Try Googling 'pattern making' or 'moldboard'.
What you describe is a mold board with half the pattern on each side. As you mention the pattern has to be made oversize for a machining allowance in certain areas and also to compensate for shrinkage. It also needs 'draft' which is a taper on ends that would otherwise be square so that these can be drawn out of the sand mold withou pulling away the corners. Then it includes other thoings like 'feeders', 'risers' and 'sprues'. These are to create channels in the sand mold for the molten metal to pass throu and allow the trapped air in the mold to escape and to provide a reservoir of molten metal adjacent to large cross-sectional areas in the casting allowing metal to flow in as these areas cool and shrink.
Google all the words in ' ' and you should dig up plenty of information. Pattern making is as much an art as a technology and it can take a lot of experience to make good patterns.
EDIT: corrected typo, pattern not patter. Google would have come up with something very different.