For process capability and control, the welding area is of decisive importance under the given technological conditions. The narrower the welding area, the more sensitive a (welding) technology is to technological deviations and thus the greater the susceptibility to errors. The high-strength steel materials, in particular, which are increasingly being used in lightweight body-in-white construction, have welding ranges that differ greatly from one another depending on the surface finish and the choice of welding parameters. The selected electrode shape also plays an essential role and influences the process reliability of resistance spot welding of these materials.
The possible welding range is, therefore, very important for process capability. Process capability can only be achieved if suitable tools and the resulting process control make it possible to comply with the welding ranges. A process capability is given if the scatter of a process, in our case the resistance welding process, does not inadmissibly influence its ability to continuously adjust components according to the technical regulations and quality criteria.
Each welding process and also each individual welding system (machine or gun) has its own scattering pattern. Whereby scatter means the deviation behaviour of a characteristic resulting from the interaction of random and systematic influences during the process.
The process capability indices Cp and CpK are key figures for the statistical evaluation of a process in production engineering. They indicate how reliably the targets set out in the specification are achieved. While the Cp value only indicates the ratio of the specified tolerance to the process variation, the CpK value also includes the position of the mean value to the specified tolerance centre. In the best case (process mean is exactly in the middle of the tolerance range) CpK = Cp; otherwise CpK < Cp. You can find more information on this in the relevant specialist literature.
For a safe welding process, the tools available, such as the welding machine and the control system, must be selected in such a way that the welding area can be safely adjusted.
You can find further information and notes in the DVS leaflets, the ISO standards and from your welding technology and Harms & Wende partner.