that the aluminum bar is centered directly on top of the
wooden disk and that the two ends of the aluminum bar
are pointing upward (Figure 2B). Also, press firmly on
the pieces for at least 30 seconds to ensure a tight initial
fit and a level attachment.
G. Then, let the epoxy cure overnight for a strong bond.
Place some weight (e.g., a small jar filled metal bolts) on
top of the aluminum bar right above the wooden disk to
press the two pieces together as the epoxy dries.
2. After the tube holder has dried, use scissors and wire cutters to remove the blades from the fan (Figure 3), which
should increase the speed of the finished centrifuge. Specifically, cut off the fan blades as close to the fan head as possible using scissors (Figure 3A and 3B), and then remove
any jagged edges by making a series of finer cuts with the
wire cutters (Figure 3C and 3D). To simplify the construction process, however, readers can skip this step and leave
the fan blades intact.
3. Next, use the wire cutters to remove the top corners of the
fan casing and two of the bottom corners that are diagonally
opposite from each other. This will leave two screw holes
that are kitty-corner from each other on the bottom of the
casing, which should provide a solid anchor for the assembly (Figure 2B). Alternatively, readers can leave all four of
the bottom corners intact and use four 1-inch-long #6
screws to secure the fan to the plywood board. Readers
can also skip this step as well, leaving the fan casing alone
and using longer (e.g., 1.5-inch-long) screws to hold the
assembly to the plywood board. Also, depending on the
fan motor used, other screws (with a larger or smaller diameter or a different length) may be necessary.
4. Then, following the same process that is described above,
epoxy the assembled tube holder onto the top of the plastic
fan head, which is the side that spins (Figure 2B), and let
the epoxy cure overnight for a solid bond.
5. Once the motor assembly has dried, prepare the plastic
bowl for the push-button switch and DC jack that will later
be inserted into the front and back faces of the centrifuge.
Although there are many ways to make the holes for these
components, we suggest the following approach, which in
our experience tends to reduce the chances of cracking
the plastic (Figure 4). Also, because our switch and jack
both had a diameter of roughly ½ inch, we used that
dimension for all of our measurements below. Of course,
for larger or smaller components, readers would need to
adjust their dimensions accordingly.
A. First, clamp a 3 × 12-inch (or longer) rectangular board
that is ½ to ¾ inches thick to a table top, so that one
end of the board extends over the edge of the table by
a few inches. Then, “hang” the plastic bowl on the corner of the board so that its front face is looking
upwards, and clamp the bowl in place (Figure 4A). This
set-up should enable readers to drill through the plastic
bowl and into (or through) the wooden board without
damaging the table.
B. Next, drill a small hole (e.g., 1/8-inch) into the flat portion
of the front face along the midline, and roughly 3/8 inch
below the bottom of the raised ridge that extends around
the bowl (Figure 4A and 4B). One way to place this initial
hole is to press the final ( i.e., largest) drill bit up to the face
of the bowl where the plug will be located (the front face),
and then mark the scratch that the tip makes on the plastic. Readers should also make sure to leave some space
between the edge of the final hole and the raised ridge
extending around the bowl to account for the washer
and nut that will hold the switch in place.
C. Then, enlarge the hole using a slightly bigger (e.g., 5/32-
inch) drill bit, and repeat this process with increasingly
larger drill bits (Figure 4C) until the hole is big enough
to hold the switch (Figure 4D). For example, given the
size of our switch, we ended with a ½-inch-wide hole.
Although time consuming, we suggest this approach
because simply drilling into the face of the bowl with a
large bit can crack the plastic.
Figure 3. Cutting the blades off of the computer fan using
scissors (A, B) and wire cutters (C, D). Figure 4. Drilling holes in the front and back faces of the plastic
bowl to hold the push-button switch and DC connection jack.