- Difficulty
- Intermediate
- Build time
- 90-140 min
- Estimated cost
- $0-$8
- Age range
- 11-17
- Workspace
- A clear table about 90 cm wide
The finish line
What you will build
At least three fingers bend through two joints, reopen after release, and hold a crumpled-paper ball for five seconds.
Learning goals
- Identify how hand pull on string tendons produces coordinated finger bending.
- Construct and explain a linear pull-to-multiple angular joints system.
- Measure how the straw-guide distance from each hinge changes performance.
- Diagnose losses caused by string-guide friction and hinge stiffness.
Before you build
Materials, tools, and safety
Reuse-material cost: $0-$3 with reused materials. Supervision: Adult help recommended for sharp or heated tools.
Tools
- Ruler
- Pencil
- Scissors
- Low-temperature glue gun or tape
- Hole punch
Low-cost swaps
- Use clean shipping-box cardboard instead of buying sheets.
- Replace hot glue with strong tape and folded tabs.
- Build three large fingers instead of five and use yarn through rolled-paper guides.
Project-specific safety
- An adult should handle craft knives and make difficult starter cuts.
- Let hot glue cool before pressing a joint or testing moving parts.
- Keep string loops away from necks and use light elastic return bands behind a cardboard guard.
Orient the build
Place the build so hand pull on string tendons is on your left and coordinated finger bending is on your right. Call the side facing you the front, the far side the back, the tabletop the bottom, and the opposite face the top.
Build it
Step-by-step instructions
Step 1
Trace and reinforce the palm
Cut one palm shape and laminate strips around finger roots.
Mark five tendon exits at the wrist.
Step 2
Build segmented fingers
Cut three sections per finger with 5 mm gaps at joint lines.
Bridge each gap on the back with strong flexible tape.
Step 3
Add tendon guides
Tape straw segments along the palm side of every finger section.
Align openings so string changes direction gradually.
Builder checkpoint: After add tendon guides, the first subassembly should stay aligned when handled gently.
Step 4
Attach fingers to palm
Tape finger roots along the palm edge with enough spacing to avoid overlap.
Check each bends freely by hand.
Watch for: If this stage binds or drifts, inspect unequal tendon length before adding more parts.
Step 5
Thread the tendons
Anchor string at each fingertip and route through all straw guides to the wrist.
Leave labeled pull loops outside the palm.
Step 6
Add return elastics
Connect light bands from fingertips to the back of the palm.
Use the lowest force that reopens joints.
Builder checkpoint: After add return elastics, operate the build slowly and confirm that coordinated finger bending begins without binding.
Step 7
Balance tendon lengths
Set every finger open and tie pull loops at equal starting tension.
Trim excess string but keep service tails.
Step 8
Test the grasp
Pull selected tendons around a paper ball, hold five seconds, and release.
Record which joints lag or stay closed.
Builder checkpoint: At the final checkpoint, At least three fingers bend through two joints, reopen after release, and hold a crumpled-paper ball for five seconds.
See the engineering
Why it works
- Input
- hand pull on string tendons
- Output
- coordinated finger bending
- Motion
- linear pull-to-multiple angular joints
- Energy losses
- string-guide friction, hinge stiffness, finger flex, unequal tendon length
Why this works
Tendon-driven joints
A string routed on one side of a flexible joint creates a bending moment when pulled. Elastic material on the opposite side stores energy and reopens the finger after tension is released.
Look for: Pull one tendon slowly and watch which joint bends first as string tension rises.
Where the energy goes
Efficiency and losses
The ideal model leaves out string-guide friction, hinge stiffness, finger flex, unequal tendon length. These effects turn some input energy into heat, sound, vibration, or unwanted motion, so measured performance will be lower than an ideal calculation.
Look for: Run the build slowly and locate the first place where string-guide friction becomes visible or audible.
Math bite
Estimate joint moment
Formula: moment = tendon force × offset
- Tendon force = 2 N
- Offset from joint = 0.006 m
Substitute: moment = 2 × 0.006 = 0.012 N·m
Result: The tendon creates about 0.012 newton-metres at the joint.
A larger offset increases moment but makes the finger thicker.
String friction means later joints receive less tension.
Make it behave
Test, troubleshoot, and tune
Controlled test
Start here: Pull one finger tendon through its full safe travel and release.
Success looks like: Three or more fingers close around a paper ball and reopen without manual straightening.
Measure: Tendon travel, closed joint angles, and hold time.
Change: the straw-guide distance from each hinge
Keep constant: finger lengths, tendon, return band, ball, palm, and pull rate
- guide near hinge
- middle offset
- farther offset
| Symptom | Likely cause | Confirm it | Fix |
|---|---|---|---|
| Only the first joint bends | Downstream string friction is high | Pull with the finger straight and observe guides | Realign straws and smooth sharp turns |
| A finger will not reopen | Return band is weak or hinge tape creased | Disconnect tendon and flex by hand | Replace hinge tape or band |
| Tendon cuts cardboard | Exit hole lacks reinforcement | Inspect wrist holes after pulls | Add straw liners and tape patches |
| The object slips | Fingers close unevenly or tips are smooth | Close slowly around the object | Equalize strings and add foam tips |
Choose your tradeoff
Reduce tendon friction before increasing pull force. Stronger return bands reopen faster but demand more input; wider fingers resist twisting but may crowd the grasp.
Keep experimenting
Try another version
Three-finger gripper
Build a thumb and two fingers with one pull bar.
Independent control
Use separate pull tabs for selected fingers.
Joint-angle study
Measure how guide offset changes bend sequence.
Build together
Classroom and access options
Classroom version
Teams can compare the straw-guide distance from each hinge while keeping finger lengths, tendon, return band, ball, palm, and pull rate. Assign builder, tester, recorder, and explainer roles; have each team predict the result before collecting three trials.
Access adaptations
- Pre-cut repeated pieces and mark fold lines with high-contrast ink.
- Use large tabs, binder clips, and tape for easier one-handed assembly.
- Use large three-segment fingers and a single combined pull bar instead of individual strings.
Reflect on the design
- How did the straw-guide distance from each hinge change the measured result?
- Where did string-guide friction affect the build most strongly?
- What evidence shows that tendon-driven joints explains the motion?
- Which change would improve coordinated finger bending without creating a new problem?
Glossary
- Tendon-driven joints
- A string routed on one side of a flexible joint creates a bending moment when pulled.
- Input
- The action or energy supplied to a system; here it is hand pull on string tendons.
- Output
- The useful response produced by a system; here it is coordinated finger bending.
- Efficiency
- The fraction of input energy that becomes useful output instead of friction, sound, heat, or unwanted motion.
Build your dreams
One build can start the next.
Share what you learned, change one variable, and help another builder understand what worked.
Explore more guidesSources and build notes
An original BrickLabClips cardboard machine with dimensionally specified construction.
- Cardboard design verification: Dimensions, fold allowances, repeated-motion joints, and likely load paths received an editorial geometry review.
Written and edited by BrickLabClips. Published 2026-07-22; updated 2026-07-22.