4. Drilling a Centered Hole for a Motor Axle

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Before we cut the aluminum replacement hub from the raw rod stock, a hole needs to be drilled to insert the motor axle. Because the final hub is relatively small, it is much easier to leave it on the nice round stock rod until work is complete.

A quick-and-dirty technique for centering a drill over a rod is to insert an end mill into the chuck that has the same diameter as the workpiece rod. Line up the edges, remove the test rod or end mill, and then insert the actual drill.

Centering a workpiece rod below the drill chuck by lining up the sides of a similar-diameter end mill.

Centering a workpiece rod below the drill chuck by lining up the sides of a similar-diameter end mill.

For visual centering purposes, an end mill is better than a drill because the end mill has a flat tip. Rotate the chuck and adjust the table so that the left and right sides of the end mill match the edges of the rod workpiece. Then, rotate the chuck by hand such that the end mill edges can be used to adjust the table back-to-front. Lock the table so that it won’t move accidentally and misalign the rod.

Now that the drill chuck is centered over the rod, it is time to drill the hole for the axle. The hole in the center of the rod needs to match the diameter of the motor shaft. The model-railroad engine motor I selected has a very narrow diameter shaft, only 2 mm.

Left: Starting a hole with a spotting drill. Right: Drilling the motor axle hole in the hub.

Left: Starting a hole with a spotting drill. Right: Drilling the motor axle hole in the hub.

A 2 mm drill can bend and flex, causing the hole to be drilled off-center and at an angle. This problem can be reduced or avoided by starting the drill hole using a spotting drill (or a center drill) on a flat, clean end. A spotting drill is designed for accuracy and rigidity. It only needs to be used to start the hole.

After the hole has been started, swap in the desired diameter drill (that matches the motor-axle diameter). If you’ve ever started to drill a hole just slightly off the mark, and then you pull out and try to drill the hole in the correct location, then you know the frustration of having a drill wander back into the wrong spot. Using a spotting drill to start a hole in the correct location applies this principle in reverse. The smaller drill will naturally want to follow the starting hole made in the correct location by the rigid spotting drill.

Another tip for preventing a narrow-diameter drill from wandering is to use a short drill. These are called “screw length” or “stub” drills. You won’t usually find screw-length drills at your local hardware store or Home Depot, but they’re available in a wide variety of diameters at most metalworking tool sellers, such as McMaster-Carr and MSC Direct.

Drilling and tapping a setscrew hole for the aluminum LEGO hub replacement.

Drilling and tapping a setscrew hole for the aluminum LEGO hub replacement.

The aluminum hub is going to be held in place on the motor shaft by a setscrew. Drill the setscrew hole and tap it to add threads.

As you can see in the above picture, I made the mistake of cutting the hub from the rod before drilling and tapping. This made it very difficult for me to generate the torque to cut the threads (by turning the tap) without the small hub turning at the same time. Oops!

After tapping, then you can cut the hub from the rod...