Make a Mechanical Hand With Plastic Drinking Straws

Intro and Materials

credit: mszymczak

Instructable user mszymczak has let us share his project with you for building a mechanical hand with plastic drinking straws. This project is a great primer for kids to spark their interest in building more complicated gadgets. First comes a drinking straw hand, next comes a Raspberry Pi robot. Mszymczak says, "The elegance of this design is that it is basically free and the assembly is pretty simple. Yet, this design achieves fully mechanical hand that rebounds back to its original state when the tendons release their tension. For this project, the goal was to build a fully functional hand using only straws...The magic of this project is that it uses no glue, welding, staples or securing other than by creative use of the source building material. Straws are incredible building materials. They come in a wide variety of dimensions but share inherent strength, flexibility, and availability attributes." Parts Needed: 25-30 Straws Tools Needed: Scissors Paper Punch Ruler (optional) Wooden Skewer (optional) Permanent Marker (optional)

Finger Digits (Phalanges)

credit: mszymczak

Think of the finger build as having two parts. You will build four fingers and four tendons. The fingers are a straw tube with cuts to allow joint movement and retain enough material to bounce back into shape. The tendons consist of a finger nail, tendon, and pull ring. The tendons are one straw piece each and are the secret-sauce to the finger working properly. All four straw fingers can be made identically. If you wish, you can make the knuckle locations in proportion to a real hand with various lengths. That simply adds more complexity to the build. You will notice that each finger consists of seven paper punches. You will make four equally spaced paper punches at the bottom of each finger. These will later allow us to create a palm to hold the fingers together. The three punches you make above the palm are at 90 degrees to the palm punches. These are the knuckles and you can hold a straw up to your hand to determine the best location for these punches. The knuckles require a bit more work to function. You need to remove half of the straw below the hold punch. You can either use scissors to make a nice V shape or just double punch offset below the first. After a lot of experimenting, I think the V shape is both faster and gives a superior 'bounce' for returning the finger to its resting position. Play around with the knuckles by flexing the finger to check for a nice easy bend. Look for plastic crimping and remove that material. The finger should snap back to a straight line when released. If not, you removed too much material. We will build the opposable thumb as separate series of steps later in this guide.

Tendons and Fingernails

credit: mszymczak

The tendon runs the entire length of the finger. Using a single straw, you will cut a fingernail, tendon, and ring pull. This is all done using scissors. Take your time. This is where a McDonalds straw shows its advantage. Use the yellow stripe to filet the length of the straw. Now you have access to cut the remainder of the shape. Use the red stripe as the perfect cutting guide. As a bonus, the tendon looks nice in red. Round the top of the finger nail into a rough spade shape. Yes, you do want the ring pulls cut on the yellow line. That will allow the finger assemblies to be pulled through the palm in subsequent build stages.

Finger and Tendon Assembly

credit: mszymczak

This is a very satisfying milestone in the build process. You will finally start to see your efforts come together and even demonstrate the mechanics in operation. At this point the fingers and tendons are almost complete. Before you can join these two pieces together, you need to make two more cuts per finger. First, cut the top of the finger along the same parallel line as the knuckle joints. Only go down about a 1/8 of an inch to 1/4 of an inch. You don't want to cut to the first knuckle, as that would destroy the finger. This cut is what will anchor the finger nail. Second, cut a section off the bottom of the finger so that the final ring pull can be grabbed. Thread the tendon by pushing the fingernail up from the bottom of the finger. It will slide up the tube easily and shouldn't pose a problem. Anchor the fingernail at the top of the finger. Grab the ring pull and gently pull it while hold the finger. With just the right amount of tension, the tendon will force the knuckle joints to collapse and the finger will bend. Use this time to remove addition material as necessary to allow the knuckle joint to operate properly. I find it best to 'break in' the knuckle joint by manually flexing them a few times using my hands instead of the ring pull.

Palm

credit: mszymczak

After the last series of steps, you may want to take a break. The following steps are less exciting and take a lot of time. In some respects, it is a long haul of cutting to get to another major milestone in the build process and it may be wise to take a break versus rush the rest of the project. The palm starts off easy enough but the addition of an opposable thumb will truly test your patience. Take your time. Begin by cutting four shorter straws to act as a matrix for the fingers. The picture below shows a suggested length of approximately 1/3 the full length of a straw.

Thumb Supports

credit: mszymczak

The thumb is horribly time consuming. In order to be fully opposable, you will need to build supports that extend out beyond the palm. To do this as efficiently as possible, you will 'overlay' the thumb support over the palm. While we used four straws for the palm, we will only need three for the thumb. More than three will prevent the thumb opposable movement. We will be striking a balance of strength and flexibility. For the each of the three straws, punch four holes in the exact same location as the palm holes. Very carefully cut the straw in half between the holes and stop about 1/4 inch past the last hole. Insert one palm straw into each thumb support. Align the holes.

Thumb

credit: mszymczak

For the thumb, simply hold a straw up to your own hand. Punch a hole for the two knuckle joints shown in the picture. Cut a V shape similar to the method used for the previous four fingers.

Hand Assembly

credit: mszymczak

To assemble the hand, we will be threading the finger through the palm and thumb supports. To do this, you will use your fingers to crimp the straw, squeeze and push. It will be a bit frustrating but with a bit of patience, it should only take about 20 minutes maximum. Just take your time. It is critical that you thread the finger through the palm with the tendon in place. You will not be able to thread the tendon after the finger is in the palm. You've been warned. The first picture shows that the thumb supports are arranged with one at the top of the hand and two at the bottom of the hand. The picture also shows that we have not yet cut the length or punched holes. This is intentional as the final cuts are not known until the first four digits are in place. Also pictured is the optional wooden skewer. This can be used to push inside the finger and return the straw back to its normal tube shape if the crimping was excessive. Don't worry about having it look perfect. All that is important is the free movement of the tendon inside the tube of the palm. If it can move freely and the fingers are roughly aligned by the fingernail, you have assembled it correctly. Having threaded the four fingers through the palm, place the free floating thumb assembly on top of the extending thumb supports at an angle roughly approximate to your own hand. You know have a rough guide as to where to punch the holes for threading the thumb.

More Thumb Work

credit: mszymczak

Thread the thumb through the hold you punched for the palm thumb supports. Cut off the excess straw extending past the thumb. In order for the thumb to be opposable, you will need to allow the thumb to collapse into the palm. Flip the hand over and punch holes for access between the thumb and finger. Cut a generous amount of material out of these locations. Check your work by making sure the thumb can collapse and fold into the palm. The picture shows the free movement of the thumb by 90 degrees. Nice.

Optional Thumb Tendon Relocation

credit: mszymczak

I like to punch a final hole into the finger adjacent to the thumb and thread the thumb tendon into that finger. This makes operating the hand a bit easier for kids. The picture below shows the thumb tendon stretching to an adjacent finger.

Have Fun and Next Steps

credit: mszymczak

Watch with amusement as kids explore the mechanics and ask questions about their own skeleton, muscles, and tendons. There is a lot going on in this simple mechanical hand. You can easily extend this project by making a simple wrist and arm. You will need to take into account linking the tendons by chaining the ring pulls with a zip tie cutting pattern. Future projects we will be posting include straw gears, straw pendulums, straw springs, straw water bell siphons, and straw bridges. Straws are great. They are an easy builder entry point for kids and the cost makes it nearly irresistible. Have fun.