|You'll need some materials and tools, they
are listed below.
Materials for these plans:
- 6 - 8 Coke cans
small scrap of wood about 6cm x 2cm x 2cm
- A balloon
tuna paste lid
- A spoke from a bicycle wheel
- 1.2mm steel
- Super glue
- A 2p coin for
- 2 x Electrical screw blocks
of thick cardboard about 10cm square
- BBQ Skewers
(1/16") brass rod
- 2.38mm (3/32") brass tube
scrap block of wood for the jig at least 75 x 75 x 20mm
- Steel wire wool
- A drawing pin
- Drill with 2mm drill bit, and 6 -
8mm HSS drill bit
- Tin Snips (you can get away without these,
but they make life easier)
- Pliers (two pairs of pliers comes in
- Needle nose pliers
- Can opener
- Half round file
- Saw to cut wood, if necessary
Some of the materials needed.
Diagram, click to enlarge
This can will be the top can that holds the cranks. Remove the top with a
can opener or scissors. You need to holes punched and drilled either
side. To make sure that they were in line with each other, I made a
paper template, you can skip doing this and just make the holes by eye,
but I think it's worthwhile making the template. When you've punched the
holes, drill them out with a 2mm drill bit.
Punch a hole in the
can, parallel to the direction that the cranks will run in with a
knife. Use this to start your cut for the viewing hole in the front of
|Step 2: Mark the centre of the top can, for the bushing
The bushing for the displacer shaft needs to sit in the centre of the
can. I marked this using a caliper set to 33mm. Draw three arcs, they
should intersect at the centre of the can. I punched a hole in the
centre of the can to make sure that the drill bit didn't slip, then I
drilled it out using a 2mm drill bit. The tubing for the bushing is
2.38mm, not 2, so you'll have to run the drill bit around the hole a few
times to widen it.
Marking the centre of the can
Step 3: A jig to glue the bushing in place.
As there's nothing physically holding the bushing in place until the
epoxy sets, you'll need a jig to hold it straight. The jig is just a
scrap of wood with a 2mm hole drilled in it. There's a 1.59m brass rod
is epoxied in this hole. Cut a piece of brass rod to about 50mm long and
epoxy it in place. Whilst the epoxy is drying, keep checking that it's
square to the wood with a square. Check all round with square.
When the epoxy has dried, you can add a spacer. The purposes of this
spacer is to hold the brass tube in the right place on the coke can, as
the bottom of the can is dome shaped. The spacer is about 9mm long and
made from the end of an ink cartridge out of a biro pen.
Checking the jig with a square
Drilling the hole for the jig
The spacer for the jig
|Step 4: Cut and glue the bushing in place.
Cut a piece of the 2.38mm tubing about 10mm long. You can cut small
diameter tubing by rolling the knife over it. Cutting the tube will
leave tiny burrs on the inside of the tube, you need to remove theses
using a knife, scrape around the inside of the cut edge tube with the
tip of the knife. Test that the piece of tube slides over a piece of
1.59mm rod, it should fall under it's own weight.
With the tube
ready, you can place it on it's jig. Then place the coke can over the
tube. The jig should hold it in place, so you can epoxy the tube in
place. Be careful not to get epoxy on the brass rod.
Brass tube fitted to the jig ready for epoxying
Cutting the 2.38mm tube
Epoxying the tube
|Step 5: Bend the cranks
The cranks are made from a bicycle spoke. I bent the cranks using
pliers. As you make them, keep checking the they running true, you can
do this by laying them on a flat surface between where the two bearing
points will be. A windowsill is ideal for this.
part of the cranks is bent out by about 14mm and the diaphragm part is
bent out by about 4mm. These two are offset by 90*
Diagram of the cranks, click to enlarge
|Step 6: The displacer cylinder
The displacer cylinder is just a coke can with a hole drilled it the
side, and the top bevel cut off.
The hole I drilled is around 6mm.
You want to remove the top with a can opener, the cut down to just below
the top bevel. Remove any burrs from the hole so they don't catch the
Hole for air, remove the bevel too!
|Step 7: A displacer
The displacer is made from steel wire wool wrapped around a length of
brass rod. Cut your brass rod to 125mm long and bend a small hook on one
end. This will keep the bottom cap from falling off.
coke cans, mark and drill the centre of these to 2mm as you did in step
3. Cut the domes out of the bottom of the cans using tin snips. You'll
need to make a depression in one of them to make space for the screw
To hold the two caps together, I took a screw block,
removed the plastic coating and sawed it in half using a hacksaw.
Now you can roll your steel wire wool around the brass rod. Keep
wrapping it until it's about 5mm smaller than the displacer can. Remove
the brass rod, and cut the roll of wool down to about 40mm using
You can assemble the displacer now. first put the
bottom cap on the brass rod then push the wire wool onto the rod. Put
the top cap on and then secure it with the screw block. When it's
finished, the displacer should be about 35mm tall. You'll probably have
to make adjustments until it will fall freely in the displacer can.
Screw connector cut in half.
| The finished displacer
Caps for the displacer
|Step 8: The diaphragm shelf
The diaphragm sits on a a little piece of wood that's epoxied to the
side of the displacer cylinder. The wood has two holes drilled in it to
transfer air from the displacer cylinder to the diaphragm.
Start by drilling a 6mm hole in the paste lid. You can position the paste
lid roughly and use this to mark the depth on your drill with a piece
of tape. You want the paste lid about 1cm away from the end of the wood
that will be glued to the coke can. You can drill the two holes now,
one in the end of the wood meeting up with another in the top of the
wood, they should meet allowing air to flow through. File the edge of
the wood that meets the into a curve to roughly match the curve of a
With the holes drilled, you can epoxy the paste lid to
the piece of wood, make sure the holes line up. Wait for that epoxy to
dry, and then epoxy the piece of wood to the side of the displacer can,
again making sure that the air holes line up. You'll have to hold the
wood while the epoxy dries.
It's important to use plenty of
epoxy to seal the engine, as it must be air tight. Even a very small air
leak will stop the engine running, so it's better to use too much epoxy
rather than too little.
The diaphragm shelf
Drilling one of the holes
|Step 9: Fit and bend the displacer rod
You can assemble the two parts of the engine now. Put the displacer in
it's cylinder, and then push the top can into the displacer cylinder.
Don't push the top can from the top, only push from the bottom through
the viewing window. Test the displacer falls freely, it should fall
under it's own weight with no obvious resistance. If it doesn't quite
fall freely, you can adjust the position of the top can until it does.
With the displacer moving as it should, you can bend the end of it to a
right angle, so that it can be connected to the cranks in the next
step. Ignore the cranks and connecting rod in the picture, that's covered later.
Bent displacer rod
|Step 10: Make the connecting rods for the displacer and diaphragm
I made the connecting rods for the displacer and diaphragm from 1.2mm
wire. The ends are wrapped around the parts they connect to, to act as
bearings. They have a zig zag bend, so that the length of them can
easily be adjusted by stretching or compressing the zig zag.
the displacer connector, I bent one end around a piece of the 2.38mm
tubing to connect to the cranks, and the other around a piece of 1.59mm
For the diaphragm I bent one end around a piece of the
2.38mm tubing to connect to the cranks, and the other end around a
piece of 1.2mm wire.
Both of these are about 45mm long
|Making the bearing point
Finished connecting rod
|Plans continued on page 2 >>>
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