The motor consists of two distinct parts. The first part is the stator, the windings that drive the magnet field around, the PCB it sits on and the ball bearings with the shaft for the rotor. The second part is the rotor itself, which is the part that will be upgraded. At some pater point I would like to attempt rewinding of the stator, too, but for now I decided to limit the project to just replacing the magnets on the rotor.
The rotor is press-fitted onto the shaft and therefore the first part of the project is to carefully remove it and the plastic/rubber disk base that the DVD disk sits on. A great tool to have here would be a small bearing separator or a very small bearing puller. I did not have anything that’s small enough and had to use a makeshift bracket, a vise and a small pin punch to hammer the shroud off the shaft. Not a perfect setup but it worked.
Some quick measurements of the shroud showed that I could just barely fit (12) 5mm x 5mm x 1mm magnets and in the end it turned out that I did not account for the small chamfer on the bottom of the shroud which ended up pushing the magnets inward just enough for the stator not completely clearing the rotor (see below).
I’ve ordered the magnets from an eBay seller called Frenergy Magnets. These are a 5x5mm, 1mm thick Neodymium N50 square magnets magnetized though their height. If you’re ordering magnets, be sure to check the direction of magnetic field. It’s not common for flat magnets like these to be magnetized differently than through their height but it is possible, so be sure to check. The magnets came from Australia but they came pretty quick, quicker than I was ready for their use!
In order to install the new magnets we have to first remove the old. I have tried to soak the rotor in less harsh solvents but in the end only acetone seemed to do the work properly. I put the rotor in a small container left over from a soy sauce on one of the last Japanese takeouts. It has to be polyethylene or else acetone will eat through the container, too. When I just poured the acetone, it was out-gassing quite apparently, so I had to put it inside a polyethylene zip bag, which eliminated the acetone stench.In approximately two days, the glue that was holding the old magnet to the shroud was gone and the old magnet itself, which turned out to be made of a rubbery stuff not unlike what you see in fridge magnets (and of similar strength), had lost all its elasticity and became very brittle. A nudge with a small screwdriver took the old magnet out.
Next step – position the magnets. There are 12 magnets in total – this was important to keep the same motor configuration so no software changes would be required. The change in the Arduino sketch for the brushless DC motor would be trivial but I just wanted the connect the upgraded motor right back to the stroboscope setup for quick testing. Since the magnets are 5mm long and the Xbox DVD spindle motor’s shroud internal diameter is 22mm, its circumference then is Pi*22mm=69.08mm and there’s approximately 0.75mm gap between each magnet.
It took several tries to manage that 0.75mm gap between each magnet just right because the magnets tend to stick together better than they stick to the ferrous material of the shroud. Also it is important to remember that the magnets are going in alternating directions of the poles. Fortunately, the magnets came with one of their poles already marked with a Sharpie. If they are not, you’ll have to mark the direction before you take the magnet off the stack they are usually shipped as. Once the magnet is placed on the metal of the shroud, it is really hard to distinguish which direction is which, so be sure to mark it when the direction is still easy to tell.
I have tried to use various separators to control the gap between the magnets but it looks like there’s some variation between the magnet sizes (these are not made to precise tolerance specs) and so I always ended up with a gap at the end that’s either too large or too small. So, I ended up positioning the magnets roughly using and then just moving them slightly back and forth estimating how evenly they are spread just by eye.Once the magnets are evenly spread around the shroud, you’ll need to glue them to the shroud. The easiest way is to use liquid Super Glue and just put a drop between the magnet and the shroud and let the capillary force suck it in. I tried to use Locktite’s Gel Control Super Glue in hopes that it will give me, well, more control but it was just collecting in blobs which felt like they were sticking out too much.
When the glue is dry, you can press the shroud right back onto the shaft it came off of. In my case, however, it was not so easy because I realized that the magnets or blobs of glue are sticking out too much and there’s some (rather soft though) grinding between the rotor and the stator. I had to take the rotor back off and scrape as much glue as I could.
When you look at the photograph of the finished shroud, you’ll see my attempts to blacken the magnets and the rotor with a marker to see which parts rub against which. After several attempts to correct the problem with a fit between the rotor and the stator I left it where I could press the shroud almost all the way down to its original place and the motor was still working. I could not, however, press it all the way down or else the friction between the rotor and the stator was more than the torque of the motor, even considering the increased torque.
So, in conclusion, this exercise showed that the motor does produce much better torque with the installation of the Neodymium magnets, even though in my case much of the extra torque was consumed by the extra friction due to poor part size match.
In my next DVD spindle BLDC motor upgrade (I still have quite a few left to experiment on), I will either use a shroud with internal diameter slightly larger than 22 mm or will take a small chamfer off of the side of the magnet facing the metal the shroud so the magnets sit little further away from the stator. Another way would be to find magnets thinner than 1mm but I’ve yet to find a supplier for those.
Anyway, there are ways to improve this procedure for better control of the gap between the rotor and the stator and I’ll be happy to post here again as soon as I have a better working upgraded DVD spindle motor.