Spot Welding

Questions and Answers

When a machine has been running good parts and a brief stop is made one expects to start back up and make good parts. The stop could be for a break, electrode change, electrode dressing, lunch or shift change. These occur every day and should not create a problem. If something is occurring what is the variable?

Some have said that the electrode cools off during the break. Possibly but it cools to merely water tower temperature of 85 deg F or if refrigerated 65 deg F. Considering that the electrode will reach red hot on the face which is 1400 deg F in less than a second. A little cooling does not matter.

One must look elsewhere. It is troubleshooting time. Something else in the system has changed and needs to be investigated. It is time to get out the current meters and force gauges. It is highly likely that in the force or voltage current world something is not being delivered as desired. Every shop should have or access to a current meter and a force gauge.

  Current Meter WA2                                   DLC Gauge

     CURRENT METER                                                           FORCE GAUGE

With these simple tools one can measure the properties being produced by the machine when it is running properly. Then one can monitor these properties during production and observe changes and take action. The main function of the machine is to produce pressure and current for a length of time “PCT”. With these tools or ones similar these parameters can be measured and compared to standards in your records.

Likely causes for changes are:

• Cylinders or force devices could need a rebuild and are sticking until they work free

• Air pressure varies in the plant during the day as other equipment comes on line and the main buss cannot provide sufficient air volume initially. A surge tank may be needed near the welder.

• There may not be sufficient power in the plant. When all of the equipment is on line the voltage drops especially when all machines fire up after common breaks or lunch. 

• The buss feeding the machine may be undersized and it shows up when everything is heated up and the demand in the plant is high.

• Cables and conductors must be connected tightly with clean connections to maximize conduction.

• Watch closely for worn cables or shunts. This will reduce capabilities.

• Controls with constant voltage or constant current are available which can help the power situation.

• Water cooling is always important in resistance welding. Make sure that all conductors, controls, transformers and electrodes are in some manner connected to flowing water.

As stated previously the sources for the reduced nugget sizes at start up will be found in a variation that affects “PCT”. It will likely be in the “PC”

For additional reading on this subject go to a Previous Article  in This Blog:
“WHY DO WELD NUGGETS VARY DURING A SHIFT?”

Reference: RWMA - Resistance Welding Manual 4th Edition
                   AWS -    AWS Standard J1.2 Guide to the Installation and Maintenance of Resistance Welding Machines

Spot welding is a form of resistance welding. The heat is generated by two opposing electrodes passing current through two or more sheets of material. The resistance to that heat flow in the materials causes them to heat up and form a weld nugget. Which when solidified bonds the two sheet together. This heating process is governed by:    

                                                Joules Law

The heat is generated by the current squared times the resistance in the materials by the time it is heated.

Spot welding galvanized material provides a unique challenge because the material has a zinc coating. Zinc melts at a lower temperature than the steel being spot welded. Zinc readily alloys with the copper alloy electrodes and coats their faces with brass and oxides. The welding interface changes dramatically as compared to welding bare steel. Resistance welding coated steel is more difficult but not impossible. Since the 1970’s all vehicles shifted to galvanized coated materials to reduce rust. Processes changed but galvanized parts are being spot welded successfully all over the world.
In general:

• Currents are a little higher
• Electrode faces must be dressed more frequently
• Electrodes are changed more frequently
• Stepper schedules are sometimes employed

Addressing the question asked, first address what is 16 gauge:

Standard Gage Chart

#16 gage material is 1.519 mm [0.060 in] thick. With this information one can research the various sources for galvanized weld schedules in the literature. Below is an excerpt from one:


AWS C1.1 Recommended Practices for Resistance Welding

16 Gage Low Carbon Steel Weld Schedule C1.1

As in all weld schedules these values will work for the material listed on a resistance welder. There is no best schedule. The machines vary, the materials vary and the specification vary therefor the actual best values will vary in every application.  However, a weld nugget can be formed with the numbers listed above. With a modest amount of adjustment to fit your welding equipment, materials and specifications, parts can be run successfully.

Equipment and material suppliers are also a source of assistance for welding galvanneal coated steel. Additional weld schedules are listed in AWS C1.1. and can be obtained through AWS. RWMA Resistance Welding Manual is also a good source of information.

Reference: AWS C1.1 Recommended Practices for Resistance Welding
RWMA Resistance Welding Manual 4th Edition

Water flow measurement is critical to ensure the components of the resistance welder and especially the electrodes are receiving the proper amount of cooling water during operation.

AWS J1.2 Guide to Installation and Maintenance of Resistance Welding Machines offers information about the setup and maintenance of resistance welders.

In the marketplace water flow frequently is 0.5 – 1.5 gal/min. to the electrodes. The transformer, control and conductors also will receive flow. Total flow to the machine is machine specific.

Installation of a flow meter, switch or indicators on the machine is helpful to insure water flow to each circuit and all critical areas. There are many acceptable flow meters and indicators on the market. Two examples are shown below.

Flow Meter                      Water Flow Indicator

               WATER FLOW METER                                                     WATER FLOW INDICATOR

The meters can ensure that full flow is going to the machine and that each cooling loop has flow. If there is a critical loop a flow switch can be installed which can ensure flow is present or send a signal/prevent operation if flow is not as desired.

Reference: AWS J1.2 Guide to Installation and Maintenance of Resistance Welding Machines

Glidcop is a tradename for a dispersion strengthened copper material classified by the RWMA. The most common form is RWMA Class 20. It is frequently used to resistance weld galvanized steel. There are other uses but this is a common usage and will be assumed the subject matter for this inquiry.

RWMA Class 20 is used in a manner similar to RWMA Class 1 and Class 2. It is most often used at or near expulsion levels of the weld lobe. It’s claims to fame are resistance to heat and anti-sticking. At various intervals during it use it is changed and dressed and reused. The other more common method in automation, the robot moves the electrodes periodically to a dressing station while the next part is loaded.

Weld Lobe

The question asked is: what is the life expectancy of RWMA Class 20? The answer depends upon what is being welded, how close is the weld schedule is to the high end of the weld lobe, how much expulsion is being experienced, how much material is being dress away?

In general, about two thousand welds could be expected. Extreme cases fewer and mild situations more welds. RWMA Class 1 and 2 may not hold up to the extreme cases of expulsion as long as RWMA Class 20.

If sticking and expulsion are not an issue RWMA Class 1 or Class 2 may function just as well.

For addition information read another article in the blog
“WHY IS RWMA CLASS 20 USED FOR SPOT WELDING GALVANIZED STEEL”

Reference: RWMA Resistance Welding Manual 4th Edition

                   AWS J1.3 Specifications for Materials Used in Resistance Welding Electrodes and Tooling

To start this off “RO” stands for Reverse Osmosis. This is a filtration process that uses a semi permeable membrane to filter out larger particles in the water. It removes most minerals. “DM” stands for Demineralized Water. It is also called Deionized water. It also removes the minerals from the water. As the name implies the ions are removed. The pro for both is they produce pure water.

AWS has a standard for water used in resistance welding equipment:

AWS Standard J1.2M/J1.2:2016 Guide to the Installation and Maintenance of Resistance Welding Machines

It states the following water requirements:

Water Quality:

1. Maintain the pH between 7.0-9.0

2. Chloride maximum content of 20 ppm

3. Nitrate maximum content of 10 ppm

4. Sulfate maximum content of 100 ppm

5. Solids maximum content of 250 ppm

6. Calcium Carbonate maximum level of 250 ppm

7. Water resistivity greater than 2,000 ohm-cm

The water should be free of contamination such as algae growth and slime. Water which has an odor or is cloudy requires attention. Do not use water additives that will attack the circulation components including the rubber hoses. The water hoses should not contain graphite or carbon black which could make them conductive.

Deionized water should not be used. It can be corrosive to the water system and can lower the resistivity of the water.
Dirt and debris such as leaves blown into the water tower can lead to restricted water flow and overheating of the welder components. Strainers and filters should be check frequently.

Water Manifold

                  COOLING WATER MANIFOLD

The above highlighted statement would eliminate “DM” Demineralize water use in resistance welding. The lack of ions in the water make the water ion hungry and the water will be aggressive and attack the various components in the welding system. “RO” systems also removed minerals and the water is ion hungry and would be aggressive and attack the components of the resistance welding system. 

The cons for both "RO" and "DM" water are they are so pure that they are ion hungry which is bad for the resistance welding environment.

Water that meets the AWS Standard would be a better choice than either “RO” or “DM” treated water.

 

Reference: AWS J1.2M/J1.2:2016 Guide to the Installation and Maintenance of Resistance Welding Machines

Have a Question?

Do you have a question that is not covered in our knowledgebase? Do you have questions regarding the above article? Click here to ask the professor.