Fully defined sketch PART #1

Updated: Aug 8, 2019

A sketch is the basic building block of any 3D parametric CAD model. We create sketches and added features to those sketch, thus converting a 2D shape into a 3D volume. The nuances covered in this blog applies to almost any 3D parametric CAD software. So lets hop in.

A proper sketch has to satisfy the following conditions,

  • It should be a closed contour. (for solid modelling)

  • It should be a fully defined sketch.

If we are working with surface modelling, the sketch can be an open profile.

In this blog post we shall discuss what does a fully defined sketch means and how to do it in a procedural manner. Since this is a big topic, I'll be breaking this entire topic into three parts.

Before entering into the topic of sketch definition, we must understand how a sketch is constructed.

A sketch consists of a collection of the following building blocks called sketch entities. They are,

  • Point

  • Line

  • Circle

  • Arc

  • Conic

  • Spline

Any sketch, however complex it may be, can be separated into the above mentioned sketch entities. Thus to fully define a sketch, we simply have to define all its sketch entities.

Geometrical attributes of the sketch entities.
Image #1

A sketch entity can have a maximum of four attributes. These attributes are nothing but the geometrical characteristics of a sketch entity. These four attributes are as mentioned in image #1.

Note that not every sketch entity has all four attributes. The attribute combination of each sketch entity is given below in the tabular column named Attribute Vs Entity. This tabular column summarizes the entire blog.

As we can see, a point has only the position attribute. A line cannot have a shape attribute, and so on.

Thus to completely define a sketch entity, we have to specify values or some sort of constraints for these attributes, which in turn will completely define the Sketch as a whole.

My next blog post will explain the intricacies involved in the relationship between the attributes and sketch entities. Which, in other words, I will be explaining the table given above, with more clarity.


- Kuralamuthan Veerapandiyan.

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