Question: Is there a CAD format, which allows to describe mechanical bonds between objects (axes, joints, springs, etc.) and/or motion of objects ?

Answer: 
A gotcha with the k-means alogrithm is that it is not optimal. That means, it is not sure to find the best solution, as the problem is not convex (for the optimisation).

You may be stuck into local minima, and hence the result of your algorithm depends of your initialization (of your centroids). A good practice in order to find a good minimum is to rerun the algorithm several times with several initializations and keep the best result.

As stated by the others, `random_state` makes the results reproducible and can be useful for debugging
Bear in mind that the KMeans function is stochastic (the results may vary even if you run the function with the same inputs' values). Hence, in order to make the results reproducible, you can specify a value for the `random_state` parameter.