Sketch in CADD Modelling

'Sketch' in a 3D Parametric modelling CAD software

Updated: Aug 8, 2019

Hello everyone! This is my first blog post and I wanted to discuss the basic question, that anybody might have, while entering the 3D parametric CAD world.

In this blog post we will be discussing about 'sketches' used in any 3D parametric CAD software, such as CATIA, Solidworks, Creo parametric, Siemens NX CAD, etc... Discussion on the 'Parametric' topic will be on a separate blog post.

Now lets dive into our topic of interest, 'Sketch'. When I was introduced to the world of 3D parametric CAD software ( here after will be referred as 'CAD Software'), I had some basic questions about sketches in general. They are,

  1. Why do we use sketch and why not 3D sculpting as in CGI softwares such as MAYA, 3DS max, Blender, etc...?

  2. What is the big deal about fully defined sketch? Is it mandatory to fully define a sketch? If yes, then what is the reason behind it?

These questions might seem simple, but they form the very basics of a properly constructed 3D parametric CAD model.

Now let's discuss them one by one.

#1 Why do we use sketch? Why not 3D sculpting as in Autodesk MAYA?

Before reading my thoughts on this topic, pause for a moment and think about it yourself.

Thinking never exhausts the mind!

It might appear complex once you think about it. So let me explain my view on this topic.

From an engineering stand point, the main purpose of any CAD model is to communicate the ideas about a component that is to be manufactured. Ideas in this case are the geometric dimensions of the component along with the tolerances and fits. Thus dimensions play a major role in CAD Modelling. Now to create an engineering drawing, if we import a complex 3D model as the starting point, the manual work to be done to completely dimension the component is huge. More over creating such a complex model is not an easy task without the exact dimensions.

A solution to this problem is to breakdown the complex model into more primitive shapes such as cylinder, prism, pyramid, frustum, etc.. so that they ease our work. and these primitive shapes can be logically interlinked with the help of Boolean operation to create a unified single volume.

Creating these primitive shapes will be much more easier with the help of sketches. Let us consider creating a model shown in the drawing. We shall also discuss two approaches to creates the same model. (i) Importing primitives as such. (ii) Creating the model using sketches and features.

3D Model #1

a. First approach: 3D Sculpting.

We will first import the primitives as a whole (In our case, two cylinders and one cylinder segment. Refer fig. 3D model#1). Now to dimension it, we have to specify the radius of the cylindrical faces and length of the cylinders, followed by the sector volume removed from the top cylinder. Fixing the primitives with respect to each other is a time consuming process. Creating the model in this manner is cumbersome and need more planning before actual modelling which takes up a lot of time.

b. Second approach: Using Sketches and features.

In this second method we follow the below mentioned steps.

  • Draw the circular base of the big cylinder as a sketch ~> Dimension it to 20 mm as diameter ~> Add the third dimension (height of 30 mm) as an 'add feature'.

  • Draw the circular face of the small cylinder as a sketch, on top of the first cylinder ~> Dimension it to 10 mm as diameter ~> Add the third dimension (height of 20 mm) as an 'add feature'.

  • Draw a sector on top of the second cylinder as a sketch ~> Dimension it with radius of 5 mm and angle subtended as 30 degrees ~> Add the third dimension (height of 17.5 mm) as a 'subtract feature'.

From the above two approaches, we can see that using sketches and features as in the second approach has a predefined set of procedures that can be used to create any complex model. In the case of the first approach, we don't have any such rules to follow. Thus making it more complicated than the second approach. For us working with two dimensions at a time is much easier to control than working with three dimensions simultaneously.

#2 What is the big deal about fully defining a sketch? Is it mandatory to fully define a sketch? If yes, then what is the reason behind it?

Fully defining a sketch is nothing but arresting the DOF (degrees of freedom) of all the sketch entities of the particular sketch. Since we are only dealing with two dimensions in a sketch, we can simplify the DOF as 'attributes' of the sketch entities. Any sketch entity can have a maximum of four attributes. Such as,

  1. Position

  2. Size

  3. Orientation

  4. Shape

Out of these four attributes 'Position' and 'Size' are primary attributes. 'Orientation' and 'Shape' are secondary attributes that are derived from the two primary attributes. Fixing all the four attributes makes your sketch a fully defined sketch.

And to answer the age old question, it is not mandatory to fully define a sketch. But it is a very good practice to do so.

Fully defined sketch,

  • Is easy to manipulate the dimensions without compromising the attributes of the sketch entities. (An under defined sketch cannot promise this.)

  • It helps in better communication among peers. Aiding in pin pointing the exact dimension that needs to verified or revised.

  • Aids in the usage of formulas and equations in the 3D model.

My suggestion to all the modelers is to fully define a sketch. It might appear as a time consuming process, but mastering it is a valuable asset.

In my next blog we shall discuss about the attributes of sketch entities in detail.

Thank you,

- Kuralamuthan Veerapandiyan.