Mini-Project 4: A Lightened Structural Plate

Weight is a constant FRC battle, and structural plates are where most of it hides. This mini-project turns a solid bracket into a properly lightened, manufacturable part.

Step 1 — Model the solid first. A core rule from gm0's pocketing guide: design pocketing last. Build the bracket's full outline and all functional holes (bearing bores, bolt patterns, mounting tabs) as a solid 0.190 in aluminum part. Get every interface correct before removing a single gram.

Step 2 — Identify keep-out zones. Sketch offset boundaries around each hole and edge — typically keep roughly 0.15-0.25 in of material around bolt holes and webbing between pockets. These offsets become the 'do not cut' regions. Doing this parametrically (offset by a variable #web = 0.2 in) lets you tune the aggressiveness later.

Step 3 — Lighten parametrically. Use the Part Lighten FeatureScript (2471's FeatureScripts) instead of hand-sketching triangles. It gives control over rib width, cut depth, and fillet radius. Round every internal corner — sharp internal corners are stress concentrators and crack under cyclic loads.

Step 4 — Respect the process. Match the pocket style to the machine: waterjet and router want continuous closed loops with a minimum internal radius the tool/jet can actually produce. A 1/4 in router endmill cannot make an internal corner sharper than 1/8 in radius (its tool radius); a waterjet can hold tighter corners but you should still avoid knife-edges. If you only have a drill press, full pockets are impossible — use a hole-saw or a grid of drilled holes instead. The geometry must be cuttable on the machine you actually own.

Step 5 — Validate stiffness. Onshape's built-in Simulation runs linear-static FEA in the browser: apply the expected load and fixtures, then read the stress and displacement. FEA tells you where it is safe to remove material, what thickness suffices, and which areas need reinforcement. If a rib shows a hot spot near yield, widen it (#web up) and re-run; if everything is deep blue with tiny displacement, lighten more aggressively. Iterate until you have the lightest part that stays comfortably below the aluminum yield with a safety margin.

The payoff compounds: trimming a few tenths of a pound off each of a dozen brackets adds up to pounds back in your budget for a climber or a second mechanism.