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

Materials:
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

Tools:
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.

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.

Comments

author
Ash792113 (author)2016-01-09

What is the pine plank for ?

author

Pine is a very soft wood. easy to cut with a jig

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jocelynh3 (author)2015-10-29

What could be a good question statment for this project?

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Darkstalker (author)jocelynh32017-01-23

How does the human hand work? Thats what it could be.

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Ethan1023 (author)2016-05-17

Quick! If you get on here in time, you can upload this ible to the "First Time Authors" contest (I would totally vote for it). It ends May 30th.

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MojtabaM9 (author)2016-05-04

like!

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Ramon21 (author)2016-04-09

That is cool

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Pameleana (author)2016-04-07

Hey! i love this project

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teagoblet22 (author)2016-02-16

I made one. planning on having my 7th grade class make them. Which they did and now 4 and 5 grades are tackling with little more prep on my part. Thank you this is a fantastic project.

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tamachii (author)2016-02-03

hello, i wanted to ask if the hand is able to grab things and rotate. i also wanted to make a full arm like including the shoulder, if i took the materials to the carpenter would they make it like that?

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belljr (author)2015-11-29

what length do you cut the fingers

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Ash792113 (author)belljr2016-01-13

it doesn't matter what u cut it just has to look like a hand :D

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Ash792113 (author)2016-01-09

Hi, My name is Kenzie Church and I'm trying to do this for my science fair project and I think this is really cool

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Fluffy_Kitty6868 (author)2015-09-15

Okay, so I need to know what the independent and dependent variables are. Is the independent variable the items you pick up and the dependent variable is how much each item weighs?

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Triumphant Warrior (author)2015-09-14

I rely want to use it for Science fair but how can I use it to do that? cool thing though but what is the science hypothesis what am I testing?

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PrinceK9 (author)2015-08-23

Plz help me my 3 finger Kat where is joined plz tell me thank you

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RyanW2 (author)2015-02-19

What is the experiment? What is your hypothosis? But pretty cool thing! Best of luck!

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che-lynneW (author)2015-01-12

what is the project? id like to use it!

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JimmyL 101 (author)2014-11-22

Can this model pick up things

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kzerpa made it! (author)2014-10-13

My son made this over the weekend for a school project on the hand. The project takes a bit of time but it's so worth it! His won't pick up anything heavy or oddly shaped although it will pick up lighter things. Either way it's pretty awesome and he's very proud of it. He can't wait to present this in class.

hand.jpg
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lulucrazy (author)2014-02-14

I need to have the variables for the science fair and I don't really know what they would be. Can you help?

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jayotte (author)lulucrazy2014-05-05

your manipulated variable could possibly the weight of an item you try to pick up with the hand (possibly using a water bottle and different amounts of water for different weights) or the position you use to pick up an item?

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instructageek (author)2013-11-03

Wow this is amazing, you inspired me to make my own Instructable! Thanks.

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grogg34 (author)2011-02-03

Sorry it took so long - here is my hand - your instructable got me 1st place in the fair!

IMG_0609.JPGIMG_0675.JPG
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rgraylint (author)grogg342012-04-11

Hey Grogg34. Congratulations! And thanks so much for posting your photos! I really am pleased to see our project help other students. It's especially fun to see pix. I like your "How does this grab you?" line. That is a great idea. Good job.

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conversegirl55 (author)2012-03-28

Oh.... ( Sorry for bothering again...) I forgot to mention what a great job you did!!!!
It looks so professional and just amazing!!!!=)

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conversegirl55 (author)2012-03-28

Hey=)
So I'm in seventh grade too and our teacher just assigned this as our next big project. Needless to say, I was pretty nervous; as the way he explained it it seemed very hard. So I went online to look for some ideas and plans, and I found this lovely design. I feel so much better, it doesn't look as hard as I imagined, although I may be underestimating its difficulty. Thank You so much, I intend to use this for the project, and (with any luck) get an A! Although this is a great design, I think I'll put a little spin on it.... do something extra or crazy like I'm always doing =) Anywhooo.... Thanks again. Thank You Thank You Thank You!!! =)

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Penelope1256 (author)2012-01-16

This amazes me every time I come to instructables. My best friend and I are going to make this for our science fair, if you don't mind. It's gonna be called "Movement of the Hand" and we are already talking about making it. We are actually compromising it right as we speak! I think we might end up giving it to the science or health teacher so we don't fight over it... Creepilish! I love this, and have been planning it since the last summer! Make more instructables please! Maybe one for next years science fair? ;)

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baybay1996 (author)2010-12-18

what is the lenght of each finger bone? i know you said they are all the same length but how long is that?

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rgraylint (author)baybay19962010-12-22

We built our design based on the size of my hands, which are pretty big. I figured a bigger hand would be easier to build. It turns out my hand was just about the right size. The middle segment of my middle finger is 1 5/16". The same segment of my little finger is 3/4".

But we didn't make different segment sizes for different fingers. You could do it, but it would be a lot more trouble to get all the right ones in the right places. So in the end we just made all our joint segments the same length - 1 1/4".

Good luck and I hope this helps...

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Purepwnage575 (author)2009-09-27

I NEED URGENT HELP!!! What would i be testing if i used this for my science fair, like what should i name it? Im in 8 grade

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rgraylint (author)Purepwnage5752009-09-27

Hi I'm not exactly sure I understand what you mean by "What I would be testing...". Do you mean what does this project demonstrate? If so you could say it shows how the skeletal and tendon structure of the hand works. You could also talk about how our muscular system is made up of flexor and extensor muscle pairs. Meaning one muscle pulls a body part one way (flexor) while another extends (extensor) to allow the movement. Then the roles reverse to move the body part the other way. I hope this helps you. Let me know if you have any other questions... Good luck!

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lockpick (author)rgraylint2010-07-01

I also have the same problem but you awnsered it. My teacher is making me say how this relates to science, any help? And do you have any tips on how to make a glove controller? Thanks. This is amazing.

author

I believe this is a demonstration of animatronics, I've tried to make one of these out of a coat hanger and some aquarium tubes but it didn't go so well, hopefully this time it will. I'm in the 8th grade too, except i was home schooled up until now so i know a lot more than I should.

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Roxy 143 (author)2010-09-02

Good

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DubRunner (author)2010-08-30

nice instructable just the thing i was looking for wanted to incorporate this to a halloween costume back pack puppet just wondering what is the grip strength on this as i want movable hands on it. Plus how do you think that it would perform inside a heavy plastic monster glove thank you.

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astrong0 (author)2010-07-30

i made mine out of bicycle chain :)

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grogg34 (author)2010-03-29

1st place in science fair thanks!!!

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rgraylint (author)grogg342010-04-07

Congratulations!  I'm really happy to hear it.  :)

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grogg34 (author)rgraylint2010-04-26

2nd place at the district science fair !!! thanks

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rgraylint (author)grogg342010-05-03

That is fantastic!  Thanks so much for letting me know.  I'm really glad our ideas helped inspire you.  Good luck!

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TFElite (author)2010-03-13

I'm making something somewhat akin to this. It's made from the kind of pipes that people use to rout wires through walls. If that turns out well, I may very well try this. The only problem is, I don't know how I will be able to cut the pieces because being a 14 year old really can't get you anywhere in terms of something that cuts like this... Anyways, do you have any tips otherwise?

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rgraylint (author)TFElite2010-03-25

I understand your trouble with limited tools.  That is frustrating.  You can actually build this project with just regular hand tools but to be honest I'm not patient enough to do that either.

The conduit idea is a good one.  I'm not sure if you mean metal or plastic conduit.  Since your tools are limited maybe you should consider using that blue plastic conduit (some people call it smurf tubing).  It is cheap (at Home Depot) and easy to cut with a hacksaw.  If you cut a triangle out of it where where the joints go it might bend like a finger.  You would have to cut it almost all the way through leaving a small piece of the plastic at the top/knuckle. 

My concern is that the plastic might be too stiff and/or brittle.  It might bend a few times then break. You could experiment with the same kind of idea on different types of materials.  Try some rubber and plastic hose - also cheap at Home Depot..  An old water hose might even work.  It would have to be a soft one though.  1/2" PVC water pipe is easy to work too.  It won't bend like a hinge but maybe you can work out a plan for that.  Cardboard or even rolled up and glued paper could work too.

If leaving a small bit of plastic or rubber at the knuckle doesn't make a good hinge, you could consider cutting your triangle all the way through and separating the fingers at the knuckles.  Then drill/poke a couple of holes maybe 1/4"-3/8" apart where the knuckle would be.  You could basically stitch the 2 finger segments together at the knuckle with a couple of pieces of stiff wire (like 12 ga house wire) or even some tywraps. The idea here is to make a hinge that moves easily. You could also try gluing a strip of fabric (blue jean denim?) along the top of the finger segments to make a hinge.  Just leave the glue out of the bendy parts.

The next challange is how to run the strings to operate the fingers.  The thinge we found is that you must have the strings at the far edges (top and bottom) of the fingers for them to  have enough leverage to move the fingers.  So the strings need to run along the top of the fingers (back of the hand) and you have to work out a way to keep them from sliding off when the fingers bend.  The wooden version allowed us to screw in eyelets.  But you can't really do that with a rubber or plastice finger.  Maybe you can bend part your finger joint hinge wire to not only act as a hinge, but also stick up a bit and make a loop for the top string to thread through.   If you used fabric for the hinge you could stitch (needle and thread) some kind of bead (check the beading store) to the fabric and leave enough room in the hole for the tendon string to fed through and slide easily.

The strings on the palm of the hand also need to be near the skin surface.  You could get some small, stiff plastic hose and tywrap it inside each finger segment.  Put them where they are against the palm side of the fingers.  You could poke 2 holes at each end of the finger segment and thread tywraps through the hole, then around the thin hose and back out the other hole. Do this at each end of each finger segment and you have a path for your palm side strings/tendons.  The trick will be to get these small tubes to all line up properly so your string doesn't bind.  It might not be a problem if your hinge is loose enough to allow a little wiggle in the joints.

These are just some ideas to get you started.  I would like to see someone put a workable hand design together that would be easy to build with hand tools (get it - "hand" tools :)  OK, that was lame.  Anyway - good luck with your experiments and don't get discouraged.  If you come up with something you like please share a picture with us.

- Robert

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grogg34 (author)2010-02-27

this is an awesome idea for the sf!!!

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rgraylint (author)grogg342010-03-25

Thanks! :)  We had a lot of fun with it.

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Chrisdalton (author)2010-02-04

I don't know if it is in your budget but here are a couple suggestions for a better model
- instead of rubber bands use surgical tubing
- make a latex hand mold and put it around the hand to make it look more realistic
- make a hand grip so that it follows the movement of your hand
if you actually did some of this it would be pretty cool

author
rgraylint (author)Chrisdalton2010-02-05

Those are some pretty cool ideas.  It would be interesting to see someone implement some of them.  We have talked a few times about making a hand grip like you describe.  That shouldn't be too difficult.  We have put a glove over the hand to make it more realistic.  It works pretty well. 

Are you thinking about building one?  Please share some photos if you do...

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tigers08 (author)2007-12-31

What a great project- I'm going to try it for my SF project next year! Good Luck!

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rgraylint (author)tigers082008-01-02

Thanks. If you build it for your SF please let us know how it works out.

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Sean_Voodoo (author)rgraylint2010-01-15

yay im doing the hand this years sf it wil be awsome i will let u know if u subscribe

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