The Working Excavator

This is my personal favorite animation effect. When you describe it to someone verbally, they often form a mental picture of an amusement park gimmick, with stiff, quick, jerky movements. What they don't expect to see is a realistic-looking model that moves almost as if a real one was shrunk down to 1/160th of its size. For this reason alone the excavator stands as the prime example of how I approach animation: things must move convincingly, or I won't do it. The first and most important key to success is slowing things down; then, when you think things are slow enough, slow them down some more!

Building the model as intended in the kit would have been almost trivial; making it articulated, however, wasn't easy at all, mostly because the kit wasn't designed to be animated. Articulating joints that were meant to be rigid required a great deal of precision and patience, not to mention a healthy supply of small drill bits. The joints rotate about brass wires that were cut just slightly longer than needed, then crimped on the ends to lock them in place—you can see them in this supersized close-up.

Hydraulic cylinders were simulated with brass tubing and rod stock, and constituted the only major cosmetic error I made: not tinning the rod stock, or using steel, for the pistons. Amazingly, I managed to get everything right on my first try, and I didn't have to purchase a second kit. Some of the parts did sustain minor damage in the modification process, but ultimately I decided that it made for convincing real-life damage.

The model has four degrees of movement: bucket, upper arm, main boom, and cab rotation. The only thing that was not practical was travel, for obvious reasons. All of the articulations, except for rotation, were accomplished with monofilament nylon thread wound around drums; all of the threads are under very gentle tension at all times to help keep the movement smooth. The hardest part was getting the monofilament routed all the way out to the bucket; I eventually solved this problem by grinding a groove in the bottom of the main boom, and covering the groove with a strip of brass. Although I shudder at the thought of re-threading the mechanism, it is nevertheless designed so that the threads can be replaced: they're not permanently anchored to the model; instead, the threads pass through holes in the parts and are tied to bits of brass wire, making them removable.

The drive motors are all surplus Swiss low-RPM gearhead instrument motors (running at less than three volts, the mechanism is virtually silent). All of the winding drums have limit switches to prevent over-travel, which could break the threads and possibly damage the soft pewter parts—particularly the delicate arms that connect the bucket to the boom. The motors can be operated simultaneously in any combination, as well as any combination of directions, just like the real thing.

THE CONTROLLER

The controller is an exercise in simplicity. The controls and power source are rolled into one compact handheld unit. Owing to the quality of the motors, they can run on a minimal amount of voltage and current, so penlight batteries provide more than enough power. Two pairs of AA batteries are connected such that a plus/minus current source is available; they're wired to four control switches, which are then connected to the limit switches on the excavator itself before ultimately reaching the motors. A connector on the the cable allows the controller to be easily disconnected during shows when the excavator isn't being manned by a qualified operator in order to prevent catastrophe at the hands of a visitor; even though the mechanism has limit switches, it's still easy to damage the model if it's operated improperly.

Below is the entire circuit. The left half comprises the handheld controller; the right half is the mechanism itself. The control switches are SPDT momentary-contact center-off toggles. The limit switches are SPDT microswitches; the diodes stop the motor at the limit of travel in each direction. The only motor that does not receive travel limit protection is the one that rotates the cab (rotation should, in theory, be protected as well, since it's possible to really screw things up by rotating the cab too far, but it was impractical to design a limit switch arrangement that permitted 500-600 degrees of rotation, which is allowed.)

THE EXCAVATOR IN ACTION

With practice, an operator can make the model mimic the smooth, fluid motions of a real excavator almost exactly (admittedly, I'm not very good at it). Loose material such as ballast can be scooped up from one location and dropped at another. Originally the excavator was joined on the layout by a functional dump truck. The excavator would remove gravel from a hole in the ground and drop it in the truck; then the truck would dump the gravel back into the hole. It may sound somewhat silly, but there were not many practical alternatives to provide a continuous supply of loose material, as well as places to put it; besides, it was fun. The video shows the excavator in action, as well as a good close look at the mechanism.

A sidebar on making small mechanical things... occasionally some discussion arises over the subject of moving parts, in particular the joints. There are those who advocate incorporating a lot of "slop" (loose play) into tiny joints supposedly as a way of making them function more easily. Unfortunately, this is the absolutely worst thing to do; the smaller the mechanism, the tighter the joints need to be. This is not opinion; this is fact based on decades of building many different functional mechanisms, including such things as scale grapples smaller than a sewing thimble, which are among the most difficult objects to make functional. Sloppy joints give rise to mechanisms that can seize up because the parts lose alignment; also, movement becomes jerky instead of fluid. Consider locomotive mechanisms: compare an entry-level cheapie with a top-of-the-line model. The former is noisy and balky, while the latter is smooth and quiet. Open them up, and you'll find out why: the cheapie has big parts with sloppy joints, while the high-end unit is tight and refined. And with that I rest my case.

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