Spot Welding

Questions and Answers

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.

M6 Weld Nuts

               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.

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

 

Spot welding stainless steel to titanium is definitely in the world of research. Both materials can be resistance welded. Data is available in AWS C1.1. It does not cover resistance welding the two together.

It is known that both materials are reactive and sensitive to heat. Titanium expulsion can be very exciting.

Expulsion

            EXCESSIVE EXPULSION

A serious shield for protection from expulsion would be in order. Stainless is austenitic and can subject to interface cracking. On the positive side they both heat up and spot weld readily.

Some digging in the research found that using an interlayer of aluminum might not work due to the formation of some undesirable intermetallic phases. Further research found that magnesium might be used in laser welding and copper was being used for friction stir welding. Both might be worth a try as a resistance braze medium if a spot weld cannot be developed using the data from AWS C1.1.

If a brazed joint is acceptable then consider that both titanium and stainless are being brazed to themselves in industry. Braze alloys exist. Contact a brazing alloy supplier and search for a braze alloy that might be suitable to join them together. A resistance welding machine can do the job depending upon the quantity design and engineering specifications.

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

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.

Low Carbon Steel Weld Schedule C1.1

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 type inline lugs

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.

Cascade Control

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 sketch CD containing 2 day program Page 415

 SERIES WELD USING FIXTURE TRANSFORMER

Reference: RWMA Manual- Fourth Edition, 19
                   Roman Manufacturing Inc
                   ENTRON Controls, LLC

 

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