2. Roughly Grinding an Aluminum Rod to the Hub Diameter

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I’m trying to replace the hub on a LEGO wheel. The hub diameter is around 8.04 mm. An 8 mm rod probably would have been fine, but I didn’t have any in stock (McMaster-Carr and MSC Direct has metric rods). If I had a lathe at the time, I could have reduced down a 3/8-inch diameter rod.

I do have a grinder. Even though it isn’t the best choice, I decided to grind down a slightly larger aluminum rod until it reached approximately the desired diameter.

Grinding down the diameter of an aluminum rod.

Grinding down the diameter of an aluminum rod.

With the grinding wheel spinning, the aluminum rod is pressed against the wheel and slowly rotated. Because a rubber tire is going to be pressed over the finished hub, the diameter does not have to be exact, nor does the reduced rod need to be perfectly cylindrical. There will be flaws in a hand-ground rod, but a rubber tire will compensate.

The scratches in the rod due to grinding can be removed or smoothed by using sandpaper. Cup a piece of sandpaper in your hand. Place the rod in the center of the sandpaper and grip firmly. Then, rotate the rod with your other hand.

After grinding and sanding the rod to the desired diameter, cut off a couple of inches in length using a hack saw or cut-off tool. Then, mill the rough-end flat. A vise with a built-in v-groove is very useful for holding cylindrically shaped material in the correct orientation during milling.

Milling flat the end of a rough cut rod. A v-groove in the milling vise holds the rod.

Milling flat the end of a rough cut rod. A v-groove in the milling vise holds the rod.

It is important for the rod to have a reasonably flat end. The rod end will be relied upon for positioning the rod throughout the process. Also, later on when the motor axle hole is drilled into the rod hub, a smooth surface will avoid the drill from drilling off-center, which might occur on a rough surface.

Now that we have a rod of approximately the correct diameter, an inner groove needs to be cut to match the original LEGO hub for the rubber tire...