The key to good control of spot weld quality is control of the process and materials. As there are dozens of factors which can influence the formation of the spot weld, it is important to set up a welding condition which has reasonable tolerance to process variation, and to aim to control other factors as far as possible.
The material itself is the first consideration. Ensure control of the material grade, any surface coating specification (e.g. type and thickness) and freedom from contamination (dirt, rust, etc). Also consider the presswork, because poor part fit-up and close sheet edge distances can lead to poor or variable quality.
Maintain the equipment in good order taking particular care with tooling and electrodes. Good control of welding parameters is wasted if the electrode alignment and the electrode tip contact face are not set up correctly. Set up an appropriate electrode dressing or replacement schedule and provide good water cooling to minimise electrode wear.
Set up welding conditions: electrode force and weld times which are appropriate to the materials and thickness being joined. There are guidelines available in standards1, text books and company specifications. These are designed to give a reasonable welding window or tolerance range. Note that substantial deviations from such conditions, e.g. low electrode forces or short weld times may produce acceptable welds but quality may be sensitive to small changes in process variables.
Keep proper records of settings and routinely confirm the performance of the machine by checking at least electrode force and welding current at intervals, e.g. daily. Keep records of periodic destructive tests. Attempt to identify and rectify the cause of deviations from the desired quality rather than compensate by excessive adjustment of settings.
- BS1140: 1993 - British Standard specification for resistance spot welding of uncoated and coated low carbon steel.
Further information
Resistance welding of sheet metals - a guide to best practice