Students will make a simple mechanical hand with a trigger connected to a hinge. Once complete, students can test the ability of their hand by trying to pick up as many straws as possible. Extra class time (and there should be plenty!) can be devoted to further testing and redesigning to make the best mechanical hand possible!

You can find the lesson plan, 1-page project sheet, and more project ideas at STEM-Inventions.com

Step 1: Materials

The Engineering Creation Kit has everything you need for this project and many more from STEM Inventions.

For this design, you will need:

(and extra for redesigning)

And a box of straws for the challenge!

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Step 2: Build the Hinge

Cut two 1/2-inch pieces of straw.

Wrap tape around the end of a skewer and thread the wood cubes with holes and the straw pieces onto it as shown. Wrap tape around the other side and cut off the excess.

The straw pieces act as spacers to prevent the fingers from colliding.

Step 3: Make the Fingers

Create the outer fingers as shown in the picture: First build a U-shaped piece that connects the outer hinge cubes, then glue regular wood cubes on as shown. The half-sticks are glued onto the sides of the cubes at a 45-degree angle.

For the center finger, glue two craft sticks and two regular cubes onto the center hinge cube, then glue two more half-sticks at a 45-degree angle.

Step 4: Make the Arm

The arm is an extension of the center finger. Sticks are glued together with at least 1-inch of overlap to ensure a nice strong bond. The arm can be any length, but longer than 6 or 7 sticks is not recommended because it is more difficult to lift objects that are further away from your body. The arm also may not be able to support it's own weight if it is extremely long.

Once the arm length is determined, add a handle and a thumb rest at the end. The handle and thumb rest that are pictured are very minimal - there is a lot of room for innovation and customization.

Step 5: The Trigger

Overlap two dowels by about and inch, then tightly wrap tape around them.

Make the trigger as shown in the second picture: two half-sticks, a cube with holes, and a regular cube. Thread it onto the end of the dowel near the handle.

This next part is tricky, so read carefully: The mechanical hand needs to be calibrated to match the user's finger length. Hook your thumb around the thumb rest and place the handle against the base of your thumb. Extend your other 4 fingers and place the trigger just under the first digit of your hand. Wrap tape around either side of the trigger to hold it in place. This can be tricky to do by oneself, so encourage students to help each other during this step.

Do not glue the trigger to the dowel! You may need to adjust its position in the next step, so just tape it for now.

Loosely tie a cable tie to hold the trigger in place. And finally, attach a rubber band to each of the outside fingers to the arm using a hitch knot. These will automatically open the hand when the user is not squeezing the trigger.

Step 6: Test and Calibrate

The placement of the trigger is important for easy operation. You may need to adjust the trigger by as little as 1/2-inch to achieve optimal range of motion. Avoid using hot glue to secure the trigger since it is more difficult to adjust.

If it's not closing all the way, then move the trigger farther away from the handle.

If it's hard to use because your hand isn't big enough, then move the trigger closer to the handle.

If the calibration is too tricky, you can always use two hands to operate: One to hold the grabber, and the other to manipulate the trigger.

Step 7: Tips and Troubleshooting

This project offers a lot opportunities to customize and redesign. Give your students at least two hours to receive instruction, build the hand, and then redesign it while participating in the challenge.

Conducting the challenge is easy. Open a full box of straws and have your students take turns trying to pick up as many as possible and move it into a separate container. Any straws that are dropped in transit don't count. Have your students count their straws and keep a record. Straws will fall out and make a mess, so tell your students that they are responsible for any straws that are dropped during their attempt.

This project has many steps and some small but important details. Having an example or two for your students to refer to will help tremendously.
If a student has a small hand, then it will be more difficult to get a good range of motion. There are two ways to solve this. One is to lower the skewer that is attached to the back of the hand. This will bring it closer to the hinge. Smaller movements near the center of the hinge have a greater effect. Another solution is to not cable tie the trigger to the arm, and instead use two hands to operate it: one hand holds the arm, and the other hand operates the trigger.
Some students may want to make enormous mechanical hands and super-long arms. Although I encourage novel designs, you may want to gently encourage your students to refrain from extremes. In my experience, ultra-large-scale designs are more susceptible to breaking under it's own weight, and there sheer amount of time required to build it leaves little time for testing and redesign.
If the hand is difficult to open, try manually stretching the rubber bands to gain more slack.

Introduction: Mechanical Hand