Mechanical Hand

Written by: gadgetguru

My most successful project to date. And I have learned that it is fun to have another hand! Not to mention that its great in costumes and gives a great deal more mileage than any other costume props I have made. In total I believe this project took less than 20 hours (once parts were assembled) and cost under $40 (the springs were expensive!).

I have now begun a more advanced version of the hand which will be larger, have more accurately scaled dimensions, better aesthetics, and, best of all, digit movement from side to side as well as both front and back. It may be quite some time before completion though. 

Before you start, let me note that I made a left hand because I am right handed and wanted the ability to retain dexterity for everyday tasks while I was wearing the hand.

Also, credit where it is due, I designed this hand after viewing the most superb hand built by Aternox over at DeviantArt, and it seems I am not the only one, as a quick poke around DA shows.

This project is becoming more common so look around the web for variants and different styles. Another of my favorites (besides Aternox's) is beautifully made by IAteSatan also over at DA.

EDIT: A short video of the hand in motion can be seen here. I apologize for the poor quality - when I have some time I will try to record and upload a higher quality video.

Step 1: Materials

Substitutions and modifications to the measurements are encouraged! Also all of these measurements are approximate in nature and suited to the hand I built, but your scale may be quite different.

Wood:
- 3/4" thick x 5" x 5" board for palm
- 3/8" thick x 3" x 2' board for wrist
- 3/4" thick dowel - 2' long gives plenty of spare, but most are sold in longer lengths

Hardware:
- 5 keyrings
- 5 wires of 1 foot each*
- Small eyehooks (40 minimum)
- Springs with a collective length of 1' unstreched - keep in mind that you are pulling against these so get an appropriately low strength
- 14 small (<3/4" wide) hinges and their screws (often included) (hereafter referred to as "small screws")
- Flat top screws (25 minimum) of short (3/4" max) length (hereafter reffered to as "large screws")
- 6 Round top screws
- 6 Washers to fit onto round top screws

Misc:
- 1 unloved glove
- Canvas or leather for wrist strap
- Buckle or fastener of your choice for wrist strap
- Thread and needle
- Decorative pieces as you like
- Wood stain of your choice or, if you want to preserve the color of the wood, linseed oil gives beautiful results

*A note on the wire - This part is under stress, will likely break, and will need to be replaced. I have tried single strand copper, braided picture wire, and discarded nickel plated guitar string. Of these the guitar string has worked by far the best. I have not yet had to replace any of the guitar string. Single strand copper went very fast. Picture wire had a moderate lifespan. String, especially nylon string (low friction), may be another good choice but I have not tested it as It did not fit the aesthetic I was aiming for as well.

Step 2: Preparing the Wood

I have drawn up schematics for steps 3 and 4 in order to guide you through the installation of the hardware. Before we get to that, however, we need to get the wood ready for the following steps.

Cutting the Wood

1. The Palm: The schematic provides the basic shape I used. Feel free to adjust this to your own desired shape. I used jigsaw to rough cut the shape and hand sanded it with a rough grit sandpaper (60) to do the finer shaping.

2. The Wrist: For me, the wrist was simply a 2' rectangle of wood cut to that length so that the end of the rectangle reached up to my mid forearm when the beginning of the palm of the mechanical hand extended about 6" from my own. Keep in mind if you change this length that there will be significant overlap of the wrist with the palm.

3. The Fingers: Take your 3/4" dowel and cut it into 14 1" long pieces. To form the ends of the fingers, choose five of these and sand these until the ends are rounded off as shown in the schematic.

Finishing the Wood (Maybe)
This is your first of two chances to add a wood stain or finish to the pieces you have cut. At this point you can choose your color and stain away. I used a Miniwax "Early American" stain. Allow the stain to dry before proceeding.

Alternatively, if you would feel more comfortable staining after you have predrilled holes for your hardware (safer in case of mistakes in drilling, but slower in terms of the procedure), then go through the next two steps (3 & 4) predrilling all the holes for the hardware, but not installing the hardware itself. If you make a mistake while drilling a finger joint, just throw it away and cut a new one. Then, after you have predrilled everything, stain the wood, allow it to dry, and install the hardware.

Step 3: Hardware: Bottom Of Hand

 

We arrive at the fun part!
 
We start with the bottom of the hand because it is here we will attach the hinges to hold the fingers together. The schematics on this page will do a much better job describing where to place parts than I will do with words. As such please follow them for the locations of the hardware. You can view a sample finger joint side view down below the image to the left. For this step, ignore the hardware on this sample joint that is marked "not represented here". The hardware you will be installing in this step is as follows:

- One hinge between each finger joint and between each finger and the palm. The only screws shown on the above image are the small screws that go with the hinges. Be sure that the rotating pin of the hinge pokes out below the bottom of the finger or palm section it is attached to. Otherwise, the pin assembly will push the hinge outwards from its screws, thus making the screws less firmly attached and creating a gap in the joint when closed.

- One centered eyehook per finger joint as well as one eyehook on the palm at termination of each finger  and 5 in a line at the back of the hand (as shown). The wire will run through these eyehooks. As such, all the eyehooks should have their holes aligned so that the wire can pass through except for the eyehook at the end of each finger which should be perpendicular to the rest (so that the wire can tie off to it).

Step 4: Hardware: Top Of Hand

 

We continue by installing the hardware on the top of the hand. Saving the springs for last and the attaching the wrist for second to last, we start out with:

- The "large" screws (larger than the hinge screws). These go into the end of each finger joint, with the head facing in towards the palm. These act as stoppers so that the spring does not pull the finger joint too far back and tilt the fingers past what would be the knuckle on a human. As such, do not screw these all of the way in: instead screw them in until the head protrudes enough that when you push the joint back against it, it is in a straight line with the palm.

- The eyehooks! On the top these act as spring mounting points. These go two to a joint (except for one on the last joint of the finger) and one on the palm at the termination of a finger. Alternately, if you do not have space to attach two eyehooks to a joint, or the distance between eyehooks would be too short for your springs, you can go one eyehook to a finger joint (as I ultimately did) and attach springs to either side of it. Screw these hooks in so that the holes are open perpendicular to the direction of the finger.

Now you can go for the wrist. Simply center the wrist board and screw it in with five of the large screws - four in a square and one in the center, as shown.

Lastly, lets add the springs. Cut your springs to length so that when you extend them to the gap they will cover, they will have just a little bit of tension. Its better to err on the side of too little tension (more spring) here because you can always cut down the length later to increase tension. Bend the end loops down perpendicular to the direction of the other loops to form the loops at either end. Use pliers to attach the loops to the eyehooks in the top finger joints. If there is no opening in the spring's loop, use the pliers to bend up the eyehook until there is enough of an opening to slip the spring's loop onto it, then bend it back into shape.

Step 5: The Glove and Wiring

 

The Glove
The glove for the hand is simple, but slightly time consuming to prepare. First of all, you must sew on or otherwise attach keyrings to the finger holes of a fingerless glove (or make a fingerless glove out of a ratty glove by cutting off the finger ends). Ultimately I used wire to attach the keyring, but it forms barbs that poke the user and I would not advise it as an ideal solution. You must also keep in mind two things. First, the keyrings will have a wire pulling on them at one point - reinforce this point to avoid problems in future. Second, the threads may fray the cloth of the glove. Sew in an extra patch of fabric around the edge if you have the time to prevent this fraying from becoming a problem.

After the keyrings are attached to the glove, the glove must be attached to the wrist. To do this, put a washer on the inside of the glove and drill it down with a short, round top screw into the wood of the bottom of the wrist. Do this at one other point on the inside of the glove so that you have two attachment points.

The Wiring
Now its time to make the hand work. For each finger, knot one end of your wire on the keyring and run the other end up through the eyehooks as shown in the pictures. When you reach the end of the finger, knot your wire so that it tugs the keyring out towards the finger just a little bit. Guitar string, while durable, will not make very nice knots. If you are using guitar string, you may have a metal cylinder with a hole through it at one end of the string. Push this onto one of the ends of the keyring and use this as your knot at the bottom of the hand. Unfortunately, I have not yet found a way to knot guitar string prettily at the top of a finger, but a slip knot serves for functionality. Repeat your wiring for each of the five fingers and your hand should be mostly operational.

Step 6: The Strap and Decorating

The Strap
The strap is really an area for your own creative mind. I originally had a canvas strap with two on one side and two loops on the other as a fastener  However, this was difficult to fasten and the hooks occasionally scratched the user's skin. Because of this, I replaced the fasteners with an old buckle I got by asking around. Any fastener of your choice will work for this, but given my experience, I would advise that you find one that is quick and simple to fasten and unfasten with one hand and will not cause pain to the user.

To attach the strap to the wood, cut a rectangle of canvas the width of the wrist and a length equal to your strap's width plus two washer diameters. Then, placing your strap under the middle of the rectangle of canvas, screw down four round top screws with washers into the four corners of the rectangle of canvas. Please refer to the above picture to illustrate this direction.

Decoration
Once you have your strap and buckle in place, your hand is ready to go. Before you go romping about the city impressing people with your newfound limb extension, consider decorating the hand with metal bids, sprockets, screws, wire, ans such. Steampunk is largely about aesthetics, so do not skimp on this step if you want to create a beautiful product.

Step 7: You're Done: Give Yourself a Hand

Or give yourself a pat on the back. A high five? A handshake? Really, the possibilities are endless. Go forth and have fun. With this handy accessory, passersby will by handing out their complements freely.

Oh well, I've had as much of that as I can handle. I hope to see some beautifully elegant hands out there. Please leave a comment and a link to some pictures of your own hand-related creations. Thanks for reading!