Constraints
Constraints
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In 3D modelling, constraints are used to control how parts fit and move together in an assembly. They ensure that components are correctly positioned and behave as intended, preventing unwanted movement or misalignment. Constraints define relationships between parts, such as keeping two surfaces together, aligning edges, or allowing only certain types of movement like rotation or sliding.
Constraints are also important in 2D sketching, where they control the size, shape, and position of sketches before they are turned into 3D models. These include geometric constraints, like making lines parallel or perpendicular, and dimensional constraints, which set exact sizes. By using constraints in both 2D and 3D, designers can create accurate and well-structured models that fit together correctly in an assembly.
2D Constraints
2D Constraints
Parallel
Parallel keeps two lines running in the same direction.
Perpendicular
Perpendicular makes two lines meet at a 90-degree angle.
Tangent
Tangent ensures a line or another curve touches a curve or circle at a single point without crossing it.
Concentric
Concentric keeps circles or arcs sharing the same centre point.
3D Constraints
3D Constraints
Work Planes
Work Planes
Work planes are like digital sheets of paper where you create 2D sketches. Most 3D CAD software gives you three main work planes to start with: one for the elevation (front view), one for the end elevation (side view), and one for the plan (top view).
Work planes help when applying constraints by providing flat surfaces, especially when a model has rounded edges. They allow you to accurately position and align parts by creating stable points for constraints, making it easier to control movement and ensure a precise fit in an assembly.
Work planes help when applying constraints by providing flat surfaces, especially when a model has rounded edges. They allow you to accurately position and align parts by creating stable points for constraints, making it easier to control movement and ensure a precise fit in an assembly.
Centre Axis
Centre Axis
Centre Axis is a constraint used to align cylindrical objects by lining up their central axis. This is useful for inserting parts like rods into holes or connecting round features. To do this, you select the centre axis (shown as a red dotted arrow), which ensures that the centres of both objects are perfectly aligned. However, the part can still slide up and down or rotate freely. To fully secure it in place, you need to add additional constraints, such as mating the bottom of the hole to stop movement or rotation.
Mate
Mate
Mate is a constraint used to attach two flat surfaces together so they touch face to face. This ensures the two parts stay connected at that surface but can still slide side to side or move forward and backward. To fully lock the parts in place and stop any unwanted movement, add more constraints, such as the centre axis, aligning faces or offsetting distances. This helps create a properly assembled and stable model.
Align
Align
Align is a constraint used to ensure that two flat surfaces are positioned at the same level, guaranteeing they are perfectly lined up. However, even when the surfaces are aligned, the components can still move sideways or back and forth. To secure them completely in place, additional constraints, such as mating surfaces or offsetting distances, must be incorporated to prevent any unwanted movement.
Offset
Offset
Offset is a constraint that allows you to position parts at a specific distance from each other while keeping them constrained. For example: The bottom of the cozzlebot's foot is placed on a cube using a mate constraint, it will be attached to the surface but can still move. Next, the front of the foot can be aligned with the cube's edge, ensuring they are lined up. Finally, the side of the cozzlebot's foot can be aligned with the face of the cube. However, an offset constraint can be applied to move the cozzlebot into its exact position, keeping it at a set distance from the edge while staying constrained.
How to answer a constraint question
How to answer a constraint question
When answering a constraint question, you should use the correct terminology:
Like any other 3D modelling question, use sketches to help explain your answer.
Use arrows and ‘colour in’ areas to help explain how you would constrain parts together.
Each part usually has a minimum of 2 constraints.
- Constraint – Align/Mate/Centre Axis
- Part - Face/edge
Like any other 3D modelling question, use sketches to help explain your answer.
Use arrows and ‘colour in’ areas to help explain how you would constrain parts together.
Each part usually has a minimum of 2 constraints.
Examples
Examples
Identify the 3D CAD constraint used at each stage. You may sketch or annotate on the graphics to support your answer. (3 marks)
Answer
- Centre Axis
- Mate
- Align
The designer used constraints to assemble both parts of the display sign. The before and after of stage 1 of the assembly are shown below.
The grey areas show the surfaces that were constrained in stage 1. (1 mark)
The grey areas show the surfaces that were constrained in stage 1. (1 mark)
Answer
- Align
State, using 3D CAD terms, how you would assemble and constrain the components shown above. Corresponding surfaces and/or edges have been indicated for you. (3 marks)
Answer
- Centre Axis
- Align
- Mate
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