1 INTRODUCTION

In the human history of making things, there have been several major steps. In oversimplified terms, there are three ways to fabricate: subtraction (hammering a flint arrowhead or whittling a stick), molding (pouring gold into a sand mold or concrete into forms), and addition (building a log cabin or a pyramid). In the nineteenth and twentieth centuries, advances in machinery—including hydraulics—increased the power for moving dirt, metal, or concrete, while new steel alloys for cutting tools and other innovations enhanced the precision of subtraction. In the realm of metalworking, however, the precision of subtraction far exceeded the precision of addition: even as recently as the 1990s, computer numerical control (CNC) machine tools as well as laser and waterjet cutters could remove material far more impressively and precisely than anything could add it.