As a naming convention the forearm parts consist of a bulk, a split, inserts and shells - the stl files are in the Forearm folder.
The bulk is typically the larger of the forearm prints and houses the servos and other components.
The split is typically smaller and is effectively a lid though may also incorporate structural supports for components and other print inserts inside the forearm.
Inserts are detachable pieces necessary to assemble pieces inside the structure beneath other components.
Shells are thin coverings that cover over openings in the main structure.
Print all the files in the Forearm folder to make the forearm. The battery cover is optional and in practice the bulk shell seals all the components adequately if well printed.
Some minor updates still required to finalise the tendon routing in the forearm and hand.
There are two main forearm structure files, Bulk and Split and additionally two shells, one to cover the servos in the palm and a pair to cover the outside of the forearm and seal over the electronics and battery compartment.
The Split print has mounting holes for the battery springs and an external hole to mount a 16mm latching push button switch to turn the hand on and off.
To download the stl files click the green button above on the right to drop down the Code menu. At the bottom click Download Zip.
The hand is made entirely by 3dprinting with additional total cost of components required for the hand around $300 USD.
The cad for the arm and hand is freely and publically available down to a parasolid level at:
The forearm and hand can operate as a stand-alone unit allowing unlimited axial rotation for rotational tasks like scrubbing.
Some notes on printing:
The parts can be printed on any extrusion printer and the hand uses less than 300 USD of components.
The parasolid CAD model can also be viewed and downloaded for free on the cloud platform Onshape: https://cad.onshape.com/documents/16c809d35214bd31c5b0324f/w/56b2fb82f7bdce505771766f/e/df78e66574dc7dbd45e4826c?renderMode=0&uiState=634d296bdb58c2464af7aa1e
Designed and tested for high speed printing using a 0.6mm diameter nozzle and 0.4mm layer thickness except for the tendon spools which benefit from printing at 0.3mm layer thickness or better.
Tested in PLA, PETG and CF nylon. ABS is not recommended for the large pieces due to warping.
To finish 3d prints to a high standard requires a fine sandpaper or emery board.
Diamond coated steel nail files are a handy tool: https://amzn.to/3VBBsOX
Though my favourite diamond file is actually intended for maintaining whetstones, just a nice big expanse of diamond dust to use :-)
- Amazon US: https://amzn.to/3eN3bvo
Additional components required are listed below.
Please consider buying via the Amazon links where available to help fund the robot development.
The biggest cost item are the 16 slim micro-servos that drive the fingers and thumb.
The discount code ROBOTSERVOS will take off 10% from the official Emax webstore: https://emaxmodel.com
There are four variants at this size with plastic or metal gears and using either digital or analog control technology.
ES3301 (approx $6.6) ES3302 (approx $9.2) ES3351 (approx $8.5) ES3352 (approx $12)
2 standard micro servos are needed for wrist flexion and extension.
Feetech SCS2332 - https://feetechrc.com/product-name_55300.html are recommended.
These are serial bus driven servos over allowing a greater range of controlled rotation.
They cost around 35 bucks each plus delivery.
If you prefer to use a standard PWM servo for simplicity they can be substituted by any servo that fits inside the envelope 23.212.128.5mm.
The trade off is the reduction of the powered range of motion of the wrist to about half.
1 standard servo needed for axial wrist rotation (optional) Feetech SCS15 - https://feetechrc.com/6v-15kg-digital-robot-steering-gear.html or any servo that fits inside the envelope 402040.5mm.
Reduces powered range of motion to about half and the loss of a continuous rotation option.
The rest of the components are relatively inexpensive:
2mm diameter steel pins - alternatively cut lengths from a wire coat hanger or in Europe order direct from https://www.visseriefixations.fr/goupilles/goupille-cylindrique-rectifiee-m6.html
Otherwise kits are available on Amazon US: https://amzn.to/3VYrnfh
M2 screw and washer kits, or also in Europe from https://www.visseriefixations.fr Amazon US: https://amzn.to/3sf4NkE
M2 brass threaded inserts M2 * 4mm long * 3.5mm Note that the type required has a plain section between the knurled ends to allow attachment of the tendon in the fingertip. Amazon US: https://amzn.to/3yYl1lY
Kite or fishing line for finger tendons - Sufix 832 is the best I know of for longevity, the 80lbs breaking strength is stiff enough to thread easily and most readily available on Amazon US: https://amzn.to/3DgjNoF
Slightly thicker 0.8mm kiteline for the finger ligaments and wrist tendons.
Amazon US: https://amzn.to/3GMKQJL
Ball Bearings 6103 - Need 50 for a hand, packs of 10 available which are enough for the sample finger. Amazon US: https://amzn.to/3Dj7cBn
Ball Bearings 263 or 262.5 - Need 6 for the wrist Amazon US: https://amzn.to/3yYlTHg
Ball Bearings 15214 - Need 4 for the wrist Amazon US: https://amzn.to/3TsMSTO
Ball Bearings 384 - Need 6 for the wrist Amazon US: https://amzn.to/3VyE28O
M3 * 70mm caphead bolt (grade 12.9 ideally) Need 2 (1 is cut down to 60mm) Amazon US: https://amzn.to/3HM0Rjx McMaster US: https://www.mcmaster.com/product/91290A297
Follow @therobotstudio Get involved at patreon.com/therobotstudio
Shield:
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
