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-The tapping screw is a well-developed, widely used fastener. The manufacturing processes of tapping screws are sequentially wire-manufacturing, forming (heading and threading), carburizing heat treatment, and phosphate coating, which can be conducted by subcontract factories. However, the manufacturing qualities of subcontract factories are sometimes uneven and resulting in difference in quality of products. The performance qualities of tapping screws are affected by the manufacturing qualities of subcontract factories. In this study, to improve the quality of AISI 1018 low carbon steel tapping screws, Taguchi method is used to evaluate the manufacturing qualities of subcontract factories. The quality characteristics of tapping screws, such as case hardness, core hardness, torsional strength, and drilling time, are investigated. The most important quality characteristic of a tapping screw is the drilling performance. It is experimentally revealed that the heat treating process (H) and the coating process (C) are the significant processes; while the forming process (F) and the wire manufacturing (W) are relatively not significant since they are well-developed manufacturing processes for tapping screws. Subsequently, the determined subcontract factories: W2, F2, H3 together with C2, evidently improve the performance measures. The drilling performance of the AISI 1018 tapping screws is effectively improved.