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Make Your Own Miniature Electric Hub Motor

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Step 8: Mechanics and Materials

Now we're getting to the mechanical design. Let's lay the ground rules for what you might need or have access to.

Rotor Material

Your magnets would like to be contained in a material which offers low resistance (reluctance) to the magnetic field, and also does not magnetize itself permanently in the presence of the magnets. Many high performance alloys of nickel, cobalt, iron, and trace metals have been invented to optimize the magnetic properties of a motor. They're expensive, require specialized heat treatment, and even specific machining processes to conform to the geometry of a magnetic machine.

We're not going to bother with that. The most common rotor material for hobbyists is just plain steel tubing. It does a good enough job, and the best part, it's cheap and readily available.  I will list sources of steel tubing in the Resources section, but as a general rule, the tubing you purchase should be:

low carbon or "mild" steel. High carbon and heavily alloyed steels have significantly worse magnetic properties.

� seamless or at minimum DOM style tubing. This is the majority of steel tubing, but keep an eye out regardless. DOM tubing has a more uniform wall thickness and no ugly weld seam to affect the roundless. It is generally made to tight tolerances. Avoid cast iron pipe.

� plain finished. A precision ground or machined and polished finish will not do you any good unless it's already precisely the diameter you need.

� Oversize (OD larger than your rotor's outside diameter) AND undersize (ID smaller than your magnet mounting surface) so you can machine it to suit and not worry about hitting the limits of your materials' physical manifestation.

Endcap Material

Since the only thing which has to support a magnetic field in your motor is the rotor, the endcap and other structural elements only have to be mechanically sound. That means you have way more choices here. Generally, it's some kind of nonferrous (not steel) metal.

� Aluminum is the number one choice. It's light, strong, easy to machine, and common. Not exactly cheap in "big", however.

� Plastics! Engineering polymers such as nylon, polycarbonate, acetal, and polyethylene in high density and high  molecular weight varieties all exhibit high strength and lightness. Plus, plastic machines like... well, plastic. Easy to shape, especially if you are new to machining.

Some plastics let your motor have the magical see-through effect. The BWD Scooter uses Lexan (polycarbonate) side plates so you can see the robot in disguise.

� If you are into that stuff, you could conceivably craft endplates out of fiberglass or carbon fiber panels. The ultimate in light weight and stiffness, but be aware of the fact that you have to attach it to the can somehow. This will be addressed shortly.

Center Shaft Material

The most important trait of the shaft is that it can't bend. I'll address shaft design shortly, but you should expect to make the shaft from some kind of metal. Larger hub motors use steel, smaller ones may be aluminum. I used an aluminum shaft on Razer's motor for weight savings and ease of machinability.

� Aluminum should be limited to the aircraft alloys: 6061, 2024, 7075, and similar. These offer higher strength than other aluminum grades.

� You can get away with a mild steel shaft such as the low 10xx alloys (e.g. 1018, 1020), but if you are already using steel, moving up to a medium carbon or alloy steel shaft wouldn't hurt. Very low alloys (1006 and similar) do not machine well - they are actually too soft to finish machine finely.

Tools

Let's be honest: a motor is a precise alignment of opposing magnetic fields. Invariably you will need access to machine tools to make them. Unless you are very crafty with your shop drill press and Dremel and can make things conceptric to within 5 thousands of an inch (0.005", or around 0.1 millimeters!)  constructing the endcaps and rotor (and shaft, and stator mount...) will require access to...

� A metal lathe. Not a wood lathe, where you hold the tool yourself, but a metal lathe. If you have made it this far, I assume you know how to operate such a machine already, because giving machining lessons over Instructables is slightly troublesome. 

You will need the ability to precisely bore an inner diameter. Boring bars, or something which can function as them, are a must.

� A milling machine, or at minimum, a drill press with X-Y table and fixturing & indexing abilities. This can be a full size Bridgeport or similar, or a miniature hobby mill like those found at Harbor Freight. Basic tooling should be available. You should have a spindle drill chuck to precisely drill holes with at the least.

� Some kind of vise. Handy to have for pushing in bearings and cans, and also for holding the stator while you wind it! Extra leverage will only help in winding.

� Measuring calipers, micrometers, dividers, etc. Because several parts need to fit closely with one another, you must have metrology tools. I get by with a single digital caliper.
 
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