"That's not in my job description!"

Last week I had a realization. After a fairly intense argument with my supervisor in regard to periotic fabrication assignments that I had been agreeing to do for him for the last 3 years. They are technically outside of my job description, but I had a been working on these assignments primarily to increase my value at the company but also because I enjoy working with my hands. Another reason why I've done the work is because I would rather be busy than to be sitting in the office with nothing to do. My supervisor is the most conceited, narcissistic, and neurotic individual I have ever met. The man legitimately believes, and has stated himself, that he can do anything and everything better than ANYONE else. Every single project I have done for him has, in his opinion, taken too long, not been up to his standards and ends in either an argument, or me hearing from coworkers that he was talking shit about me. So, he asked me if I would like to build a device for him that he sketched out that we would use to lift and move an exchanger. I said "Sure I'll take care of it". Over the next week I couldn't stop thinking about what the ultimate outcome would be: me finishing the project, him being dissatisfied with the outcome, him talking shit about me, and me taking it personally no matter how hard I try not to.
So I declined. I sent him an e-mail, blind copied his boss (who I have a good relationship with), and respectfully declined. He replied and CC'd his boss (who he didn't know was already included) and my lead operator and told me that he needs a drawing on his desk by Monday for him to review (It was Thursday, I don't work Fridays and I wasn't due back to work until Monday). I responded re-explaining that I didn't want to have anything to do with the project, and that it is not part of my job description. He didn't reply. This last week, something else came up, he told me I "must not be too bright" and we got into a huge argument. He started threatening my job and telling me that I really blew it with that project and that my actual boss (his boss) was not at all happy about my e-mail. The argument ultimately turned into an argument about what my job description includes. According to him, my job description says I have to do anything and everything he tells me to do, however I had just read it that morning and it said nothing of the sort.
After it was all said and done I realized that the less I do, the easier my job will be. Since I haven't yet been rewarded in the form of a decent raise any more than the laziest of workers there, and there are no positions above me available unless someone quits, there's no reason for me to be going out of my way to do extra jobs that I wasn't hired to do. The whole problem comes down to this: the more I do, the more my theoretical job description expands, making my job harder in the long run because those additional "freebies" end up getting written into my job description and become requirements. It's sort of depressing actually. Logic would say "the more you do the more you'll be paid/rewarded" but sadly, that's not how it works. The truth is actually more like "the less you do, the easier your job will be, and you won't get paid any less than anyone else". So long as your doing the bare minimum, and not slacking off in any of those areas that are absolutely required by your job description, you are far better off. You won't burn yourself out and you won't get too attached to your work so you won't take it too personally when your boss takes a dump on you and stomps you down, and hey you may not even have that happen because he won't see you as a threat. Here's the thing, you can still be proud of yourself at the end of the day because you are still doing the job you were paid to do.
Everything about this realization is completely contrary to everything I've believed all my life in regards to work ethic, but I'm realizing that in most cases, this is the cold, hard, sad truth as long as you are an hourly employee. The only place that good work ethic and being a high achiever is rewarding is when you're getting paid on commission or when you run your own business. That's my realization, and I know it's nothing too profound, but I sure wish I would have realized this a bit sooner. Before I worked here, I ran my own business, and when you run your own business hard work pays off, you have a direct correlation between your work output and the money you bring home. I guess the take away is to know the rules of the game that your playing, and play the rules to a "t". Focus on the rules so you don't get lost in the game. So next time you get an "offer" to do something that's outside of your job description, think it through and make sure it's in your best interest and not just your boss trying to push work off on you that you're not going to be rewarded for.