Solutions for Projection Weld Nut Failures

The process of welding nuts and bolts onto stamped sheets, usually zinc-coated, is increasingly used in the metalworking industry, especially in the automotive, truck, machinery, and agricultural implement markets.

This condition is related to gains in productivity, reduction of rework, and restrictions imposed by MIG/MAG processes, which depend on a qualified welder and isolated environments due to UV rays, fumes/welding spatter.

In this scenario, resistance welding equipment must be equipped with resources that can increase reliability and reduce the occurrence of non-conformities, especially with part detachment.

In this regard, equipment with controls for welding current, pressure between electrodes, and upper head position (displacement) is recommended, with functions controlled by limits that indicate trends, making it possible to segregate out-of-specification parts and carry out statistical monitoring, significantly increasing the capability of the welding process.

The management of quality records also plays a strategic role in the resistance welding environment, with the establishment of a hierarchy for access controls and parameter editing, to ensure process quality and traceability.

Sampling plans/audits with dimensional tests (torque) and destructive tests (pull-out/detachment) increase the robustness of this process through constant monitoring. In this sense, when the welding equipment incorporates AQL (Acceptance Quality Level), efficiency and effectiveness increase substantially, by ensuring that the test is carried out within the defined frequency and parametrization for the respective process. The verification of welding current/pressure between electrodes also contributes to process qualification, especially if these records are entered directly into the welding equipment. "Acceptance Quality Level (AQL) - Defined as the maximum defective percentage (or maximum defects per 100 units) which, for sampling inspection purposes, can be considered satisfactory as a process average."

Another important approach to highlight is visual controls and control plans for the critical characteristics identified during the quality planning of the welding process. These "instructions" are generally posted at workstations so that operators can carry out the process within the established requirements. "A control plan is an active document that describes the methods adopted to control critical characteristics to provide results that meet customer requirements."

The possibility of configuring control plans on the welding equipment itself, where the operator will systematically visualize all critical processes and respective controls to be adopted, associated with the resources of sampling plans, enhances and qualifies the welding process.

Our SPX resistance welding machine integrates all these control functions and uses "Industry 4.0" concepts, such as remote access with email triggers to those responsible whenever out-of-specification events occur, for example: Maintenance stops, non-conformity occurrences, or productivity below planned.

References:

SANTOS, F. B., MAINIER, F. B., Relevant Variables for the Quality of the Electric Resistance Spot Welding Process Used in the Automotive Industry. Available at: Relevant Variables for the Quality of the Welding Process.pdf. Accessed on 18/06/2021.

AIAG, Automotive Industry Action Group. Special Process: Welding System Assessment, CQI-15.