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Originally posted by jaydee116
or maybe i just showed how ignorant I am on the subject. 
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LOLOL. Grain size, hardness, and yield strength in metals are intimately related topics. Density of copper (or other metals) is virtually unaffected by these items and does not have a direct influence on machineability.
Annealing is only one half of the equation, too. The other half is controlled cooling, though I don't have any feel for its impact with copper. In alloyed steels and iron, the rate of cooling can affect the structure and material properties dramatically. As an example, the paper industry uses "chilled iron" rolls. The roll shells are formed in a single pour, but the exterior is rapidly cooled. While chemical composition is the same throughout, the outer ~3/4" is white iron (very hard, low tensile strength, low thermal expansion, extremely difficult to cut) the remainder is gray iron (soft, decent tensile strength, good conductivity, relatively ductile, easy to cut). The exterior resists wear while the interior gives strength and thermal conductivity. The impact of carbon on steel is similar in that higher carbon content allows much higher hardnesses to be reached, but only with the right cooling rate. Hence the variations of air cooled vs water cooled vs brine etc. Just to make certain laypersons don't figure any of this out, metal workers and engineers call it "annealing and quenching" rather than heating and cooling.
Specifically with regard to thermal conductivity and grain size, I'd point out that the very act of making a block destroys some of the effort. Any machining operation done at "low" temperature results in cold-working. This is basically causing the metal to yield, which severs existing grain boundaries leaving smaller grains behind. This is why a soft metal is often harder after it's been machined. It's also why the hardness of a wire increases once you've bent it back and forth a few times (and why it will eventually fail in a brittle nature, even though the raw material may have been ductile).
If you were talking about steel, I could point to some TTT diagrams of relevance. As it is, I'll shut up now and leave it to those with better information at hand, like 8-ball.