Sketching: The Foundation of Every Part

Every part starts as a sketch

A sketch is a 2D drawing on a flat plane (or a flat face of a part) that defines the shape you will later turn into 3D. Almost every feature — extrude, revolve, cut — starts from a sketch. Good sketching habits are the difference between a model that updates cleanly and one that breaks every time you change a number.

Pick a plane and draw

Onshape gives you three default planes: Top, Front, and Right. Start a sketch on one of them (or on a flat face of an existing part). Then use sketch tools: line, rectangle, circle, arc, slot.

Constraints: telling the sketch how to behave

Raw lines are floppy. Constraints lock down relationships so the sketch behaves predictably:

• Coincident — two points meet.
• Horizontal / Vertical — a line stays level or upright.
• Parallel / Perpendicular — relationships between lines.
• Equal — two lengths or radii match.
• Tangent — a line meets a circle smoothly.
• Concentric — circles share a center.

Dimensions: setting the numbers

Dimensions set actual sizes — a line is 2 in long, a hole is 0.196 in in diameter (the standard #10 clearance hole), two holes are 0.5 in apart. In FRC you will use inches constantly because COTS structure uses inch hole patterns.

Fully define your sketches

A sketch is fully defined when every point and line is pinned down by constraints and dimensions. Onshape colors a fully defined sketch black; an under-defined (floppy) sketch shows blue. Aim for black. A fully defined sketch updates predictably and won’t shift unexpectedly when you edit it later. This discipline is one of the biggest markers of a strong CAD designer.

A concrete FRC example

To model a 2x1 in box tube cross-section:

1. Start a sketch on the Front plane.
2. Draw a rectangle and dimension it 2 in by 1 in.
3. Add a coincident constraint to put one corner on the origin (anchoring the sketch).
4. Confirm it turns black — fully defined.

Later you will extrude this into a tube and add the hole pattern.

Tips for beginners

• Anchor to the origin. Constrain at least one point to the origin so the sketch can’t float away.
• Constrain before dimensioning. Add geometric relationships first, then numbers.
• Keep sketches simple. Several small features are easier to edit than one giant complicated sketch.
• Use construction lines (toggle a line to ‘construction’) for reference geometry that won’t become a real edge.

Work through the sketching exercises in Stage 1A of FRCDesign.org to drill these until they are automatic.

Key takeaways

• Almost every 3D feature begins as a 2D sketch on a plane or face
• Constraints (coincident, parallel, equal, tangent, etc.) define relationships; dimensions set actual sizes
• Aim to fully define every sketch (black in Onshape) so the model updates predictably and never shifts unexpectedly
• Anchor sketches to the origin and keep them simple to make editing easy later

This lesson was adapted from learnfrc.com.