I think the problem you are having is that you are not able to tell the rpms this is what happened to me before. You may be running the machine way too fast.
With coil machines you have the sound a guidance but with rotaries you need to guess a bit more.
I say try reducing the voltage drastically and increase it by tiny increments until you are happy with the result.

If you run it too fast you'l chew skin like bubblegum.