PNX RAX-03 – Open-Source 5-Axis 3D-Printed Robotic Arm

PNX RAX-03 – Open-Source 5-Axis 3D-Printed Robotic Arm

PNX RAX-03 is my third-generation experimental robotic arm platform, designed around FDM 3D printing, NEMA 17 stepper motors, custom gearboxes, and replaceable printed mechanical components.

Rather than building the robot around commercial actuators, I designed the main mechanical system myself, including:

• Joint housings

• Links

• Bearing support structures

• Motor mounts

• Joint covers

• Output structures

• Gearbox integration

• Custom 36:1 gearboxes

The goal of RAX-03 is to create a robotic arm platform that can be printed, assembled, modified, repaired, and experimented with using commonly available components.

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Current Project Status

The current release has completed:

• Full mechanical assembly

• Gearbox assembly

• Motor installation

• Joint rotation checks

• Full-arm assembly verification

• Basic operation testing

Current validation level: Assembly Tested / Basic Operation Tested

Long-duration endurance testing, maximum payload testing, repeatability testing, gearbox lifetime testing, and structural fatigue testing have not yet been completed.

Please begin with low-speed, no-load testing after assembly.

This is an experimental development platform, not a finished industrial robot.

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Motor Configuration

PNX RAX-03 uses five NEMA 17 stepper motors.

• Base: NEMA 17, 60 mm

• Shoulder: NEMA 17, 60 mm

• Elbow: NEMA 17, 48 mm

• Wrist 1: NEMA 17, 40 mm

• Wrist 2: NEMA 17, 40 mm

Larger motors are concentrated near the base and shoulder, while smaller motors are used toward the wrist to reduce distal mass.

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Recommended Print Settings

All printed parts of the current prototype were produced using:

• Material: PETG

• Nozzle: 0.4 mm

• Walls / Perimeters: 4

• Infill: 15%

• Infill Pattern: Gyroid

Some fits are intentionally tight to reduce joint play.

Do not immediately drill, sand, file, or enlarge a part simply because the fit feels tight.

Try the intended assembly method first.

Kapton tape can be used for small tolerance corrections where necessary.

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Bearings

Different joints use different bearings.

• 6914ZZ — 70 × 100 × 16 mm × 1

• 51117 thrust bearing — 85 × 110 × 19 mm × 1

• 6006 series — 30 × 55 × 13 mm × 1

• S6807DD — 35 × 47 × 7 mm × 1

• S6806DD — 30 × 42 × 7 mm × 2

The bearings are selected according to the geometry and load path of each joint and should not be treated as interchangeable.

Several 6 mm BB balls are also used as part of the joint-axis reinforcement/support structure.

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Required Hardware

Prepare the following hardware before assembly.

Heat-Set Inserts

• M3 × 5 mm heat-set inserts

• Prepare multiple pieces

• It is recommended to keep approximately twice the expected quantity available

6 mm BB Balls

• 6 mm BB balls

• Multiple pieces

• Used for joint-axis reinforcement/support

Socket-Head Screws

• M3 × 12 mm — 40 pcs

  • Gearbox → joint cover

• M3 × 20 mm — 6 pcs

  • Base rotation fixing section

  • Maximum usable length: approximately 32 mm

• M3 × 16 mm — 18 pcs

  • Joint → motor output section, excluding base

  • Maximum usable length: approximately 20 mm

• M3 × 16 mm — 18 pcs

  • Link → joint cover

Use correctly sized M3 metal flat washers with normal socket-head screws.

Do not apply excessive tightening force to printed PETG parts.

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Countersunk Screws

• M3 × 8 mm — 23 pcs

  • Joint honeycomb covers

• M3 × 12 mm — 24 pcs

  • Gearbox → motor

  • Used when a rubber pad is installed between the motor and gearbox

• M3 × 15 mm — 32 pcs

  • Gearbox upper → gearbox lower housing

• M3 × 8 mm — 24 pcs

  • Output section → bearing section

  • Excluding base

• M3 × 15 mm — 6 pcs

  • Base output section → bearing section

If no rubber pad is used between the motor and gearbox, use M3 × 8 mm countersunk screws for the motor connection instead of M3 × 12 mm.

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Prepare Extra Hardware

I strongly recommend preparing approximately twice the calculated quantity of small hardware and consumable parts.

This includes:

• Screws

• Heat-set inserts

• Washers

• BB balls

• Kapton tape

• Small printed parts

Extra parts are useful for reassembly, lost screws, failed insert installation, tolerance adjustment, and replacement.

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Custom Gearbox

RAX-03 uses my custom:

PNX CHG-3617 – 36:1 Compound Helical Gearbox for NEMA 17

Gearbox project:

https://www.printables.com/model/1811248-pnx-chg-3617-361-compound-helical-gearbox-for-nema

The gearbox modules should preferably be fully assembled before installing them into the robot joints.

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Gearbox Lubrication — Required

Apply lubricating grease to the gear teeth before closing the gearbox housing.

Do not operate the gearbox dry.

Use a grease compatible with PETG or the plastic material used for the gears.

Apply a thin and even layer to the actual meshing surfaces, including:

• Input sun gear

• Compound planet gears

• Upper sun gear

• Ring gear contact surfaces

The gearbox does not need to be filled completely with grease.

The objective is to maintain a consistent lubricant film on the gear teeth.

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Gearbox Planet Alignment

The gearbox uses three compound planet gears positioned at 120° intervals.

Before assembly, mark the three planet positions with a permanent marker or similar tool.

If one gear feels significantly tighter than the others, do not force the housing closed.

Check for:

• Stringing

• Burrs

• Elephant foot

• Incorrect planet position

• Input shaft eccentricity

• Gear tooth defects

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Pre-Assemble the Gearbox Modules

It is much easier to build the complete:

Motor + Gearbox + Output Module

before installing it into the joint.

Recommended sequence:

1. Inspect printed gears

2. Apply grease

3. Assemble internal gearbox components

4. Assemble output/bearing section

5. Close gearbox housing

6. Install NEMA 17 motor

7. Check gearbox rotation manually

8. Install completed module into the robot joint

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Installing the Motor/Gearbox Module Into the Joint

The completed motor/gearbox module may not slide straight into the joint.

This is intentional due to the compact joint geometry.

Insert the motor housing from the cable side first.

Then:

1. Insert the cable side of the motor housing

2. Tilt the module

3. Rotate it around the joint axis

4. Adjust the angle and position

5. Align the gearbox output

6. Seat the complete module into its final position

In short:

Cable Side First → Tilt → Rotate → Align → Seat

Do not immediately grind or enlarge the joint housing because the module does not enter in a straight line.

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Tight Fits Are Intentional

Some interfaces are intentionally designed slightly tight to reduce mechanical play.

This may include:

• Bearing seats

• Motor housings

• Gearbox module interfaces

• Joint-axis interfaces

• Link connections

Do not immediately modify these parts using:

• Drills

• Files

• Sandpaper

• Knives

• Reamers

First verify the correct orientation and assembly method.

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Kapton Tape for Tolerance Adjustment

Small FDM dimensional differences can be corrected using Kapton tape.

Typical uses include:

• Small bearing-seat clearance

• Slightly loose gearbox fit

• Minor shaft clearance

• Small joint interface gaps

Apply one layer at a time and test the fit.

Do not compensate for clearance by excessively tightening bolts and deforming the printed structure.

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CA Glue

A small amount of CA glue may be useful during assembly.

Typical uses:

• Holding small printed parts

• Temporary positioning

• Retaining Kapton tape

• Preventing small parts from falling out during assembly

Avoid permanently gluing bearings, gearboxes, or major joints that may need future disassembly.

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Fastener Tightening

Use metal washers with normal socket-head screws.

Do not overtighten bolts against PETG.

Recommended method:

1. Start all screws loosely

2. Do not fully tighten one screw first

3. Tighten opposite screws gradually

4. Keep the mating surfaces even

5. Stop once the parts are securely seated

Countersunk screws can generate outward wedging forces in printed holes, so they should also not be overtightened.

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Cable Routing

Route motor cables before closing the joint covers.

Check that:

• Cables do not contact gears

• Cables do not contact motor shafts

• Cables cannot be pinched by rotating joints

• Cables are not excessively tensioned at maximum joint angles

• Screws and covers do not crush the wires

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Final Mechanical Inspection

Before applying motor power, check:

• Correct bearing installed at each joint

• 6 mm BB-ball reinforcement properly assembled

• Heat-set inserts are secure

• Socket-head screws use metal washers

• No screws are excessively tightened

• Gearboxes are properly lubricated

• Each gearbox rotates manually

• No repeated binding occurs at specific angles

• Joint play is not excessive

• Links are not twisted during assembly

• Motor cables are not pinched

• Joint covers do not interfere with moving parts

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Recommended First Test

Do not begin at maximum speed or with a heavy payload.

Recommended sequence:

Manual Rotation Check
→ No-Load Low-Speed Test
→ Forward / Reverse Test
→ Individual Joint Test
→ Gradual Speed Increase
→ Full-Arm Low-Speed Test
→ Light-Load Test

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Documentation

Detailed assembly manuals are included in:

• English

• Korean

Please read the assembly guide before starting the build.

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Project Goal

PNX RAX-03 is not intended to be presented as a finished industrial robot.

The project is intended to explore a complete robotic system developed around:

Gearbox
→ Joint Structure
→ Bearing Support
→ Links
→ Motors
→ Electronics
→ Sensors
→ Control Software

The goal is to create an open-source experimental robotic arm platform that can be printed, assembled, tested, modified, repaired, and improved.

The current release has completed full mechanical assembly and basic operation testing, but further long-term durability and performance testing is still required.

Build it, test it, modify it, and improve it.

Customization & Control System

If additional hardware such as distance sensors, cameras, LiDAR modules, limit switches, or other sensors is required, download the included STEP files and modify the corresponding parts to suit your own hardware.

The mechanical design is intended to serve as a customizable platform rather than a fixed sensor configuration.

A control box is not included with PNX RAX-03.

This is intentional. You are free to design your own control system and enclosure according to your requirements, including your choice of:

• Microcontroller or motion controller

• Stepper drivers

• Power supply

• Sensors

• Communication hardware

• Emergency-stop system

• Wiring and connectors

• Control-box layout

The provided files primarily define the robotic arm's mechanical platform, leaving the electronics and control architecture open for modification and experimentation.

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New Revision — 6810 Bearing Joint Upgrade

A newer mechanical revision of PNX RAX-03 has now been developed for the higher-load joints.

The updated design introduces a 6810 deep-groove ball bearing (50 × 65 × 7 mm) in the revised output-support structure.
The supplier specification for the selected 6810 bearing lists a dynamic load rating of approximately 6400 N.

This revision was created primarily to improve the shoulder and elbow joint support structure rather than simply increasing bearing capacity.

Main Changes

• Larger output-bearing diameter

  • Previous compact joint-support structures used smaller bearings and smaller support diameters.

  • The new 6810 bearing increases the inner support diameter to 50 mm and the outer diameter to 65 mm.

• Wider load-transfer path

  • Joint loads are transferred through a larger-diameter output structure.

  • This reduces local stress concentration in the printed PETG parts and distributes bending loads over a wider area.

• Improved shoulder-joint fastening layout

  • The previous shoulder-axis design concentrated six heat-set inserts close to the joint center, at a radius of approximately 12–14 mm.

  • The new design moves the fastening pattern outward and distributes the connection over a much larger flange area.

  • This reduces the force carried by each individual insert and decreases local compression of the PETG around the insert locations.

• Improved elbow-joint fastening layout

  • The revised elbow interface uses six evenly spaced fastening points on a larger bolt circle.

  • The larger fastening radius improves resistance to joint bending moment and torsional load.

• Reduced localized BB-ball pressure

  • The previous structure used localized 6 mm BB-ball support in areas where concentrated contact pressure could leave visible compression marks in the printed material.

  • The new 6810-bearing structure replaces that localized support approach in the revised joints with a larger, continuous steel bearing interface.

• Better joint stiffness distribution

  • The larger bearing and wider insert pattern allow the joint housing, output flange, and link interface to share load over a larger diameter.

  • The design is intended to reduce local deformation and improve long-term structural stability.

Why This Revision Was Made

The previous version was functional, but the shoulder and elbow joints concentrated too much load near the center of the joint.

Under higher bending loads, this could produce:

• Local PETG compression around inserts

• Visible pressure marks from BB-ball support points

• Higher force per fastener

• Greater sensitivity to printed-part deformation

• Lower resistance to joint tilting and bending moment

The 6810 revision addresses these issues by increasing the bearing diameter and moving the structural load path outward.

In simple terms, the new design changes the joint from a small central support structure into a larger flange-style support structure.

Compatibility Note

The 6810 revision uses different bearing geometry and revised joint/output parts.

Do not assume that old and new bearing-support parts are interchangeable.

When building the 6810 version, use the matching updated:

• Gearbox output parts

• Bearing-support parts

• Joint housings

• Link/interface parts

The corresponding STEP files should be used as a matched set.

V6 File Packages

Two separate V2 file packages are provided for the 6810 revision.

• Full V6 Package

  • Contains the complete updated RAX-03 mechanical file set for the new revision.

  • Recommended for new builds or users who want to start directly with the latest version.

• V6 Replacement Parts Package

  • Contains only the parts that were changed for the new 6810-bearing revision.

  • Intended for users who already have the previous version and only want to replace the updated sections without downloading or reprinting the entire robot.

Choose the package that matches your current build status.
For a completely new build, use the Full V6 Package.
For an existing RAX-03 build, use the V6 Replacement Parts Package to update only the revised components.

Validation Status of the 6810 Revision

The 6810 version is a newer mechanical revision and should currently be treated as an experimental upgrade.

The design objective is to improve:

• Shoulder and elbow joint stiffness

• Load distribution

• Insert durability

• Bearing support diameter

• Resistance to local PETG deformation

Long-term endurance, maximum payload, bearing-life, and fatigue testing of the revised 6810 structure are still required.

Further revisions may be released as testing continues.

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Hall Sensor HOME System Update

This update adds a non-contact Hall sensor HOME system to each joint of the robot arm.

Hall Sensor

The HOME sensor used in this design is the WSH131-XPAN3 omnipolar Hall effect sensor.

• Sensor: WSH131-XPAN3

• Type: Omnipolar Hall switch

• Output: Open-collector

• Detects both north and south magnetic poles

• No magnet detected: HIGH

• Magnet detected: LOW

Because the sensor is omnipolar, the magnet polarity is not critical. However, the physical orientation of the Hall sensor still matters, so it is recommended to verify the sensing direction with the magnet before final assembly.

Magnet

The standard magnet used for each HOME sensor is:

N35 neodymium disc magnet, Ø5 × 5 mm

One Ø5 × 5 mm magnet is used per joint by default.

The Hall sensor is installed on the stationary part of the joint, while the magnet is installed in the rotating part. As the joint rotates, the magnet passes through the sensor’s detection area and creates the HOME sensing region.

Increasing the Magnetic Detection Margin

If one Ø5 × 5 mm magnet does not provide reliable detection because of assembly tolerances or a larger sensor-to-magnet gap, additional magnets can be stacked axially to increase the total magnetic length.

Configuration	Magnet arrangement	Total size
Standard	1 × Ø5 × 5 mm	Ø5 × 5 mm
Extended	2 × Ø5 × 5 mm	Ø5 × 10 mm
Extended	3 × Ø5 × 5 mm	Ø5 × 15 mm

For example:

Standard
[N35 5×5]
2× length
[N35 5×5][N35 5×5]
3× length
[N35 5×5][N35 5×5][N35 5×5]

The magnets should be stacked in the same axial magnetization direction.

Increasing the magnet length does not mean that the sensing distance will increase exactly 2× or 3×. The purpose is to increase the available magnetic field and provide more tolerance for the sensor-to-magnet distance and assembly variation.

Installation

1. Install the WSH131-XPAN3 Hall sensor in the fixed sensor mount.

2. Insert one N35 Ø5 × 5 mm neodymium magnet into the corresponding rotating joint part.

3. Slowly rotate the joint and verify that the sensor reliably changes to LOW when the magnet passes the sensing area.

4. If detection is weak or inconsistent, add another Ø5 × 5 mm magnet.

5. If necessary, up to three magnets can be stacked to create an effective Ø5 × 15 mm magnetic assembly.

6. After reliable detection is confirmed, permanently secure the magnet in the printed pocket.

It is recommended to leave a reasonable detection margin instead of operating at the absolute maximum sensing distance.

HOME Position Calculation

The Hall sensor is not used as a simple single-edge limit switch.

During the HOME sequence, the controller measures both edges of the magnetic detection region.

HIGH → LOW
Detection start point A

LOW → HIGH
Detection end point B

The HOME position is then calculated from the center of the detected region:

HOME=A+B2HOME=\frac{A+B}{2}

The joint then moves back to this calculated center position and sets it as the software zero position.

This method reduces sensitivity to small differences in magnet placement, Hall sensor mounting, and switching distance, providing a more repeatable HOME reference than using only a single detection edge.