In the modern manufacturing industry, approximately 70% of the product cost is determined during the design phase. This reality compels designers to manage not only functional requirements but also manufacturing constraints and cost dynamics. This study addresses the classical construction rules for machining processes and the digital capabilities introduced by Industry 4.0 through a holistic approach. The first section examines the cost locking paradox and the economic impacts of standardization, revealing the effect of design decisions on business profitability. The second section details geometric design rules for drilling, milling, turning, and grinding operations based on established design for manufacturability methodologies. The final section discusses how classical physical constraints are overcome by modern technologies. Specifically, 5-axis machining kinematics, AI-supported cost prediction using Bi-LSTM algorithms, and Digital Twin applications that prevent errors in a virtual environment are presented as the new competency areas for modern designers. Consequently, this book chapter serves as a hybrid guide aiming to transform the designer from a drafter who merely creates geometry into a production architect who manages the physical and digital interaction between material, tool, and machine tool.