Materials To Make Robot Hand:
- 2 days - Because you have to wait at least overnight for the silicone to dry, you will have to allow for drying time. If you
do the reinforcement step 7, then you will need a few more hours for that to dry, too.
- Flexible drinking straws (colored, if possible). They form the channels for the strings (tendons). Get multi-colored
ones if you can find them.
- String. Any strong string will work. My favorite is nylon "mason's line" which comes in various bright colors at my local
building center. It is best to use heat (a lighter or candle) to cut nylon string because it melts the end, preventing it from
unraveling. Alternately, you could wrap some tape around the end. You'll need 5 pieces, each about 20" long.
- 100% silicone (rubber) caulk, clear. This is usually used to make waterproof seals in bathrooms and other wet
places. It is available in every building center and hardware store. It comes in 10 ounce tubes and requires an
inexpensive dispenser, known as a "caulk gun," to actually squeeze the caulk out. In this project the silicone rubber
holds everything together, gives the hand some thickness, adds an interesting texture, and its elasticity straightens
the fingers and thumb. Caulk is messy to work with. Have rags available to wipe your hands. It releases acetic acid
(known more commonly as vinegar) during its curing (hardening), making for a smelly operation. You'll need one 10
oz. tube, and you'll have lots left over.
If you have never used caulk, you have to cut off the tip of the nozzle then stick a coat hanger or something down in to
break the foil seal at the other end of the nozzle. Often there is a tip cutter and poking wire attached built into the caulk
gun for this. The more of the tip you cut, the bigger the hole at the end, and the more caulk will come out at one time.
Cutting off too much tip will make it hard to control in step 4. You can always cut off more of the tip and make the hole
bigger, but you can't make it smaller once you have cut too much off.
Squeeze the lever to dispense. Different caulk guns have different ways to release the pressure so the caulk stops
coming out. In the one pictured you stop it by turning the notches so they are not on the bottom any more. To store the
unused caulk, cover the nozzle with aluminum foil or plug it with a nail. You might have to cut more of the nozzle off to
get it flowing again. As for cleanup, wipe off as much of the caulk off your hands with a dry rag before trying to wash it
because it's not very water soluble.
A correspondant named Jonathan from Canada found a beige colored caulk for more realistic skin color, and even a
red caulk for Halloween effect.
- Thin cardboard
I use one side of a cereal box so as to recycle, but you can use "poster board." You will need a piece of cardboard at
least 6" by 8". Use sandpaper or something to scratch and scuff up the print side of the cardboard. This is so the
silicone sticks to it.
- Single-edge razor or other razor, scissors, rags, newspaper
You can substitute an "Exacto" knife or utility knife for the razor. Whatever you use, it must be sharp. The rags are for
wiping silicone caulk off your hands. The newspaper keeps caulk from getting on the table.
Print out, cut out hand pattern, strings and cardboard.
Click here and print out the pattern. Cut Out Hand Pattern
Some browsers--especially Netscape--change the scale and the size of the printed pattern. If the printout says
something like, "Scaled-60%" try another browser. Also, the printout has a scale check. It says 2" line to line or 5 cm
line to line. Make sure it's accurate. Look at the pattern. The 5 pictures of straws indicate where straws will be glued
on. The 3 dashed lines going across each finger and thumb indicate where the joints will go. The two zigzagged lines
on the thumb indicate where reinforcement will go (explained at the end of step 4).
Carefully cut the outline of the hand on the solid lines.
Cut 5 pieces of string, about 20" long. Cut out a big side of a cereal box (or have the poster board ready to go).
Glue pattern, cardboard and strings together.
In this step, we will sandwich the end of the strings and caulk between the pattern and the cardboard. Have rags
handy and work on newspaper. The squeezed-out caulk --known as a "bead" --will be shown as light blue in the
Lay the cut out hand print side down on a piece of white scrap paper. The scrap paper has to be white because it
helps the pattern to show up even though it's on the bottom. On the blank side of the hand, squeeze a "bead" of caulk
--about the diameter a little bigger than a pencil,--from the tip of each finger (and thumb) to the wrist, as shown at right.
Just follow the faint parallel lines (shown in grey in the illustration) that show through, from the finger tips to the wrist.
Make sure you have not missed any spots with the silicone--or put it on too thin--especially at the finger tips. It's better
to have too much than too little. The extra silicone that might squeeze out is easy to cut off with scissors.
Lay string on the finger parts of the beads of caulk, as shown above. The strings then radiate out from the finger tips,
as shown on the left. The short, jagged, double red lines indicate a "break" where there should be string, but it won't
all fit in the picture. You can push the string slightly with a scrap of cardboard so it sticks to the caulk.
Pick up the piece of cereal box cardboard (orange in the illustrations) and gently press the scuffed-up print side of
cardboard onto the silicone (the caulk will keep them together). Flip the sandwich so the pattern is on top. It will look
like the illustration above.
With a ruler or some other straight object, push the pattern down a little, all over. Flatten out the caulk in between so
that it almost squeezes out from the fingers of the pattern, but not quite. The illustration at left is an edge view of before
and after the squeeze. If a little accidentally squeezes out, don't worry. Remember, the solidified caulk cuts off easily.
Glue straws onto the pattern.
Run small beads of caulk (much smaller beads than before) from the tips of the fingers to the wrist again, using the
straw pictures as a guide . This time the pattern is facing out. Then push straws onto the beads of caulk as indicated
by the pattern. Don't worry about the ends of straws sticking out. Make sure the straws stay flat on the cardboard--no
Cover the straws with caulk.
You might want to practice this step on a scrap of cardboard that is as wide as a finger, with a straw glued on.
Hold the caulk gun at about a 45 degree and keep the tip touching or very close to the work. As the caulk comes out,
weave back and forth across the width of the finger to cover the straws. Pause on the sides each time so the caulk can
fill in there.
In this is a closeup of the "weave" you make with the silicone. The tip spends most of its time to the side of the straw,
just a little on top. The angle of the gun is mostly downward and the tip presses the tip, so the silicone is well-adhered
to the paper pattern.
As the cross-section illustration of a couple of fingers shows, most of the caulk will be on the sides of the straw, with a
thinner layer on top.
The beads of caulk should run together. The illustration to the left does not show the beads running together (it only
shows the zigzag motion with a pause on the sides). But if you look at the rest of the illustrations below, the light blue
shows how the caulk should look. Don't worry if gaps appear, however, because you can easily fill them in when the
Cover the thumb straw as you did the fingers, but notice two sets of zigzag lines on the pattern of the thumb. These
indicate where caulk should be piled twice as high as usual on top of the straw to keep the string from pulling out
there. Step 7 details why this reinforcement is needed, and you can do it then if you don't get it in now.
It is almost impossible to smooth the caulk. I like the textured ripple pattern that results from weaving back and forth
with the caulk gun, so I leave it that way.
By the way, a correspondant named Jonathan from Canada says that a little bit of dish soap on your own hands keeps
the silicone from sticking so you can smooth it out. I have not tried it yet, but it seems like a great idea.
You can't do any more until the caulk "cures." Figure at least 12 hours for that to happen.
Cut straws and cardboard
Cut off the overhanging parts of the straws, being careful not to cut the strings. Cut around the outline of the hand.
Good scissors should just cut right through any caulk is in the way. With a razor or scissors cut the 3 short lines in
between the fingers (drawn red in the illustration). These represent the webbed part of our hand. If you look at the back
of your hand where the lower joints of your fingers are, then look at where the hand actually separates into fingers, you
can see that our hands are partly webbed.
Cut notches for finger and thumb bending.
Cutting notches makes the fingers bend at certain places, like real knuckles on our fingers. Using a sharp
single-edge or other razor, slice notches at each joint (knuckle). Make each cut at about a 45 degree angle. The
notches should bottom out almost--but not quite--touching the paper pattern. It is better to not make the notch deep
enough (you can always cut more out) than to make it too deep.
The illustration below is a cross sectional side view of part of a finger. A layer of caulk is on top. Below that is a green
Then comes the pattern, another layer of caulk, and the cardboard on the bottom. Notice that the notch does not cut
quite all the way through the straw.
Thread the strings through the straws to the wrist. You can go ahead and try out your hand by pulling the strings, but
be gentle with the thumb until you've had a chance to reinforce it in step 7.
Reinforce the back (optional). Sometimes, after extended periods of use, the cardboard that is the back of the hand
might start to crack where it bends. It seems to happen at the lower-most knuckles. I prevent it or fix it by applying a
thin layer (1/8" or a few millimeters) of silicone, as seen below. I use a textured, swirling pattern. I don't know of a way
to apply it smooth.
Now get your friends and make a robot hand!
Weird Science Kids
fun cool exciting easy science experiments and
Eduacational Toys for kids