3. Milling an Inner Groove in a Rod to Use as a Replacement LEGO Wheel Hub

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The original LEGO wheel hub has a groove that mates to the rubber LEGO tire. This centers the tire onto the hub and prevents the tire from sliding off.

A lathe with a cutoff blade would be the perfect tool for cutting the groove in the hub. Since I didn’t have a lathe at the time, I made the groove on a milling machine. The process is to cut away a small flat section, rotate the rod, cut away another small flat section, and so on.

A milling vise with a v-groove holds a rod on the milling machine. A workpiece stop made from a clamping set (step blocks, a holddown bar, stud, tee nut, and flange nut).

A milling vise with a v-groove holds a rod on the milling machine. A workpiece stop made from a clamping set (step blocks, a holddown bar, stud, tee nut, and flange nut).

For this to work, the rod must remain in a consistent location after every turn. A milling vise with a v-groove insures that the up-and-down and back-to-front location of the rod stays the same. Assembling a backstop out of step blocks (“42-piece clamping set” #82578 from MicroMark) makes sure the side-to-side position of the rod stays consistent. Thus, the only change between each cut is the rotation about the rod’s axis.

Milling an inner groove on an aluminum rod.

Milling an inner groove on an aluminum rod.

Here is the step-by-step process:

  1. With the milling machine turned off, insert the rod into the milling vise’s v-groove.
  2. Push the rod until the nice flat end presses against the clamped backstop. (Now you know why we needed to mill the rod end clean.)
  3. Tighten the vise.
  4. Start the milling machine.
  5. Mill back-to-front and then front-to-back at the desired groove depth.
  6. Stop the milling machine. Never reach into a running milling machine -- you'll injure yourself! Never move a workpiece on a running milling machine -- you'll hurt yourself, damage the workpiece, and/or break the end mill!
  7. Loosen the vise.
  8. Rotate the rod approximately 1/8 turn.
  9. Repeat step 2 to step 8 through several complete revolutions, targeting any high points (corners of facets), until the groove appears sufficiently round.
Aluminum rod with faceted inner groove due to crude milling operation.

Aluminum rod with faceted inner groove due to crude milling operation.

The groove would be nice and smooth if it had been cut on a lathe. However, using the crude milling technique, the groove will consist of a series of small flats or facets. Depending on how rough the facets are, the LEGO rubber tire may not care. But, if necessary, a narrow metal file can smooth away the flats.

An aluminum rod showing the transition to a LEGO hub through various machining processes.

An aluminum rod showing the transition to a LEGO hub through various machining processes. I like this picture.

The end of the aluminum rod is beginning to look a little like a replacement LEGO hub. The picture above shows the workpiece going through the transitions from an oversize stock rod.

Next, the rod must be drilled to accept a standard DC motor axle...