A spin coater made from hard drives.
A spin coater is used to coat a thin film on the substrate. Like a dip coater, commercially produced units are too expensive for the average people to buy, but the operating principle is very simple.
People have tried to make spin coaters using computer fans, but commercial ones needs to spin up to 10,000 RPM or so. For such a high speed, a method is needed to fix the substrate when it's being spun. Gluing the substrate to the rotor is messy and will eventually clog the motor, therefore commercial units uses a specially designed chuck to hold the substrate, either by clamping the substrate or using a vacuum.
A hard drive can spin up to 7200 rpm or even futher depending on the load and the supplying voltage. Using a ESC motor controller (commonly used for driving BLDC motors in quadrotors or UAVs), we can control the rotor speed without building the complicated driving circuit. The hard disk spindle has a non-rotating shaft, which can be used to pull the vacuum we need.
This isn't new, of course,
Step 1: Motor Modification
To hold the sample in place, a vacuum pump is connected to the bottom of a modified motor. The vacuum tubing is glued to the shaft using instant glue.
The hdd motor is drilled through its axis, which is made of hardened steel, and requires the use of a carbide drill bit.
A O-ring is fitted on the rim of the motor.
The O-ring costs 10 cents where I live.
The drill bit is 3mm in diameter and costs about 10 USD.
Step 2: Assembly
ESC handles the control of the motor.
Arduino converts the knob position to PWM and controls the ESC.
Knob position is displayed on the LCD display.
Since I don't have a 3D printer to make the case, I simply used double sided tape.
Step 3: Conclusion
1. Depending on the ESC, you might be able to set different operating modes, breaking on/off, fast/slow startup for the motor.
2. If balanced well, small slides could spin to max speed, which is roughly 8500 rpm for this motor.
3. One should be able to design a better chuck using a larger O-ring and giving more suction.
4. For the first few runs and during drilling of the motor shaft, some bearing oil will spill out due to the heating during drilling or the pressure difference, but it didn't seem to matter to the operation. The suction force is enough to bend a cover slide to the point of breaking.
P.S. : If I get a 3d printer, I'll make a better vacuum connector, sample holder, and a nice casing for it.
I've included the source code for the arduino pro mini. Since I prefer direct programming, the code will be only readable to those who know something about AVR. However if you know how to send PWM, it can be easily done using the Arduino IDE.