A working model of an excavator from a single NXT set
About this creation
Last week i made this NXTcavator.
Again it is from a single NXT, as long as you use the claws instead of the bucket.
See for some action my YouTube video: NXTCavator!
Build it yourself from the 300+ photos at Photobucket: NXTCavator, but it is not _so_ easy, make sure you start at picture 8715 and follow the order of the numbers, the picture order is not alway proper. The first few dozen pix relate to the alternative arm, the last ones relate to the grabber with the claws of the NXT.
I used the same (slightly adapted) home-brewn Java program to control it via BlueTooth as i used for the forklift.
The base is very compact, rigid, heavy and stable. Guess i was lucky, it took me half an afternoon. The arm however, took me several evenings to get right. I realised that a digging/grabbing action could be done using a single motor, because a single motor can go down in two ways: one forward and one backward. Feared the motor power would be insufficient but the NXT proved me wrong there :)
The disc gear wheel with its center hole was used to guide the cable from the brick in the base to the arm motor. This allows for over 360 degrees of swinging.
When rebuilding, please make sure your cable to the arm motor runs freely through the disc wheel center hole, else i suspect the cable gets eaten away by the inner teeth of the disc wheel. I had the cable clamped in between the hollow slit in the motor and the axis that holds the motor to the disc wheel. The cable then makes a sharp rectangular bend down through the center of the disc wheel. Guess this is much clearer from the pictures i took.
When playing with this model, make sure you do not rotate it too long in one direction as the cable will ultimately be torn apart easily: the power of the motor driving the worm driving the disc gear wheel is incredible!
Mounting the arm motor immediately onto the disc wheel, i had to move the angular gear that drives the worm wheel to the outside, else the motor would rotate into it and get stuck. So i lengthened the axis a bit, and shifted the gear wheel to the other side of the gear wheel on the swing motor. Fortification of the free end of the axis was needed. This was a real puzzle too, and i used the possibly weirdest piece of which there is a single one in the NXT set. The result still has rather tight clearances, as is apparent from a clip in the video. Actually the motor slides over the worm wheel, for which i found no solution, it works anyway.
One other evening was spend on the counterweight/engine compartment. This contributes in my opinion to the typical excavator look. It turned out that the excavator can be considered "zero-tail-swing", as no tail part exceeds the wheel base when swinging.
Then i added the drivers seat with cushioning and "captain" arm rests, belts to "buckle up" the driver, the dashboard, the cabin with roll bars. Also the mirror and the four spot lights were added, like the orange tail lights and steps for the driver to get in. The drivers seat nicely masks the arm motor and gives the driver a centered high position with a clear view. From the remaining gears and axes i squeezed out a trailing roller for flattening the ground. It is not in the video, but i will try to get it in pictures.
I am very satisfied with the outcome and the color scheme, having the arm and cabin in light colors and the rest in dark grey and black mostly. Best of all: it works reasonably well, though again operating it like a pro is hard, actually much harder than the forklift!