2. Vertical Motor Mount Made Of Legos And Screws

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The Flip-Flop Robot features a single motor in the center of a metal candy container to provide forward and reverse movement. There is a wide variety of ways of mounting a gearmotor inside of a box. In this case, I’m trying to avoid machining a fancy motor mount. Instead, Legos and screws are used.

Overhead view of Flip-Flop robot, with focus on motor-mount screw details.

Overhead view of Flip-Flop robot, with focus on motor-mount screw details.

The vertical Lego motor mount has two screws -- one on each side of the top beam. The first step in making the motor mount is to drill holes in the top Lego block.

Drilling a hole in the end of a Lego Technic beam.

Drilling a hole in the end of a Lego Technic beam with a Dremel rotary tool.

This is fairly easy to accomplish, because the studs atop the Lego beam already have hollow tips that guide the drill. Use a Dremel with a 1/8-inch diameter drill bit. Drill a hole through the studs on both ends of a six LU (Lego Unit) long beam. To be clear: You only need to drill two holes, one on each end.

Removing an arc from the internal tube using a tungsten-carbide cutter bit.

Removing an arc from the internal tube using a cutter bit.

Unfortunately, drilling by hand usually results in a slightly angled hole. This is almost guaranteed to occur when drilling the Lego beam, as the side of the inner tube tends to deflect the drill bit.

I promised myself that I wouldn’t cheat and use a milling machine or drill press for this robot. However, that doesn’t stop you from doing so. A drill press ensures that the drill stays straight (more or less).

If you see a fully intact tube (rather than the arc portrayed below the dotted blue line in the above photograph) then that solid portion of the tube will prevent the screw from passing straight through the hole.

If the drill still doesn’t do the job for you, you can switch to a Dremel cutter (such as #9901 Tungsten Carbide Cutter) to cut the notch on the side of the tube. A cutter has sharp edges, unlike a drill bit that has a sharp point but dull edges. Therefore, a cutter can rapidly chip away at anything pressed against it.

Positioning the Motor Mount Straight and Flat

Look way back at the robot’s side view on the previous page. The yellow arrow points to a slight gap between the mount and the side of the candy tin body. Clearly, the gap provides room for the nuts.

Less obvious but more important, the gap moves the base of the mount away from the slightly curved bottom sides of the candy tin. If the motor mount rested on the curved floor, it would be tilted. Instead, the gap ensures that the mount is sufficiently far away from the edge that is rests on the flat part of the tin.

Positioning a Lego beam using a flat Lego as a spacer.

Positioning a Lego beam using a flat Lego as a spacer.

The trick to creating an even, straight gap is to place a flat Lego piece between the Lego beam and the candy tin wall. Then, tape them together with low-adhesion blue masking tape to keep them from moving while marking the locations for screw holes.

It also helps to place the Lego beam stud-side down. That results in the smallest distance between the pen and the candy-tin floor.

Three tools for making marks.

Three tools for making marks.

There isn’t much room to fit a marking pen inside an upside-down Lego stud hole. I tried three different common household items:

The nail worked okay, but it was difficult to aim. Neither the mechanical pencil nor the permanent marker was able to leave a mark on the coated candy-tin surface.

Silver coating to accept hole mark.

Silver coating to accept hole mark.

I peeled back the tape and painted a bright spot with a felt-tip permanent Sharpie in the approximate location of one of the holes. The Legos were placed back into position and the mechanical pencil was tried a second time.

Nail dent and pencil mark.

Nail dent and pencil mark.

As you can see, the pencil mark shows up much better on top of a marker patch. As for the nail dent, it is a bit crude, but the divot will help guide the drill bit.

Wheel-Well Opening

After drilling holes in the marked locations, the Lego motor mount and wheel can be roughly mounted with screws in the pair of holes. However, the mount can’t be tightened down all the way because the wheel will press against the bottom of the container. We need to cut an opening for the wheel.

Mark the panel hole and drill out the corners.

Mark the panel hole and drill out the corners.

Roughly sketch a rectangle around the wheel. Then, drill out the four corners.

Cutting a wheel access in a metal candy container with a Dremel cut-off disc.

Cutting a wheel access in a metal candy container with a Dremel cut-off disc.

When the container is turned over, the four corner holes serve as guides as to where the panel needs to be cut out. A Dremel cut-off disc is reasonably fast at slicing each side of the opening. In addition, the four drilled holes avoid the need to cut corner-to-corner, lessoning the chance that you'll overshoot.

Rough burrs and sharp edges should be filed down and taped over.

Rough burrs and sharp edges should be filed down and taped over.

Hand machining sheet metal results in ragged edges that may scratch or injure. File down the burrs manually or with a Dremel grinding tip. Then, tape over the thin metal edges for finger and rust protection.

The right side of the above photograph only shows two sides taped over. The bare sides are pictured to demonstrate how smooth the edges can be ground or sanded. After this photo was taken, I placed tape on the remaining sides.

This completes the steps necessary to mount the single motor in the center of the robot. Nevertheless, there is still some work required on the robot’s body before we can proceed to the circuitry. The bumpers and switches need to be mounted.