Classification of spot welds using power factor time profiles

It has long been suspected that information about the quality of a resistan ce spot weld might be contained in the behaviour, over time, of the seconda ry circuit current draw (or a monotonic function of the current draw) as re presented by a 'power factor' curve (PFC). We have studied the effectivenes s of a statistically-based algorithmic method (which we call 'template matc hing') for 100% inspection of welds, using realized PFCs. Such a method is valuable for use in real- time process adjustment or maintenance decision m aking. We have started to develop a 'catalogue' of parameters that relate P FCs to weld quality, for example exogenous factors such as current, pressur e, welding tip geometry and wear, part material and thickness etc. Prelimin ary calculations using data given to us by a major automotive fabrication p lant are extremely promising, showing classification errors of less than 5% per weld. This is a result, which- if shown to hold over a variety of weld ing processes and conditions- would provide for extensive cost reduction or even re-design of parts with regard to the location and numbers of welds.