Triangle Frame

Open with a demo: push on a hand-held rectangle of cardboard strips pinned at corners, then add one diagonal. The rectangle collapses; the triangle does not. Tie it to robots: an unbraced 2x1 elevator tower flexes under a NEO-driven carriage and ruins your shooter aim. Tell them every good FRC frame hides triangles inside it.

Show the cross-section in Fusion. Stress that 2x1 with a 1/16 wall is the team default; 1/8 wall exists but is heavy. Mention that real tube comes pre-drilled in some lengths from WCP/AndyMark but we model plain stock. Common mistake: students model a solid bar, not a hollow tube. We want the wall so it looks and weighs right.

Key teaching point that mirrors frcdesign: design the SKELETON before the geometry. The triangle of construction/centerlines is your single source of truth. Demo: Create Sketch, pick XY, draw three lines forming a closed loop. If the loop closes, the profile shades — that confirms it is closed. Students often leave a tiny gap; zoom in and check.

Walk through Sketch Dimension (keyboard D). Dimension the base length, then use the angular dimension between two lines for the brace angle. Anchor one vertex to the origin with a coincident constraint so the whole triangle cannot float. Repeat the mantra: blue is underdefined, black is fully defined. An underdefined skeleton will shift later and break your tubes.

In Fusion, make each tube a separate Component so you can joint them later and get a real BOM. Demo: Assemble > New Component (Activate it), sketch the 2x1 rectangle on a plane perpendicular to a skeleton line, extrude along the line. Emphasize Component vs Body now — this is the Onshape Part Studio equivalent. Students forget to activate the new component and everything piles into one.

This is the lesson's core skill. At a corner where two tubes meet at, say, 60 degrees, each tube end is mitered so they share a flush seam. Easiest Fusion method: model both tubes overlapping past the vertex, then use Modify > Combine (Cut) with one tube as the tool body, or use a sketched cut line on a construction plane bisecting the angle. Common mistake: cutting the full angle on one tube instead of splitting it — the seam ends up lopsided.

Demo Combine carefully. Target body is the tube you want to keep; Tool body is the overlapping neighbor. Set Operation to Cut and check Keep Tools so you do not delete the other tube. Do this corner by corner. Students panic when Combine eats a whole body — that means they swapped Target and Tool, or left Keep Tools unchecked. Undo and retry.

In Fusion, Joints replace Onshape Mates. A static frame uses Rigid joints (vs Revolute for spinning, Slider for linear). Demo: Assemble > Joint, select a face/edge on each tube, set type Rigid. If a part jumps, use Capture Position first. Beginners reach for As-Built Joint here, which is fine for already-positioned parts — mention both. Stress: a frame should have zero DOF when done.