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Engineering Project

Designing an Adaptive Fin-Ray Gripper

A mechanical design study and prototyping log for a compliant biomimetic two-finger gripper that naturally wraps around irregular geometries.

•Author: Robotics Engineering Student•Status: In Progress•
#CAD#Soft Robotics#3D Printing#Mechanical Design#Prototyping
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Design Concept & Biomimetic Principle

Traditional rigid robotic grippers require complex multi-axis tactile sensing and closed-loop force control to manipulate delicate or irregularly shaped objects without crushing them.

The Fin-Ray Effect (inspired by the biomechanics of fish fins) leverages passive compliance:

  • When a lateral force is applied to one side of the triangular rib structure, the tip bends towards the applied force rather than away from it.
  • This creates an automatic conforming wrap around objects of varying diameters and shapes without requiring individual joint sensors.
       Force (F) --->
         \           
          \──┐       Rib structure deflects inward:
           \ │       [Conforms tightly to object]
            \│
             ▼

Mechanical Architecture

The gripper consists of three subsystems:

  1. Compliant Fingers: Dual flexible triangular beams with cross-rib struts angled at 15°.
  2. Mounting Base: Rigid frame designed to mount directly onto an ISO 9409-1-50-4-M6 robotic arm flange.
  3. Transmission: Lead-screw or symmetrical linkage actuated by a single standard metal-gear MG996R servo motor.

3D Printing & Material Selection

Selecting the right material is critical for repeatable compliance and fatigue resistance:

Material Shore Hardness / Rigidity Advantage Disadvantage Suitability
TPU 95A Flexible / Shore 95A High layer adhesion, excellent fatigue resistance Challenging to print with bowden extruders Recommended for Fingers
PETG Semi-Rigid Impact resistant, easy to print Too rigid for delicate passive compliance Good for Gripper Frame
PLA Rigid High stiffness, easy dimensional accuracy Creeps under sustained static load Gripper Mount only

Slicing Settings for Flexible Fin-Rays (TPU 95A):

  • Extrusion Temperature: 228°C
  • Bed Temperature: 50°C
  • Print Speed: 25 mm/s (slow and steady to prevent filament buckling in the extruder)
  • Perimeters / Walls: 2 walls (allows sufficient elastic deformation)
  • Infill: 0% inside finger beams; struts provide 100% of structural rigidity

Verification Plan & Grasp Benchmarks

To evaluate the design, the following standardized grasping test bench is planned:

  • Spherical objects: Ping-pong balls (lightweight, fragile) and tennis balls.
  • Cylindrical objects: Aluminum cans, marker pens, and test tubes.
  • Irregular geometries: 3D-printed polyhedral shapes and soft fruit models.

Key metrics to record in future iterations:

  • Maximum payload before slip ($g$)
  • Actuation stroke duration (seconds)
  • Deflection recovery after 500 cyclic compressions