This is not a milling machine made from plastic alone. Chris Borges’s three-axis CNC mill uses a 3D-printed outer shell as formwork, then fills that shell with concrete reinforced by threaded rods and square tubing. Linear rails, lead screws, stepper motors and an air-cooled spindle provide the machine’s motion and cutting hardware.
The result is a fascinating hybrid construction: printed geometry provides the custom shape, while concrete and steel provide mass and structural support. A build video demonstrates the machine cutting aluminum, but the available documentation does not establish industrial accuracy, repeatability or production-level cutting performance.
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How the printed shell becomes a machine frame
The important idea is the relationship between the materials. The printed shell is primarily a mold and enclosure. Concrete supplies bulk mass and compressive structure, while threaded rods and square steel tubing reinforce the casting. Conventional precision components are then mounted to or incorporated into the finished structure.
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Hackaday’s report describes the project and its construction in detail: read the original coverage.
Reported machine layout
Borges built a three-axis machine using:
- Linear rails on all three axes
- Lead screws for axis movement
- NEMA 17 stepper motors
- An air-cooled spindle
- A pivoting counterweight supporting the spindle assembly
- A concrete-filled central structure
- Threaded rods and square tubing inside the frame
The available coverage does not verify the machine’s overall dimensions, axis travel, spindle wattage, speed range, controller, lead-screw pitch or workholding system. Those specifications should not be inferred from photographs or video.
Why concrete is useful in a CNC frame
Cutting forces can make a lightweight machine vibrate. That vibration may appear as chatter, poor surface finish, tool wear or dimensional errors. A massive, rigid stationary structure can reduce unwanted movement, although mass alone does not guarantee accuracy.
Concrete is attractive for a custom build because it is comparatively inexpensive, can be cast into complicated shapes and adds considerable weight. The printed shell can incorporate cavities, mounting areas and reinforcement channels without requiring a large welded fabrication project.
However, this is an engineering rationale—not a published performance test. The source material does not provide vibration measurements or a stiffness comparison with an aluminum-extrusion or steel frame.
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Can it really cut aluminum?
Yes, aluminum cutting is demonstrated in the build video. The narrower and more accurate conclusion is that this particular machine has been shown performing an aluminum-cutting operation. That does not prove it is suitable for every type of aluminum machining.
The video does not, in the available reporting, establish:
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- Maximum depth or width of cut
- Feed rate, spindle speed or cutter specifications
- Maximum workpiece size
- Dimensional accuracy or repeatability
- Spindle runout or surface-finish quality
- Long-duration reliability
- Whether it can machine steel, stainless steel or other hard metals
For that reason, “cuts aluminum” should be treated as a proof of concept rather than a complete capability rating. The creator’s video is titled “I Printed A Better CNC Mill”.
Where the design may struggle
Concrete casting and alignment
A concrete pour introduces risks that do not exist in a simple bolted frame. Voids can form around reinforcement, shrinkage or cracking can occur, and rails or inserts can shift before the material cures. Embedded hardware can also be difficult to remove or replace later.
Any mounting surface for a rail or bearing must remain accurately positioned. If the concrete-filled structure is heavy enough to exceed the capacity of the bench or supporting frame, the machine can become unsafe even if the main body is rigid.
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Motion geometry
A rigid frame can still produce inaccurate parts. Rails must be parallel, lead screws must be aligned with the moving assemblies, the gantry must be square, and the spindle must be trammed to the work surface. Backlash, bearing preload, printed mounting surfaces and thermal movement also matter.
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The spindle
Hackaday identifies the air-cooled spindle as an apparent weak point. The spindle determines available torque, runout, tool compatibility, heat management and speed stability under load. A strong frame cannot compensate for a spindle that lacks suitable power or precision.
NEMA 17 motors
NEMA 17 describes a motor frame size, not a guaranteed torque rating. Whether these motors are adequate depends on the lead-screw pitch, supply voltage, acceleration, friction and cutting load. Their presence alone does not establish the machine’s performance envelope.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Is this a practical project to copy?
It is better viewed as an advanced maker or educational project than as a first CNC build. A prospective builder would need more than a 3D printer:
- A printer capable of producing the required large structural parts
- CAD and mechanical-design experience
- Accurate drilling, squaring, alignment and tramming skills
- Experience with concrete casting and reinforcement
- Electrical wiring and stepper-driver knowledge
- CNC control software and calibration skills
- A rigid workbench or stand designed for a heavy machine
- Safe control of the spindle, chips, dust, noise and rotating tooling
The project reportedly includes a bill of materials and 45 pages of step-by-step assembly instructions. That is valuable documentation, but it does not necessarily mean that every CAD or STL file is currently downloadable. The video description indicated that parts were still to be released when it was indexed, so current file availability should be confirmed from the creator’s own channels before purchasing materials.
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- 【One More Axis, Infinite Possibilities】The 4th axis module expands your capabilities, enabling cylindrical machining, double-sided objects, and 3D shapes. With a rotary work area of 3.6" diameter x 7.9" length (9.2cm x 20cm), Carvera Air supports true 4th-axis simultaneous machining, perfect for crafting detailed 3D models.
- 【Quick Tool Changes for Complex Projects】Speed up your workflow with the quick tool changer, switching tools in just 10 seconds. Whether you're milling wood, plastics, or metals, transitions between tools are seamless, making it ideal for advanced multi-step projects.
- 【Precision Meets Automation】With auto-probing and leveling, Carvera Air ensures precise calibration and flawless cuts on uneven materials. The closed-loop spindle control (0–13,000 RPM) and spindle runout <0.01mm guarantee exceptional accuracy and surface quality, even for high-complexity machining.
- 【Smart Control with Cross-Platform Software】Operate your CNC effortlessly with WiFi or USB connectivity. The intuitive Makera CAM software supports Mac OS, Windows; The Controller supports iOS, Android, Mac OS, Windows, and Linux, and integrates with popular CAD/CAM tools such as Fusion360, SolidWorks, and VCarve Pro.
Build it or buy a hobby CNC?
The concrete-filled design makes sense for a builder who wants a custom, heavy machine and enjoys solving alignment and fabrication problems. It may be suitable for carefully chosen work in plastics, wood, composites and light aluminum, subject to actual testing.
A supported commercial hobby CNC is usually the better choice when the priority is getting to the first cut quickly, obtaining replacement parts, following established setup procedures or working from published specifications. An aluminum-extrusion machine is easier to modify and square, although it may need additional bracing or mass to control vibration. A welded-steel machine can provide a stronger fabrication platform but requires welding, stress management and accurately prepared mounting faces. A small benchtop mill conversion offers a metal machine base, but typically brings its own limitations in travel, backlash and cost.
For commercial options, readers can compare a supported Carbide 3D Shapeoko, the Bantam Tools Desktop CNC Milling Machine or modular hardware from OpenBuilds. Current prices and specifications should be checked directly with each manufacturer.
Safety considerations
Concrete mass does not make an improvised CNC machine inherently safe. A rotating cutter can eject tools and chips, while aluminum swarf, dust, noise, entanglement and electrical faults create serious hazards. A suitable enclosure, emergency stop, grounding, spindle controls, limit protection, secure workholding and appropriate eye and hearing protection are essential. The machine should be tested conservatively before attempting metal cuts.
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1Scan for outdated or missing drivers - takes under a minute2Clear out junk files and repair common Windows errors3Fix the driver behind crashes, sound loss and screen glitchesThe broader lesson
The most useful takeaway is not that 3D printing has replaced machine tools. It is that 3D printing can create custom formwork and structural geometry that would be inconvenient or expensive to fabricate conventionally. Combined with concrete, steel and purchased precision hardware, a printed shell can become part of a much more substantial machine.
A related project on Hack Club Blueprint shows that other builders are adapting the same general concept. That adaptation is evidence of interest in the idea, not proof that Borges’s original design is standardized, officially supported or guaranteed to reproduce the same results.
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