"Consistent performance of welding equipment and quality of welds can only be achieved by accurate setting of the squeeze force."

The manufacture of vehicle bodies and other sheet metal assemblies is highly dependent on resistance welding. This process is more than 100 years old and probably the most frequently used of all welding processes. The spot weld itself is completed in a fraction of a second with complete reliance placed upon the welding gun and its control system to accurately reproduce the welding parameters necessary for an acceptable weld. That critical fraction of a second may make the difference between life and death, but certainly will make the difference between a cost effective or non-cost effective production line.

 There can be a high degree of tolerance in the parameters used, but in such cases as the welding of high strength steels, coated materials or in safety critical applications, more careful attention to operating parameters is required. Even in routine spot welding a Squeeze Analyser can be used to optimise production rates and reduce the incidence of defects and electrode wear.

 The force and electric current characteristics of a resistance weld are shown in figure 1. The weld cycle has three stages:- squeeze time, weld time, hold time

1. The electrodes come together, bring the components together and apply a force. This force builds up and requires a certain length of time, called the "squeeze time", to build up to a value where welding can take place.

2. The welding current is applied for the "weld time" which typically lasts for a few cycles of the mains frequency.

3. The electrode force is maintained during the "hold time" whilst the weld nugget is allowed to solidify and cool.

Figure 2 shows how the weld time can be started at different times relating to the force cycle. In the middle example, the welding current comes on too early and the squeeze time is too short to allow sufficient force to build up between components to produce a satisfactory weld. Many welding defects can be attributed to welding with too short a squeeze time.

The lower example shows a welding cycle where the current is applied late and the peak force has been established for some time. Although acceptable welds may result from this sequence, time is wasted unnecessarily, and in volume production this can add significant costs.

In the top figure, the squeeze time is adjusted so that the current is initiated just before the peak welding force is achieved. This produces the best quality weld at the highest production rate.

In practice it is disruptive and expensive to monitor important parameters during the short welding duration and most parameters are pre-programmed in one way or another. Correct calibration of the equipment is therefore essential to maintain weld quality.

Modern programming systems for spot welding equipment enable the welding force current value and the relevant time sequence to be programmed. On closer inspection the programmed sequence actually performed by the welding gun may differ from the intended welding cycle. This is because of delays in the control system due to mechanical inertia, performance of the pneumatic force cylinder and other mechanical losses which modify the intended time sequence. It is essential to calibrate not only the forge force and the welding current but also to set the squeeze time correctly.

For example, two similar welding guns and control systems can be installed on a production line and programmed to produce the same weld cycle. When the welding force and current are measured during the spot welding process it is not unusual to find that the squeeze time of one gun is different from the other. Although the welding parameters demanded by the controller are nominally identical, the slight mechanical differences in the gun manufacture and installation cause the resulting weld sequences to be different. It is important to adjust the welding programme so that the actual sequence of events at the gun is correct and not rely on programmed parameters.

In addition, degradation of equipment over time from such causes as electrode wear or increased flex in welding arms will also alter the actual sequence from the programmed expectations. Regular adjustment of the welding programme maintains efficiency.

Early work carried out by The Welding Institute for Volvo in Sweden resulted in the Squeeze Analyser which enables both the electrode force and the squeeze time to be correctly set with negligible disruption to production lines. The Squeeze Analyser measures and displays the force when the welding current is initiated (the squeeze force) and also the peak welding force in the cycle. The measurement of both these forces is carried out on a live welding gun and no electrical connections to the control system are required. The welding supervisor can then adjust the squeeze time until the squeeze force is just below the peak force when optimum conditions apply. This technique avoids any requirements to monitor time itself and it allows the effect of any delays in the control system to be removed.

The key forces are displayed on an illuminated bar on the Squeeze Analyser, shown schematically in figure 2. Short squeeze times are indicated by a large gap between the squeeze force and the peak force. Long squeeze times result in the squeeze force and the peak force being identical so that no gap in the illuminated bar occurs. Ideal squeeze times show as a small gap (one unlit light emitting diode) between squeeze and peak force. The simple visual display of the Squeeze Analyser enables the supervisor quickly to assess the operating values of a spot welding installation. In practice, it takes minutes to calibrate a gun correctly for optimum operating conditions. The actual values of the forces are also indicated on the front panel display.

Once the importance of the welding sequence involving the squeeze time and weld time is understood, it can be seen that a simple measurement of the peak force of a resistance welding gun is not sufficient to calibrate the equipment correctly. Consistent performance of welding equipment and quality of welds can only be achieved by accurate setting of the squeeze force.

DIVERSE Technologies in Cambridge have developed the latest version of the Squeeze Analyser to incorporate further measurement ranges, improved user interface and lower battery consumption.

Tel. +44 (0) 1223 84 44 44