c. 1966-1969 [and later; this expression appears in several Star Trek shows and films], Dr. McCoy in Star Trek to the erstwhile Captain Kirk: “Dammit Jim, I’m a doctor, not a(n) {engineer, mechanic, bricklayer}!” This one is somewhat unique in that it started out as a snowclone, where Bones altered the Y as it fit the situation. I think the exclamation point is obligatory, to further separate X from Y: not only is McCoy not a bricklayer, it is absurd to consider him one. The exclamation point–or intonation mimicking the original, when spoken–is also necessary to evoke the original usage, rather than a simple comparison of two unlike things. X and Y should be semantically distinct from each other, but sometimes the difference between them is subtle for the sake of humor.
Other variations seen on the web:
“I’m an ingenieur, NOT a bloody locomotive driver!”
“I’m an architect, not a project manager!”
“I’m an accountant not a magician!“
The Snowclones Database 
http://memory-alpha.org/en/wiki/I%27m_a_doctor%2C_not_a…
First of all, I don’t recall McCoy ever saying “Dammit, Jim” on the original show in any context. He might have said that line in one of the movies (I can’t recall for sure), but since he never spoke the “I’m a doctor, not…” line in the movies, the “Dammit Jim” part is a misquote, though it’s well-integrated with the snowclone.
Second, the “bricklayer” line happened when he was asked to heal a patient that very much resembled a rock. And despite his misgivings, he did manage to bind the rock’s “wound”, using a substance (at least superficially) resembling bricklayer’s mortar.
Third, he also used the line “I’m a doctor, not an escalator” in one situation where he was trying to climb a slope.
Agreed, the “dammit Jim” has been added for pragmatic effect in common usage, much like the “that would be” of the old Newlywed Game quote.
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I actually got to use this one at work. Jim asked me about something about whether we could legally spend money in a certain way, and I have a PhD…
I’m a genius, not an engineer! (The Man in the Iron Mask)
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“I’m an engineer, not an ornithologist!” appears in “The Outer Limits,” in “The Cry of Silence,” which aired in its 2nd season—so in 1964. Anyone have any earlier ones?
(Incidentally, only two episodes earlier, Jimmy Doohan starred—another Star Trek connection.)