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Stage 3 — Design & CAD

Window: Week 2 through Week 3 (subsystems freeze independently). Gate: CAD Lock (per subsystem). Precondition: Prototype Down-Select passed for that subsystem.

Design converts winning prototypes into a manufacturable, integrated robot. The rules here exist because small teams cannot afford re-machining a plate three times.

Tooling

  • Primary CAD: Onshape. Real-time collaboration, no license/PC limits, works on Chromebooks. Free FRC plan.
  • PDM / versioning: Onshape versions + named releases per part.
  • File naming: <subsystem>-<part>-<rev> e.g. intake-side-plate-A.
  • Alternative: SolidWorks if the team has an existing PDM workflow.

Master Model / Top-Down Assembly

A single skeleton part defines the robot envelope. Every subsystem references the skeleton for its interface dimensions.

Skeleton contains: - Frame perimeter (bumper zone limit). - Drivetrain wheelbase and wheel positions. - Battery + roboRIO + PDH mounting anchor points. - Elevator or scoring-mechanism envelope. - Height / extension limits per game rules.

Change the skeleton, every subsystem updates. Only the Robot Lead (student) + Mechanical Mentor edits the skeleton, and edits require gate-review sign-off after CAD lock.

Part Numbering

Every part gets a number when released. Format:

<team>-<year>-<subsystem>-<seq>-<rev> e.g., 9999-2026-IN-0004-A (intake, part 4, revision A)

Subsystem codes: DT drivetrain, IN intake, SC scorer, EL elevator, CL climber, EE electrical.

Rev letter increments on any dimensional change after first release. A rev bump kills the previous rev's fabrication tickets.

Part Release Process

A part is not "released" (i.e., cannot be ordered or machined) until:

  1. Part fits in the master-model context — no interference in assembly.
  2. Drawing exists if the part is machined (tolerances, material, quantity, finish).
  3. Reviewed by a second CAD person (student or mentor) — the CAD lead does not self-approve.
  4. Part number assigned per convention above.
  5. Entered in the BOM with vendor, stock link or drawing file, quantity, and lead time.

Part release checklist template: templates/part_release.md.

Subsystem CAD Lock Gate

Each subsystem hits CAD Lock when:

  • All parts released.
  • BOM complete, orders placed for long-lead items.
  • Master-model interfaces verified — no floating unconnected components.
  • Weight budget checked. Center-of-gravity checked.
  • Motor selection, gear ratio, and travel calculations documented.
  • Reviewed at CAD Lock gate meeting (see 07_gate_reviews.md).

After CAD Lock, all changes go through Engineering Change Request (templates/ecr.md). No ad-hoc "just tweak that plate."

Typical Freeze Cadence (6-week season)

Subsystem CAD Lock target
Drivetrain end of Week 2
Primary scoring mechanism end of Week 3
Secondary mechanism / endgame mid-Week 4
Electrical panel mid-Week 4

Drivetrain freezes first because everything mounts to it.

Deliverables (CAD Lock Gate)

  • Complete subsystem assembly in master model.
  • Every part numbered and drawing-released if machined.
  • BOM with vendor, cost, quantity, lead time — practice-bot + comp-bot + spares.
  • Weight + COG analysis.
  • Motor / gear-ratio calculation notes.
  • Second-reviewer sign-off.

Common Failure Modes

  • No master model. Two subsystems independently claim the same volume; discovered at assembly.
  • CAD lead self-approves. Small errors ship. Always second-reviewer.
  • No part numbers. "Which side plate?" becomes an hourly question by Week 4.
  • BOM in someone's head. Long-lead item forgotten, arrives after event 1.
  • Weight discovered late. Robot over 125 lb the day before bag/ship. Track it every gate.
  • "Just a small change." Every unlogged change is a landmine at integration. Force ECR.

Roles

  • CAD Lead (student, senior): owns master model, approves releases, runs CAD gate.
  • Subsystem CAD Owner (student): designs their subsystem, produces drawings.
  • Second Reviewer (student or mentor): required signoff, not the same as the CAD Lead for that part.
  • Mechanical Mentor: blocks bad ideas, teaches tolerances, watches weight budget.

Handoff to Stage 4 (Manufacturing)

CAD Lock triggers: - BOM handed to purchasing student for order. - Drawings handed to fabrication student for machining queue. - Part numbers appear on the manufacturing kanban.

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