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 a stepper motor, a ball screw and a 5 kN load cell.
by Stefan Hermann (CNC Kitchen)
WorkshopScience
- difficulty
- ●●●●○
- time
- a weekend-plus
- license
- GPL-3.0
- repo
- repo ACTIVE159 stars
●●●●○ · a weekend-plus · GPL-3.0 · 159 stars · repo ACTIVE
WHAT YOU’LL NEED
- 3D printer + filament — printable parts — files are in the repo
- Electronic parts — full list with part numbers in the repo BOM
- Dev board / microcontroller — runs the project firmware
Partner
COMPAREE VERDICT
Open Pull is a 5 kN universal test machine — the kind factories pay 14,000 dollars+ for — built from maker-grade parts. Stefan Hermann of CNC Kitchen uses it on camera across dozens of 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, the firmware compiles, and the parts list is clear. The catch: the electronics are still on a breadboard and the README says 'work in progress' in the second sentence. There are no assembly instructions, no calibration guide, and no documentation on zeroing the load cell or tuning the stepper movement. If you have built test rigs before or are comfortable reverse-engineering from CAD and video, this is a usable foundation. If you need step-by-step directions or a polished interface, you will spend more time writing documentation than building hardware. The most likely mistake is assuming the breadboard setup will 'just work' — plan to debug signal noise on the HX711 and write your own data-logging interface.
IN THE REPO
NOT IN THE REPO
- —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 NEMA17 stepper, ball screw, HX711 amplifier and AEP TC4 5 kN load cell.
- —README explicitly marks project as work in progress; electronics are breadboard-stage, not production PCB.
- —No assembly instructions or calibration procedure documented.
- —GPL-3.0 license permits commercial use with source disclosure.
Can I build this?
Build at your own risk. Projects involve tools, electronics and sometimes mains voltage — follow the creator’s safety notes.
Partner · KickstarterHeyGears G1: 10M+ colors and transparent parts in one print, plus UV printing on flat objects. Figures, parts and labels — no painting.
Videos
CNC Kitchen video showing Open Pull in use; no assembly walkthrough, but demonstrates operation and typical test specimens.
Gallery
Start here
Navigation into the creator’s own docs — we don’t rewrite the guide, we route you to the source.
- 1.Read the README and watch the video above to see the machine in action.
- 2.Download CAD files from the repository to plan your build and measure clearances.(Fusion 360, STEP and STL all provided.)
- 3.Source the AEP TC4 5 kN load cell first; availability varies and it sets the build timeline.
- 4.Print frame parts and assemble mechanics before starting electronics.(Ball 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 is the long-lead part and costs $80-120; buy it first or the rest of the build will wait.
- Electronics are breadboard-stage with no PCB or enclosure — plan to prototype and debug HX711 signal noise yourself.
- No assembly instructions in the repository; you will reverse-engineer from CAD and the video.
- No calibration procedure documented; you will need to write or find code to zero the load cell and convert raw ADC counts to force.
- README states 'work in progress' — expect to adapt firmware if you want features beyond raw serial output.
- Ball screw requires precise alignment; if the carriage binds, the stepper 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?
Most 3D-printed tensile specimens, small metal fasteners, adhesive joints and thin sheet materials. It will not test structural steel or thick composites.
Is there a user interface or do I need to write one?
The Arduino firmware outputs raw data over serial; you will need to write or adapt a logging interface if you want graphs or CSV export.
How accurate is the force measurement?
The HX711 ADC is 24-bit, but absolute accuracy depends on load cell calibration, which is not documented. Expect good repeatability for relative comparisons, not lab-grade absolute readings.
Community builds
No community builds yet — be the first, we feature the best ones.
Discussion1
FROM THE COMPAREE TEAM
5 kN is half a ton of pulling force, and the whole frame is 3D-printed. What would you test first — filament strength, layer adhesion, or something else entirely?
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.
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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.
CompareeTEAM5d agoedited
Practical notes from our verification: the repository contains complete CAD and firmware, but the README opens with 'work in progress' and there are no assembly instructions — you will reverse-engineer the build from the Fusion model and the video. The electronics schematic is in Eagle format and shows a breadboard setup (no PCB), so expect to prototype and tune the HX711 amplifier yourself. The video linked above shows the machine running across dozens of CNC Kitchen episodes, so you can see real test specimens snap before you commit to the build. The single biggest unknown is calibration — there is no documented procedure for zeroing the load cell or converting ADC counts to force, so plan to write that yourself or adapt existing HX711 examples. Photo note, 22 August 2026: this page had no images. We added 3 photographs of this specific build, each credited above; nothing here is a stock shot or another project.