Update: After hearing about the arm joint getting broken while being played with, I have modified the design to make it stronger. I increased the size of the connecting rods substantially. I adjusted the elbow joint so that it can fold completely. And I enclosed the joints to keep them from sliding up and binding. I have printed the new version and it works quite well as shown in the first two pics. It is the "Little Digger V2" file. My little "Iron Man" from the first video has been testing it. V2 video here: http://www.youtube.com/watch?v=0RYpjmRyl90
I haven't tried scaling this yet. The tolerances would scale as well, so I am not sure how well that would work.
End of Update.........
I wanted to make a fun toy for my kids for christmas, so I came up with this!
This is a pre-assembled excavator called the 'Little Digger'. The arm and bucket have 3 moving joints. The base rotates for Real digging action and the wheels turn. All moving parts have been designed to print in place using the power of the 45 degree angle and a few very small bridges.
After getting all of the moving parts worked out, I went to work on the details, adding tread and rims to the wheels, a drive train on the bottom, lights, a cab with a seat and a steering wheel, weights on the back of the turret, teeth on the bucket, hydraulic cylinders on the arms, and of course the 'Little Digger' name down the arm.
I created this using tinkercad and have made it public so that others can modify it and make something new. One thing that you can do is change the name on the arm. You can see in my picture that I made one called "Jack's digger" for my son. Or you could take it apart to see how the different parts work. I think this link should work for getting there: https://tinkercad.com/things/eoIsmBEg9vA-little-digger
Here's a quick video of my kids playing with a few of them:
Print one out for Christmas!
print it :) I used .2mm layers with 10% fill. At 100mm/s perimerters and 260 mm/s infill, it took 95 minutes to print.
Carefully remove it and test the three joints in the arm. They should already move freely or move with a little bit of wiggling. Note: the little connectors in the arm joints can be delicate if they are twisted.
Next take the upper and lower portion of the body in separate hands and twist. Again, this should move freely to begin with, but may take a little motivation depending on how well your printer works.
Finally, you need to get the wheels turning. After my last adjustments to this model, I was able to get them turning by just grabbing them and turning. If they won't turn for you, try using a pair of pliers to get a better grip. I wanted to keep the wheel axles tight so they would turn well without binding. once they are turning, I recommend spraying a little bit of in the cracks around the wheels. I uses CRC Heavy Duty Silicone. After the silicone spray is worked in and has dried, they should turn very well.
If you get carried away and break an arm connector, I have successfully repaired one by melting it back together with a soldering iron. With that said, my 4, 6 and 8 year olds have played with them a lot without breaking one. I was the one that got carried away :)
**** If you find that you need a little looser tolerance, a good way to adjust it is to adjust the size of the object. The tolerances scale proportionately to the size of the object.