Dimensioning in a 3-D CAD system

Dimensions transform the artistic inspiration into reality

THE FABRICATOR® FEBRUARY 2009

February 1, 2009

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Columnist Gerald Davis explains that preparing a 3-D CAD model without dimensions may look great, but it doesn't really do anybody any good. For example, a 3-D CAD model with dimensions helps quality control inspectors.

 Precision molded dimensions figure 1

Figure 1: Modeled dimensions make sense to a CAD jockey, but they may not make much sense to those in fabricating.

How important are dimensions to 3-D CAD modeling? Depends on your design objective, I guess.

The 3-D CAD software that I frequently use will allow me to model products without using any dimensions at all.The ability to model without dimensions is a characteristic of most of the competing software as well.The result of such free-form modeling may look great, but it really leaves the actual size of the product in a subjective state.If eye-balling it is close enough, then all is well.

Don't Rely on ESP

Some manufacturing processes directly use the 3-D CAD model for programming manufacturing equipment. For example, I've used a "3-D printer" to create plastic models that closely resemble the 3-D CAD model; no mechanical drafting or blueprint was needed. As with other types of "rapid prototyping" equipment, a 3-D printer manufactures a part by depositing thin 2-D layers of plastic to build up the final 3-D shape. Some final sanding and finishing are required, but if all you need is a shape to hold in your hand, this is a pretty amazing tool.

When it comes to quality control for more traditional manufacturing processes, it really helps the inspectors to have a dimensioned drawing that can be used to verify that the product has been fabricated to the correct size.For an example, see Figure 1. Without a dimensioned drawing, it might take an act of extrasensory perception to read the mind of the designer to determine if the part is right.

Off-the-shelf components offer up another compelling reason to have dimensioned drawings available. For instance, if I'm going to use a toggle switch, I really need to know what the mounting provisions should be. Accordingly, dimensioning will always be an important skill to have when it comes to 3-D CAD modeling.

Two Dimensions of Dimensions

I make a distinction between dimensions that are used when modeling the part and dimensions that appear on a drawing for quality control.First, let's look at dimensioning techniques for modeling, and later consider ways to leverage those dimensions so that the creation of the mechanical drawing is nearly automatic.

With the 3-D CAD software that I'm using, I can create a 3-D solid in two simple steps.First, make a sketch.Second, extrude the sketch to create a solid.

As I explained last month ("Sketching in a 3-D CAD environment," The FABRICATOR, January 2009, p. 26), my sketching process begins with just a quick series of mouse strokes to add lines and arcs to represent the general idea in mind.I don't immediately worry about dimensions or scale. As it is convenient, I'll add sketch relationships and dimensions.

I generally use a simple point-to-point dimensioning style in my modeling sketches.I want to minimize the distraction of operating the software and keep most of my focus on my design intent. Later, when I shift modes and put on the draftsman hat, I'll focus on the dimensioning standard.That's when I've found ordinate, and other stylistic dimensioning tools, to be most valuable.

Design intent can be difficult to nail down as a definite and constant target.It includes speed and efficiency of modeling, future editing, and perhaps review by others.Perhaps the most important goal while adding dimensions to a model is to establish relationships among features in the model.It is conceivable that the design intent would include dimensioning the model for the convenience of creating a mechanical drawing, but that has not generally been my experience.

The software allows me to draw the modeling sketch with other, previously completed model items visible. If I choose, I can select features from those external entities as part of my dimension.I can also choose to select features that exist only within my current sketch internal entities.

When deciding whether or not to use external references, remember that the project's design intent is key.A model that automatically adjusts itself as changes are made requires external references.On the other hand, a model that stands entirely on its own merit, regardless of the context in which it is used, will use only internal references in its sketches.

As I mentioned last month, I frequently try to use sketch relationships with a minimal reliance on dimensions in my modeling sketches.I will put dimensions on features that I will likely need to adjust.I use sketch relationships to force other features to follow the changes in the controlling dimensions. This generally results in a minimum amount of clutter and confusion.I've found that this approach makes it easier to edit and adjust the model as the design evolves.

Fabricating Dimensions

When it comes to drafting a mechanical drawing for manufacturing, most 3-D CAD software will give you a lot of help, but it probably won't do all of the work for you.What it will do is create dimensions on the drawing with minimum effort.What the software probably won't do well is to automatically comply with your drafting standard.The drafting standard specifies which dimensions to include or omit, where to place the dimensions, and what symbols to use.

You've probably heard the joke about the importance of standards they're so important that everybody should have them!All kidding aside, which standard is "best" depends on the manufacturing network that you're working within.

If you're uncertain about which drafting standard to use, I recommend that you consider studying ASME Y14.5M-1994 Dimensioning and Tolerancing, particularly if you are creating drawings that will be used to support the traditional roles of purchasing, manufacturing, inspection, and general product documentation.By making an effort to comply with a widely accepted standard like Y14.5, your work will be processed more efficiently and with less second -guessing by a wider group of fabricating professionals.

The 3-D CAD software that I use will set up the 2-D projected views of the model quickly.Isometric views, sectional cuts, hidden lines all of that is a breeze.Adding the dimensions to the mechanical drawing, on the other hand, can be an interesting experience.

Automatic Drawings: Myth or Reality?

I'm a big proponent of 3-D CAD modeling and then using that model to create a 2-D mechanical drawing for fabricating.A great benefit of using the 3-D CAD model to drive the mechanical drawing is that changes made to the model automatically propagate into the 2-D mechanical drawing.

Although it is possible to have edits to the 2-D drawing make changes to the 3-D model, it is my habit and preference to edit in 3-D. It is much easier to visualize the context and consequences of the changes, particularly when several components must fit together.

The specifics of how to place dimensions on your mechanical drawings depend entirely on the 3-D CAD software that you're using. I am going to offer some general suggestions and leave it to you to discover the exact menu clicks to make it happen.

In an effort to automate the production of drawings, the software will allow me to import the modeled dimensions into the projected views on the mechanical drawing. That often results in a confusing "birdnest" of dimensions that would not make a lot of sense to a fabricator (see
Figure 2
).That's due largely to the fact that, while the model was being modeled, the CAD operator didn't put a lot of effort into the way dimensions would appear on the mechanical drawing.While this auto dimensioning tool to import the modeled dimensions might look great in a software demo, the reality is that sorting out the tangle of dimensions can require more labor than it is worth.

I frequently use the slow and tedious technique of re-creating each of the needed projected dimensions on the mechanical drawing. The software makes this easy just click on the feature and a dimension automatically appears.This can be time-consuming, but the software will help with tools to automate the ordinate or baseline dimensioning.

One benefit that I've experienced from this redundant dimensioning effort is that it forces me to closely review the features of my 3-D model.I frequently discover errors or omissions in my work.A drawback is that I often omit a feature and later get a call from the fabricating crew to fix the error on my drawing.

Next month I'll take a closer look at using sketching and dimensioning techniques to create 3-D solid bodies, such as revolves and extrudes,which are the building blocks for a lot of product designs.

Gerald would love to have you send him your comments and questions. You are not alone, and the problems you face often are shared by others. Share the grief, and perhaps we will all share in the joy of finding answers. Please send your questions and comments to



Gerald Davis Design and Consulting

Gerald Davis

Contributing Writer
Gerald Davis Design and Consulting

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