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ARX Hand Project MK0M

An advance 3D printed mechanical prosthetic

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ARX Hand Project mainly started as an interest in creating a robotic 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 robotic hand using 3D printing.

Currently, I've made numerous developments over the years with many interesting techniques and methods I had had to use to within the project, so worth looking through my project logs as it may help with other projects as well.

ARX Hand Project currently has 3 branch designs based on the MK0 hand, so I've split the branches into individual pages for clarity between design specific developments.

The project is still a working in progress so some things may not yet be documented or finished. I've yet to release the design, as there are quite a number of things that I would like to fix before making it publicly available.

ARX MK0M

MK0M focuses on developing an advanced fully mechanical design that maintains a high functionality whilst maintaining ease of assembly. All major parts can be printed in PLA with no supports, and with little to no post print processing required. Assembly takes around 15-30 mins with around 23 printed parts needed for the hand. 

ARX MK0 Variants 

  • MK0 - https://hackaday.io/project/167785
    Main variant which focuses on a bringing a fairly advanced low cost robotics hand that's compact in design.
  • MK0S - https://hackaday.io/project/169583
    Servo variant which focuses on being an easy to develop robotic hand using standard servos which can be easily sourced and controlled
  • MK0F - https://hackaday.io/project/173565
    Figure variant which is non motorised and static design. Mainly focused for artistic uses such as in stop motion, display purposes, sketching, etc.

Design Intentions

Currently, many available 3D printed mechanical prosthetic devices struggle to maintain a balance of functionality, aesthetics, and ease of printing with FDM technology. Some designs may bring a fully 3D printed that's easy to print and assemble but lack a more human aesthetic. Other designs may have the human aesthetic but have difficulties in making it easy for printing. ARX Hand Project hopes to bring a better balanced design to the community with a futuristic humanoid prosthetic design that's been heavily optimised for easy printing using FDM technology. Many new and unique 3D printing design techniques have also been introduced to help aid in assembly together with better functionality.

Design Aims

  • Fully mechanical design
  • Can be printed in with PLA or PETG
  • Easy to print - No supports needed nor any fancy materials to be printed
  • Easy to assemble - Roughly 15-30 mins to assemble without the need for drilling, tapping or part cleanup
  • Materials are easy to source - Uses common screws, fishing line, elastic cord and 3D printer filament
  • Maintain good balance of functionality, aesthetics, and ease of assembly and printing
  • Design is durable and is capable of withstanding impacts, drops and long term use
  • Able to assist with lifting and grasping of objects - Able to lift 8kg

Design Overview

The design brings 4 actuating fingers with a manually adjustable thumb. Design uses a whippletree style of actuation to allow for each closing finger to be independent of one another for better adaptability when grasping. in addition to the whippletree, an integrated pulley system is used to provide a configurable 2 or 3 times greater mechanical advantage for better grasping strength. Cord opening and additional screw holes are added to the back side to allow for motors or extra parts to be mounted if needed. Hand parts can be printed using any printer capable of printing PLA, and can be printed with or without supports. Additional materials required are some self tapping screws, 1.75mm & 2.85mm/3mm nylon filament, 1mm diameter elastic cord and 0.5mm to 0.8mm diameter braided fishing line.

Hand Design

  • V3.4 Finger Design

    V3.4 builds on top of the initial design for V3 finger design. The aesthetic design remains the same, but many optimizations have been made to the design to improve overall printabilty and assembly. 

    Subtle features have been added within the model to reduce effects of 3D printing imperfections from impacting the fitting of joints. Rounding to edges have been added to reduce overshooting artefacts on surfaces from ghosting, ringing and nozzle pressure build up. Seam adjustments have been added to avoid seams being placed between joint surfaces. Clearances on the joints are still fairly lenient to allow for easier printing, but clearances may be reduced down to 0.1mm if required.

    For a two joint finger, fingers require the use of 1.75mm and 2.85mm/3mm nylon filament to act as pins between joints. Nylon should ideally be used as it provides a low friction, low wear joint. The flexibility of nylon also...

Read more »

  • 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 × 1.75mm Diameter Nylon Any nylon that's 1.75mm will do. No need to print with nylon. At least 2.4 inches (6cm) is needed

View all 9 components

  • 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 7 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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