Spot Welding
Questions and Answers
In this scenario we will base the schedule on the governing metal thickness (GMT– the thinnest outside sheet), which is 1mm. For simplicity there was a chart readily available at 0.045 in material, which is 1.15mm.
This AWS C1.1 schedule calls for a 1/4inch weld face, 750 lbs force and 13000 amperes for 11 cycles.
Since the materials are dissimilar in thickness adjustments will need to be made to balance the heat and get the nugget to form evenly at the interface. This heat balancing is done with electrode face changes or electrode material changes. One wants to cool the thick side to move the nugget toward the thin material. Or you can heat the thin sheet.
For additional information see article:
WHAT IS POOR HEAT BALANCE IN RESISTANCE WELDING
The issue of pressure vs force is they are directly related. Force is measured in pounds in the above schedule and the pressure applied is that same pounds applied to the ¼” weld face (pounds/in2).
Reference: AWS C1.1 Recommended Practices For Resistance Welding
This question can be answered with two types of equipment. A package or fixture type transformer can be manufactured with either one or two secondary outputs.
FIXTURE TYPER TRANSFORMER TWO SECONDARYS
When two secondarys are used they by transformer design will fire with the same voltage. Thus is the current output will be the same as long as the two current paths are exactly the same. For the welds to be alike the materials being welded and electrodes must be alike and in the same condition. If any of these items vary the current flow will begin to vary between the two paths. If the currents vary too much the welds will start to vary. Generally this is not an issue.
However if one ignored the imbalance it could become a real issue and one weld or the other could become discrepant if material thickness, materials, coatings, cable condition, electrode wear, or water cooling were not maintained properly. Therefor for a fixture transformer material thickness, material and coating are an issue that must be considered in double firing spot welding.
A second type of equipment that could be making single or double welds might be a Cascade welder. This is a control arrangement where multiple guns are being fired on a machine system individually by one control. Each gun has its own schedule but they are sequenced so that they fire in sequence to prevent multiple guns from firing at the same time. In this scenario material thickness and coatings are not a big issue because an individual schedule can be set for each weld to match the circumstances.
FOUR CASCADE CONTROL
This Cascade Control would be used to fire four individual guns with one control.
In place of this unit, four individual controls could fire the same four weld guns singly. If sequencing were desired the addition of a PLC or other timing function would be needed to control their sequence.
Below is a schematic of an application of a fixture type transformer in use.
SERIES WELD USING FIXTURE TRANSFORMER
Reference: RWMA Manual- Fourth Edition, 19
Roman Manufacturing Inc
ENTRON Controls, LLC
This question can be answered with two types of equipment. A package or fixture type transformer can be manufactured with either one or two secondary outputs.
FIXTURE TYPER TRANSFORMER TWO SECONDARYS
When two secondarys are used they by transformer design will fire with the same voltage. Thus is the current output will be the same as long as the two current paths are exactly the same. For the welds to be alike the materials being welded and electrodes must be alike and in the same condition. If any of these items vary the current flow will begin to vary between the two paths. If the currents vary too much the welds will start to vary. Generally this is not an issue.
However if one ignored the imbalance it could become a real issue and one weld or the other could become discrepant if material thickness, materials, coatings, cable condition, electrode wear, or water cooling were not maintained properly. Therefor for a fixture transformer material thickness, materials and coating are an issue that must be considered in double firing spot welding.
A second type of equipment that could be making single or double welds might be a Cascade welder. This is a control arrangement where multiple guns are being fired on a machine system individually by one control. Each gun has its own schedule but they are sequenced so that they fire in sequence to prevent multiple guns from firing at the same time. In this scenario material thickness and coatings are not a big issue because an individual schedule can be set for each weld to match the circumstances.
FOUR CASCADE CONTROL
This Cascade Control would be used to fire four individual guns with one control.
In place of this unit, four individual controls could fire the same four weld guns singly. If sequencing were desired the addition of a PLC or other timing function would be needed to control their sequence.
Below is a schematic of an application of a fixture type transformer in use.
SERIES WELD USING FIXTURE TRANSFORMER
Reference: RWMA Manual- Fourth Edition, 19
Roman Manufacturing Inc
ENTRON Controls, LLC
In North America the common unit of measure for force is pounds (lb). AWS publishes force in units of Kilo Newtons (kn) and pounds (lb). Of course other parts of the world and even N America may use different but equivalent units of measure.
All of these force units (lb) may be converted into pressure units (psi) based on the area the force is being applied to.
Many electronic force gauges measure force and have the ability to read in the unit of measure desired.
DIGITAL FORCE GAUGE
Pressure and Force are frequently confused in resistance welding. They are related terms. The force applied produces pressure at the face of the electrode. For example approximately 1600 lb-force will produce 32,000 psi/pressure on a ¼” electrode face.
Reference: AWS C1.1- Recommended Practices for Resistance Welding
TUFFALOY CATALOG, Rev 2017
In this scenario we will base the schedule on the governing metal thickness (GMT– the thinnest outside sheet) , which is 1mm. For simplicity there was a chart readily available at 0.045 in material, which is 1.15mm.
This AWS C1.1 schedule calls for a 1/4inch weld face, 750 lbs force and 13000 amperes for 11 cycles.
Since the materials are dissimilar in thickness adjustments will need to be made to balance the heat and get the nugget to form evenly at the interface. This heat balancing is done with electrode face changes or electrode material changes. One wants to cool the thick side to move the nugget toward the thin material. Or you can heat the thin sheet.
For additional information see article:
What is poor heat balance in resistance welding?Reference: AWS C1.1 Recommended Practices For Resistance Welding
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