Introduction: Science Fair - Articulated Hand < $20
My niece Sydney needed help for a 7th grade Science Fair project. I helped another friend's daughter (Kaitlin) build an articulated hand for a project in the past and suggested we resurrect the project. The build is fun for the kids and I wanted to try some enhancements on the original idea.
Step 1: Materials, Tools & Time
You can get everything you need at Lowe's or Home Depot.
1 - 3/4" hardwood dowel
1- 4" x 1" Pine plank (I'm calling it '1x4' here)
5 - bags of #216-1/2 small screw eyes (eyelets)
1 - roll of braided nylon string (one color for ligaments. We used yellow)
1 - roll of braided nylon string (another color for tendons. We used pink)
1 - bag of 1/4" wide rubber bands (like the Post Office uses)
1 - cup hook (open eyelet)
1 - can of white spray paint
2 - 3/4" sheet rock screws
Various hand tools (you already have them if you are a maker! :)
We used a small drill press, 4" side grinder, jig saw (my Dad taught me this was called a "Saber saw") and a chop saw with a trim blade. These are time savers, but you could do it all with hand tools if you can spend the time.
Assuming you have the power tools, you should be able to easily build a hand in about 8-10 hours. That includes taking time to teach your young experimenter how to safely use tools.
Step 2: The Design Template
We started by drawing the outline of my hand on a piece of paper. We could have used Sydney's but her hand is small and building smaller is harder. My hand is big enough that the 3/4" dowel makes good finger bone segments. Draw the line (black) with all the fingers then draw a line (red) to cut off the fingers and thumb. Notice the thumb carpal bone (1st from the wrist) is cut off too. The 1st thumb segment will be shaped from a scrap of the pine 1x (one by) and screwed to the bottom of the hand. Cut out your hand template and transfer it to the pine one by. Extend the wrist lines long enough to make it to the elbow. Cut out this shape with the jig saw and put it aside. Keep your scraps!
Step 3: Cut the Wooden Fingers - NOT Yours!
Rather than trying to work out specific lengths for the various finger bones, we just decided to make them all the same. We setup the chop saw with a stop block and C clamp to lock in the length.
Step 4: Gotta Have the Safety Talk!
Sydney and I went over the rules multiple times with her trying fake cuts with the saw unplugged. You might be tempted to do it all yourself, but understand using the tools is a BIG DEAL to the kids. You know your child so decide for yourself, but if you spend plenty of time teaching they should be OK to at least cut a few. The safety rules are, wear goggles, use one hand to tightly hold the stock FAR away from the blade. Put the other hand on the saw handle and do not take your hand off it until the cut is finished and the blade stops spinning. Be aware of one more thing - when using a stop block to cut small pieces. When the blade breaks through the bottom, the loose piece you just cut is now at the mercy of the spinning blade. You have to be careful that it doesn't fly out like a little wooden bullet. In retrospect it would have been a good idea to screw an extra piece of wood to the block in such a way that it would pin the newly cut piece down once it is free. Cut extra finger segment pieces because you will goof some up in the drilling process.
Step 5: Drilling Ligament Holes ('Sydney the Orthopedic Surgeon!')
I used the chop saw to cut a 90 degree notch in a scrap. We then used this and a speed square to mark a center line on each finger bone segment. Once all the parts were marked we lined up the first one with the drill bit and c-clamped the 90 deg notch block to the drill press table to make it easy to drill all the holes accurately and quickly.
Sydney and I spent more 'no power' training time tell I felt like she was good to use the drill press. She drilled nearly all the holes.
Plunge the drill bit slowly! If you go too fast the bit will flex and creep to the outside of the dowel. It isn't critical that they be EXACT, but do your best.
Step 6: Give Your Grinder the Finger (well All 5 Actually)
We used a 4" side grinder to cut a 45 degree angle on both ends of all but five of the finger bone segments. Five of them should have the 45 degree angle on only one end to make the finger tips. We used a 100 grit sandpaper disk to make the cut. You could also use a bench grinder but be careful that it doesn't grab.
I wanted to let Sydney use all the tools, but this one is really dangerous - especially when grinding on such small parts. I helped her hold the grinder and let her do some random carving on a scrap of the 1x4. She got to use a cool, noisy, dust throwing tool, but I kept control of it.
I did all the finger grinding myself - just the wooden ones, not mine! :) Of course you could use a vice to hold each piece. But my fingers are a lot faster. Just be REALLY careful if you aren't experienced with this tool. Be ready for it to jump every time you hit the switch. You can't see it in this shot, but I had my right ring and little finger touching the table to stabilize the grinder. Even though it is just two fingers, bracing like this really helps you control the tool and keeps you from touching the spinning wheel to any of your fleshy parts.
It is very important that you watch how close you get to the holes with your 45 degree slope. If the sloped part extends into a hole that finger joint will not stay straight when the hand is assembled. The plan calls for the ligament strings (the ones that hold the fingers together) to squeeze the finger joints together under the force of a rubber band. As long as the holes are on the flat part of the finger bone the tension will hold the joint straight. If the hole extends into the sloped part the tension will bent that joint as it is squeezed together. You can see where I goofed one up here by cutting the slope into the hole.
Step 7: The Palm and Forearm
I cut a triangular piece of the 1x4 scrap, pre-drilled holes and used sheet rock screws to attach it to the bottom of the hand in the thumb 1st joint spot. Don't skip the pre-drilling step or you will split the wood! Think about the angle you want the thumb to work toward that palm and how far that joint folds under when you fold your own thumb under. Look back at the part of the thumb you cut out on the template to get an idea.
Once I screwed this piece to the hand I used the 4" side grinder to shape and smooth the entire piece. You need to cut a 45 degree flat on the underside of where the fingers attach. This slope will mate up with the slope on the first bone segment of the fingers. You also need to grind off a slope on the inside of the thumb attachment point. Just think again about where the holes are going to line up with the finger joints. Remember that the flat part of the bones must rest against a flat spot on the hand when the fingers are straight. The sloped parts of the joints must rest against the sloped part of the hand when the finger is folded.
Next Sydney laid all the pieces out to check the fit. We selected the best joint pieces for the first joint of each finger. We labeled that end of each joint so we could make sure they stayed in the right spot. We used I-index, M-middle, R-ring, L-little. We spaced out these first joints so they looked right on the hand and marked the center of each hole on the hand. I used a set punch to mark the ligament holes on the flat surface (not the sloped part!) of the hand piece. The set punch keeps the drill bit from wandering as it starts to bite the wood. Then I drilled the holes from the inside toward the outside of the hand. The holes came out right behind where the knuckles would be if you made a fist. Be sure the holes slope as much as possible toward the elbow. This will allow the ligaments to pull smoothly and hold the finger joints together.
Sydney used sandpaper to polish up the hand piece while Dargo chewed on some stock I was saving for my next project.
'Dargo! Give me that!'
Step 8: Ligaments and Tendons
Drill a pilot hole on the inside near far end of each joint piece (except the finger tips) and screw in the small eyelets. The finger tip pieces get an eyelet closer to the inner slope - just inside of center. Drill a pilot hole in the center of the end of the fingertip and add the eyelets. Check out the pictures and notice the orientation of the eyelets for each spot.
We decided to add eyelets and tendons to the back of the fingers on this version of the hand. This makes it more complex, but more realistic. If you place your ligament holes just right they will hold the fingers straight so the top tendons won't be necessary. If you decide to add them, or have to add them because some fingers won't stay straight, then pre-drill and add the eyelets. We experimented with the positioning and it is pretty critical. Put the eyelet in the first finger segment close to the far joint. Put the eyelet in the middle of the middle segment. Put the fingertip eyelet close to the inside of the segment. Experiment on your own. Your design may vary.
Tie a piece of fishing line or your nylon string tightly to some upright in your shop and string the finger segments on it. Pull it really tight and tie it off. Separate the pieces on the string and paint away.
Step 9: Some Assembly Required
Lay out your pieces to test the fit. We folded the end of a paperclip over tight enough that it would easily pass through the holes. Then I cut off the end on a sharp angle to make a hook. Burn the end of your nylon string so it won't fray. You will need to do this after each cut.
Push the hook thorough the 1st hole and hook the ligament string. Use pliers to pull it back through. Be careful with your free hand so you don't get hook your skin. Feed the ligament through the hand out the finger, across the tip, back up the finger and through the hand. Feed the string through a rubber band then tie each ligament in a knot to make a loop that ends around the center of the back of the hand. Pull the rubber band tight and see how much tension it takes to hold the finger straight. String the ligaments for the rest of the fingers. Pull each rubber band to find which finger takes the most force to hold it straight. Mark that spot and pre-drill a hole and screw in the cup hook (obviously on the back of the forearm). Pull all the ligament rubber bands tight and hook them over the cup hook.
Start by stringing the tendons on the top of the hand first. Tie to the fingertip eyelet first. Feed it through the eyelets on the middle and first finger segments then through a rubber band. Feed it back through the eyelet on the first segment and temporarily tie it to the middle segment eyelet. Leave some slack and don't make this knot permanent until you have run the inside tendons and tested everything. Hook the rubber band around the cup hook. The top tendon rubber bands should barely be tight at all.
Fold each finger and notice where the fingertip hits the palm. Mark this spot for each finger, pre-drill and install an eyelet. Tie a tendon string to the fingertip eyelet and feed it back through the rest. Tie all the inside tendon strings together near mid-forearm. You might want to attach a single rubber band to this group of strings and pull them lightly to an eyelet at the elbow. This will keep the inside tendon strings from getting tangled and make them easy to grab separately.
Step 10: It's Really Fun/Weird to Play With!
With very little practice, you can make it do all sorts of hand signs. It's also really weird feeling to shake hands with it.
You don't have to paint the hand, but it looks a lot nicer. After painting white you can draw the carpals and forearm bones (radius and ulna) on the back of the hand/forearm piece with a sharpie. Then you can paint in (black) all the parts that aren't bone and the finished product will look more like a skeleton - for science fair purposes as least. The student should include some pictures of themselves cutting the pieces and building the hand. If you paint the hand to show the bones then you can label them and call them out on their info foam board. By using different colors for the ligament and tendon strings you can easily call them out in the documentation too.
Kaitlin wants to do a more advanced hand for her high school project. We are talking about using copper water pipe, soldered joints and maybe muscle memory wire for the tendons. If that works out I'll post the project. (EDIT: I just posted a couple of pix of Kaitlin with her original hand and her prize ribbon.)
The hand was a BIG hit with the students and teachers at both Kaitlin and Sydney's schools. Their friend and uncle are a pretty big hit with these two girls too. I enjoyed teaching them about tools, shop safety and *making* interesting things that you dream up yourself.