Stator number 3 was thinner than the previous versions, and so the magnetic force much greater. This caused the previous plywood and 1mm steel plate to delaminate.
A local engineering firm wanted to charge me R250 per disk, which I thought was a bit ridiculous. So I ordered plain disks flame cut to 250mm diameter, which I think cost about R40 per disk - the local company charged me R60, which was Ok - for now.
To make the holes accurately using only a drill press, I needed a template. I found (thanks Johan!) a place called the Fab Lab, which offers innovators free access to perspex laser cutting. So I whipped up some DXF files, and went off to cut them. Its truly amazing watching the laser cutter work - its like a plotter with a mirror where the pen would normally be. A laser beam is reflected off the mirror, and depending on the settings, either cuts or etches the perspex. Apparently there are Fab Labs all over the world - really worth visiting if you can find one.
I made a disk with 1mm holes where the 12mm holes need to be, and clamped that to the steel disks (which were noticably less exactly round), drilled pilot holes, and then used bigger bits to work the holes up to 12mm. It worked really well, although the holes in the perspex template were a bit damaged in the process.
I also cut a disk to act as a holder for the magnets, to hopefully stop them flying out by centrifugal force (pedants will insist there is no such thing as centrifugal force, but hey, you know what I mean). Notice how I cracked the perspex - but using chloroform to glue it together worked well. The magnets are currently stuck to the steel plate by magnetism alone, which keeps them in place even when spinning at speed.
Eventually, the magnets will be epoxied in place, as per the first rotor - but until then the perspex is convenient, and comes apart easily.
These two rotors attract each other with phenomenl force, I guess enough to crush a finger, although I do not plan to investigate that empirically. So far I have avoided any major accidents. Disassembling the rotors takes skill, courage, and cunning. Quite often I have pulled the magnets out one by one (I use the perspex holder on one rotor only) before attempting it. The most recent method is to lever the rotors apart slightly, and then use a strip of plywood to turn the seperator nuts a bit one at a time, until the outer rotors is far enough away from the inner one to pull off by hand.
Below is the sequence of operations currently used for making the rotors. First, a perspex template is used to accurately position the magnets.
Its hard to do this without the magnets smacking into the rotors. This can cause the magnet to lose some of its force, and in extreme cases the magnet can chip or crack. Its almost impossible to glue a chip back onto a magnet!
Next a wooden disk is bolted into the centre, after being lightly wiped with some vaseline or release wax. Finally, masking tape is used to seal the edge and provide a dam for the epoxy.
Its important to have the epoxy evenly spread, and so a level should be used to check the rotor is level before the epoxy is allowed to set. A heat gun is useful to help the epoxy flow evenly, and on probably essential if the room temperature is below 20°C
Once the epoxy is set, the wooden disk and the tape is removed, and the sharp edge should be trimmed off to avoid injury. Finally, the magnet surface is roughend with fine sandpaper, and painted with an etch primer, followed by an overcoat.
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