Your problem is in your 3rd sentence
There is a lot of wear and tear in the operation of the machine. so much so it broke the first time, he did fix the problem and it is very hard to connect a wear and tear issue to a faulty repair. Think of this way. the Machine had broken Prior to him working on it. He did seem to go above the call of duty by doing more work, You mentioned "warranty" so i am not sure why you paid him more than once.
Really your only recourse imo is to just never go back. If someone asks your opinion of the mechanic your response should be, he did some work for me in the past but i dont think i will let him do work for me in the future. Avoid openly criticizing him or he could open up an action against you.
If he does try and sue you, your best recourse would be to try and reach a compromise, trying to defend the action often costs more than settling. Do not forget that it costs him money to work on your machine.
IMO you should have paid him. Since you did not like the work you simply should have found a new mechanic.
I am not a lawyer nor do I not represent myself as one.