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CAD & Design·Lesson 20 of 31

Mini-Project 3: A Pivoting Arm on 2x1 Tube

Design a single-jointed arm pivot using in-context layout geometry, a dead-axle hex pivot, and a hardstop, using a dead-axle clamping approach like the one documented in FRC 6328's 2023 A-frame arm.

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Pivoting arms are everywhere in FRC. We will build a single-joint arm that pivots on a fixed (dead) axle, using a layout sketch to drive the geometry. For the pivot hardware we borrow the dead-axle clamping idea that FRC 6328 documents in its 2023 A-frame arm (note: 6328's example is a multi-jointed A-frame with a custom MAXSpline gearbox — we are taking just the dead-axle clamping pattern, not the full mechanism).

Step 1 — Layout the pivot and reach. In a Part Studio, sketch a line representing the arm at its stowed angle and another at its scoring angle, sharing the pivot point. Dimension the pivot height above the belly pan and the arm length to your target reach. Add #stowAngle and #scoreAngle variables so you can sweep the range later. Keep the swept arc inside the legal extension/height envelope for the season's game (always re-check the current FRC Game Manual section on robot extension — e.g., the 2026 REBUILT manual limits extension beyond the robot perimeter).

Step 2 — Build the tube arm. Extrude a 2x1 tube along the arm line. REV MAXTube 2x1 or AndyMark pre-drilled tube both carry the #10-on-0.5-in hole grid, so mounting plates land on existing holes. Use Tube Converter to add the lightening pattern.

Step 3 — Pivot hardware. Use a dead axle: a fixed 0.500 in hex shaft clamped to the side plates, with bearings pressed into the arm so the arm rotates on the axle. Place flanged hex bearings (1/2 in hex ID) at the pivot. Model the side plates from 0.190 in aluminum with a bore for the bearing OD and a bolt pattern to the frame.

Step 4 — Add a gearbox interface. Drive the pivot with a planetary stack (MAXPlanetary or VersaPlanetary) feeding a chain or belt reduction to the arm. Sketch the sprocket/pulley centers and use Belt & Chain Gen to generate a correct chain run with the right center distance and tooth count — this avoids the classic 'chain is one link too short' mistake.

Step 5 — Hardstops and CG. Add a physical hardstop block at #stowAngle. Then open mass properties at both stow and score positions and read the center of mass — arms dramatically shift CG. CAD computes CG automatically; check that the COM stays inside the wheelbase at full extension so the robot will not tip. If it drifts outside the track, shorten the arm or move heavy components inboard.

This mirrors how top teams iterate: gusset-and-tube construction (a style 6328 favors) lets you nudge pivot and arm geometry after the first prototype without a full redesign.

Key takeaways

  • Drive arm geometry from a layout sketch with `#stowAngle`/`#scoreAngle` so you can sweep the motion range and check the legal envelope
  • Use a dead-axle hex pivot with flanged hex bearings and generate the chain run with Belt & Chain Gen to get center distance right
  • Read mass-properties center of mass at both arm extremes and keep it inside the wheelbase to avoid tipping

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Lesson quiz

Required

Answer all 3 questions correctly to complete this lesson.

1.When a pivoting arm built on 2x1 tube uses a 'dead axle' pivot, what does that mean?

2.Why are dead axles generally preferred over live axles for a heavily-loaded arm pivot?

3.On a 2x1 tube arm, which standard hex shaft sizes are most commonly used for the pivot?

Answer every question to submit.