Dawg, its hard to think of carved wood being a precision part. But in patternmaking its a pretty precise ordeal. The actual patterns and core pieces themself are in fact larger by some percentage, than the part it will make. So a lot of time and cost goes into making the patterns. For a part to be made or cast in aluminum, the patterns are about 2% larger than what the finished part will be. This is called the "shrink allowance". You also have to add extra thickness for areas that will be machined. When the molten aluminum is poured in to the molds and cores,its around 1350°F, and as with all things when it is hot, it expands, so an allowance has to be made for that. All the features of the part to be cast have to be enlarged by the shrink allowance factor, in order to compensate for the thermal expansion process. For a cylinder head for example, you have to make a negative core for all the intake and exhaust ports, and a negative cavity for the water jacket, and the exterior features of the head as well. Then the combustion chamber side of the head and the valve cover area,plus a way for all these individual cores to interconnect and make the complete core box. You can easily end up with close to 20 seperate core pieces just to make one head. You should also make a pattern master, because the actual parts you use in the sand and core box don't last long, because the sand is very abrasive and wears it out quickly. So its a little more involved, and the more attention to detail you put into it the pattern, the better the part will turn out, but the more costs you add to the patterns.
