Sketches, Constraints and Features
Learn how every part begins as a 2D sketch, how dimensions and constraints make a sketch fully defined, and how features such as extrude, revolve, fillet, hole and pattern turn sketches into solid parts.
In this lesson you will
- Draw sketches using lines, rectangles, circles and construction geometry
- Apply dimensions and geometric constraints to fully define a sketch
- Create solid parts with extrude and revolve
- Add fillets, holes and patterns to finish a part
Lesson
Almost every part you design starts the same way: a flat 2D drawing called a sketch, placed on a plane or on a face of an existing part. You then use a feature to turn that flat shape into a 3D solid. Learning to sketch well is the most important CAD skill you will build this week.
Sketch tools
- Line and rectangle for straight edges such as plates and brackets.
- Circle and arc for holes, rounded ends and shafts.
- Construction geometry: dashed reference lines that guide your sketch but do not become part of the solid. They are perfect for laying out a grid of hole centres.
- Use or project existing edges, so a new sketch lines up with a part you already made.
Dimensions and constraints
A dimension sets a size: a length, a diameter, an angle or a distance between two points. A constraint sets a relationship: this line is horizontal, these two circles are the same size, this point sits on that line. Together they describe exactly what you want, not just what you happened to draw with the mouse.
| Constraint | What it does | FTC example |
|---|---|---|
| Horizontal / Vertical | Keeps a line level or upright | Edges of a mounting plate |
| Coincident | Joins a point to another point or line | Closing the corners of a shape |
| Equal | Makes lines or circles the same size | All screw holes the same diameter |
| Concentric | Gives circles the same centre | A shaft hole inside a bearing pocket |
| Parallel / Perpendicular | Sets the angle between two lines | Sides of a bracket |
| Tangent | Makes a line meet an arc smoothly | A rounded end on an arm |
A sketch is fully defined when every line and point is locked by dimensions and constraints, so nothing can be dragged out of place. In Onshape, under-defined sketch geometry shows in blue and fully defined geometry turns black. Aim for black every time: a fully defined sketch behaves predictably when you change a dimension later.
Features turn sketches into solids
| Feature | What it does | FTC use |
|---|---|---|
| Extrude | Pushes a sketch straight out to a depth | Plates, brackets, spacers |
| Revolve | Spins a sketch around an axis | Pulleys, round spacers, knobs |
| Fillet | Rounds a sharp edge | Safer corners, stronger inside bends |
| Chamfer | Cuts an angled edge | Lead-in on a hole or a part that slides into place |
| Hole | Creates standard holes, including counterbores | Screw holes that match real hardware |
| Linear / Circular pattern | Repeats a feature in a row, grid or circle | Rows of mounting holes, bolt circles |
Extrude can create a new part, add to an existing part, remove material from it or keep only the intersection. Removing is how you cut holes and slots: sketch the shape on a face, then extrude it through the part as a removal.
- 1SketchOn the Top plane, draw a centre-point circle at the origin and dimension its diameter.
- 2ExtrudeExtrude it as a new part to the spacer length you need.
- 3Cut the boreSketch a smaller concentric circle on the top face and extrude it through as a removal.
- 4FinishAdd a small chamfer to both ends of the bore so a screw or shaft slides in easily.
Finally, check your units. Many FTC kits are designed in millimetres, while some parts and tools use inches. Set the document units to match the kit you use most, and type the unit after a value when you need the other one.
Key terms
- Sketch
- A 2D drawing on a plane or face that is used as the base for 3D features.
- Dimension
- A number that sets a size, distance or angle in a sketch or feature.
- Geometric constraint
- A rule that sets a relationship between sketch entities, such as horizontal, equal or concentric.
- Fully defined sketch
- A sketch where every entity is locked by dimensions and constraints so nothing can move unexpectedly.
- Feature
- An operation such as extrude, revolve or fillet that creates or changes 3D geometry.
- Construction geometry
- Reference lines and circles in a sketch that guide placement but do not form part of the solid.
Practical exercise
Model a fully defined mounting plate
- 1In a new Part Studio, sketch a rectangle on the Top plane with one corner on the origin.
- 2Dimension its width and length, and confirm the whole rectangle turns black.
- 3Add four equal circles near the corners and dimension them from the edges so the sketch stays fully defined.
- 4Extrude the plate to a thickness your team would realistically use, with the holes cut through.
- 5Add fillets to the four outside corners and rename the sketch and features clearly.
- 6Change the plate width and check that the holes move correctly with it.
Knowledge check
Answer every question, then check your answers.