Made of welding rod, timber, and nichrome wire, this battery-powered device frees up foam cutting to be mobile and unrestrained by a platform.

1. IDIN Build-it: Hand-held foam cutter
GOALS:
o Participants learn how to use basic woodworking tools
o Participants have made a nichrome wire foam cutter.
o Participants know how to measure simple dimensions
precicely.
TIME:
60 min
PREPARATION:
Collect materials and tools
Make a foam cutter yourself to discover what participants might find difficult. This is
helpful to have available for participants to explore while they are making the cutter.
Review and/or print Build-It instructions
Have a first aid kit and safety glasses for participants on hand
ROOM SET-UP:
A clean, flat surface such as a table for participants is useful to have for easier assembly.

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TOOLS MATERIALS*
Item Amount Cost
Saw
Hard wood
40mm wide x
40mm thick
150mm
long
$1
Hand crank
brace and
3mm bit
Filler rod
3mm thick
400mm $2
Tape measure
and marker
A piece of a
straw
~20mm
long
minimal
Hacksaw or
Bolt cutters
Nichrome
wire
32 gauge
250mm $0.10
Pliers
Brass shim
(very thin
brass sheet)
25mm x
50mm
$0.10
Vice D-batteries 2 $4
Wire cutters Strong tape
300mm-
450mm
minimal
Metal file Total Cost = $7.20
Hammer

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1. What is a Build-It?
A Build-It is a lesson where people make a small technology and learn some workshop
skills. Everyone will build together, step by step. As you build the technology, you will
learn good techniques for handling metal and wood. Each member will have an
opportunity to learn these hands-on skills by using some tools and materials. At the end
of the Build-It, you will have had some practice using tools and also have a technology
for you to take home.
2. What is a foam cutter?
This foam cutter allows people to cut
interesting shapes from foam without
needing electricity or a large machine.
Foam parts can be used when casting metal
parts. Using the foam cutter, someone could
make the part they want out of the foam, and
then put it into the sand casting frame, so
create the mold.
Unlike when using a solid part to make the
mold, the foam can be left inside the mold.
When the molten metal is poured in, the
foam evaporates, leaving only the solid cast
part behind.
Demonstrate how to use the foam cutter. Show how to switch it on and off with the switch
and straw.
3. Setting up the Build-It
ENCOURAGE PARTICIPANTS TO ENGAGE FULLY
• Ask, “Why?” “Where?” “What?” “How?” at any time during the session
• Engage everyone in the building activities and be sure that each person in the group can
both observe and participate in learning and doing all of the different techniques
DIVIDE INTO PAIRS AND INTRODUCE THE STATIONS
• Half of the pairs will make the wooden base with one facilitator; the other half of the
pairs will make the metal pieces with the other facilitator. Then, the pairs will switch
and those who were making the wooden base will make the metal pieces, and vice versa.
• Single-gender pairings often work best for allowing all members of the group to
participate evenly. Explain which tools need to be shared by the larger group, and where
they will be located.

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4. Preparing the wooden frame
Tools Materials
Steps
1. Cut a 150mm piece from the
40mm thick x 40mm wide piece
of wood
SHOW THE PARTICIPANTS HOW TO PROPERLY MEASURE AND CUT THE WOOD.
o Begin by explaining the details of the measuring tape. If there are both imperial and
metric units, clarify which you are using and what the different-sized lines mean (1cm,
1mm, etc.) Then, mark the plank
o Situate yourself to begin cutting, with your feet firmly planted and your saw, arm, and
shoulder are all in a line. Explain how to use a backwards stroke on the first cut of the
saw. You can use your thumb to help guide and steady the saw.
o It may be necessary to have teams share wood and/or saws.
o Walk around to ensure each participant has used the saw.
2. Place the wood so that the rings
lie horizontally. Identify which
way the rings are lying by
looking at the edge you cut.
Drilling through horizontal
rings will keep the wood from
splitting.
We are using hard wood so that the risk of splitting is reduced even more. Hard wood will
also hold the rods in tension for a long time without slowly getting squished, as soft wood
would tend to do.

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10mm
10mm
20mm
10mm
3. Keeping the rings horizontal,
on the top, mark halfway along
the width.
4. Mark 10mm from each end.
Mark an additional 10mm from
the right end's mark.
Which end is the “right” end
doesn’t matter, but this
description will be used to help
distinguish between sides.
5. Drill using a 3mm drill bit.
Drill through, but do not wiggle
the bit! Making the hole larger
could make the rod slip
through. This is easiest to be
careful with a drill press, but
possible with a hand drill.
5. Adding the metal rods
Tools Materials
Steps
1. Cut three filler rods: 80mm,
150mm, and 180mm
Filler rods are strong because they have a steel core, but very conductive because they have a
copper coating. The copper has little resistance, so the electricity takes the easy path along
the copper to reach the resistance wire, and does not heat up in the process! The copper also
helps prevent rusting.

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2. File the edges of the filler rod
so they are slightly rounded.
This helps the filler rod slide
into the hole easier. This is very
easy to do with a belt sander
and is fine to do with a hand-
held file.
3. Insert and hammer in the
180mm filler rod into the single
hole, on the left end until it
sticks out 15mm.
Support the wood very near the
hole (on a table or vice), but
allow empty space for the rod
to come out. Hit the rods into
the holes before you bend the
rods, so the rods are straight
and easy to get in.
4. Hit the 150mm filler rod in the
middle hole, on the right end.
Hit the 80mm filler rod in the
far hole, on the right end
Be careful to not drive the filler
rod too far – if you have to
drive it back the opposite
direction, it will loosen the
wood’s grip on the rod.
5. Slide a 20mm piece of a straw
onto the 150mm rod.
The straw is an electric
insulator and will act as part of
the "switch" for the foam
cutter.
6. Mark at 10mm and 20mm
down the two long filler rods
(150mm and 180mm rods)
180mm rod 150mm rod
80mm rod
10mm 130mm 20mm
180mm rod 150mm rod
80mm rod
10mm
10mm

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7. Bend the top 10mm of the
180mm filler rods outward
Clamp the rod so that the top
10mm is sticking out of the vice.
Bend with pliers.
The purpose of bending the
wire is to give the wire a place
to sit and not slip off the rods.
8. Raise the rod to 20mm and
bend the 180mm filler rod
inward (the opposite
direction), so the rod looks like
an "S"
9. Repeat with the 150mm rod.
10. You will have the basic form of
the foam cutter! Make sure the
bent areas are bent in opposite
directions.
If they are not, then adjust
them with the pliers. If you bent
them the wrong way, just spin
the rods around so they look
like the picture.

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11. Bend the two short pieces of
filler bar coming out of the
bottom so the tips are tipped
toward the middle and each
other.
This helps create a firmer
contact between the batteries
and filler rod.
6. Make the switch and assemble
Tools Materials
Steps
1. Cut the 20mm x 40mm brass
sheet.
It should be about the same
height as the top part of the
80mm rod.
We are using brass sheets because brass is strong, somewhat springy, and very conductive
(all things that are good for a switch). Copper sheets are not used because they bend very
easily and do not hold their form.
2. Bend the shim around a piece
of rod.
This helps you shape the shim
so that it will fit tightly around
the 80mm rod.

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3. Slip the shim off the other rod,
and squeeze the rolled piece
slightly with the pliers.
This will make sure it will fit
snugly on the 80mm rod
4. Slide the brass shim onto the
80mm rod, making sure it can
swing nicely, but not slide off
too easily.
5. Bend the brass shim around the
long rod.
Go slowly and sort of "pull" the
shim to make sure it is wrapped
tightly around the rod.
6. Add a bend in the
opposite direction at the
end of the tab to make it
easy to engage/disengage.
7. You have everything except the
wire and power source, the
batteries.
Make sure you leave the straw in
the "OFF" (closed) position for
the next steps, when you add
the wire so you do not get
burned!

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8. Add the batteries between the
rods by wedging the battery
ends next to the rods first,
keeping the middle where the
batteries meet sticking up in
the air.
Doing this before you add the
wire is nice so that the wire
doesn't get messed up while you
put the batteries in.
9. With it flat on the table,
carefully push downward
where the batteries meet.
The batteries will "snap" into
place when they form a straight
line.
10. Pull a piece of tape around the
wooden handle and the two
batteries.
Pull it tight!
11. Cut a 250mm long piece of 32-
gague nichrome wire.
Other wire should not be used in
this project! See the note below.

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The nichrome wire is special because it will not corrode with high temeratures and has
a consistent electrical resistance.
A 32-gague nichrome wire is used because the resistance from a ~90mm length
(Resistance=3Ω) corresponds to the battery power supply (Voltage=3V) and a desirable
current output (I). In other words, the demand of the wire matches the supply from the
two batteries and creates enough heat to melt the foam.
When you find the wire is not cutting the foam as easily as it usually does, it could be the
voltage coming from the batteries has dropped. This has created an imbalance in your V=IR
equation! Your current has dropped to match the voltage drop. To counteract this and
increase your current output, reduce the resistance. To do so, simply squeeze the rods in
towards each other and wrap the wire around the rod a couple of more times. This reduces
the length of the wire, which reduces its resistance, and increases the heat released (I) by the
wire, cutting the foam faster.
12. Wrap the wire around one of
the long rods, 5-10 times.
Alternate wrapping above and below.
13. Twist the end around the rest
of the wire and snip any long
wire remaining.
14. Squeeze the rods toward each
other to be ~90mm apart and
begin wrapping the wire
around the other rod.
The wire should be tight! The
filler bars should be squeezed to
be 10mm closer at the top than
the bottom.
Be careful to not kink the wire – it will
create unwanted "hot spots!"

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As before, alternate wrapping
above and below the wire
stretched across, as before – it
will make the wrapping tighter
and prevent it from loosening
from slipping.
15. Twist the end and snip any long
wire remaining.
16. Unlatch the switch, move the
straw and click it back in to
turn the foam cutter on!
Build-Its are published by IDIN. The
International Development Innovation
Network (IDIN) is a consortium funded
through USAID’s Higher Education
Solutions Network (HESN) that supports
leading universities to catalyze the
development and application of
innovative science, technology, and
engineering approaches and tools to
solve some of the world’s most
challenging development problems.
The foam cutter featured here was designed with critical contributions from D-Lab staff,
Dennis Nagle. Participants in IDIN contributed as well. The document was created by Benji
Moncivaiz in collaboration with IDIN representatives at UC-Davis and Olin College.