Hand-held Foam Cutter_v2(1)

Build-it: Hand-held foam cutter

Cost of whole system

Nichrome wire $0.05

Filler rod $2

Brass shim $0.05

Wood $1

D-batteries $4

Tape Minimal

Total cost $7.10

What is needed to make a foam cutter

Tools

Materials

Saw Item Amount Cost

Brace and

bits

(7/8" and

3/8")

Hard wood

1.5" wide,

1.5" thick,

6" long

$5-10 for

2"x2"

3'-5' long

Tape measure

and marker

1/8” thick

filler rod 16 inches

$4 for a

3-foot rod

Hacksaw or

Bolt cutters A straw 1 minimal

Pliers

Nichrome

wire, 32

gauge

10 inches

$20 for

700-1000

feet

Vice

Brass shim

(very thin

brass sheet)

1" by 2"

$30-50

for 12-15

of 6"x12"

sheets

Wire cutters

D-batteries 2 $4 for

pack of 2

File

Strong tape 12-18" $5 for 55-

yard roll

Hammer

GOALS:

o Participants have made a nichrome wire foam cutter.

o Participants know how to measure simple dimensions precicely.

o Participants have practiced more detailed machining and bending.

TIME:

60 minutes

PREPARATION:

Collect materials and tools

Make a foam cutter yourself to discover what participants might find difficult

Review and/or print Build-It instructions

ROOM SET-UP:

A clean, flat surface such as a table for participants is useful to have for easier assembly.

Prepare the wooden block 3

Prepare the wooden block

Time: 10 minutes

Tools Materials

Steps

1. Cut a 6” long, 1.5” thick, 1.5” 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 (1”, ½”,

etc.) Then, mark the plank

o Situate yourself to begin cutting and 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.

1

Prepare the wooden block 4

0.5

"

0.5

"

1"

0.5

"

3. Keeping the rings horizontal, on the top, mark halfway along the width.

4. Mark 0.5” from each end. Mark an additional 0.5" 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 1/8” 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.

Add the metal rods 5

Add the metal rods

Time: 20 minutes

Tools Materials

Steps

1. Cut three filler rods: 3”, 6”, and 7”

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!

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. Hit the 7” filler rod in the single hole, on the left end.

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.

7" rod 6" rod

3" rod

2


4. Hit the 6” filler rod in the middle hole, on the right end. Hit the 3” 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 1.5" piece of a straw onto the 6" rod.

The straw is an electric insulator and will act as part of the "switch" for the foam cutter. It should go onto the straw before you bend the rod and add the wire.

6. Mark at ½" and 1" down the two long filler rods (6" and 7" rods)

7. Bend the top ½" of the 7" filler rods outward

Clamp the rod so that the top ½" is sticking out of the vice. Bend with pliers.

0.5

"

0.5

"

5" 1"

0.5

"

7" rod 6" rod

3" rod


8. Raise the rod to 1" and bend the 7" filler rod inward (the opposite direction), so the rod looks like an "S"

The purpose of this bent area is to give the wire a place to sit and not slip off the rods.

9. Repeat with the 6" rod

10. You will have the basic form of the foam cutter! Make sure the bent areas are bent in opposite directions.

11. Cut a 10" long piece of 32-gague nichrome wire

A 32-gague nichrome wire is used because the resistance from a ~3.5" length (R) corresponds

to the battery power supply (V) 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.

Alternate wrapping above and below.

13. Twist the end and snip any long wire remaining.

14. Squeeze the rods toward each other slightly and begin wrapping the wire around the other rod.

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.

You are almost there! Your foam cutter is just missing a switch and batteries.

Make the switch and assemble 9

Make the switch and assemble

Time: 15 minutes

Tools Materials

Steps:

1. Cut the 1" x 2" brass shim.

It should be about the same height as the 3" 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 3" rod.

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 3" rod

4. Slide the brass shim onto the 3" rod, making sure it can swing nicely, but not slide off too easily.

3


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 are left with a cutter with everything except the power source, the batteries.

8. Add the batteries between the rods by wedging the battery edges next to the rods first, keeping the middle where the batteries meet sticking up in the air.

9. With it flat on the table, carefully push downward where the batteries meet.

The batteries will sort of "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. Unlatch the switch, move the straw and click it back in to turn the foam cutter on!

Hand-held Foam Cutter_v2(1)

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Transcript of Hand-held Foam Cutter_v2(1)