ONSHAPE: SKETCHING & CONSTRAINTS

In a parametric software like Onshape, sketching is a fundamental process used to create and define 2D profiles or shapes that serve as the foundation for 3D models. When sketching, users can draw lines, arcs, circles, and other geometric entities on a 2D plane while also applying constraints to control their relationships and dimensions.

Onshape's parametric modeling capabilities allow users to define and apply constraints to the sketch elements. Constraints are rules that govern the geometric relationships between different sketch entities, ensuring they remain interconnected and react intelligently to changes. By setting constraints, such as horizontal, vertical, parallel, coincident, and equal-length, users can easily control the position, orientation, and size of sketch elements in relation to each other.

These constraints act as design rules, making the sketches dynamic and adaptable. When users modify a dimension or move a sketch entity, the constraints automatically adjust the rest of the sketch, ensuring the model stays consistent and accurate. This powerful feature enables rapid design iterations, making it easy to experiment and refine ideas.

Sketching in Onshape involves creating 2D profiles while applying constraints to control the relationships between sketch elements. This parametric approach ensures that modifications to the sketch propagate intelligently through the model, making the design process more efficient, flexible, and iterative. With constrained sketches, Onshape empowers users to create complex 3D models with precision and ease, making it an invaluable tool for designers and engineers alike.

Starting a Sketch

A sketch in Onshape can be thought of as a 2D projection of a 3D object onto a plane. This means that you are producing a drawing much in the same way that you would if you were drawing an orthographic projection. 

Step 1

When you click to start a sketch in Onshape the first thing you will need to do is select which plane the sketch is on. This is the plane that you want to extrude your sketch from to turn it into a 3D object. You should also notice your entire toolbar changes once you are in a sketch. These are all the different operations and constraints that pertain to sketching. You want every sketch that you produce to be completely defined with constraints to ensure that if you make any adjustments they are reflected through out your sketch correctly. This is the main benifit of parametric modeling so use it to its fullest.  

Step 2

Orient your screen so that you are looking perpendicular to the plane you are drawing on. This can quickly be done by hitting the [N] key to normalize your view. I also turn off the planes in my view so they do not get in the way of drawing by hitting the [P] key. 

Step 3

Sketch the 2D profile of your object. It is good practice to have the origin as part of your sketch to ensure that you can easily constrain your drawing.

Step 4

Fully constrain your drawing by using constraints. In Onshape if your sketch is blue it is undefined, if it is black it is fully defined and if its red it is over defined.  

Constraints

Constraints on a sketch in Onshape are crucial because they define geometric relationships between sketch entities, such as lines, arcs, and circles. By applying constraints, designers can maintain precise control over the shape, size, and position of sketch elements, ensuring design intent is preserved throughout the modeling process. Constraints enable easy modifications to the sketch, automatically updating related geometry, fostering design flexibility, and accelerating the iteration process, ultimately leading to more efficient and accurate 3D modeling.

A coincident constraint aligns two points or entities at the same location, ensuring they move together if the sketch or model changes. It helps maintain precise relationships and achieve accurate geometry in the design.

A concentric constraint aligns two circles or arcs with a common center point, ensuring they remain concentric as the sketch or model is modified. This constraint helps maintain circular relationships and enables precise alignment in the design.

A parallel constraint ensures that two lines or edges remain equidistant and never intersect, creating parallel alignment in the design. It helps maintain consistent spacing and geometric relationships between elements.

A tangent constraint ensures that two curves, such as arcs or circles, touch each other at a single point, creating a smooth and continuous transition without overlap. This constraint helps maintain a seamless connection between curves and is useful in creating organic shapes and precise design interactions.

A horizontal constraint ensures that a line or edge remains parallel to the X-axis or horizontal plane. It helps maintain horizontal alignment in the design, ensuring that the line does not tilt or deviate from the intended horizontal orientation.

A vertical constraint ensures that a line or edge remains parallel to the Y-axis or vertical plane. It helps maintain vertical alignment in the design, ensuring that the line stays upright and does not deviate from the intended vertical orientation.

A perpendicular constraint ensures that two lines or edges meet at a right angle (90 degrees), creating a perpendicular relationship between them. It helps maintain precise right-angle alignments in the design, which is often crucial for various geometric shapes and construction.

An equal constraint ensures that two dimensions, such as lengths or radii, have the same value, making them equal in size. It helps maintain uniformity and symmetry in the design by ensuring that related elements have identical dimensions.

A midpoint constraint ensures that a point lies exactly at the middle of a line or segment. It helps maintain symmetry and precise positioning in the design by ensuring that the point is equidistant from the endpoints of the line or segment.

A normal constraint ensures that a line or edge is perpendicular to a selected surface or curve at the point of intersection. It helps maintain a right-angle relationship between the line and the chosen element, facilitating accurate design interactions and alignments.

A pierce constraint ensures that a point lies on the intersection of two or more curves or edges. It enforces precise positioning of the point at the common intersection, helping to create smooth and accurate connections between different sketch entities or elements in the design.

A symmetric constraint ensures that sketch elements or features are mirrored or reflected about a specified axis or line. It helps maintain symmetry in the design, enabling designers to easily create and modify balanced and harmonious shapes.

The fix constraint is used to permanently lock a sketch entity in place. When you apply the fix constraint to a sketch element, it becomes immovable and unaffected by any changes made to the sketch. This constraint is particularly useful when you want to maintain specific elements of your sketch as reference geometry or when you want to prevent unintentional modifications to certain parts of the design.

The curvature constraint is used to create continuous transitions between sketch splines or conics with surrounding geometry. It allows for a defined multicurve sketch element in your design.   

Why is Fully Defining Your Drawing Important?

Fully defining a technical drawing is crucial for several reasons, and it plays a fundamental role in the design and manufacturing processes. Here are some key reasons why it is important to fully define a technical drawing:


A fully defined drawing provides clear and unambiguous information to all stakeholders, including designers, engineers, manufacturers, and inspectors. It eliminates confusion and ensures that everyone interprets the design intent accurately.


By including complete and precise measurements, tolerances, and geometric constraints, a fully defined drawing ensures that the manufactured part or product matches the designer's specifications. This precision is essential for functionality and compatibility with other components.


Complete definitions of dimensions, tolerances, and material specifications help manufacturers understand how to produce the part or product. This information is critical for selecting appropriate manufacturing processes, tools, and materials.


Fully defining a drawing allows for interchangeable parts, meaning that components produced to the same specifications can be used interchangeably in an assembly. This is essential for mass production and repairability.


A well-defined drawing serves as a basis for quality control procedures. Inspectors can use the drawing to verify that manufactured parts meet the required standards and specifications.


Accurate and complete drawings reduce the likelihood of errors and rework during the manufacturing process. This, in turn, helps to minimize costs associated with wasted materials and time.


A fully defined technical drawing serves as a legal document that outlines the intellectual property and design rights associated with the product. It provides a basis for patent applications and protects the designer's ideas.


Full definition helps preserve the designer's original intent. As the design moves through different stages of development and manufacturing, a fully defined drawing ensures that the final product aligns with the initial concept and specifications.


If changes or revisions are necessary, a fully defined drawing makes it easier for designers to identify and update specific dimensions or features without affecting other parts of the design unintentionally.


Fully defining a technical drawing is essential for effective communication, precision in manufacturing, quality control, legal protection, and overall efficiency in the product development process. It forms the foundation for successful collaboration among different stakeholders and ensures that the end product meets the required standards and specifications.