I'm quite aware of what an excavator is, I was hoping for you to show how far you've come creating one, so we could give you some pointers on how to make it better.
Is there a reason you want to use physics to create it?
What are your plans for it to do in the game after you have created it?
In what way do you want the player to control it?
does the player control the bucket, the first arm, the second arm and how?