Constraints
Constraints
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When you're working on a 3D model, constraints are the rules that control how different parts fit together and move. They help keep everything in the right place, so parts don’t shift around or end up misaligned. You can use constraints to lock two surfaces together, line up edges, or allow only specific movements, like rotating or sliding.
Constraints are also important in 2D sketching. They control the size, shape, and position of your drawing before you turn it into a 3D model. There are geometric constraints (like making lines straight, equal, or perpendicular) and dimensional constraints (which give exact measurements). Using both makes sure your final model is accurate and everything fits together the way it should.
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.
Tangent
Tangent
The tangent constraint is used to line up curved or cylindrical parts side by side. You select the curved edges (like the edges of a cylinder), and they’ll snap together. Just like other constraints, you’ll usually need to add more constraints to lock the parts in place fully.
Orientate
Orientate
The orientate function lets you position one part at a specific angle in relation to another. It’s handy when parts need to sit at a certain tilt or direction, rather than just flat or straight on.
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/Tangent/Orientate
- Part - Face/edge/Vertex
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
Describe, using 3D CAD terms, how to assemble and constrain the axle and main arm onto the base so that the main arm will sit at 20° to horizontal. (4 marks)
Answer
- Centre axis the axle through the hole of the main arm and the base (1 mark)
- Mate one of the inner sides of the axle to one of the outside flat sides on the base (1 mark)
- Orientate the main arm to an angle of 20 degrees (1 mark)
- Offset 13mm the centre of the main arm to the inner edge of the base (1 mark)
Describe, using 3D CAD terms, how to constrain the three door handle components. (4 marks)
Answer
- Centre Axis the handle to the rose. (1 mark)
- Offset the rose 15mm away from the handle (1 mark)
- Mate three separate faces (very top square, left/right side and bottom side) of the spindle to the faces on the handle. (2 marks for 3 correct mates)
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