# M2. One joint end to end: the wrist and joint 6

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**Whole task:** print, assemble and wire the wrist, joint 6. Prove its homing sensor triggers, then jog joint 6 to a commanded angle and measure it.

## Why the wrist first

The official building instructions say the robot "needs to be built from wrist to the base. … Any other order will not work", because every wire passes down through the robot ([building instructions](https://docs.source-robotics.com/PAROL-docs/1.1/assets/pdf/PAROL6_building_instructions.pdf#page=7), page 7). The wrist is the first official assembly (pages 20 to 23). So M2 is also the real first step of [M3](/lore/m3-whole-arm), and nothing gets torn down afterwards.

Stock firmware homes all six joints in one sequence ([Homing](https://docs.source-robotics.com/PAROL-docs/1.1/05-operation/homing/)), so a single joint cannot finish homing. M2 proves a sensor trigger plus a jogged move, and homing stays in M3.

## Parts

From the [building instructions](https://docs.source-robotics.com/PAROL-docs/1.1/assets/pdf/PAROL6_building_instructions.pdf), pages 21 to 23 and 97 to 98:

- The printed wrist part.
- The EG 10:1 planetary gearbox, Stepper 1 (the 20 mm, 16 N·cm motor; [Drives, ratios and resolution](https://docs.source-robotics.com/PAROL-docs/1.1/03-technical-data/drives-and-ratios/)), and Sensor 1, the joint 6 inductive sensor.
- Bearing 1, the AXK3552 thrust bearing (page 22).
- Screws: 3 × M4 10 mm and 4 × M3 8 mm. The gearbox's coupler uses its own set screws (page 21). An M3 14 mm screw and nut clamp the sensor if it is loose (page 22).
- The shaft coupler on the gripper holder, and the M3 × 14 mm homing screw, which must trigger the sensor (pages 97 and 98).
- An ST-Link and the PAROL6 programming adapter ([Firmware](https://docs.source-robotics.com/PAROL-docs/1.1/board/08-firmware/)).

[Print timelapses](/lore/timelapses): [Plate 0: wrist and E-stop box](/lore/timelapses#timelapse-plate0-wrist-estop).

## Concepts

- **Planetary gearbox ratio and step math.** Joint 6 has 64,000 steps per joint turn, so one step is 0.00563° ([Drives, ratios and resolution](https://docs.source-robotics.com/PAROL-docs/1.1/03-technical-data/drives-and-ratios/)).
- **Backlash.** The EG series has 15 to 20 arc-minutes; the cheaper MG series has more ([Drives, ratios and resolution](https://docs.source-robotics.com/PAROL-docs/1.1/03-technical-data/drives-and-ratios/)).
- **How an inductive sensor sees a steel screw head.** Joints 1, 4 and 6 home with inductive sensors triggered by small screws ([building instructions](https://docs.source-robotics.com/PAROL-docs/1.1/assets/pdf/PAROL6_building_instructions.pdf#page=23), page 23).
- **Couplers, set screws and blue threadlocker** ([Couplers and threadlocker](https://docs.source-robotics.com/PAROL-docs/1.1/07-maintenance/couplers/)).
- **Why PETG.** The parts around the motors reach 60 to 70 °C in long use, which PLA does not survive ([Printing](https://docs.source-robotics.com/PAROL-docs/1.1/build/printing/)).

## How-to

- [Building instructions](https://docs.source-robotics.com/PAROL-docs/1.1/assets/pdf/PAROL6_building_instructions.pdf#page=20), pages 20 to 23 and 97 to 98.
- The official [wrist and forearm assembly video](https://youtu.be/eCXA4BP3y8M), 0:00 to 1:59. Its on-screen note at about 0:20 says to attach the stepper shaft to the gearbox coupler on both sides.
- Firmware upload with an ST-Link and the adapter, 24 V disconnected ([Firmware](https://docs.source-robotics.com/PAROL-docs/1.1/board/08-firmware/); [setup and calibration video, from 0:31](https://youtu.be/rv8zt6-3rvo?t=31)).
- The direction check and fix ([Firmware and first run](https://docs.source-robotics.com/PAROL-docs/1.1/build/firmware-and-first-run/), section 4; [setup and calibration video, from 3:21](https://youtu.be/rv8zt6-3rvo?t=201)).
- [Jogging](https://docs.source-robotics.com/PAROL-docs/1.1/05-operation/jogging/).

## Practice

- The print fit check: test the bearing seat, and ream it if tight ([Printing](https://docs.source-robotics.com/PAROL-docs/1.1/build/printing/)).
- Set screws and threadlocker.
- Adjusting the sensor gap against the homing screw.
- Extending the motor and sensor leads with heat shrink.

## Done check

1. Joint 6 turns smoothly by hand through the gearbox, with no clicks.
2. The test firmware's `#LIMIT` reports the joint 6 sensor as the homing screw passes it. The motor turns freely on the test firmware, so keep fingers clear ([The test firmware](https://docs.source-robotics.com/PAROL-docs/1.1/board/09-test-firmware/)).
3. On the main firmware, a commander or Python API jog of +30° measures 30° on a printed protractor. The API refuses planned moves while the robot is not homed, so this must be a jog.
4. The direction matches the official figure, or gets fixed with `direction_reversed`.

## Risks to settle before filming

- The main firmware may not compile on the current PlatformIO STM32 toolchain. See [pull request 19](https://github.com/Source-Robotics/PAROL6-Desktop-robot-arm/pull/19).
- **needs\_source:** whether the commander finds a serial port on macOS. Our untested reading of its sender is that it builds port names for Windows and Linux only. The fallback is the [Python API](https://docs.source-robotics.com/PAROL-docs/1.1/programming/02-python-api/).

## Terms

- **ST-Link**: the STMicroelectronics programmer that uploads firmware to the board, through the PAROL6 programming adapter or the four named header pins only ([Firmware](https://docs.source-robotics.com/PAROL-docs/1.1/board/08-firmware/)).
- **planetary gearbox**: a compact gearbox on the motor. PAROL6 uses the EG precision series, 20 to 1 on joints 2 and 3 and 10 to 1 on joint 6 ([Drives, ratios and resolution](https://docs.source-robotics.com/PAROL-docs/1.1/03-technical-data/drives-and-ratios/)).
- **transmission ratio**: motor turns per joint turn. Joint 6's gearbox is 10 to 1, so one motor turn of 6,400 microsteps moves the joint 36° ([Drives, ratios and resolution](https://docs.source-robotics.com/PAROL-docs/1.1/03-technical-data/drives-and-ratios/)).
- **backlash**: the play in a drive before the output moves. The EG gearboxes have 15 to 20 arc-minutes of it ([Drives, ratios and resolution](https://docs.source-robotics.com/PAROL-docs/1.1/03-technical-data/drives-and-ratios/)).
- **PETG**: the print material for the parts. It softens around 80 °C, where PLA fails near the warm motors ([Printing](https://docs.source-robotics.com/PAROL-docs/1.1/build/printing/)).
- **threadlocker**: an adhesive for screw threads. Blue goes on every metal-to-metal screw and on all set screws ([Mechanical assembly](https://docs.source-robotics.com/PAROL-docs/1.1/build/mechanical-assembly/)).
- **inductive sensor**: the contactless sensor that homes joints 1, 4 and 6, wired brown to 24 V, blue to GND and black to a LIMIT terminal ([Electronics and wiring](https://docs.source-robotics.com/PAROL-docs/1.1/build/electronics-and-wiring/)).
- **set screw**: a headless screw that clamps a coupler to a shaft. The gearbox's coupler takes two ([building instructions](https://docs.source-robotics.com/PAROL-docs/1.1/assets/pdf/PAROL6_building_instructions.pdf#page=21), page 21).

## Video

[The M2 video](/lore/video-m2) is planned.

## Sources

1. [PAROL6 building instructions](https://docs.source-robotics.com/PAROL-docs/1.1/assets/pdf/PAROL6_building_instructions.pdf)
   Source Robotics. The wrist-to-base build order on page 7, the wrist on pages 20 to 23, and the J6 homing screw on pages 97 and 98. Accessed 2026-11-07.

2. [Drives, ratios and resolution, PAROL6 docs 1.1](https://docs.source-robotics.com/PAROL-docs/1.1/03-technical-data/drives-and-ratios/)
   Source Robotics. The joint 6 motor and gearbox, 64,000 steps per joint turn, and EG gearbox backlash. Accessed 2026-11-07.

3. [Printing, PAROL6 docs 1.1](https://docs.source-robotics.com/PAROL-docs/1.1/build/printing/)
   Source Robotics. Why PETG, and fitting bearing seats. Accessed 2026-11-07.

4. [Firmware, PAROL6 docs 1.1](https://docs.source-robotics.com/PAROL-docs/1.1/board/08-firmware/)
   Source Robotics. Uploading the main firmware with an ST-Link and the programming adapter, 24 V disconnected. Accessed 2026-11-07.

5. [Firmware and first run, PAROL6 docs 1.1](https://docs.source-robotics.com/PAROL-docs/1.1/build/firmware-and-first-run/)
   Source Robotics. Jogging and fixing a joint that turns the wrong way. Accessed 2026-11-07.

6. [Electronics and wiring, PAROL6 docs 1.1](https://docs.source-robotics.com/PAROL-docs/1.1/build/electronics-and-wiring/)
   Source Robotics. Inductive sensors on joints 1, 4 and 6 and their wiring. Accessed 2026-11-07.

7. [Jogging, PAROL6 docs 1.1](https://docs.source-robotics.com/PAROL-docs/1.1/05-operation/jogging/)
   Source Robotics. Joint jog in the commander. Accessed 2026-11-07.

8. [Couplers and threadlocker, PAROL6 docs 1.1](https://docs.source-robotics.com/PAROL-docs/1.1/07-maintenance/couplers/)
   Source Robotics. Set screws, and blue threadlocker on every metal-to-metal screw. Accessed 2026-11-07.

9. [Mechanical assembly, PAROL6 docs 1.1](https://docs.source-robotics.com/PAROL-docs/1.1/build/mechanical-assembly/)
   Source Robotics. Threadlocker on metal-to-metal screws and all set screws. Accessed 2026-11-07.

10. [Homing, PAROL6 docs 1.1](https://docs.source-robotics.com/PAROL-docs/1.1/05-operation/homing/)
    Source Robotics. Homing is one sequence over all six joints. Accessed 2026-11-07.

11. [The test firmware, PAROL6 docs 1.1](https://docs.source-robotics.com/PAROL-docs/1.1/board/09-test-firmware/)
    Source Robotics. The motors turn as soon as the test firmware starts. Accessed 2026-11-07.

12. [PAROL6 Robot arm Wrist and Forearm assembly](https://youtu.be/eCXA4BP3y8M)
    Source Robotics on YouTube. The wrist assembly, from 0:00 to 1:59. Accessed 2026-11-07.

13. [PAROL6 setup and calibration](https://youtu.be/rv8zt6-3rvo)
    Source Robotics on YouTube. Firmware upload at 0:31 and the direction check at 3:21. Accessed 2026-11-07.

14. [PAROL6-Desktop-robot-arm pull request 19](https://github.com/Source-Robotics/PAROL6-Desktop-robot-arm/pull/19)
    Source Robotics on GitHub. The main firmware and the current PlatformIO STM32 toolchain. Accessed 2026-11-07.

## Lore on this page

- [The PAROL6 milestones](/lore/milestones)
- [M3. The whole arm moves](/lore/m3-whole-arm)
- [Build steps by milestone](/lore/build-steps)
- [M1. Bench it: power, one motor, a safe stop](/lore/m1-bench-it)

## What links here

- Agent guide → Joel builds a PAROL6 robot arm, one sourced and filmed milestone at a time. → [Read page](https://parol6.build/llms.txt)

- Build steps by milestone → The official how-to pages for each milestone, in build order, gathered from the milestone pages. → [Read page](https://parol6.build/lore/build-steps)

- Change log → What changed in the files served here, newest first. → [Read page](https://parol6.build/log)

- Concepts by milestone → The mental models each milestone teaches, gathered from the milestone pages, with the glossary for single terms. → [Read page](https://parol6.build/lore/concepts)

- Full agent guide → Joel builds a PAROL6 robot arm, one sourced and filmed milestone at a time. → [Read page](https://parol6.build/llms-full.txt)

- M2 video: One joint end to end → The video for milestone 2 is planned and not yet recorded. → [Read page](https://parol6.build/lore/video-m2)

- M3. The whole arm moves → The third milestone builds the rest of the arm from the wrist down to the base, tests the wiring, then homes the arm, jogs every joint and checks the witness marks. → [Read page](https://parol6.build/lore/m3-whole-arm)

- Milestone videos → Each milestone ends in one video, and every video is planned until Joel publishes it. → [Read page](https://parol6.build/lore/videos)

- PAROL6 build → Joel builds a PAROL6 robot arm, one sourced and filmed milestone at a time. → [Read page](https://parol6.build/)

- PAROL6 wiki | PAROL6 build → The PAROL6 build wiki: the milestones, their videos and the official sources behind them. → [Read page](https://parol6.build/lore)

- Parts by milestone → What goes on the bench at each milestone, gathered from the milestone pages and the official bill of materials. → [Read page](https://parol6.build/lore/parts)

- Print timelapses → Every PAROL6 print plate as a sped-up video from the printer's camera, with its parts, material, layer count, print time and milestones. → [Read page](https://parol6.build/lore/timelapses)

- Source change history (log.md) → The generated change log as Markdown. → [Read page](https://parol6.build/log.md)

- The PAROL6 milestones → Seven milestones take the build from a bare control board to a robot arm that plays a Eurorack skiff. → [Read page](https://parol6.build/lore/milestones)

## Unlinked mentions

- [Concepts by milestone](https://parol6.build/lore/concepts) → Planetary gearbox ratio and step math, backlash, how an inductive sensor sees a steel screw head, couplers and threadlocker, and why the parts are printed in PETG.

- [Parts by milestone](https://parol6.build/lore/parts) → The printed wrist part, the EG 10:1 planetary gearbox, Stepper 1, Sensor 1, the AXK3552 thrust bearing, the shaft coupler and the M3 homing screw, plus an ST-Link and the programming adapter.

- [Print timelapses](https://parol6.build/lore/timelapses) → Material: Orange PETG HF, 40% infill, tree supports.¶