Spot Welding
Questions and Answers
Voltage and current drops can be caused from two areas. One is the plant environment. The other is the wire mesh welder. If the plant does not have sufficient power or utility buss feed to the wire mesh welder problems can arise. As the plant turns on equipment during the day the plant voltage can drop and the input voltage to the welder can suffer. Secondary voltage and current will drop.
Along with this, pneumatic cylinders can be affected in the same manner. If sufficient air plumbing is not supplied, air pressure can drop during the day and the resistance can change if force is not maintained.
Internal changes on the welder which can affect the weld conditions are temperature, and resistances.
If the cooling water is not effectively cooling some component, it will heat up and become more resistant. If a joint connection heats up, it also may become loose and become more resistant. The conductors will be more resistant if hotter. This means less current due to higher resistance. A frayed cable or broken shunt will have a higher resistance and not conduct properly. An overheated transformer does not perform as well. Proper cooling of all components is imperative.Copper is a very conductive material and therefore can require different procedures. It can be resistance welded to similar materials. Factors that will be present are:
• Currents will be high to generate heat.
• Projections may be a good design idea to concentrate heat in a small area.
• Resistance brazing may be a method to simplify the attachment.
Sometimes increasing the current and weld time seems to hit a wall as far as nugget growth goes. It is very difficult to grow a nugget larger than the face size of the electrode.
A method that some find successful to increase the face size is to use weld impulses. At the end of the weld cycle schedule an off period of one or two cycles then repeat the weld schedule. The length of the impulses may need to be adjusted to prevent overheated and expulsion.
A larger electrode face size is also an option. This will require a completely new weld schedule. The larger weld face will encourage a larger weld nugget.
Reference: RWMA – Resistance Welding Manual 4th Edition
Nugget diameters are published for various materials and thicknesses in many locations.
The American Welding Society AWS C1.1 - “Recommended Practices for Resistance Welding” is one of these. Weld schedules including nugget diameters are listed for various materials. How to measure a nugget is also shown in the AWS C1.1 standard.
This blog also describes the method to measure a nugget:
HOW DO YOU MEASURE WELD NUGGETS?
WHAT IS THE FORMULA TO CALCULATE NUGGET DIAMETER IN RESISTANCE WELDING?
There is no formula to calculate a weld nugget.
Yes, resistance welding electrodes should always be water cooled.
The water for the electrodes should come directly from the water manifold. The flow rate should be optimally 5.5 L/minute (1.5 gallons/minute) at a water temperature of 18 – 29 deg C (65 – 85 deg F). Flow should never be less than 2 L/minute (0.5 gal/minute). Additional flow and water specifications can be found in AWS Standard - J1.2 “Guide to Installation and Maintenance of Resistance Welding Machines” Water flow tubes should be located within 6 mm (¼ inch) of the hole bottom. This prevents steam pockets from forming. Water flow on the exit side should be monitored with flow indicators or gauges. 
FLOW INDICATOR PROPERLY LOCATED WATER TUBES
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