Spot Welding

Questions and Answers

Resistance brazing is not spot welding so the electrode face geometry does not have to concentrate the heat into a small area. Frequently the electrode face is machined to conform to the surface that it is in contact with. Frequently the electrode will contact the entire work piece surface. In some cases the electrode faces is milled out. The work piece is nested into the face of the electrode in order to insure proper part placement and alignment. A small vacuum is sometimes pulled through the center of the electrode to hold the part in place before the electrode closes. This is very common for small part brazing.

The electrode material is frequently RWMA Class 2. It could be ETP copper and in many cases carbon electrodes are used.

Refractory faced electrodes2

When resistance brazing contact materials, refractory faced electrodes RWMA Class 11, 13 and 14 as well as Class 2 are commonly used.

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.

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.

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

Weld Fixture Exhibiting Non Magnetic Stainless Tooling and Insulated Pins and Tools with Water Cooling

Reference: RWMA Manual Chapter 19

 

Improper weld current selection can cause problems during resistance welding. The problems normally show up in the form of expulsion, indentation, electrode sticking, mushrooming or low weld strength. There is an ideal current range for every resistance weld set up. This will produce good weld nuggets with little or no expulsion, modest indentation and minimal electrode wear or sticking. Whenever you push the extremes of the current in the weld window either high or low you risk problems.

Electrode face mushrooming is the normal wear mechanism of a spot welding electrode. To slow this process or prevent excessive mushrooming one must control several factors in the welding process.

 Mushroomed electrode

EXTREME MUSHROOMING

Back up electrodes are used for projection welding electrodes. They are normally used on the lower side and are flat faced. There is no need for a defined weld face since the projection concentrates the weld current and heat into a desired spot for nugget creation.

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