Spot Welding
Questions and Answers
Pressure is one of the major components of the welding process Pressure, Current & Time (PCT). In most weld schedules pressure is called out as a force value. Pressure is this same value as force but applied to the surface area of the electrode contact face.
See the article - How are force and pressure related?
The relationship between pressure and the heat generated during resistance welding is well established. It is represented by the figure below.
Increasing pressure will reduce the overall resistance and decrease the heat generated. Conversely reducing pressure will increase the resistance and increase the heat. The question asked was how does force affect the process. Pressure is that same force applied to a specific surface area (lbs/in2). Therefor increasing or decreasing force has the same effect as pressure.
To answer this inquiry we will start with a previously published question and expand upon it.
“WHAT IS THE DEFINITION OF RESISTANCE WELDING”
Per the definition of the Resistance Welding Manufacturers Alliance:
To expand upon this PRESSURE is provided by mechanical means of pneumatic or hydraulic cylinders, servos, cams and sometimes manually. Normally it is measured in pounds, kilos or equivalent. It is applied in sufficient amount to contain and control the weld to a specific desired area, before the current is applied. CURRENT is supplied by the control and transformer. The transformer does just that transforms relatively small current input from the buss to many thousand amperes needed for resistance welding at people safe low voltages. The control determines the amount of CURRENT and the TIME.
Heat is generated according to JOULES LAW.
The heat is generated by the current squared times the resistance multiplied by the time. It is the current flowing through the area to be joined for a length of time and the resistance at that joint which generates heat to make a joint. The force holds all of this together and forges the joint as it forms and cools down.
AC resistance welding would use AC current and transformers for the power in resistance welding. Most rocker arm and press welders have been manufactured with AC power supplies. The equipment can also be DC direct current. In the last thirty years MFDC mid frequency direct current is has become the go to equipment for automated robotic systems and many other applications.
There are many articles published in this forum about the advantages and disadvantages of the AC, DC and MFDC systems. One can find these by performing a search on the home page of this forum.
Reference: RWMA – Resistance Welding Manual 4th Edition
This question is getting into the area of physics and varies with the frequency of the input power.
The core is based upon flux density and can be calculated based upon physics calculations found in text books presented here.
Any further discussion is beyond the scope of this forum.
Contact a resistance welding transformer manufacturer for additional assistance.
The answer to this question is – it depends upon the geometry/design of the part, the nut being welded and safety. M6 weld nuts come in many shapes and sizes.
Assorted M6 Weld Nuts
Workpieces also vary in shape and size. Clearance for the electrodes will be determined by the geometry of the parts involved and the fixtures and tooling being used. There is no set value for spacing other than clearance to load the parts safely and not too much clearance to make the travel time too long for the electrodes to close.
After part design and clearance are determined what is left is safety and time. Time considerations are determined by the welding and loading equipment capability and the plant investment in tooling, automation and other factors.
Safety of personnel operating the equipment is a factor that must be considered in the design and layout of the equipment. Various safety codes could apply to the operation and determine pinch point, light curtain, lock out or other safety needs. A starting point to determine these needs could be ISO 12100 Safety of machinery — General principles for design — Risk assessment and risk reduction.
Automated or manual nut welding operations all require evaluations for safety and part clearance to determine the proper electrode opening. No two operations are alike therefor there is not a set answer to this question.
For economics and safety one would not open any more than is necessary to load the part. Safety must be built in to keep the operators hands out of the pinch points. This reduces electrode travel time and reduces room for an operator to be hurt.
Reference: ISO 12100 Safety of machinery — General principles for design — Risk assessment and risk reduction.
The answer is no the base plates should not be hot. If they are the reason must be determined.
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.
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.
Inductance is an electrical field induced in any magnetic material in or near the throat of an AC resistance welder. Insulated or not any magnetic material in or close to the throat of the AC machine can be affected. It will absorb energy and heat up. This includes the fixture itself or its components (bolts & nuts) or base plate. To prevent this, fixtures, base plates and other components should be fabricated from nonmagnetic materials (aluminum, copper, nonmagnetic stainless or insulators). Inductance is a common problem in the field. Many tool rooms are not aware of the problem (inductance). To check your tool, test your tool or components with a magnet to see if it is fabricated with magnetic material.
Insulation of the fixture is also necessary to prevent any current from flowing through the fixture and heating it up by resistance heating. This means insulating bolts, nuts, and washer with washers and sleeves as well as the mating surfaces underneath or between.
The other housekeeping issue is weld flash. A buildup of weld flash can lead to current flow on an otherwise insulated fixture. A stray current path of this sort can lead to heating of the fixture and loss of power at the welding operation.
The proximity (location) of the weld base to the welding should also be considered. Normally the electrodes handle the greatest heat and cooling functions. If the weld fixture base is too close it may come in contact with large amounts of heat and it may also need some form of heat dissipation. This could be a chill block or direct water cooling.
Weld Fixture Exhibiting Non Magnetic Stainless Tooling and Insulated Pins and Tools with Water Cooling
Reference: RWMA Manual Chapter 19
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