Drilling Screw Holes in a Cylinder on a Milling Machine with a DRO

Previously, I showed how to make eight solid plastic cylinders on a lathe to eventually make reliable Lego wheel couplers. Using a precisely-calculated layout, a milling machine, and a digital readout (DRO), we will drill a bunch of screw holes.

The first step is to find the center of the cylinder. I’ve cheated by drilling a centered hole in one of the cylinders using the lathe, because that’s an easy operation on a lathe.

When in use, nothing touches the outer diameter of the coupler. Therefore, it doesn’t matter if the motor shaft hole is exactly center with respect to the outer diameter. The only critical dimension is the relationship of the hub screws to the center hole, and even that has some wiggle room. Therefore, although this next trick is not incredibly precise, it is perfectly acceptable for this piece.

The cylinder is placed in a small vise that has a V groove to hold round objects securely. Underneath the cylinder is a flat scrap piece of wood so that the drill tip won’t cut into the vise. A 4 mm rod is inserted into the drill chuck to match the hole in the cylinder. If the unpowered drill can slide into the hole, then the cylinder is centered. If not, adjust the position of the x-y table that holds the vise.

Step 1: Align drill bit using cylinder with lathe centered hole

Step 1: Align drill bit using cylinder with lathe centered hole

Note: I used an upside-down drill instead of a rod. I cringe at this, because the flutes are not being held as evenly. But, I didn’t have a plain 4 mm rod.

At this point, the digital readout is cleared to read 0, 0. The cylinder with the guide hole is removed, as I don’t want to alter it until all the other cylinders are finished. This allows me to realign the milling machine using the original cylinder and hole if something goes wrong in the meantime.

Start with a new cylinder.

Step 2: Insert fresh cylinder

Step 2: Insert fresh cylinder

Lock the x-y table in place. Drill the 4 mm hole in the center for the motor shaft.

Step 3: Drill center hole that matches diameter of motor shaft

Step 3: Drill center hole that matches diameter of motor shaft

Remove the 4 mm drill bit and swap in a 7.5 mm end mill. By not moving the x-y table, the 7.5 mm hole will be centered with respect to the 4 mm hole. We’re using an end mill because it has a flat tip. A 7.5 mm drill bit has a tapered tip that would cut partially into the section of the 4 mm hole that is supposed to grip the motor shaft.

We only want the end mill to cut 4 mm dip into the piece, unlike the drill bit that we drilled all the way through with. The cross-axle protrusion in the center of the Lego hub will eventually fit into this space. We need a trick to measure the depth of the cut.

With the end mill inserted in the drill chuck but without powering up the mill, rest the tip of the 7.5 mm end mill on top of the cylinder. Clear the z-axis of the digital readout to zero. At this point, the tip sitting on top of the cylinder represents a depth of 0 mm.

Raise the end mill. Start the milling machine. Plunge the end mill into the cylinder until the z-axis shows a depth of 4 millimeters. Cool trick, huh?

Step 4: Mill depression for Lego hub cross axle hole

Step 4: Mill depression for Lego hub cross axle hole

Remove the end mill and swap in a #43 drill bit. Obviously the power should be off at this point. Obviously.

Unlock and move the x-axis of the milling table 8 mm to the left according to the digital readout. Lock the axis and drill the first hub screw hole.

Step 5: Drill first screw hole

Step 5: Drill first screw hole

Unlock both table axes and move to 4, 6.93. Lock the table and drill the top hole.

Step 6: Drill second screw hole

Step 6: Drill second screw hole

Unlock and move to -4, 6.93. Lock the table and drill the bottom hole.

Step 7: Drill third screw hole

Step 7: Drill third screw hole

Sweet! See how useful a digital readout is?

Repeat this process for the remaining cylinders.

It is very important that each cylinder has all of these operations performed before removing it from the vise. The moment you unmount a workpiece, you can never get it mounted exactly the same again. By drilling all the holes in immediate succession, with a digital readout, we can be sure that they are positioned accurately with relative respect to each other.

On the next page, we will finish the coupler by drilling the setscrew holes on the sides.