Think of all the things you could do with a mechanical arm, including impress your teacher by using your mechanical arm to pick up the eraser next time it drops. Building a mechanical arm for a science project does not have to be overwhelming. You can build one that is robotic and very challenging or you can go with something mechanical that is a bit simpler. These can be made of almost anything such as wood, plastic, cardboard or even straws. Here is a simple design that you can use for your science project.
Make a wooden platform for your mechanical arm. A simple square with legs will be the most stable, so saw your wood into an eight-inch square. Glue or hammer two-inch legs at each corner. Drill a hole in the centre with a one inch diameter. The base should be heavy enough that the support piece you insert in the next step will not pull it over. If it is too light, you may need to attach some small weights to it to keep it from toppling. Hammer the umbrella nail into the base near one of the corners but leaving it up about one-half inch rather than nail it flat.
Insert the one-inch wooden dowel upright through the hole in the base so that it can rotate 360 degrees. If needed, sand the dowel and the rim of the hole so that the dowel can rotate smoothly. Aside from rotating, this piece will not move in any other way. Instead, it will stabilise the arm that will project out of it.
Glue an empty thread spool horizontally to the upper end of the wooden dowel. Your string or fishing line will need to move smoothly over the spool.
Hinge a lever to the upright piece just under the empty spool so that it is free to move up and down. You can create a hinge by drilling a hole and inserting a brad into the wood. The lever and upright piece should now be hinged together so that you can mechanically move the lever up and down and rotate the entire assembly -- except the base -- 360 degrees.
Attach your string or fishing line to the lever simply by tying or glueing it halfway down the arm. The string should extend over the spool to the support piece. Pulling on the string will move the arm up, and easing the string will let the arm down. When you are not using the string to manipulate the arm, wrap it around your umbrella nail in the base to hold the arm steady.
Glue the magnet or clothespin to the end of the mechanical arm. The magnet will work on its own but you will need to open and close the clothespin yourself.
Decide what function you want the mechanical arm to do. This will determine what you place at the end of the lever arm. If your mechanical arm will pick up something magnetic, you need a magnet on the end, but if you want to pick up a pencil, a clothespin will do.