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A homebrew two-stage tentacle uses cable pairs, flexible conduit and stacked vertebrae to create four-way bending without a conventional joint at every segment. Each stage bends in two perpendicular directions, giving four degrees of freedom overall. Sonya Vasquez’s 2016 Hackaday series presents the mechanism, a manual cable controller and the final assembly as a reproducible maker project rather than a commercial kit.
What “two-stage, four-way” means
The tentacle is divided into two bending stages. Each stage has two orthogonal degrees of freedom—for example, one cable pair can produce motion in one plane and a second pair in the perpendicular plane. Combining those motions lets the stages form a range of curved poses. In an idealized model the stages can be controlled independently; in a physical build, friction, cable stretch and material limits couple the motions.
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This design intentionally does not provide twist along the core. That constraint suits a puppet-like tentacle motion, but it is a characteristic of this build, not a general rule for continuum mechanisms.
How the cable drive bends the tentacle
Opposing cables create each bending axis
For every bending axis, two cables run on opposite sides of the vertebrae. Turning a pulley shortens and tensions one cable while releasing its partner, so the tentacle curves toward the shortened line. Reversing the pulley reverses the curve.
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A cable can pull but cannot push. The complementary line is therefore essential for motion in the opposite direction and for returning control to the neutral position.
What the conduit does
The cable conduit resists compression and keeps each line on a controlled path. That support lets cable tension create a repeatable bend instead of simply buckling the line. Fixed terminations and sensible pre-tension help the mechanism hold a pose, although friction at every vertebra prevents a mathematically perfect arc.
Core and vertebra materials
| Part | Material or option described in the series | Selection considerations |
|---|---|---|
| Central core | Replacement automotive speedometer cable | Vasquez discusses a cited 0.125-inch cable as a practical option, while noting that larger builds may twist under their own weight. |
| Central core alternative | Flexible shaft for a rotary tool | A larger or stiffer shaft may suit a heavier tentacle. Diameter, bending flexibility and torsional stiffness must match the intended size. |
| Vertebrae | Delrin (acetal) plates adapted around hobby motor hubs | Delrin is used for repeatable fabricated plates; obtain dimensions and thickness from the original design files rather than guessing. |
| Control lines | Flexible wire rope | Choose a line that bends through the routing path while carrying the required tension. |
| Multistage conduit | Continuous-length extension spring | The spring acts as flexible conduit where cables must pass through moving sections. Smaller sizes may require a custom spring fabricator. |
These are the author’s project selections and observations, not universal specifications or controlled performance comparisons. Current stock, dimensions, grades and prices require confirmation from vendors and the project files.
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Build sequence
- Define the envelope. Decide the tentacle’s length, mass and required bend radius before choosing the core diameter. A shaft that is adequate for a short, light build may twist in a longer one.
- Lay out the two stages. Place the orthogonal cable paths so each pair acts on its intended axis. Keep the geometry consistent from vertebra to vertebra.
- Fabricate the vertebrae. Cut the Delrin plates and adapt the hobby motor hubs as described in the series. Use the supplied laser-cut drawings or 3D models for hole locations and clearances.
- Install the core and conduit. Center the flexible core, route each cable through its conduit and ensure the conduit can flex without collapsing or binding.
- Attach and pre-tension the cables. Secure both ends of every pair. Remove excess slack without overloading the plates, hubs or cable terminations.
- Build the controller. The second installment describes a manual hand controller that tensions one line while releasing its complement. Verify each control’s neutral position before connecting it to the tentacle.
- Tune one axis at a time. Move a single control slowly through its range, check for rubbing or uneven travel, then repeat for the perpendicular axis and the second stage.
- Check the complete assembly. Confirm that the mechanism returns predictably, that no cable goes slack during the intended motion and that the core remains stable under the tentacle’s weight.
Choosing between a speedometer cable and a rotary-tool shaft
The series does not publish a controlled test that ranks these options. Use the project’s observations as a sizing guide:
- Speedometer cable: accessible and suitable for a smaller build when its flexibility and torsional stiffness are sufficient.
- Flexible rotary-tool shaft: worth investigating for a larger or heavier build that could twist around a smaller cable.
- Decision rule: prioritize resistance to unwanted twist, then confirm that the core can still achieve the required bend radius without forcing the vertebrae or conduit.
The cited 0.125-inch speedometer cable is a build reference, not a guarantee that every cable of that size will behave identically.
Routing cables through two stages
Single-stage routing is comparatively simple because each cable can terminate near the section it controls. In a multistage tentacle, lines for a farther stage must pass through an earlier stage that is itself moving. The conduit therefore needs enough flexibility to follow that motion while resisting compression from cable tension.
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Vasquez identifies continuous-length extension spring as a flexible conduit substitute and mentions custom spring fabricators for smaller dimensions. Match the spring’s inside diameter to the cable and inspect the full range of motion for snagging before applying high tension.
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The project favors repeatable methods—laser-cut plates, 3D-printed parts and common hardware—while allowing alternatives such as wood when the builder’s goals and tools justify them. Material substitutions can change stiffness, friction and hole accuracy, so they should be treated as a redesign rather than a drop-in replacement.
The final installment lists a bill of materials, laser-cut vector drawings in both pre-offset and original forms, STL models for 3D printing and original CAD models for the tentacle and controller. Use those files for dimensions, offsets and material compatibility; a summary cannot safely substitute for the model data.
What the three-post series covers
| Installment | Focus |
|---|---|
| Opening guide | Two-stage mechanism, cable physics, core, vertebra and conduit choices. |
| Cable-controller post | Manual hand controller and practical cable-tension management. |
| Final assembly post | Assembly and tuning, plus the BOM, vector drawings, STL files and original CAD models. |
The series was published in September and October 2016. Treat it as a design reference: present-day component availability, prices and service status are not established by the tutorial.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Troubleshooting the first motion tests
The tentacle bends but will not hold a pose
Check for excess slack, slipping cable terminations and a conduit that compresses under tension. Increase pre-tension only after confirming that the vertebrae and hubs are aligned.
One direction works and the reverse direction does not
Inspect the complementary cable for slack, a crossed route or a snag. Because each line can pull only, both members of the pair must remain connected and free through the full travel.
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The core twists instead of bending cleanly
The core may be undersized for the tentacle’s length or weight. Compare a stiffer or larger flexible shaft with the existing speedometer cable, while preserving the bend radius required by the vertebrae.
The stages interfere with one another
Look for friction where the farther-stage cables pass through the nearer stage. Recheck conduit flexibility, cable alignment and clearances at maximum bend in both stages.
Is this project a good starting point?
It is a useful starting point if you want an accessible, manually controlled animatronic mechanism and can fabricate repeatable plates or printed parts. It is less suitable if you need axial twist, quantified repeatability or validated load and speed ratings: the series supplies design guidance and files, not independent performance benchmarks.
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