Spot Welding

Questions and Answers

Squeeze is the first of the three standard resistance weld sequences:
SQUEEZE
WELD
HOLD

The purpose of squeeze time is to allow the electrodes to close on the part and build up full pressure in order to contain the weld before the current initiates. Short squeeze time can lead to expulsion and the electrodes being damaged, running hot, inconsistent nuggets and erratic weld quality. Long squeeze time extends the total weld time and could mean fewer parts per shift. Selection of the proper squeeze time is important for the efficient production of quality parts.

Every welding machine or weld gun operates differently be it powered by pneumatic cylinders or servo. So no single rule can be set. One must determine at each weld station how long it takes for the electrodes to close and build up full force after the control initiates the squeeze sequence.

Tooling and fixtures are used in virtually all resistance welding operations to hold parts in place for welding. They must be kept clean of flash and debris and be safe for operators. Heat buildup should be minimized. If a fixture is heating up several possible reasons come to the forefront: Inductance, Insulation, weld flash and location.

Metal in Throat

                    BAD                                            GOOD

This exhibits the part in the throat area but magnetic tooling in the throat causes the same problem. It absorbs energy and heats up during the welding process and forces the welder to work harder to make the desired spot weld.

To answer this question the first assumption is that the question is not concerned about a fixture or holding type clamp. The question is directed towards the electrodes on the welding gun or press welder that closes on the part and hold it in place to deliver the current and force and forge the weld. This answer will talk about the copper alloy used as the resistance welding electrode materials.

In other articles RWMA Group A Class 1, 2, 3, 4 are described. In addition RWMA Group B Class 20 is described. With the exception of the Class 4 all are used regularly as electrode materials in various applications. Their properties vary and it is their properties that decide their use in the various applications. The more conductive materials, Class 1 find applications where conductivity is most important. The stronger materials are applied where strength is the overriding factor, Class 3. Class 2 is used in most applications, which are steel. It is a middle of the road material with good conductivity and strength.

As long as the metal thickness, material and stack up are the consistant the same schedule can be used. This is an accurate statement with several exceptions.
First, the arrangement of the group of welds must be considered. All published weld schedules specify a minimum weld spacing to minimize shunting currents. Shunting current is current that can flow through the first weld rather than the weld being made. If the spacing is greater the resistance is higher and the current will not flow through the original weld. All of the current will flow through the desired weld area and produce a second good weld. If too close together more current is required to compensate for this wasted current.
Most published weld schedules list a value for minimum weld spacing between individual welds to prevent current shunting through the first weld.

Low Carbon Steel Weld Schedule C1.1

This spacing will vary by the material and its thickness. Whether you are designing fixed automation or deciding the space between robot welds a minimum spacing must be adhered to for good weld practice.

The AWS recommended temperature range for spot welding cooling water is 65-85 degrees F. One should make a decided effort to stay within this range for the protection of the transformer and control.

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