APPENDIX C: THE BASICS OF RESISTANCE WELDING
UB25 LINEAR DC RESISTANCE WELDING CONTROL
990-665
C-5
4
Program a weld schedule, then make your first weld. Always observe safety precautions when
welding and wear safety glasses. For a complete procedure on making welds, refer to
Chapter
4, Operating Instructions
.
5
Use pliers to peel the welded materials apart. A satisfactory weld will show residual material
pulled from one material to the other. Tearing of base material around the weld nugget indicates
a material failure NOT a weld failure. Excessive electrode sticking and/or "spitting" should
define a weld as unsatisfactory and indicates that too much weld current, voltage, power, or time
has been used.
6
If the parts pull apart easily or there is little or no residual material pulled, the weld is weak.
Increase the weld time in 1ms increments. Increase weld current, voltage, or power if a
satisfactory weld achieved using 10 ms of weld time.
NOTE
:
Actual weld strength is a user-defined specification.
7
Polarity, as determined by the direction of weld current flow, can have a marked effect on the
weld characteristics of some material combinations. This effect occurs when welding materials
with large differences in resistivity, such as copper and nickel or when welding identical
materials with thickness ratios greater than 4 to 1. The general rule is that the more resistive
material or the thinner material should be placed against the negative (-) electrode. Polarity on
the Power Supply can only be changed by reversing the Weld Cables.
Weld Strength Testing
Destructive tests should be performed on a random basis using actual manufacturing parts. Destructive
tests made on spot welds include tension, tension-shear, peel, impact, twist, hardness, and macro-etch
tests. Fatigue tests and radiography have also been used. Of these methods torsional shear is preferred
for round wire and a 45-degree peel test for sheet stock.
Weld Strength Profiles
Creating a weld strength profile offers the user a scientific approach to determining the optimum set of
welding parameters and then displaying these parameters in a graphical form.
1
Start at a low weld current, voltage, or power, making five or more welds, then perform pull tests
for each weld. Calculate the average pull strength. Increase weld current, voltage, or power and
repeat this procedure. Do not change the weld time, weld force, or electrode area.
2
Continue increasing weld current, voltage, or power until any unfavorable characteristic occurs,
such as sticking or spitting.
3
Repeat steps 1 through 3 for different weld forces, then create a plot of part pull strength versus
weld current, voltage, or power for different weld forces as shown in the illustration below,
Typical Weld Strength Profile
.
4
Repeat steps 1 through 3 using a different but fixed weld time.
