Identification of threaded fastening parameters using the Newton Raphson Method

Threaded fastenings are a common assembly method, and account for over a quarter of all assembly operations. These operations are very difficult to automate, with threaded to align and position the screw with respect to the hole, and to apply axial torque until the appropriate tightening forces are achieved. Screw insertions are typically carried out manually with the purpose of joining one component to another. They permit easy disassembly for maintenance, repair, and relocation. There is little research reported on automating screw insertions, with most automated assembly research focusing on the peg-in-hole assembly problem. This paper presents an on-line parameter estimation strategy employing the Newton Raphson Method (NRM) for threaded fastenings. The motivation for the study is to employ the estimated parameters to develop strategies for monitoring threaded fastenings. The monitoring problem deals with predicting the integrity of the self-tapping screw insertion process, based on the torque vs. insertion angle curves generated during the insertions. A technique for estimating three unknown parameters (friction coefficient, hole diameter, and screw major diameter) during a typical screw insertion is presented.