Project Details

ARX MK0S

This is the servo variant of the ARX Hand Project which focuses on being an easy to develop robotic hand using common RC servos that can be easily sourced and controlled. 
 

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.

• MK0M - https://hackaday.io/project/169585
  An advance fully mechanically driven design with wippletree actuated fingers and 3 or 2 times mechanical advantage pulley system

• 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, scale modelling, etc.

Design Intentions

This design is more of a fun low cost design I made that may be used in STEM education for inspiring future engineers and makers. I may work further on the design for other possible purposes.

Design Aims

• Uses commonly available servos
• Can be printed in with only 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, ease of assembly and aesthetics

Design Overview

The design brings 5 degrees of actuation using servos along with 11 degrees of freedom in joints. It uses 5 micro/mini RC servos which can be controlled using an Arduino, a PWM driver or any servo driver. The use of servos brings the ability to have the fingers actuated to a set position through standard servo control. The servos can also be modified if there's a need for greater control and feedback of the fingers. Lifting and holding capabilities are dependent on the servos used, so ideally a high torque servos should be used if possible. Currently, the design will be compatible with the SG90, MG90S or any similar sized servos. 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, 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 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 allows for a compliant spring effect of joints, as well as to aid with friction fit joints through deformation of the material. 

  For closing actuation, fingers are actuated through a single cord of 0.5mm to 0.8mm diameter braided fishing line attached using a screw. For opening actuation, fingers are opened using an elastic material attached on the back side. The method of attachment of the elastic material allows for a number of options that available or easily accessible. Elastic cords can be used through tying knots at ends, and elastics can be printed or made through casting/injection moulding. Detachable cords also allow for easy replacement or disassembly of fingers. Currently, 1mm elastic cords are recommend for the design.

  Fingers can be printed without supports for little to no post processing needed for assembly. Supports may however be required if there isn't enough adequate cooling for overhanging areas. Additionally, TPU grips can printed to improve functionality of fingers with grasping objects. Soft PU grips can also be made through casting, however, this method has not been tested and may require slight changes to design. 

• V1.2 Thumb Design
  Thumb design features the same design improvements as the V3.3 fingers with improved assembly and printability. Only slight difference to the V3.3 fingers is the greater overhang angle that results from the thumb's unactuated position. Thumb has a 3 position ratcheting mechanism which is user adjustable to allow for different grasping positions. 
  

• V1.3 Palm Design
  

  The palm accommodate 5 micro/mini servos that are roughly 12x23mm in case dimensions. There are no restrictions on case height, so a decent number of different popular and commonly available servos can be used. Currently, the design has been tested with SG90 and MG90S servos. 

  Pulleys are used for actuation within the design and require an almost full 180 degree rotation of the servos. Larger rotation servos may be used, however, pulleys will require a redesign to make effective use of available rotation. Pulleys can be fitted onto the servos with or without having to use the original servo horn that are included. When printing the pulleys without using the original servo horn, printing at 0.1mm layer height is a must to improve XY consistency.

  Palm design has been split into flat plates to allow for easy printing of complex geometry and allowing for additional plates to be adhered or screwed on. Palm plates don't require support, but may require them if there's inadequate part cooling whilst printing. Index-middle and thumb can be swapped or removed for easy repair. Plates are screwed together using self tapping screws.

• V1 Wrist Design
  Wrist design features a 180 degree rotating joint with 7 ratcheting positions. The torque required to ratchet between positions is user adjustable up to 4 stages and can be further increased/decreased by swapping out a single printed part. Design is created to be fairly reliable and durable with the use of nylon bearing surfaces provided from 3mm nylon filament, and reinforced structure from multiple screws. Center rotating axis has a open channel for any cords or wiring to be routed through. 

3D Printing

The ARX Hand Project can be printed using any standard FDM printer that can print PLA. Majority of prototypes I have printed are in PLA using a fairly modified RepRapPro Huxley 3D printer purchased in 2014. The design has been based around 0.2mm layer height with a 0.4mm nozzle, so using different parameters may affect part fit. Some additional small geometries have been added to the models to compensate for some effects caused by 3D printing.

No supports are needed when printing out all parts, however, if there's inadequate cooling of the undersides of joints, supports may be needed. Parts can be printed without brim for materials like PLA and PETG, but bed adhesion should be made as strong as possible for finger joints as they are prone to warping due to their length. The use of a removable flex bed is recommended to allow for easy part removal if print is adhered strongly to the bed.

Typical print specifications:

• 0.4mm nozzle
• 0.2mm layer height
• 30mm/s outer perimeter
• 40mm/s inner perimeters
• 60mm/s infill
• +-0.1mm XY accuracy
• 0.2mm Z resolution (layer height)

Print materials:

• PLA or PETG for hard plastics
• TPU for rubber parts (not a requirement)

Disclaimer

Although I have some experience in designing things from being self taught, I do not have any degree or qualification as of writing. This design therefore should not be used as a medical device or to be used in any situations where it may cause injury or harm. I can not be liable to any injuries, harm, or issues resulting from the design and therefore should only be used at your own discretion.
 

License

This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.