I have a fascination with the mechanics of the human body, the hands in particular due to their complexity of movement. I have long considered how I might recreate those movements in a way that might be useful, a tangible manifestation of my own interpretation of the organic masterpiece.
The opportunity arose when I started working a new job. A young lady I work with was born with no left hand and after getting to know her and hearing of her poor experiences with less than optimal prosthetic design, I was inspired to have a go.
This build is a prototype to test the movement capabilities of the design I have come up with and is the foundation for ongoing improvements leading to what I hope will be a very functional, practical, and capable device.
Step 1: Gather Your Tools and Supplies
- sheet steel ( 22 gauge, 0.03”, 0.76mm)
- sheet brass (0.01”, 0.25mm) )
- brass tubing (1/8”, 3.18mm interior diameter)
- 1/8” diameter aluminum pop rivets
- 1/8” diameter steel rod
- cold bluing compound (WARNING: hazardous material) *optional
- lubricating oil
- small plastic baggies
A laser cutter or a 3D printer would have made this much easier as would many other expensive and more advanced tools. I myself am on a budget and work mainly with hand tools so I tried to come up with a design that would accommodate those of us who are still a ways out from putting together our dream shop.
- computer with Autodesk Fusion 360 installed
- permanent marker
- sheet metal hole punch
- tin snips, sheet metal sheers
- center punch
- ruler (transparent is ideal)
- protractor (transparent is ideal)
- assorted pliers
- needle files
- mill file
- hack saw
* a bench grinder, belt sander, and/or a dremel tool will greatly reduce the time required to complete the project but snips and files will get the job done
Step 2: Establish Your Design
Before you put pencil to paper or stylus to pad, if you are creating a device to enhance the life of a particular individual be sure to sit down with them and listen to their needs, desires, and feelings about the project. A crab-like claw will pick things up just fine and may work for some, but such a device for others may feel dehumanizing and discourage them from regular use simply because of how the thing looks and how other people react to it. The function of your design should reflect the needs of the individual and the aesthetics should reflect their personality. Form must follow function, but design the function giving consideration to the desired form. This project is focusing primarily on function to establish the greatest range of motion for the prosthetic.
There are many very powerful digital tools available to quickly and accurately layout your designs, and just as many methods, machines, and materials to provide an endless array of options for fabrication. I find often I must save myself from being overwhelmed by all the choices and limit my approach (limits often imposed on me by budgetary restrictions) and though not always practical, I like pencil and paper and I enjoy the punching, cutting, bending, and filing all done by hand. I have chosen flat steel and simple joints and levers which have forced me to seek out creative solutions to achieve a near full range of motion in the hand.
That being said, using a CAD program can be indispensable depending on the complexity of the project. After laying everything out on paper I built the entire thing digitally using Autodesk’s Fusion 360 program. The 3D image I was able to create greatly aided my ability to visualize the function of the mechanical hand and to recognize issues before I went on to fabricate the physical object.
Here you can view and download my digital construction:
Step 3: Transfer the Design From Paper to Steel
Once you have finished your design (or if you plan to recreate my design) you can begin the layout process on the steel using your scribe, calipers, hammer and center punch, ruler, and protractor. Take your time and as always, measure twice and mark and cut once. I find it useful to trace for the inner and outer curves the metal punch die that I used to punch out the disks for my washers. I also mark all the center punch marks that will have holes punched with a permanent marker in order to distinguish them from reference marks that I had made to aid in the layout process.
The outside edges of each piece are important, but your main focus should be to properly place the holes. Poorly placed holes will frustrate your final assembly efforts and impair or restrict movement in your final piece.
NOTE: for each tab that is to be bent, leave a space in your design the width of your sheet metal between the main piece and the tab. This space will compensate for the bend itself. Without it your components will not line up when you try to put them together.
Step 4: Prep Brass Washers and Bushings
You can make your own custom washers using a thin brass sheet and a sheet metal hole-punching tool. I punch out a small disk of brass using the largest die (in the case of my tool, 9/16” or 14.3mm) and then use the 1/8” die to punch out the center hole. Each die has a centering point so the impression from the large die makes it easy to center the smaller die (that is why I punch the disk then the hole in that order). It is a very tedious process, making your own washers, but the task is simple and somewhat mindless so while you are cranking them out you can listen to or watch something in the background to help pass the time.
Equally tedious, if not even more so, is making your own little bushings from the brass tubing. I use a little hobby hack saw to cut off small slices of the tubing (between 1/32” and 1/16” or 1-1.5mm). In order to make the bushings flush with the steel your using, I have made a little tool out of the sheet steel in which I punched multiple holes to match the outside diameter of the tubing (in my case 5/32” or 3.97mm) . I press a rough cut bushing in each hole and I use the mill file to take off the tops until they are all flush with the steel.
These processes are slow and painstaking, but the satisfaction of small baggies filled with uniform washers and bushings you have made yourself is palpable.
Step 5: Cut Out the Pieces … Stay Organized!
Using your metal shears I recommend getting each piece down to a more manageable size by first making a rough cut around the component, then proceeding to your finer cuts. On your finer cuts, get as close to your lines as possible but know that you can always come back with a file and clean up the edges. The more you can remove with your shears the less work you have to do with the file, but cut too much off and you will have to remake the whole component. Patience is paramount!
To expand on my previous point, be careful not to strain your hand. I would suggest you layout a few components at a time, cut them out and finish them before you move on to the next set of components. A marathon session with the shears can leave your dominant hand hurting for the next day or three.
Keep your components separated! As many of your pieces can be very similar, it is best to divide them up according to their placement (I divided mine in to individual baggies by fingers and thumb).
*don't forget to punch out the steel spacers you will need for assembly! Use the same process as you did to make the brass washers.
Step 6: Finish the Edges and Punch the Holes (In That Order)
Once you have completed the finer cuts on each of your components, use your needle files to refine your edges and finish out the nooks and crannies the shears could not reach. To prevent cuts from the sharp edges left on the steel by the shears and to improve the smoothness of operation of the final assembly, I round the edges of each component.
When you have completed the filing you can begin punching holes to fit your bushings.
All the cutting and filing and bending can leave your pieces bent and twisted. Use the hammer and pliers to flatten them out.
Step 7: From 2D to 3D
Don’t bend your tabs the wrong way!!!
Before you begin bending tabs, for each component make absolutely sure of its orientation in the final piece. This will vary depending on whether you are making a right or a left hand.
When bending your tabs, line up the end of your pliers with the marked line and grip firmly. Do your best to keep the piece and the pliers square so your tabs do not end up twisted one way or the other.
Step 8: Order of Assembly
Now that all your pieces are done, what you have on your hands is a complicated 3D puzzle that must be assembled in the proper order. The order is important because out of order, your tools (i.e. your pop rivet gun) will not be able to get into position to mount the remaining pieces. Take your time and think through your process. If you make a mistake, carefully drill or file out the rivets and begin again from where things went wrong.
I take the rivet, put the brass washer on, then the first bushing, then the first steel piece, then the second washer, then the second bushing, then the second steel piece, then the last washer. After all those pieces are in place I pop the rivet.
As far as the order I used, I assembled the thumb and each individual finger completely. Once those were completed I mounted the thumb to the index finger. I then mounted each finger assembly to the plate on the palm side, and when all were attached I added the final plate on the back side of the hand.
Step 9: Testing Range of Motion
When you have everything assembled, check to make sure all joints are moving smoothly and be sure there are no obstructions.
Step 10: Protecting the Piece
To protect your piece you can use a thin coating of oil, or if you are feeling ambitious you might use cold bluing compound as a more long term solution.
WARNING: cold bluing involves hazardous chemicals - read all safety material and follow all directions provided to avoid negative outcomes.
Step 11: Admire Your Handiwork (pun Shamelessly Intended)
The human hand is a lovely thing, a biological work of art. The mechanical interpretation of said thing is beautiful in its own right!