There are a number of different types of weld-nut available commercially; some are covered by national standards such as British Standard BS 7670 'Steel nuts and bolts for resistance welding' (DIN equivalents). These are usually solid, machine-threaded nuts with formed solid projections, although some types are formed from sheet metal with embossed projections within an integral flange. In each case, there are general guidelines for achieving best results.
The welding equipment should be of an appropriate size, for the particular nut, to allow preferred welding conditions to be used, which should be within the equipment's normal working range. The welding head should be free-moving in its bearings and have a reasonably low inertia, so that good follow-up characteristics, i.e. head movement, can occur when the projections collapse during welding. The machine frame and electrode assembly should be rigid so that flexure, and thus misalignment, is avoided under load.
The electrode surfaces should be flat and well aligned. Class 2 copper alloy electrodes, such as copper/chromium/zirconium, may be used but harder, tungsten/copper inserts are normally used to minimise wear of the electrode surfaces. The lower electrode should be fitted with an insulated location pin, preferably ceramic, or ceramic coated, and air-supported or spring-loaded in the electrode assembly.
Welding parameters depend on the nut type, size and sheet thickness to be welded. As an example for square nuts to BS 7670, the following conditions for welding to 1mm sheet may be used as a guide to setting up.
|Nut size||Electrode force, kN||Weld time, cycles||Approximate welding current, kA||Typical minimum torque to failure, Nm|
A destructive torque test is normally required to verify that a minimum specified torque can be achieved at the settings selected. This should be supported by a destructive peel or push-off test to check that a piece of the material can be pulled from the sheet at each projection. This ensures that a tough weld is achieved, even if the actual torque value required for an application is low.
|Weld splash in threads:
||Slight weld splash is common and should not stick to threads, especially if a good location pin is used. Heavy splash is likely to be caused by low electrode force, short squeeze time, poor head follow-up or excessive current.
||Collapse of the nut body can result from long weld time. A short time, high current, high force condition may help.
||Poor location of the components or an inadequate location pin can lead to misalignment of the holes in the nut and sheet.