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Mechanical, Build & Pneumatics·Lesson 18 of 47

COTS vs. Custom, and Prototyping

Knowing what to buy and what to build, then proving it works in cardboard and wood before you cut metal.

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COTS vs. custom

COTS (Commercial Off-The-Shelf) parts are pre-made components you buy from vendors like REV Robotics, AndyMark, WestCoast Products (WCP)/VEXpro, CTR Electronics, and The Thrifty Bot (TTB). Custom parts are ones your team designs and fabricates.

Buy COTS when:

  • The part is hard to make well (gearboxes, swerve modules, bearings, gears, sprockets, belts).
  • A proven design exists (KitBot chassis, COTS intake/launcher kits).
  • Time is short — COTS skips design and machining.

Go custom when:

  • Nothing off-the-shelf fits your geometry or weight budget.
  • The part is simple to make (a plate, a bracket, a standoff).
  • A custom part gives a real competitive edge worth the time.

Most strong robots are a hybrid: COTS drivetrain/gearboxes/modules plus custom plates, mounts, and mechanism geometry. The skill is spending your limited build time only where custom design pays off.

Prototyping

Never build a mechanism in aluminum first. Prototype in cheap, fast materials to learn what works:

  • Wood, plywood, cardboard, and 3D prints to mock up geometry.
  • Versa-frame/MAXTube scraps, zip ties, and clamps to hold a quick test rig together.
  • Test the real game piece against rollers, flywheels, or grippers to dial in compression, wheel speed, and angles before committing to a final design.

For intakes and shooters, prototype to find the right surface speed and compression. For arms and elevators, prototype to dial in motors, gear ratio, and speed, since those need precision and a wrong ratio means rebuilding.

CAD and the design workflow

Many teams design in Onshape (free for FRC/FTC teams, cloud-based, runs in a browser on Macs and Chromebooks), which has large libraries of COTS parts you can drop into an assembly. The workflow: prototype → confirm key numbers (ratios, speeds, compression) → CAD the real design around COTS parts → fabricate → assemble → test → iterate. CAD lets you check fit, weight, and motion before cutting anything.

Assembly best practices

  • Dry-fit subassemblies before final torque.
  • Use threadlocker/nylocks on every fastener that sees vibration.
  • Route and strain-relieve wires away from moving parts.
  • Label and bag spare parts; build a thorough pit kit.
  • Leave access for battery swaps and common repairs.

Key takeaways

  • Buy COTS for hard-to-make precision parts (gearboxes, swerve modules, bearings) and fabricate custom plates/mounts/geometry
  • Prototype mechanisms in wood, cardboard, and 3D prints with the real game piece before cutting metal
  • Design around COTS parts in CAD (e.g., Onshape), then iterate: prototype → confirm numbers → CAD → build → test

Go deeper

Lesson quiz

Required

Answer all 3 questions correctly to complete this lesson.

1.According to the FRC Game Manual, which of the following best describes a COTS (Commercial-Off-The-Shelf) item?

2.What is the main advantage of building a custom part instead of using a COTS part?

3.What is the primary purpose of prototyping a mechanism before committing to a final design?

Answer every question to submit.