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A knife holder for a child

A knife holder for a child

The context 

Lina is a 5 years old girl when we meet her for the first time. She was born with a malformed right hand equipped with two fingers. One of these is weak. She cannot flex both of her fingers, but only move them laterally as a kind of needle nose pliers. And she is very comfortable for most of everyday life activities… except for some actions that she cannot perform. And an example is holding a knife. Her parents are used to help her cutting meet, and she is used to push rice to the fork (which is handled by her left hand) with her two fingers.  

But now she has to have lunch at school, or sometimes diner in  a restaurant, and she probably feels not so comfortable in front of other children.  Thus Eric her father asks e-Nable community if someone can help in developing a kind holder for Lina.

Modeling of a fitting socket

After a first meeting with Lina and Eric, we consider that a standard e-Nable hand cannot fit the need of Lina, and we decide to develop a very dedicated knife holder.  It will be made of a socket in which Lina can insert her right hand, and some specific shapes where she can fix different kinds of knives.

Hand casting 

The first operation is to get a model of Lina’s hand. This is done by casting her hand in pink Alginate to get a positive plaster copy.  

Shape modification

Unfortunately she bent her wrist during casting operation. After the 3D scanning and reverse engineering process that leads to a clean digital model, the next operation to get a proper model usable to make a comfortable socket is then to unbend the CAD model. This is performed thanks to a specific “flexion” function available in SolidWorks. This allows to put the hand inline with the forearm, and also to put the two fingers closer to each other.

Design of the socket

The next step is to design the socket. We come back in our collaborative CAD software, Onshape, to draw a few sections and build a “loft” that approximately fits Lina’s hand. Then two offset operations of surfaces lead to the inner and outer shapes of the socket, with 3mm gap dedicated to put a comfortable 3D fabric that can absorb moisture and can be easily removed for washing. 

Designing the knife holder

The input information for designing a knife holder it to have an idea of the types of knives that will be used with this device. Eric, Lina’ farther, provided two CAD models (he drawn by himself) of knives available at home: the “knife to push” and the “knife to cut”.


Then we started with the idea of fixing the knife handle in a flexible interchangeable part that should fit both types of knives, and to add a simple slot to center the blade. In the first prototype the blade has been tied with adaptive strap made of Velcro type band (ID-Scratch).

After validation of the orientation and position of the knife by Lina, for the second version the strap was replaced by a simple neodymium magnet. You will find below a photo of the first version in test (pushing knife), and several CAD views of the last version.

And then…

Lina is happy, she can eat without asking for help to her classmates;

Eric is happy, Lina eats now without pushing noodles with fingers.

Eric asked for advice because (and that’s also good news!)

  • he is in the process to learn how to design with Onshape. He already made a new insert (the blue part on screen shots above) that will fit another knife handle.
  • And he is in the process of buying a 3D printer and we hope him to become a new maker within e-Nable France community 🙂


The CAD models are available for inspiration, adaptation to other cases, and hopefully for improvements under, if you have an account (free for non profit activities and public models), you just have to search for “Team Gre-Nable : knife_holder” among the public models. 

Please just let us know if you design an adaptation of this!


Terminal Socket for Keyboard Typing & Smartphone Use

Terminal Socket for Keyboard Typing & Smartphone Use

Open source project:

This design, like all those made by Team Gre-Nable, is open source. We explain in these articles why and how they were done, the reasons for our technical choices like the means and tools we used. All 3D models are available under the Onshape environment (free access for public models). This design is accessible on Onshape by using the Search tool with team : manchon Jean“.

Jean’s Situation …

Jean is an adult suffering from agenesis of the right arm. He has his elbow and forearm only 9cm long, with very conical form and ending with a small “bud” … he has become accustomed to use among others to typing his MacBook’s. And we must admit that it is impressive with his “dexterity”. But the more and more intensive use of his computer for his work ends up hurting him and generating pain from his little “finger”. He would like to protect his skin while maintaining his dexterity he has acquired in the usage of the keyboard. He has already consulted two or three professionals who made him special appliances, which ultimately did not suit him. He then contacts Team Gre-Nable.


Molding & Casting

An alginate molding session provides a plaster model of Jean’s forearm and elbow. Note on the image below many bubbles that have been trapped in the hair of his arm during molding, which generates these small “balls” distributed on the surface of the plaster model. These were removed very easily with a cutter blade on the plaster. We can distinguish very well in the upper part the “bud finger” Jean used until now to type on the keyboard of his computer. In the lower part of this molding, the slight restriction of section is due to the presence of an elastic band we place intendely to clearly mark the location of his elbow during molding. This restriction will be removed (smoothed) between the 3D scanning operation and the CAD volume reconstruction.

Plaster model cast from the alginate mold.

Comparison of 2 brand/types of 3D scanners.

This plaster model is digitized with a 3D scanner. We took this opportunity to compare the iSense (see also  with a high-end HandyScan 700 professional scanner. HandyScan 700 .

The Handyscan will be considered as the reference tool, with a reported accuracy well below 0.05mm on this type of object.

Results of scan shows the iSense giving efficient results for this application (accuracy less than 0.7mm, average around 0.5mm) in most areas with few variation of curvature, however, in more rugged areas, errors can reach nearly 2mm. But since we foresee the insertion of a comfort glove with thickness of 3mm (blue 3D fabric visible on the featured image near the Jean’s elbow: the process of designing this comfort glove is fully described in his post), and that we can also count on the adaptability of the flesh in contact with the socket, it seems that the quality of the iSense will be sufficient to digitize this model in plaster.

Please note that a direct digitization of the arm would probably have generated larger dimensional variations. We will keep the accurate scan made with the Handyscan as we have it.

A CAD model will be created based on this 3D scanning (obviously a mesh file), and exported in STEP format for use by Onshape online webapp.


Socket Design

Two new models are generated thanks to VXelements application (associated app with Handyscan scanner of Creaform)), with offsets of surfaces of 3mm then of 5mm. We wanted to test these features in VXelements (and we were very satisfied with the results) nevertheless these surface offsets and the generation of the new volumes could have been done with other modeling tools, either on the STL model (with Meshmixer for example) , or on the reconstructed STEP model (Fusion360, Onshape, etc …). You can read our post “Création d’un Multi-tool holder“for an example of using the “Surface Offset” function in Onshape.

The three volume models obtained, which we will call “ arm “, “ arm + 3 ” and “arm + 5” are imported into Onshape in STEP format. A boolean (volum) subtraction operation between “arm + 3” and “arm + 5” makes it possible to obtain a socket with a thickness of 2 mm thick, 3 mm away from the arm.


ARM and initial socket


Clearance around pinky

An additional offset of the surface, followed by some cuts and re-assemblies of volumes allow us at this stage to release a significant clearance at the end of socket, which will avoid the contact between the Jean’s finger and the socket. This protection is the main element of his design “specifications” transmitted to us, so we pay special care for this!

Then we add an artificial “finger”, which will be equipped with a flexible tip ( made of NinjaFlex) to allow a soft touch with keyboard’s keys.

Socket and finger, external view.

Preparing 3D fabric, and clearance provided around Jean’s bud to prevent any injury.

Socket and finger, section view on.

3D fabric : finger clearance.

Adaptation for touch screen

Like most of us Jean uses a smartphone or tablet and thus a touch screen.Could he take advantage of this artificial finger to manipulate the apps on this type of screen ?

I had studied this issue few months ago, and I came to the conclusion that it should be possible. Indeed most of the current touch screens are capacitive, and speaking without too technical terms (which I would not control for that matter!) the touch screen detects a variation of potential generated by a slight leak of electrons when the finger touches the sensitive surface. It “would be enough” therefore that the end of our plastic finger (in this case the foam cap) would be connected to a sufficient electrical mass to provoke a slight electrically discharge during contact with the screen. The solution is …

  • drilling a small channel inside the artificial finger to insert a flexible electrical wire,
  • to replace the current printed tip (made of Ninjaflex) with an conductive tip retrieved from a smartphone stylus,
  • and to connect the wire to a metal mass, and possibly touching the skin of the user.

Channel to end of the tip for the wire

Wire exiting toward conducting tip

Former stylus tip placed on the finger

Electric mass made of aluminium foil (kitchen product)

Electric wire as an alternate solution

The smartphone stylus tip was taken on a stylus of this type.  Tests show that direct contact with the user’s skin is not necessary and good news as well ! What’s more, aluminum foil is not essential either. The system works very well with just a turn of wire in the socket, wire which is located about 3mm (the thickness of foam “3D fabrics” of comfort) of Jean’s arm.

Results of first tests

The second meeting with Jean (after the molding) was really satisfying.
remaining point was to imagine a way to maintain the socket well in place around his forearm. The first tests were done by placing two pieces of adhesive Velcro on the edge of the socket, and placing another strip of Velcro around his arm. The following videos taken during the first minute of use in each context (keyboard and touch screen) show that Jean will undoubtedly succeed in appropriating this new tool … if it is already done from this moment.


Very first try, Jean typing on his Mac.

Jean’s first test on his touch screen. See how smoothly he zooms with his left thumb and right “index”!

Holding the socket on the arm

The very conical shape of the Jean’s forearm does not allow effective hold by simply clamping in this area. The first tests showed the feasibility of maintaining a strap around the arm, but we are looking for a solution that would be easier to handle, and that would avoid a localized tightening certainly not comfortable.

It seems then that the proposal made by Dominick Scalise to use a fabric sleeve for some prostheses would fit here.
A first test is done by cutting a sock … it seems to work well, and we propose to Jean, by email, to test himself this solution with the socket it uses.  

Meanwhile, we also offer a socket version printed in TPU (semi-flexible material) rather than PLA, to further improve comfort. Let’s not forget that John wants to use this device every day for several hours. 

In our third meeting, Jean introduced us to the fixation solution he found : he replaced the idea of ​​the sock with an ankle (usually used in the case of a sprain) that provides homogeneous and very efficient clamping. Just find the right dimension, and he will use or not the complementary elastic straps that come with it.

The TPU version seems to seduce him on the comfort side, and he plans to reduce the length, or even cut a slot on the side … what we do on the spot.

Printed release with TPU with slots. Please note the “glove” made of 3D fabric.

Hold with ankle (it may be necessary to use the size below)

Matteo’s Release – Technical Aspects

Matteo’s Release – Technical Aspects

Matteo’s arm: Technical Aspects

Matteo’s arm has been designed mainly based on the Unlimbited Arm , and more precisely the Alfie Edition.

This page will document why and how we proposed some modifications of the UnLimbited Arm. The main set of modifications intends to improve “grip” efficiency.

We then also propose to modify some other details:

  • modification of the forearm shape to make it better aligned with the arm cuff when the elbow is open
  • cuff printed flat and bended in several steps (easier to print and stronger result)
  • insertion of a fun magnetic functionality in one finger.
         Note: The STL files and size information are provided on Thingiverse.

How to get a firm grip with the Phoenix hand mounted on an Unlimbited Arm?

As we said in the “story page“, the main goal was to improve the grip efficiency to allow the user to take a variety of objects. The main ideas are in 3 categories:

  • minimize friction so that a maximum effort is actually transmitted to the fingers;
  • use an efficient whippletree [John Diamond] to ensure almost all fingers come to contact with the manipulated object;
  • put on almost every potential contact surface both a soft material (flexible/deformable) and a high friction coating.

These 3 points are described more in details below.

Reducing friction along the tendons paths

In the Unlimbited arm mounted with Phoenix hand, all tendons go through a complex path from the fingers to the tensioner situated on the cuff. Thus a relatively high friction occurs. For example, if we have a look in the hand, the standard “S-shaped” path (red line on the figure) is needed for the tendon to path around the hand of a standard patient with his(her) palm in the prosthetic hand. But for a person without a wrist, there is enough place in the hand to go straight to the base of fingers (yellow line of the figure). This simple modification can drastically decrease friction within the hand. The first idea is thus to drill the yellow holes and leave the tendons cross the hand straight. The second idea in the same area is to insert PTFE tubing in these holes. This PTFE tube is 2mm outer diameter, and around 1mm inner diameter. It has been sourced from a hobby shop (RC models).

Direct path for tendons in the hand (click to enlarge)

For the thumb, a continuous PTFE tubing drives a kind of Bowden large radius path (yellow dotted line on the picture). This thumb tendon tube has been maintained in place in the hand by PLA deposited with a 3D printing pen (3Doodler for example). Another gluing or welding solution may be used as well.

Thumb tendon path with PTFE tube (click to enlarge)

Finally along the forearm, a PTFE tubing can also be inserted for each tendon (and we will see below that we only need two: see the red zone on the CAD image below).

Holes for PTFE tubes in forearm (click to enlarge)

Integrating a whippletree

On the Unlimbited Arm Alphie version, the fingers are associated by pairs (see stringing process). We expect that a full whippletree system as proposed by John Diamond would be more powerful to ensure the best possible contact of all fingers on the manipulated object.

As we have enough room inside the hand, the second idea is then to place the whippletree in the hand, so that the tendons are as short as possible, with minimum friction. It is simply made of a whippletree bar and a fork. Only one tendon then goes from the hand to the cuff. Note that this tendon should be very strong and rigid as it has to withstand the force of 4 fingers simultaneously. A fishing line made of steel core coated by Nylon (outer diameter 0.5mm) is used. This main tendon is fixed to the fork by a M2 or M2.5 screw (still too long in the picture) and a bowline-knot.

Whippletree integrated in palm cave (click to enlarge)

Note the small hooks at both ends of the whippletree bar: the may receive two elastics that will connect to the central screw, to make a balance in order to ensure that the movements of the two pairs of fingers initiate simultaneously despite a possible difference of friction in pins joints or difference of stiffness of the dental elastiques. They were not required on Matteo’s hand.

Whippletree – 3D design


The video below demonstrates how our whippletree is functionning.


Combining flexible material and adhesion coating

Case of fingertips

The last phalanx of each finger should be as sticky as possible on the manipulated object. Makers from e-Nable community often use Micro Gel Fingertip Grips, but when we ordered the smallest size available (size 3), it appeared that they were too big for the small hand we made for Matteo. They just fitted its thumb size, not other fingers. Moreover, we didn’t want to add thickness over the fingers, as we expected to keep the fine design of the Phoenix hand and fingers. So we decided to develop a different solution to improve the grip.
Tip-top assembly

After numerous trials, we converged to a combination of rigid PLA with flexible PLA, and a layer of sticky material. We thus separated the distal phalanx in two parts. The first half is made of rigid PLA in order to withstand the movement force (the tendon is attached to this part), and the tip of the finger is made of flexible filament (after trials with a variety of flexible filaments, we used Ninjaflex for the final version). The printing parameters need to be carefully tuned[1] so that the fingertip is rigid enough to support the catching force, but has a deformable surface to maximize the contact area with the manipulated object. The assembly of rigid and soft materials have been initially made by the use of a double extruder printer. But we preferred to publish a version that can be made with a single extruder. So the two parts of the distal phalange are printed separately and assembled by taking advantage of the flexible part. It results in a strong assembly but removable tip if needed. No glue needed here. The new parts have been called “tip” and “top” in the STL files list.

Each distal is split to create a “tip” and a “top”
(click to enlarge)

The next step will be to take advantage of the removable tip part to hide the tendon knot inside this assembly. This is freeing more surface to put adhesion coating on the inner side of the distal phalanges.

Finally in order to improve friction coefficient, these soft fingertips have been immersed in PlastiDip to be covered with a kind of rubber shell.

Case of the palm

The same idea of placing a soft and sticky material is also applied to the palm. We started from the hand generated by the .scad file of the Unlimbited arm, removed the rigid palm, and made a new one that will be fixed by a number of 2.2mm x 6mm screws. We set up the thickness and added a few bosses distributed quite like in a human hand and that will be printed with flexible PLA.

Palm cover made of PLA and Flex

This palm is printed with three 100% infill layers of PLA (that is 0.6mm) to provide a certain rigidity, then the filament is replaced by NinjaFlex to continue printing the flexible layers and flexible bosses. After printing the palm is covered with a thick layer of PlastiDip.

Blue =PLA, green = Flex

On the proximal phalanx

Finally small square pads made of Ninjaflex and coated with plastiDip are glued under each proximal phalanges. The following figure shows the look of the final hand.


Put fun and useful add-in: a super power for Matteo

As most of children we were sure that Matteo would like to be a super hero, with super power. We also wanted to provide him with a additional possibility to catch small iron objects with his new hand. This has been done by including a small neodymium magnet into the “tip” part of the forefinger. A void has been reserved in this part, and a pause in printing process allowed to insert the magnet. The magnet size for the forefinger was diameter 3mm and length 6mm. We also made some tests with a diameter 6mm length 4mm included in the thumb tip. Both version work well, the thumb being more powerful (thanks to the bigger magnet), but the forefinger seems to be easier to use.


Split view of the forefinger tip.

Pause during print for inserting neodyme magnet

Both STL files are provided in “Thingiverse Matteo release“. Magnets have been provided from this shop (

Matteo picking a badge with the forerunner

Forearm modifications

The main modification we did on the forearm model is linked to a misalignment between forearm and arm cuff.

The problem we faced with

After printing and bending first forearm and cuff, and trying to make the assembly, we discovered that we had to make the forearm quite flat to be able to insert pins.

Original forearm

Pins axes distance

Cuff forearm misalignment

cuff arm aligned


This seems to be due because the distance between pins axes is designed to be equal on cuff and forearm before the bending operation, as we can see on the same video (second screen copy). And because of the S-shape of cuff arms, the width of cuff will become bigger than the width of forearm after bending, which makes it difficult to assembly, and also generates a misalignment (see third screen copy aside) of the arm and forearm on the final prosthesis which may hurt the arm in case of intensive use.

Solution proposed

That is the reason why we decided to modify the forearm pins distance, so that the final distance after bending of the forearm is the same as on the cuff. As a result, the forearm becomes well round-bended, and moreover the misalignment is suppressed.


The final arm of Matteo is shown on the fourth picture beside. One can see that the forearm is well round bended, and well aligned with the cuff shape. The STL file provided includes this width modification.

Cuff modification

As the idea of printing parts flat before thermal bending with hot water or hair dryer seems to be a very good process providing both easy printing and really increased strength (no more delamination of PLA layers under load), we wondered why it has not been done for the whole arm cuff, and decided to try it.

Thus starting from the good shape of the “UnLimbited” cuff, we redesigned it with regular thickness to make the two arms bendable, and flattened to make it printable. Then a new jig has been designed to ensure the original dimensions after bending. The green part on the second picture is the new jig. Red arrows show the process of bending the S-shape of the two arms of the cuff.

Flat printable cuff

Bending cuff arm

Note also that we propose to slightly bend the lever on the cuff, which pulls the tendons (see on third figure aside, yellow area). This is just to avoid having this lever prominent from the arm. The drawback of this modification is that it slightly decreases the distance between rotation axis and pulling point, thus it requires a bit more angle of the arm for the same movement of fingers.

Lever and tendons

And finally, a little problem we discovered too late to make the modification on Matteo’s arm, is the thumb tendon is too long when the hand is completely open (see third figure, green area), and we fear that it may catch something (door handle, etc.) when Matteo is playing or running in the house… A solution could be to also put a short PTFE tubing or anything else to close this area, as there is no movement of the cables here except during the tuning phase.

Notes :  Slicing parameters: with a nozzle of 0.4mm and a layer thickness of 0.1mm; 2 perimeters, 3 top/bottom solid layers, 45% infill.