984… just kidding.

In the engineering drawing, we are using terminators to clearly represent the end of the dimensional lines. Based on ISO 129-1:2018, recommended types of terminators are as shown below:

As you can imagine, the sizes and shapes of all of these elements are defined by ISO 129-1:2018. But as I already told you, all of these rules are already incorporated in the CAD software, and as long as you choose your preferred standard in the software settings, you should be fine.

Let us now take a look at an example of the engineering drawing, slightly more complex than the previous example:

In addition to the elements mentioned above on the drawing, we can also encounter the following elements:

5. Leader line – used to direct a dimension, note, or symbol to the intended place on a drawing.

6. Reference line – is created in conjunction with the leader line.

7. Property indicators (symbols) – are used to replace words to simplify the drawing. More about the property indicators in the following text.

8. Reference letter – is used when we have alternate dimensions for different usage. For example, we have the exact geometry of the plate with two different hole diameters. In that case, we don’t have to create a new drawing with the different hole sizes, but we can mark this dimension with the capital letter. Then, the letter would be placed in the table and the values for the different hole sizes.

Property indicators (symbols)

Diameter

Radius

Square

Spherical diameter

Spherical radius

Arc length

Thickness of thin objects

Depth

Cylindrical counterbore

Counter sink

Rules for dimensioning

While defining dimensions on the drawing, we should keep in mind some rules:

All dimensions on the drawing should be in the same specified units e.g., millimeters. If we use other units for specific features, the corresponding unit should be written with the nominal value.

2. All the relevant dimensions of the object should be defined on the drawing.

3. Each feature is dimensioned only once on the drawing. If we need to dimension something twice for informational purposes, we can use auxiliary dimensions. The auxiliary dimensions are placed into parenthesis ().

4. The comma should be used as a decimal separator.

Dimensioning methods

Now that we understand the basics, let us investigate the methods we can
use to dimension the shape and form of our objects.

Chain dimensions

Chain dimensioning is the method where dimensions are placed from one feature to another feature. This method is commonly used. We should keep in mind that each tolerance builds on the next one. This method is usually used when the parts function requires that the features are related to each other. When using chain dimensions, it is important to provide clear references and maintain a logical and consistent order. Proper alignment and spacing of dimensions are essential to avoid confusion and ensure the accurate interpretation of the chain of dimensions.

Parallel dimensions

In the parallel dimensioning method, all the dimensions originated from the same base. This method is commonly used for machined parts. Also, it is used when the size or feature should be controlled from a common reference plane and less tolerance buildup is desired. When employing parallel dimensions, it is important to ensure that the dimension lines are not excessively long or extend beyond the feature being dimensioned. Proper spacing, alignment, and consistent placement are key considerations to maintain a clear and organized drawing.

Running dimensions

The running dimensioning is exactly the same as a parallel method, but a different graphical representation is used (dimensions are placed in a continuous manner along an uninterrupted length or feature, typically along the edge or centerline of an object.). Running dimensions are commonly used for dimensions along linear features, such as lengths, widths, and heights. It is particularly useful when there are numerous repetitive features or when dimensions need to be placed in a compact and organized manner.

The starting location of the dimension is defined with the origin symbol:

Another way of representing the running dimensions is as shown below:

Combined dimensions

The combined method is the practice of utilizing multiple dimensioning methods within a single engineering drawing.

By combining different methods within the drawing, we aim to optimize clarity, readability, and space utilization, while providing comprehensive and accurate dimensions for different aspects of the design.

Equally spaced features

When features are equally spaced, we can simplify add dimensions as follows:

Repeated features

When we have multiply features with the same dimensions, we can dimension only one feature and note how many times it is repeated:

Symmetrical object

When we have to dimension symmetrical object, we can simplify the drawing by creating only half of the object and indicate the symmetry line:

Restricted areas

When we need to define special requirements to a restricted portion of the feature (for example, surface-hardening) we have to indicate that area on the drawing:

Countersink and chamfer

Chamfer 30°:

Chamfer 45°:

Functional and non-functional dimensions

When we are dimensioning an object, naturally, some dimensions will be more important than others. Some dimensions will be critical to the correct functioning of the component, and these are called functional dimensions. Other dimensions will not be critical to correct functioning, and these are called non-functional dimensions. Functional dimensions are obviously the more important of the two and, therefore, will be more critical when making decisions about the dimension value.

Closing words

Dimensioning on the engineering drawing is probably the most challenging part of the drawing creation. You can see that there are many basic rules, and that is not even the hard part. The next step to upgrade your dimensioning skills is to learn about the tolerances.

With time and practice, these rules will embed in your work routine, and you will not think about them. When that happens, make sure to read this article again. As a real expert, you want to make sure that you are communicating the right message every time someone checks your drawing. You are not only communicating design details on your drawing; you also communicate your expertise and your company brand. With that being said, make sure that you always communicate the right message.

Now you have an excellent overview of the dimensioning elements and methods. However, I suggest you go through the text once more and identify areas you think need more understanding and clarity. Then, once you have identified those areas, start building up your knowledge in those areas.

To make it easier for you to find related posts, check the “Further reading” chapter below. Do you have any questions or need something to be clarified better? Leave a comment below, and I will give my best to adjust the post accordingly.

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