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ARX Hand Project X1M

An advance modular 3D printed mechanical prosthetic

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ARX Hand Project mainly started as an interest in creating a prosthetic hand using 3D printing to see what I could be achieve with just using my RepRap Huxley 3D printer. From being inspired by the e-NABLE community, I had a desire to create a capable and yet low cost prosthetic hand using 3D printing.

Basing my design off of the core mechanics of the e-NABLE Raptor Hand, I wanted to bring a more advanced design that allowed for greater modularity to adapt for a range of possible design requirements. Additionally, the modularity of the design allows for experimentation of different materials, parts and configurations.

ARX Hand Project currently has 3 branch designs based on the X1 hand, so I've split the branches into individual pages for clarity between design specific developments.

ARX X1M

Design Overview

X1M focuses on developing an advanced mechanical design that maintains a high functionality whilst being modular and maintaining ease of assembly. The hand mainly functions off of a configurable whippletree block that drives two removable finger modules (index & middle, ring & pinky) when actuated. Driving the actuation is a configurable 1 to 3 stage block and tackle pulley system to provide mechanical advantage or shorter actuation for use with motorised attachments. Additionally, alternative attachments may be placed on the back of the palm using the additional mounting holes. The thumb on the hand is also a removable module for a fixed thumb or an actuated thumb. Wrist can be rotated 180 degrees with 7 indented positions that can be adjusted for varying stiffness.  

Design Features

  • Mechanical prosthetic design
  • Easy to print - No TPU prints
  • Easy to assemble - Roughly 30 mins to assemble with minimal need for drilling, cutting or sanding
  • Materials are fairly easy to source - Uses common screws, fishing line, elastic cord and 3D printer filament
  • Design is durable and is capable of withstanding impacts and drops
  • Able to assist with lifting and grasping of objects - Able to lift 8kg
  • Modular hot-swappable finger and thumb sections
  • Configurable whippletree and pulley system allowing for 1 to 3 stages
  • Additional print features (custom brim, supports and part reinforcements) are added to aid printing
  • Back of the palm has additional mounting holes for extra attachments
  • Compatible motorised attachment in works
  • Wide range of rubber finger tip options available

Hand Design

  • Finger Modules

    The fingers of the hand are separated into two modules with one module consisting of the index and middle, and the second module consisting of the ring and pinky. The modules can easily be swapped out by removing a few screws and unhooking the tendon line.

    The ability to swap out the finger module aids with allowing for ease of customisation and experimentation of alternative parts. The fingers of the module can have fewer joints, posable, fixed or can be designed to be application specific. Alternative finger mechanics like TPU joint fingers or compliant fingers can also be experimented within the same design as a module.

    Current fingers used in the modules also bring their own set of customisability and experimentation, as the pins, tendons, elastic and rubber grips can be adapted to whatever possible requirements or lack of source materials. Every part of the finger can be disassembled and replaced, so each part can be replaced with alternative materials or designs. Shown bellow are a range of possible elastics (TPU, silicone, latex and nitrile) as well as a range of possible finger tip materials (silicone cast, finger protector, TPU print, rubber feet, Sugru)

  • Thumb Module

    The thumb module allows for swapping out between a fixed thumb or an actuated thumb. Both thumbs have a 3 position ratcheting mechanism which is user adjustable. The module is only held by one screw which allows for easy replacement in case of any damage to the thumb or to swap to a different type. Additionally, application or user specific thumbs can be designed.

  • Palm

    Whippletree system designed into the palm consists of a rotating block with a channel for each pair of fingers, index-middle and ring-pinky.  Each channel allows for slipping between the pairs of fingers to provide independence between fingers when either is obstructed when closing. When either pair is fully obstructed, the rotating block allows for further independence between finger pairs.

    For the pulley system used in the palm, mechanical advantage can be configured by adjusting the method of cord attachment as shown in picture bellow. With a 2x mechanical advantage, the cord is attached to the main palm body, and routed through the whippletree block. For 3x mechanical advantage, an additional pulley is added to the cord path and the cord is attached to the whippletree...

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  • 1 × PLA Filament for printing Requires at least 100g of filament to print a single hand. Other materials like PETG may be used
  • 1 × (Optional) NinjaFlex May be used for printing elastic and/or grip pads
  • 1 × 1mm Elastic Cord Needed if not using NinjaFlex for elastics. At least 25cm (10") is needed
  • 1 × 0.5-0.8mm Braided Fishing Line At least 80cm (32") is needed
  • 1 × 2.85mm/3mm Diameter Nylon Any nylon that's 2.85mm/3mm will do. No need to print with nylon. At least 30cm (12") is needed

View all 8 components

  • Update soon

    Supercell08/23/2021 at 06:31 0 comments

    New design. Motors. Video

  • General Impact Testing

    Supercell10/15/2020 at 15:18 0 comments

    A quick impact test using a half full 2L bottle (equivalent to 1L) dropped from 1m (3.3ft) above. The test doesn't push the limits of the design by any means, but it mainly checks to see if there's any potential weak spots for general day to day use. Will test more extreme impacts at a later date when I'm able to get a better camera and mounting attachment for the hand.

    All parts are printed with 2 perimeters and 10% infill except for the wrist components which are 100%.

    Slow motion shot at 240fps.

  • Object Pickup Test

    Supercell09/13/2020 at 20:48 0 comments

    Testing out the capabilities with the MK0M design in picking up objects off the desk using rubber finger cots on fingers. The design really needs a proper method of adding rubber grip to the finger tips as it would not be possible for the hand to repeat this test without.

  • Building an MK0M

    Supercell09/12/2020 at 03:00 0 comments

    Here's a video of how an MK0M is built. This is the first time for me editing videos so it may look jumpy or weird.

  • Lifting Test

    Supercell09/10/2020 at 18:07 0 comments

    Testing the hand to see if it's capable of lifting 8kg. Fairly easy task for the hand, and I think it would be possible for it to lift even more. Will have to design something to attach to the cord as my own fingers may not fair well with a thin fishing line holding up more weight. Without the pulley system that's in the hand, there would have been no way for my fingers to bare 8kg. 

  • 07/09/20 Update

    Supercell09/07/2020 at 14:56 0 comments

    MK0M has been updated to suit either 2.85mm or 3mm diameter filament. They will allow either compatibility depending on availability of materials that can be sourced locally. Nylon is still the recommended material of choice for its durability, flexibility, low friction and low wear. Alternative materials may be used but will impact on performance and reliability.

    I'm currently working together with the e-NABLE NIOP team to hopefully bring the MK0M to masses with existing e-NABLE sockets that have been developed. The time-frame for availability is uncertain but should be available in a month. So far with progress, I've made a compatible sleeve that fits their existing Quick Connect system of sockets. Transition between wrist and hand isn't particular great at the moment but may have changes in future.

  • Second design revision

    Supercell08/04/2020 at 00:44 0 comments

    Second revision helps reduce the issue that the previous prototype which had issues with strength when holding uneven loads across all 4 fingers when using a whippletree system. To help with the issue, instead of a single connection point for the main actuation cord for the whippletree block, a pulley system is used to help distribute the actuation force across the block. The pulley also has an added benefit of adding mechanical advantage to the system to help improve overall strength. 

    To integrate a pulley system into the design without increasing additional part cost, the pulley system will have to function without ball bearings, and will mainly use sliding elements in the design. To use a thin fishing line that's around 0.5-0.8mm, high loads should not be focused on a small area, as it will result in wear of the surface. Without ball bearings, the cords will be sliding on the pulley surfaces, so therefore the effective diameter of the pulleys shouldn't be small as it would result in a low surface area. For pulleys that need to be small, they can be made in to pulley wheels that slide on an axle. Pulley wheels can use nylon as a low wear and low friction axle, and can be designed wide to distribute the load out onto a large surface area of the axle.

    The additional mechanical advantage added to the system from the pulleys greatly increases usability of the whippletree. For how much mechanical advantage the system should have would depend on the actuation length it would result in. Having more mechanical advantage to provide greater strength is ideal, however, it adds increased length to the total length required to fully actuate the fingers. Without mechanical advantage, the fingers require around 20mm of actuation. For 2, 3 and 4 mechanical advantage, 40mm, 60mm and 80mm of actuation length is required respectively. The maximum actuation length that would be suitable for manual actuation was uncertain so a decision was made to implement a configurable 2 or 3 times mechanical advantage as it was the easiest to integrate into the system.

  • MK0M Initial Prototype

    Supercell05/21/2020 at 17:24 0 comments

    Initial prototype for a fully mechanical design. This prototype mainly looks into the viability and properties that a whippletree mechanism has on the actuation. 

    The whippletree system that's designed into the palm consists of a rotating block with a channel for each pair of fingers, index-middle and ring-pinky.  Each channel allows for slipping between the pairs of fingers to provide independence between fingers when either is obstructed when closing. When either pair is fully obstructed, the rotating block allows for further independence between finger pairs.

    The use of a whippletree allows for fingers to be independent of one another which give a more variable and conforming grip around an abject. For example in the case of holding a ball in the palm, the index and pinky will be able to surround the ball more tightly than if the fingers were dependant on one another. 

    The whippletree design does however bring one downside in that it doesn't handle loads very well at extremities at pinky or index. An example where the hand may have issues is when it may be holding a bat outwards which places most of its weight closer towards the index. This issue might be due to how the whippletree can behave like a lever with reduced mechanical advantage when one side of the load is fixed. It might be possible to help reduce the effect by distributing the main actuation cord further out on the whippletree block.

    Overall, the design showed promise as the whippletree was able to allow for a better conforming grip but at the cost of reduced strength against unevenly distributed loads across all 4 fingers. With the reduced strength, a pulley system for increased mechanical advantage is a must.

View all 8 project logs

  • 1
    Preparing Pins

    Materials:
    - 2.85mm/3.0mm/3.175mm filament or rod (preferably made of nylon but other materials like wood, metal or other plastics may be used)
    - 1.75mm filament (preferably nylon but other plastics may be used)

    Tools:
    - Side cutters (also called flush cutters)
    - A saw may be needed if cutting metal pins

    Prints:
    - Template_m.stl

    Steps
    - Following the markings on the template_m.stl, use the guide to cut pins to specified lengths and quantities

View all instructions

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