Micro Centrifuge

Needed a micro-centrifuge for 1.5 ml and 0.5 ml tubes with about 5,000 rpm spin speed and capacity for 6 or more tubes at a time. Online DIY builds for such centrifuges left a bit to be desired so decided to design a simple but very usable device with professional level centrifugation performance from motors I had at home.

The biggest challenge is in making a properly balanced centrifuge rotor that holds the tubes. On eBay used rotors are available for low prices. A set of two gray plastic centrifuge rotors (one for 0.5 ml tubes the other for 1.5 ml tubes) rated for 7,000 rpm were purchased for $12.

1. Centrifuge Rotor
   
   1. The purchased plastic centrifuge rotors seemed in excellent shape and looked unused. They were about 4 inches in diameter. One of them held six 1.5 ml tubes, the other held twelve 0.5 ml tubes. Would have been great if they also fit 0.2 ml tubes which are increasingly common for PCR.
   2. The rotors also had a label on the bag stating "Rated for 7,000 rpm or less".
2. Motor
   1. Three inch long high-speed motors from my parts bin seemed perfect for these centrifuge rotors. Seemed to remember that these were 110 V AC motors but was not too sure.
   2. I hooked one of these motors to the Variac power supply that I had built a few weeks back and gradually increased the voltage. They seemed happy up to a 100 volts AC.
3. Motor Spindle
   
   1. The diameter of the motor spindle was too small to fit the centrifuge rotor so would need a collar on the motor spindle to which the rotor would be attached. Used a piece of a wooden dowel as the collar by first making a through hole exactly through the center of the dowel. I followed the method shown in this YouTube video and also described below.
      
      1. Drill press table was checked to be square with the drill press table. A piece of hardwood (about 2 inch thick) to hold the dowel was clamped to the base of the drill press.
      2. A hole was made into this wood with a spade drill bit slightly smaller than the size of a dowel (for a 1/2 inch diameter dowel so used 1/2 inch bit).
      3. The drill bit was removed without disturbing the clamped wood.
      4. A bit slightly smaller than the diameter of the motor shaft was inserted into the chuck of the drill press.
      5. A 2 inch long piece of the 1/2 inch diameter dowel was then pressed into this hole in the wood.
      6. Again without disturbing the clamps or the wood with the dowel in the hole a new hole was drilled into the center of the dowel.
      7. The dowel was removed and cut into half to give two 1 inch long pieces. One of these pieces was a backup.
   2. Shaping the dowel on the motor like a lathe
      1. The dowel was press fit onto the shaft of the motor. About 36 AC volts was applied to the motor with the homemade variac power supply
      2. The dowel was shaped while spinning with files till it fit the hole inside the centrifuge rotor. The shaping also balances the spindle so that it spins true.
      3. The rotor was attached by a friction fit to the spindle and spun at about 50 volts AC to make sure that there was minimal vibration from uneven spinning.
   3. Permanently attaching the dowel to the motor shaft
      1. The dowel was carefully tugged off the shaft.
      2. A spot of wood hot glue was placed inside the dowel hole and the dowel was slid onto the shaft
      3. The open end of the dowel was also sealed by filling the hole with hot glue.
      4. The dowel was warmed up by bringing a soldering iron close to it to and then brushed with acrylic varnish till no more varnish could be absorbed by the wooden dowel.
      5. I tried using a heat sink tubing over the dowel but the rotor did not fit anymore. I would have to reshape the dowel again so I removed the heat sink tubing.
4. Determining optimum voltage for the motor
   1. A tachometer was used to read the rpm at different voltage. A small piece of reflective aluminum tape was stuck onto the rotor to generate a reflection signal for the tachometer. At 60 Volts I got about 6000 rpm. So I decided that 60 volts AC would be a safe voltage to use for this centrifuge (speed would drop when the centrifuge rotor was loaded with tubes) as the rotor was rated for use at 7000 rpm or less. At 60 V AC, the motor drew about 300 mA at startup and about 100 mA while running with the rotor but without tubes.

I struggled with the decision to include a speed controller and a timer. A mechanical timer would have been great but the lowest price I could find was about $20 and they were rather large. I toyed with the idea of getting a broken toaster oven and extracting the timer from it. Maybe one will show up on freecycle and I will add the timer later.

Also for speed control I tried using a fan control, it worked, but somehow did not like that set up. I did have a few AC transformers, the ideal sized one had two primary 120 V coils (for use with 120 or 240 volts) and two 6.3 volt secondary coils rated at 1 Ampere (for 6 or 12 V). I removed the secondary winding's from this transformer with the expectation that I would wind my own secondary coil with outputs of 30, 45, and 60 volts. I needed 32 gauge magnet wire to replace the 24 gauge used in the original winding's so that I would have more space to fit in the additional turns. This seemed like too much work and I did not have the 32 gauge wire, so finally, I decided to make a single speed centrifuge with no timer.

To get 30-60 volts from the 12 volt transformer I could have used a diode-capacitor-voltage multiplier to get the required output but I was stupid enough to have removed the secondary winding by then. While I was looking for another small transformer I had the genius idea to use the primary coils of my "damaged" transformer. By connecting the two primary coils in series and connected the two ends of these coils to 0 and 120 V I would get 60 volts at the junction of these two coils. Voila!

Tried using this configuration and it worked very well. Phew!

I built a box approximately 5 inches square as that left enough clearance around the rotors. Used remnants of plywood pieces from other projects.

1. Box base: Cut out a 5 inch square out of 1/2 inch plywood for the base of the box. Drilled four holes at the corners for four rubber feet I had saved from a non-working coffee grinder.
2. Motor holder: This had three parts. A central plate to attach to hold the motor assembly and form the base of the top compartment of the centrifuge. A small disc to attach to the motor and another disc (spacer disc) which would attach the motor holding disc to the central plate and also provide the needed clearances for the centrifuge rotor. The pictures make it clear.
   
   1. Central plate. Used a hole saw on my Harbor Freight drill press to cut out 1.5 inch hole (slightly larger than the diameter of the motor) from the center of a 5 inch square piece of 3/4th inch plywood.
   2. Motor holding disc. The motor had bolt holes at the top, flanking the shaft. To support the motor I would need a top plate. This top plate would need to be smaller than the recess at the bottom of the centrifuge rotor. I could made a 2.5 inch diameter disc and still clear the rotor recess. So I used a 2.5 inch hole saw to cut out two discs from a 1/4th inch MDF hardboard. The extra disc was a backup or potentially for another centrifuge. I used a piece of thin cardboard to figure out where the holes were on the top of the centrifuge for the two bolts and then used these as a template to drill two small holes for the two bolts which would attach this wooden disc to the motor. The hole in the center was already created by the hole saw. Did a test fit to make sure everything connected.
   3. Spacer disc. The top of the motor had to be a certain height from the top of the central plate otherwise the rotor would rub against the central plate. I measured this and found that an additional half inch height would be required to clear the rotor. I cut a 2.5 inch disc from a 1/2 inch plywood and then cut out the cross-sectional shape of the motor (kind of like a circle flattened on two sides) on my band saw. Did a test fit with the motor holding disc and this spacer disc. Looked good, so bolted the motor to the motor holding disc, then applied wood glue to the bottom of the motor holding disc and to the top of the spacer disc, slid the spacer disc on and clamped it till the glue dried.
   4. The whole assembly (motor and two discs) were then glued onto the central plate. The motor was tested to see if it spun easily without vibration.
3. Sides of the case were cut from 1/4th inch plywood (harvested from the back of an oak book case). One piece for the back and two pieces for the side, all 5 inches in height. Three 1.5 inch x 1 inch pieces of wooden strips were glued onto the two sides and the back edge of the base plate (the bottom of the centrifuge with holes for the rubber feet) to support the side walls. The side walls were then glued and attached with a screw to these wooden pieces. The first coat of black acrylic paint was applied.
4. Front of the case. This was measured from the the partially assembled box and cut from the same 1/4th inch plywood. The front would not be glued but held by screws so that it could be removed for repairs and upgrades.

The steps to connecting all the components are shown below. First, I attached the transformer to the central plate next to the motor then wired the AC input through the switches to the transformer and finally wired the motor to the transformer pins.

1. Attaching the transformer to the base plate. I had already soldered two wires to the motor for testing so shortened one of these and attached that to one end of the primary coil of the transformer. The transformer was a tight fit between the central plate and the base (fortuitous accident!) so I only had to prevent it sliding around. The transformer had two existing holes through the metal core. I did not have thin and long enough screws so used two brass colored nails (for use with frame hangers) and nailed the transformer to the central plate. The nails were covered with plastic tape before nailing them through the existing holes in the transformer to prevent shorting of the transformer core (would create eddy currents).
2. Power cord for AC input. Had a left over power cord from a hair trimmer so attached this through the back wall of the centrifuge box by drilling two connected holes and expanding it into a small rectangular hole. The free end of the power cord was pulled through.
3. Attaching the switches. I decided to use an AC 1-pole 2-way switch and an AC momentary-on switch. The two way switch would connect the transformer directly to the motor or through the AC momentary-on switch. This would allow me to pulse the rotor for quick spins or continuously. I cut two holes for the switches in the front panel. The central pole of the two way switch was wired directly to the Live wire from the AC power cord. One terminal of the switch was soldered to the outer connection of the transformer and the other end was soldered to one pole of the Momentary-ON switch. The other pole of the Momentary-ON switch was also soldered to the same outer connection of the transformer. The two separate primary coils on the transformer were bridged with a wire from the remaining connection from the motor.The wiring diagram shows this a bit more clearly.
4. Check. So 120 V AC Live goes from the power cord through the switches to the one of the outer connections of one of the two 120V transformer primary coils. The outer end of the second 120 V transformer coil is connected to the motor and to the Neutral wire from the AC cord. The inner connector of the two 120V transformer primary coils are joined together and connected to the other pole of the motor. The two coils of the transformer are now in series and will see 120 V across the joined coil. The central tap will give the required 60V to the motor. Had tested this before to make sure it all works and also tested this before attaching the front face plate to the centrifuge case. The AC power is not isolated from the mains so be extra careful.

Drilled screw holes to hold the central plate (which holds the motor and the transformer and of course supports the rotor) to the side supports that hold the side walls to the bottom plate. The central plate was then attached by drywall screws to the side supports. Did not use glue in case I needed to access the wiring later. The front of the centrifuge was then attached by two screws to the central plate.

Cut a piece of 1/4th inch thick acrylic for the lid. Glued Mylar transparency film to the acrylic so that alcohol etc from the centrifuge tubes would not affect the acrylic.

Laser printed some labels and glued these on and apply a final coat of acrylic varnish to seal everything.