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YOU CAN BUILD THE MACHINE FACTORIES USE TO TEAR MATERIALS APART — OPEN PULL UNIVERSAL TEST MACHINE

The machine that pulls materials apart until they snap, built from two geared stepper motors, two lead screws and a 5 kN load cell.

by Stefan Hermann (CNC Kitchen)

FULL CAD BOM FIRMWARE DOCS

WorkshopScience

Built withArduino3D printing

difficulty
●●●●○
time
a weekend-plus
license
GPL-3.0
repo
repo ACTIVE162 stars

WHAT YOU’LL NEED

Videos
1
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COMPAREE VERDICT

Open Pull is a DIY universal test machine that pulls samples apart with up to 5,000 newtons, built from maker-grade parts. Stefan Hermann of CNC Kitchen uses it in many of his videos to test 3D-printed parts, layer adhesion and filament strength, so you can watch the machine work before you build. The CAD is complete (STL, STEP and Fusion 360), the Arduino firmware and schematics are in the repo, and a community web controller is linked from the README. The catch: the README is marked work in progress, the electronics are schematics plus a contributed first-version carrier PCB that may still contain errors, and there are no written assembly or calibration instructions. If you have built test rigs before or are comfortable working from CAD and video, this is a usable foundation. If you need step-by-step directions, expect to work a lot out yourself. Plan time for wiring and checking the HX711 load cell readings against a known reference.

GOOD TO KNOW

  • —Complete Fusion 360 CAD, STEP and STL files in the repository.
  • —Eagle schematics for electronics and Arduino firmware included.
  • —Bill of materials present, built around two geared NEMA 17 steppers, two trapezoidal lead screws, an HX711 amplifier and an AEP TC4 5 kN load cell.
  • —README explicitly marks the project as work in progress; the electronics come as schematics plus a contributed version-1 carrier PCB (Eagle) that its designer warns may still contain errors.
  • —No assembly instructions or calibration procedure documented.
  • —GPL-3.0 license permits commercial use with source disclosure.

Parts to buy

9 items

From our check of the build. Exact quantities and part numbers are in the creator’s BOM.

  • 2x geared (14:1) NEMA 17 steppersFind
  • 2x 10x2 trapezoidal lead screws with nutsFind
  • Bearings and couplingsFind
  • AEP TC4 5 kN load cellFind
  • HX711Find
  • Arduino NanoFind
  • 2x A4988 driversFind
  • 24 V 5 A supplyFind
  • Hardwarefrom the repo files

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Can I build this?

PrintPrinted parts, specimen grips, alignment tools and test specimens; STL, STEP and Fusion 360 files of the whole assembly are provided.
Buy2x geared (14:1) NEMA 17 steppers, 2x 10x2 trapezoidal lead screws with nuts, bearings and couplings, AEP TC4 5 kN load cell, HX711, Arduino Nano, 2x A4988 drivers, 24 V 5 A supply, hardware.
Tools3D printer, soldering iron, multimeter, breadboard or perf board for electronics, hex keys.
SkillsIntermediate 3D printing, Arduino firmware upload, basic electronics assembly; CAD reading helpful for assembly; no PCB experience required but debugging analog signal chains expected.
TimeA weekend to assemble the frame and mechanics, then a few evenings to wire the electronics, flash the Arduino and calibrate the load cell.
Cost$$: no cost figure is published. The main purchases are the 5 kN load cell, two 14:1 geared NEMA 17 steppers, two 500 mm trapezoidal lead screws with nuts and bearings, and a 24 V supply.
Safety5 kN is half a ton of force; clamp specimens securely and keep hands clear during tests. The machine runs from a 24 V supply — use a closed, properly fused mains PSU.

Build at your own risk. Projects involve tools, electronics and sometimes mains voltage — follow the creator’s safety notes.

Videos

CNC Kitchen video showing Open Pull in use; no assembly walkthrough, but demonstrates operation and typical test specimens.

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Gallery

CNC Kitchen, Printables model 81214 (CC BY)
CNC Kitchen, Printables model 81214 (CC BY)
github.com/CNCKitchen/Open-Pull

Start here

Navigation into the creator’s own docs — we don’t rewrite the guide, we route you to the source.

  1. 1.Read the README and watch the video above to see the machine in action.
  2. 2.Download CAD files from the repository to plan your build and measure clearances.(Fusion 360, STEP and STL all provided.)
  3. 3.Choose and order a 5 kN load cell (the author used an AEP TC4) before you print the grips.
  4. 4.Print frame parts and assemble mechanics before starting electronics.(Lead-screw alignment is easier to check without wiring in the way.)

Resources

Documentation, files and community threads for this build — we link straight to the original sources and never rehost the creator’s files.

KNOWN ISSUES

  • The load cell sets the machine's range: the author used an AEP TC4 5 kN cell. Pick and order it early, and design your grips around its threads.
  • The README still says the electronics exist only as schematics and images, but Electronics/Eagle-Files now holds a contributed version-1 carrier PCB (Eagle board) for the Arduino Nano, HX711 and two A4988 drivers. Its notes warn it is a first version that may still contain errors, so check it against the schematic before ordering, or wire the parts yourself.
  • No assembly instructions in the repository; you will reverse-engineer from CAD and the video.
  • Zeroing is built in (M12 tare), but there is no written procedure for calibrating the load-cell scale factor; check it against a known reference such as the crane scale the author lists in the video description.
  • The README is marked work in progress and the author calls the firmware rudimentary: control is over serial, or through the community web controller linked in the README.
  • The two lead screws must run parallel and in sync; if the crosshead binds, the steppers will skip steps and ruin test data.

Can this replace a commercial UTM for certification testing?

No. Commercial machines are calibrated to traceable standards; this is a workshop tool for comparative testing of prints and materials, not certified measurements.

What can 5 kN actually test?

Anything that breaks below 5,000 newtons (about 500 kg of pull). The repo ships ISO 527-2 tensile specimens, layer-adhesion specimens, a test hook and a compression jig, which is what the author uses it for: 3D-printed parts and filaments.

Is there a user interface or do I need to write one?

The stock firmware is driven by serial commands (M10-M14), but the README links a community web controller (OpenPull-Web-Controller by Iqwertz), and the repo includes Excel sheets for analysing the data.

How accurate is the force measurement?

The HX711 is a 24-bit ADC, but the accuracy depends on how you calibrate the load cell. The README has no written calibration procedure; the author lists a crane scale among the parts you need, so plan to check your readings against a known reference. Treat results as comparisons between your own samples, not certified values.

Community builds

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Discussion1

FROM THE COMPAREE TEAM

5 kN is about half a tonne of pulling force on a machine you build at home. What would you test first: filament strength, layer adhesion, or something else entirely?

CompareeTEAM1mo agoedited

Practical notes from our verification: the repository contains complete CAD (STL, STEP and Fusion 360) and firmware, but the README opens with 'work in progress' and there are no written assembly instructions, so you will work the build out from the CAD, the BOM and the video. The drive is two geared (14:1) NEMA 17 steppers turning two trapezoidal 10x2 lead screws, which have to run parallel. The electronics are an Arduino Nano, two A4988 drivers, an HX711 load cell amplifier and a 24 V 5 A supply, documented as Eagle and Fritzing schematics plus a contributed first-version carrier PCB whose designer warns it may still contain errors, so expect to wire and tune it yourself. The firmware is controlled over serial and already has a tare command (M12), a community web controller is linked from the README, and results are analysed in the Excel sheets in the repo. The biggest unknown is calibration: there is no written procedure for converting load cell readings into force, so plan to check it against a known reference. The gallery shows two CAD renders by CNC Kitchen and the wiring schematic from the repo. Correction (4 October 2026): we re-checked this page line by line against the project's own repository, documentation and videos, and fixed errors in earlier versions.

Stefan Hermann (CNC Kitchen)

Stefan runs CNC Kitchen, a YouTube channel and blog focused on 3D printing, filament testing and practical engineering. He built Open Pull to generate repeatable test data for the filament and part reviews that appear across his videos.

GitHub

Star the project on GitHub

DISCLAIMER

  • Comparee is not the author of the projects featured here. All rights to each project belong to its creator — every page links to the original source, and we never host creators’ files.
  • Information is provided without warranty and may become outdated as projects evolve. Prices are indicative bands only — always check the creator’s parts list for current costs.
  • Building and operating any project is at your own responsibility. Protective equipment, safe workshop practice and compliance with local regulations are the builder’s responsibility.
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