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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10m

m

10m

m

20mm

10m

m

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

10m

m

130mm 20mm

180mm rod 150mm rod

80mm rod

10m

m

10m

m

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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.