Welding Handbook

Table of Contents

REVISION RECORD

PREFACE

SAFETYHazard AwarenessEnergy SourceSafeguard DevicesWeld FlashPneumaticsHydraulicsWelding Presses/Pinch PointsLaser Class Descriptions

ARC WELDINGIntroductionMIG Weld DetailsApplicationSilicon BronzeSupportTroubleshootingStud WeldingBarrier CoatWeld GunQualitySupport

BRAZINGIntroductionApplicationTorchRegulatorTorch/MixerTipsSurface Repair

LASER SYSTEMSIntroductionOperation DetailGas Delivery

RESISTANCE WELDINGIntroductionCaps, ShanksCablesShuntsElectrode BackupElectrode HoldersTransformersControlsDensification UnitAVC and C-REGWeld GunsEqualizing and Pinch TypeIntegral or Package GunsAir Over Oil GunsPortable GunsWeld SchedulesStepper ControlsSuggested Weld SchedulesTooling ApplicationsWelding StandsTabletopMechanical Handling SystemPortable GunAutomatic FixturesRobotsHoseManifoldsSecondary CircuitPolarityProgrammingStepper ProgramEdge WeldsWeld Nugget SizeElectrode Tip ContactContact ResistanceFlashingExpulsionTip Geometry

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ChillersBeam Delivery SystemShielding GasWeld JointsSystem ControlsSet-up VariablesSafety EnclosureLaser ToolingEvaluationCalorimeterOptics CleaningBeam Delivery AlignmentMode BurnTeach Tool

APPENDIXA1 Resistance WeldingA2 Arc WeldingA3 Brazing/SolderingA4 Laser WeldingA5 ElectricalA6 PneumaticsA7 HydraulicsA8 General

Support/QualityTool VerificationDestructive TestingNondestructive TestingWeld AnalysisPreventive MaintenanceInstrumentation

UTILITY DISTRIBUTIONIntroductionElectrical SafetyElectrical DistributionBuss DuctBuss PlugsBolted Pressure SwitchesFusesIsolation ContactorsApplicationFab. Plant ExamplesAssembly Plant ExamplesCompressed Air SafetyCompressed Air DistributionMain Shut-off ValveAir PreparationPressure RegulatorLine LubricatorPiping ExamplesDirectional Control ValvesCooling Water SafetyCooling Water DistributionSystem TypesManifold ExamplesCatwalk PipingHydraulic SafetyHydraulic SystemHU-56 Troubleshooting

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Revision Record

REV. NO. DATE SHEETS INSERTEDBY/DATE

A 10/15/91 2

B 6/1/92 10

C 8/2/93 12

D 11/15/96 81

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Preface

The GM Automotive Welding Handbook is a revised andexpanded publication of the 1985 General Motors ResistanceWelding Handbook. More than 55 years ago, in 1939, a manualentitled "Spot Welding with Portable Tools" was published anddistributed within General Motors. It was the first publication withthe purpose of explaining the resistance welding process usedfor assembly of automotive sheet metal.

As technology advanced, many other handbooks were writtenand distributed within the corporation. In 1955 the firstResistance Welding Handbook was published. It was updated in1963. By 1983, with more advancement of solid-statetechnology, a second update of the Resistance WeldingHandbook was done by the Manufacturing DevelopmentDepartment, Production Engineering Activity of the Fisher BodyDivision. It was published and distributed in 1985.

By 1983, with more advancement of solid-state technology, asecond update of the Resistance Welding Handbook was doneby the Manufacturing Development Department, ProductionEngineering Activity of the Fisher Body Division. It waspublished and distributed in 1985.

As would be expected, not long after the distribution of the"redbook," technology moved again. A new resistance weldingtechnique known as high frequency/direct current (HFDC) wasintroduced. And laser welding was going beyond theexperimental state. In 1989 it was decided that the "redbook"should have an update.

This book, titled Automotive Welding Handbook (AWH), is thenext step. Since different welding techniques are in commonuse within General Motors, it was decided that the newhandbook go beyond resistance welding.

Notice that this is titled "handbook," not "training manual." Itprovides reference material written specifically for maintenancepersonnel; people who have been trained in areas of automotivewelding. The book has been published as a loose-leaf binder. It

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allows for future updating by section as technology andapplications advance. This could allow for updated sections tobe distributed individually as required. Recordkeeping at eachGM facility would be a necessity for this to be practical. It is thegoal, however.

This "handbook" is a product of the Weld Team at the NAOManufacturing Center. It is available for use by all divisionswithin the GM Corporation.

Weld TeamNAO Manufacturing Center

480-109-135(810) 947-0020 8-227-0020

November 1996

Automotive Welding Handbook, Preface


Safety

Automotive welding processes are an important part of theGeneral Motors Mark of Excellence.

Equally as important as the processes are the safety techniquesapplied during operation and maintenance of the systems.Everyone has a responsibility to work safely.

This handbook's safety section presents only an overview ofsafety concerns and approaches. It does not preclude individualplant safety agreements. It does offer points to be considered bypersonnel who may be involved with system operation andmaintenance.

The flowchart which follows presents a thought pattern whichmay be used in a situation requiring maintenance involvement.In some cases a complete system lockout may be the properapproach. The flowchart can help outline the proper steps tofollow.

The chart is taken from the lockout manual published by:

U.A.W.-G.M. Health and Safety Center1030 Doris RoadAuburn Hills, Michigan 48326810-340-7800

Further discussion and/or information regarding the flowchart orlockout manual may be obtained through each plant's local JointHealth and Safety Committee or by contacting the addressnoted above.

Automotive Welding Handbook, SAFETY


Safety

Hazard Awareness

The following flowchart is taken from the UAW-GM LockoutEnergy Control manual. It is designed to create a thoughtpattern for each machine or system maintenance operation inwhich you may be involved. It helps define the hazardidentifiaction process, potential exposure and then hazardelimination.

The next two pages which follow are also from the LockoutEnergy Control manual. They extend beyond the initialguidelines. The intent is to state the process in more detail.

For a complete review of the lockout procedure, use the LockoutEnergy Control manual.

Automotive Welding Handbook, SAFETY, Hazard Awareness


Automotive Welding Handbook, SAFETY, Hazard Awareness


Safety

Energy Source

The flowchart is designed to create a thought pattern before, during and afterany lockout procedure.

Below you will find a brief description or action for each step in the flowchartlockout of the electrical system.

1. Job Assignment2. Define Work Area3. Identify Energy Sources by the Equipment's Components4. Is Lockout Needed to Secure Job Area?5. Find All System Components in the Work Area6. Check to See if Energy Source Can Be Turned Off7. Get Qualified Personnel to Make the Decision8. Follow Shutdown Procedures: Turn Off, Lockout, Test9. Should Be Discharged or Disconnected by Qualified Personnel10. Attempt Manual Start11. Look for Action or Movement12. Attempt Manual Start (Again)13. Look for Action or Movement (Again)14. Check for Any Other Energy Sources15. Do the Indicated Work16. Remove Lockout Devices From Disconnects17. Restart Equipment18. Job Finished

Automotive Welding Handbook, SAFETY, Energy Source


Safety

Safeguard Devices

A fence with an interlocking gate is asafeguarding device. To open the gatethe plug must be removed. When theplug is removed the electrical controlsystem is opened and machine motion isstopped.

In some applications, a gate latch pincannot be removed unless the controlinterlock is pulled out first.

Fig. 1-4a, Interlocking Device

This a later style safety gate. interlock. Itis shown with the key removed. Beaware that in some cases motion, only,may be stopped when the key is pulled.Complete system energy removal mayrequire additional steps of procedure.

Automotive Welding Handbook, SAFETY, Safeguard Devices


Fig. 1-4b, Interlocking Device

Fig. 1-5, Robot Emergency Stop Buttons

Automatic welding systems are designed and constructed with emergencystop buttons.

Located in areas of open access they can be used when the operator orsupport personnel observe an unsafe condition, Anyone can hit the red



mushroom-shaped "E-Stop."

It should be noted that E-Stops are required as the resource for anabsolute emergency stop. It must not be used as a "handy" systemshutdown. E-Stops do not select "convenient" places to stop the system.An E-Stop improperly employed may cause a very lengthy down-time inorder to clear the system for restart.

Fig. 1-6, Flashing Light

Some systems are constructed with visual awareness flashing light safetysignals. When the red light is flashing it is to make local personnel awareof a potential hazard.

Audible awareness devices ,Such as horns, bells and buzzers are oftenused where flashing lights may not be visible.



Fig. 1-7a, Photoelectric Cell Fig. 1-7b, Light Curtain

When safety fences and gates are not applicable, a "light" curtain orphotoelectric cell can be used to provide a "safety screen."

A pressure sensitive mat maybe used to sense the presence of anoperator or any other personnel in the area.



Safety

Weld Flash

A safety concern of resistance spot welding is when the molten metal is ejectedfrom the weld spot. Weld flash or sag is usually caused by welding too "hot." Forexample, when an electrode tip is changed or dressed and the stepper is notreset, the concentration of weld current at the weld spot will probably result inmolten metal exploding outward from the weld zone. Weld flash can burn skinand severely damage eyesight. Protective eyeglasses with side shields aroundspot welding operations is a requirement.

Long sleeve shirts and long trousers must be worn to protect against weld flash.Synthetic materials such as nylon or rayon should not be worn. When heated byweld flash, they can melt onto the skin they are covering.

Fig. 1-8, Spot Weld Station

Automotive Welding Handbook, SAFETY, Weld Flash


Flash curtains, protective screens or barriers protect personnel passing a weldstation. Proper eye protection with side shields should always be worn bypersonnel in the weld area.

A full face welding hood is worn to protect the welder's eyes from the arc glare orradiant energy generated by the arc or oxyfuel process. This protects thewelder's vision. ANSI standard Z87.1 defines lens requirements for specificwelding operation (see page A8-1). Long trousers and long sleeve shirts orleather protectors must be worn here also.

The floor should be kept clean of hoses and loose parts strewn about. Solventsshould not be used or stored near welding operations.

Fig. 1-9, Arc and Oxyfuel Welding Concerns

Automotive Welding Handbook, SAFETY, Weld Flash


Safety

Pneumatics

Mechanical motions that are air powered take place with theinitiation of compressed air to the machine. Transfer motions willnot stop and hold unless all compressed air sources have beenremoved. Emergency stop buttons are provided to interrupt asequence in an emergency only. Stored compressed airpressure and potential motion may not be totally removedby hitting the E-Stop.

When working on a compressed air activated machine alwaysturn off the air supply first. Then bleed off the total remaining airpressure from the system.

Never direct a compressed air stream toward yourself or anyother personnel.

Note also that re-energizing compressed air service must bedone carefully and with no personnel within the machine.Air-actuated system motions may occur even though theelectrical control has not been turned on.

Automotive Welding Handbook, SAFETY, Pneumatics


Safety

Hydraulics

The cylinders and valves of a hydraulic system are much thesame as those used on air systems. Designed for the higherpressures of a hydraulic operation, they operate with a pumpingunit which delivers the hydraulic oil to them at much higherpressure and volume.

The HU-56 hydraulic unit is usually used with press weldsystems to supply the hydraulic oil to the weld guns. Wheneverit becomes necessary to work on the hydraulic system, allpressure must be removed from the system before starting thework. The HU-56 unit has a Dump Valve that is used as asafety valve to remove all pressure from the system before anymaintenance work is started.

Automotive Welding Handbook, SAFETY, Hydraulics


Fig. 1-11, Hydraulic Unit HU-56

When the pump motor is shut off, the dump valve isde-ener-gized and the pressurized oil is dumped into the tank.Whenever the pressure is relieved, an indicating rod on theunit's accumulator will be up. Never work on the hydraulicsystem unless the rod is up. Opening a system mounted bypassvalve will also relieve the pressure and provide insuranceshould the system be restarted at the wrong time.



Fig. 1-12, Air Over Oil Accumulator



Safety

Welding Presses/Pinch Points

Entry into a welding press must never be done without firstfollowing the energy control procedures. Lockout, tagout andplatten support are required. Manual safety swing blocks andlockout devices that deactivate the welding press motor andcontrols are provided for the safety of trained maintenancepersonnel when work is required inside the press.

Vise grips or pliers can be used for electrode shank or tipremoval. Avoid putting your fingers between the electrodeswhen dressing or replacing them.

Fig. 1-13, Safe Electrode Removal

Automotive Welding Handbook, SAFETY, Welding Presses/Pinch Points


Safety

Laser Class Descriptions

CLASS IThese lasers are considered safe by all present measures ofpotential hazard. No individual, regardless of exposure time,would be injured by a Class I laser. Class I lasers have powerlevels well below 1.0 milliwatt.

CLASS IIThese lasers are limited to visible beams, where exposurewithin the normal blinking response of the eye would not behazardous. A true Class II laser beam has an upper limit of 1.0milliwatt total emitted power. This class does not produceharmful effects to skin, eyes or any other portion of the body.

CLASS IIIThe power level range of these lasers is between 1.0 milliwattand 5.0 millivvatts. Class III includes both visible and non-visiblebeams. Class III lasers can produce a hazard if viewed directly.Prolonged exposure 'on skin can be hazardous. Normally,reflections from far range viewing will not present any danger.

CLASS IVPower levels of these lasers range from 5.0 millivvatts tothousands of watts. This class is extremely hazardous to eyesand skin. Reflections of a Class IV laser are dangerous for evenfar range viewing conditions. Special care must be taken toensure that proper safety practices are being followed. Thesebeams are usually invisible.

Laser systems have conspicuously displayed signs that makecaution or danger statements.

The color of the sign and what is stated on the sign explain theparticular hazard.

"Yellow" for Caution"Red" for Danger

Automotive Welding Handbook, SAFETY, Laser Class Descriptions


Fig. 1-15a

Labels for MEDIUM POWER VISIBLE LASER having a totalpower output below 5.0 mW.

Fig. 1-15b

Automotive Welding Handbook, SAFETY, Laser Class Descriptions


Arc Welding

Introduction

The term "arc welding" applies to a group of welding processes that use anelectric arc drawn between the metal and the tip of an electrode as the source ofheat to melt and join the metals.

The electrode may or may not provide a filler metal in the process.

Although many arc welding processes are done manually, some may be donewith robots or automatic systems.

Automotive Welding Handbook, ARC WELDING, Introduction


Fig. 3-1, Manual Arc Welding

A manual arc welding process, called Shielded Metal Arc Welding (SMAW),uses a covered electrode rod. The core of the covered rod conducts theelectrical current to the arc and provides filler metal for the joint. The rod'scovering shields the molten metal, protecting it from the atmosphere during theprocess. The protection comes from the gases that form during the melting ofthe electrode and the combustion/decomposition of the shield itself.

The electrode and actual parts being welded are part of the welding circuit. The



electrode is usually the "positive" side of the circuit. The parts being welded areusually grounded directly to the weld controller which is also building-grounded.

Since the filler metal comes from the electrode, the electrode must be manuallyreplaced as needed during the welding process.

Fig. 3-2, Typical Metal Arc Welding

Some applications of arc welding use an automatic wire feed where the weldpool is shielded from atmospheric contamination by a flow of shielding gas.Three basic shielding gases, argon, helium and carbon dioxide, can be used todisplace the nitrogen, oxygen and water vapors present in the atmosphere whilethe weld is being made.

The electrode, or filler metal, is a light-gauge wire which is automatically fedthrough a nozzle to the workpiece. The feed rate can be adjusted at the weldercontrol panel. Officially known as Gas Metal Arc Welding (GMAW), it is alsoknown as MIG welding. It can be properly applied manually or automatically forassembly operations.



Fig. 3-3, Gas Metal Arc Welding

Plasma Arc Welding (PAW) operates on the principle of establishing a pilot arcbetween a tungsten electrode and the shielding gas orifice. This pilot arc ionizesthe orifice gas. The ionized gas then ignites the primary arc between the metalbeing welded and the nozzle electrode. This process develops higher weldingtemperatures than conventional arc welding processes. Welding can also bedone without adding filler metal.



Fig. 3-4

If filler metal is used, care should be taken to keep it (rod or wire) free fromcontamination in storage as well as in use.

The hot end should not be removed from the shielding gas protection during thewelding process.

The plasma arc cutting process uses a high-temperature constricted arc which isconcentrated onto a small area of the metal. The high-temperature arc heats thegas to a plasma beam.

Melted by the heat of the plasma beam, the metal being cut is removed by thejet-like gas stream from the torch nozzle.



Fig. 3-5, Plasma Arc Cutting Nozzle



Resistance Welding

Introduction

A resistance spot weld is a small localized weld made between overlappingpieces of metal.

Fig. 2-1a

In making a resistance spot weld, electrical current is passed from one electrodetip through the metals to be joined and into the other electrode tip. Resistance tothe flow of current at the faying surface, or interface between the metals, heatsthem. Based on the amount of current which flows and the length of time it isallowed to flow, the metals become molten.

Automotive Welding Handbook, RESISTANCE WELDING,Introduction


Fig. 2-1b

When they become molten and flow together, the current is stopped. The heatdeclines, the nugget cools and the metals become one. A resistance weld hasbeen made.

There are three basic elements involved in making a resistance spot weld:HEAT -- PRESSURE -- TIME.

Heat is obtained from the resistance of the metal to the flow of electric current atthe faying surface.

Pressure is obtained from the weld gun cylinders through the electrode tips.

Time is the length of the time the weld current flows.



Fig. 2-2

Resistance to current flow will generate heat in an electrical conductor.

The amount of heat generated depends on the amount of current flowing, thelength of time the current flows and the resistance of the conductor to the currentflow.

In resistance spot welding the metal being joined is a conductor in the electricalcircuit. During the flow of welding current through the circuit, resistance isencountered which generates heat. The majority of the heat will be at the weldinterface, or faying surface, because the resistance at that point in the circuit isthe highest. The resistance of the other components in the circuit is lowerbecause they are made of highly conductive copper alloy.



Fig. 2-3

Electrode tip pressure brings the metal between the electrodes together andprovides electrical contact to the metal so that a weld can be made.

If the electrode pressure is too low, a poor contact will result. This causes alarger amount of heat to be developed at the contact points between theelectrode tips and the metal. The excessive heat produces excessive wear onthe electrode tips and "too hot" of a weld. More importantly, it minimizes theacceptability of the weld nugget itself.



Fig. 2-4

When correct welding pressure is applied, most of the heat will be developed atthe interface where the weld is to be made. A proper nugget will be formed.

At this point it should be noted that weld cap alignment to the metal beingwelded is also very important. The discussions which follow are presented withthe fact that the cap alignment is normal, 90, to the metal. Under "plant floor"conditions, any misalignment greater than 5 is not acceptable.



Fig. 2-5

The length of time the weld current is allowed to flow is known as the weld time.

Weld time and weld current flow are complementary portions of the weldingprocess. Heat transfer is a function of time. Therefore, the development of aproper weld nugget size requires a minimum length of time regardless of current.Proper welds may be accomplished with shorter weld times, but the current mustbe higher. But very high current may result in expulsion of product metal duringthe weld. For weld quality, this is not acceptable. Proper weld time must not besacrificed to improve tool cycle time.

Fig. 2-6



Fig. 2-7

A welding transformer transforms the high voltage, low current primary powersupply to the required lower voltage, higher current secondary weld circuit.

The primary voltage to the welding transformer is turned on and off by a weldingcontactor that is controlled by a timer.

The timer controls how long the welding current flows. An initiating switch startsthe operation.

The secondary circuit of a welding system conducts the welding current from thewelding transformer to the panels to be welded.



Fig. 2-8

The electrode tips, welding cables and welding transformers are the maincomponents of the secondary circuit.

Most of the maintenance of a welder's electrical system will involve keepingthese components and their connections in good working condition.

There are different methods of making resistance spot welds. Basically, themethods are related to the way current flows through the parts being welded.

Three of the most common methods used in automotive resistance spot weldingare DIRECT WELDING, SERIES WELDING and PROJECTION WELDING.Direct welding is the type of welding done by single weld guns. It is also theprincipal type of welding used in assembly and fabricating applications.

Series welding is a method of welding which can be used on lighter gauge baremetals.

Projection welding permits thick metal to be welded to much thinner metal, suchas a weld nut to a body panel. It also permits a group of welds to be made in asmall area at one time.



Fig. 2-9, Direct Welding

In direct welding, all of the current from the transformer passes directly throughthe single weld nugget being formed. No other path exists for the current tobypass (shunt) the weld nugget. Only one weld is made per transformersecondary. A high conductivity copper path exists from the transformer to theweld nugget.

Direct welding is preferred when welding heavier or galvanized metal.



Fig. 2-10, Series Welding

Series welding refers to the path the welding current takes from the transformerthrough the metal and back to the transformer.

Two welds are made per transformer secondary. From this comes the name"series" welding.

One weld is in series with the other.

In series welding, however, a portion of the secondary current bypasses (shunts)the weld nuggets being formed. This shunt current passes through the panelsbeing welded and not through the faying surface. For proper welds, the currentpath between the caps must be the path of least current flow resistance.Caps-to-metal-to-backup must be an excellent alignment. Series welding shouldnot be used on galvanized metals. The galvanized coating itself can provide avery low resistance shunt path.



Fig. 2-11, Projection Welding

A projection is an embossment on one of the workpieces at the location where aweld is desired. The workpieces are placed between electrodes in the projectionwelding machine. Then pressure and current are applied as in spot welding.However, because the contact area between the work and the electrodes ismuch greater than the contact area at the end of the projection, most of theheating will occur at the projection where the weld is desired.

As the projection becomes molten, welding pressure causes the projection tocollapse and form into the panel as the weld is taking place.

Although direct, series and projection welding are the three basic styles ofresistance welding, there are a number of acceptable modified applications.

One is known as "direct-in-series." Its picture is shown below.



Fig. 2-12a, Direct-in-Series Welding

Another type of welding is known as "over/under" welding. It is used forapplications where it is not possible, or practical, to make a required direct weldwith a single transformer. The picture below shows the "over/under" technique.There is further discussion of this technique on pages 2-164 and 2-165.



Fig. 2-12b, "Over/Under" Welding

Closed-section structural auto body members are becoming an important part ofautomotive design and assembly. Since conventional resistance weldingrequires two-sided access, another approach has been developed. It is knownas single-sided spot welding (SSSW). The SSSW approach uses conventionalresistance welding equipment with modifications to the gun arms, electrodes andtransformer size/type. The picture below is an example of an SSSW application.



Fig. 2-13, Single-Sided Spot Welding

Product design and demand changes have regenerated the use of a fewadditional styles of assembly weld processes. They are all discussed in detail inPED 106. A brief overview is presented on the following pages.

BUSS BAR WELDINGThis refers to the method of conducting the weld current to the weld guns andinto the metal. As shown in the sketches, the product receives many welds fromone weld control/ transformer system. It's just a matter of placing the parts in thefixture and allowing the control system to sequence the closing of the weld guns.Weld current is efficiently carried to the weld gun conductor cables through solidcopper buss runs attached directly to the transformer. A few operation concernsfollow.

Metal combinations in the weld path should be the same.1.

No more than one gun can fire at a time.2.

Water cooling of the buss may be required.3.

Constant current weld control mode is strongly suggested.4.

Another style of buss bar welding uses the buss bars to feed individual cablesservicing both arms for multiple guns in a single fixture. Again, the guns aresequenced in firing order.



Fig. 2-13b, Buss Bar Welding

POGO WELDINGNot too much different than buss bar welding, pogo welding has the robotmoving a single gun. A few big points here are:

Used where there is no access for a pinch type gun1.

Single phase DC weld power is recommended2.

Special purpose robots are required3.

PM of backups and tool cleanliness is very important4.

Fig. 2-13c, Pogo Welding



PEDESTAL/ROBOT WELDINGThis is just a matter of the robot moving the part through the tool. Concerns hereare:

Robot must be able to handle it's carrying fixture and the part beingwelded

1.

This is usually a respot operation2.

Fig. 2-13c, Pedestal/Robot Welding



Resistance Welding

Caps, Shanks

Successful execution of any resistance spot welding operation depends on theelectrode tips.

Water-cooled resistance welding electrode tips conduct the welding current,apply the pressure and dissipate the heat.

Fig. 2-14, A Typical Series Weld Setup Using Straight GunsWith Solid Backups and Backup Buttons

If the application of pressure did not have to be considered, electrode tipselection could be made entirely on the basis of electrical conductivity.

Because of the mechanical force required for metal contact and electricalconductivity during the welding operation, an electrode tip must withstandconsiderable stress at very high temperature.

Electrodes are manufactured in various sizes and shapes and are made ofdifferent alloys.

Automotive Welding Handbook, RESISTANCE WELDING, Caps, Shanks


The metal being welded and the design of the body section determine the sizeand shape of the electrode to be used.

Fig. 2-15, Electrode Tips, Caps and/or Shanks AreAvailable in Many Sizes and Shapes

CLASS II or CLASS III copper alloy electrodes are usually specified for use withmost spot welding operations. They have more strength and better wearresistance than pure soft copper.

Another factor affecting the selection of electrode tips is "sticking" or "pick-up."This is caused by too small a contact area or too high a current for the areaavailable. The result is a tendency for the metal being welded to stick to theelectrode tip.

This is especially true when welding galvanized metal with new electrode tips.

*R,W,M.A, ALLOYSCOPPER BASE ALLOYS

R.W.M.A. CLASS I (Zirconium Copper)Class I alloy is a good substitute for pure copper as an electrode material. It is



specifically recommended for welding aluminum.

R.W.M.A. CLASS II (Chromium - Zirconium Copper)Class II alloy is resistance welding electrode material specifically recommendedfor high production seam and spot welding of cold- and hot-rolled steel.

R.W.M.A. CLASS III (Beryllium Copper)Class III alloy is generally recommended for projection welding because it hashigher strength than Class II.

Any electrode made with Class III copper alloy is assumed to contain beryllium.All grinding or discing of Class III electrodes should be done in a special booth,in accordance with the recommendation issued by the General Motors IndustrialHygiene Department.

* Resistant Welding Manufacture's Association.

Class I weld caps, also labeled as G.M. Style MWZ, are recommended for robotand fixture welding.

DSC weld caps, labeled as G.M. Style MWP, offer reduced "sticking," goodstrength and wear resistance. However, any alloyed electrode tip isaccompanied by some sacrifice in electrical conductivity.

Weld schedules can sometimes be altered to weld "hotter" with DSC alloyedelectrode tips. That is, higher currents and shorter weld times than with Class II,or G.M. Style MW, can be used.

Most electrode "sticking" may be overcome with some form of pulsation welding.Refer to weld schedule section, pages 2-100 through 2-109.

Fig. 2-17



Water freed" and "return" in the electrode holder must be correctly connected sothat the water passes through the deflector tube first. See pages 2-22 and A1-4& 5 for proper deflector tube clearance.

Electrodes have identifying marks to distinguish their alloy group andmanufacturer.

Fig. 2-18, Typical Marks for Different Alloy Electrodes



Electrode tips come in various shapes. But generally only three different sizesare used in GM plants: size #1, #2 or #3.

The #1 size electrode cap and/or shank is 1/2 inch in diameter. If is used forspecial applications or after hem welding.

It should never be used with over 670 lbs. of force.

The #2 size electrode cap and/or shank is 5/8 inch in diameter. It is commonlyused for all welding up to 1200 lbs.

The #3 size electrode cap is 3/4 inch in diameter. The #3 size shank orone-piece electrode is 7/8 inch in diameter. It is used when there is more than1200 lbs. of welding force.

Fig. 2-19, Commonly Used Electrode Cap Styles

See appendix A1 for pictures and numbers of electrode caps.



Fig 2-20, Electrode Caps and Shanks

Electrode caps (sometimes referred to as "tips" or "points") eliminate the need toreplace the complete electrode. Available in different sizes and shapes, the capfits on the "cap end" of the shank. Weld gun force, part clearance and electrodeangulation are determining factors when selecting the size and shape of theelectrode cap.

Electrode shanks may be seated and retained in the arm or holder by a numberof methods. They include a tapered seat in the arm itself for direct inserting ofthe shank, or a locked copper insert, a soldered copper insert, a threadedcopper insert. See Appendix A for details.

Electrode caps and shanks may be inspected for taper and size by using theproper ring and/or plug gauge.



Fig. 2-21a

Fig. 2-21b

Proper cooling of the electrode tip is very important. The most common cause ofelectrode wear is high temperature resulting from improper water-cooling.



A 1/2 gallon per minute water flow rate is needed for each electrode tip. This isparticularly important on heavy metal and galvanized welding operations.

Equally important is the water deflector tube length. It should extend to within 1/8inch from the bottom of the reinforcement lip. The "inlet" water should come outof the tubing for maximum cooling efficiency.

When electrode shanks are replaced in electrode holders, they should bebrought together under welding force or tapped gently with a hammer to insurethat the electrode is seated properly to seal the water circuit and assure goodelectrical conductivity. Water leaks around the seat of the electrode maysometimes occur. These leaks should never be stopped with insulating material,such as "teflon" tape, etc.

Fig. 2-22, Proper Water-Cooling (See A1-4 for dimensional detail)

Worn electrode tips may be replaced with new tips or reconditioned on either alathe, a drill press or with a portable pneumatically operated electrode dresser.

Mushroomed or worn electrode tips are dressed by removing as little of theelectrode tip as necessary to restore it to its original shape.



Fig. 2-23

Lightweight and easy-to-operate electrode dressers require no special trainingfor the operator.

Pneumatically operated and usually with several speed options, most models ofdressers accept a variety of cutters designed to dress specific electrode styles.



Fig. 2-24

When applying a tip dresser, it should be positioned properly, operated only untilthe electrode cutter has bottomed and no further cutting is taking place.

Weld caps can be purchased with a variety of contact faces. They are used forspecific weld applications. But even if the tip contact face is correct for the task,there are situations which can result in improper welds.



Fig. 2-25a, Offset Centerlines

Shown in the above illustration are two properly dressed electrode tips withfaces square with their shanks and parallel to each other. But with theircenterlines offset, the result of their misalignment is a reduced effective contactarea and a small weld.

Fig. 2-25b, Welding Surfaces Are Not Parallel

Shown in the above illustration, two properly dressed tips have faces squarewith their shanks. But the welding surfaces are not parallel.

The result of this misalignment is indentation and a small weld. Misalignment oftip surface is not always dependent on the alignment of tips, because often thecenterlines of tips are at an angle with each other in order to get into a closeflange condition. Actual tip faces should be made parallel, regardless of theangle.



Fig. 2-26

Correct alignment of the electrocle tips is essential for good tip life and qualitywelds. Loss of alignment while operating may indicate that some part of thewelding gun or head is not tightly secured. When it becomes necessary toremove a worn electrode from the holder, caution should be taken so that theelectrode seat in the holder is not damaged.

Electrode tip shanks are held by the electrode holder in a tapered hole. Check tosee if the shank is held directly by the gun arm or by a tapered insert which isthreaded into the arm.

Before attempting to remove an electrode tip, shut off the feed and return watersupply valves or pinch off the water lines.



Fig. 2-27a

"Vise grips" may be used to remove the electrode tips directly from the arm byusing a twisting action. But if the arm has a threaded insert, remove the insertand shank, together. Then set the insert in a vice and tap the shank from theinside of the insert. Using threaded inserts eliminates shank-change damage tothe expensive gun arm. Shank insert details are in Appendix A1



Fig. 2-27b

The above "attempted" electrode tip shank removal method may damage theshank and the electrode holder.

A welding machine contains the electrical, mechanical and control systems. Itthen becomes the function of the electrode tips to conduct the welding currentunder the required mechanical pressure in order that a proper relationship bemaintained between HEAT -- PRESSURE and TIME.

It might be concluded that electrode tips do all the work. In a sense, this is true.The need for care in the selection of electrodes does not mean that allunsatisfactory welds indicate poorly maintained and/or selected electrodes.

Electrode tips cannot produce good welds unless the welding machine and itscontrol furnish the correct HEAT, PRESSURE and TIME.

Roll and seam spot welding are terms used for a series of resistance weldsmade between disc-shaped electrodes. In roll spot welding the spots are spacedapart. Roll seam welding defines the same process, but one which causes thespots to overlap. The electrode discs are usually made of Class 2 RWMA alloyand are between 1/4" and 3/4" thick. Disc diameters can be between 4" and 12".



The discs are removable to permit dressing. Cooling water flows only throughthe disc mounting shaft.

By adjusting the travel speed and the weld cycle time between welds, specificspot weld spacing can be done.

Fig. 2-28, Roll Spot and Seam Welding



Resistance Welding

Cables

Flexible connectors, called secondary cables, are needed to complete thesecondary circuit between the transformers and the moving electrode holders.These secondary cables are either air-cooled or water-cooled and insulated withan outer cover.

Fig. 2-29, A Typical Series Weld Setup Using Straight GunsWith Solid Backups and Backup Buttons

Automotive Welding Handbook, RESISTANCE WELDING, Cables


When the welding gun is actuated, the electrode holder and tip move with thewelding gun cylinder rod.

The selection of any particular type of welding cable will depend on its specificapplication.

From a current-carrying standpoint, the factors used to select the cable arecurrent and duty cycle, and the length of cable required.

Fig. 2-30, Air-Cooled Cable

Cable size and length are referred to separately. Size denotes thecross-sectional area of the copper rope, in circular mils. 1000 MCM isconsidered to be the "standard" size.

Air-cooled cables offer greater flexibility and faster installation (water hoses andfittings are not needed) than water-cooled cables. However, experience hasshown that if air-cooled cables are sized very close to, or below, actual weldcycle current loads, they can get pretty hot. When they get hot, even though theystill conduct, they'll swell and become shorter in overall length. This can preventproper gun closing pressure and allow an improper, flashed weld.



Fig. 2-31a, Water-Cooled Cables

Single-conductor water-cooled cables are usually used as jumpers when thethermal capability of standard 1000 MCM air cooled cables may be exceeded.350 MCM is the standard size of water-cooled jumpers. They will heat up rapidlyif the water supply is shut off, or is inadequate. Check the Cooling Water sectionof Chapter 6 for further information.

Fig. 2-31b, Kickless Cables

Low reactance (kickless) water-cooled cables are recommended for all portablegun installations. These two-conductor cables are constructed in such a mannerthat the tendency for the cables to "kick" when welding currents are passedthrough them is essentially eliminated. Also because of the low reactance, theyare often used for fixture welding where the gun is at a greater than normaldistance from the transformer.



Resistance Welding

Shunts

Fig. 2-32, Laminated Shunts

Laminated copper shunts are constructed of thin strips ofcopper. They are almost always used with integral transformerguns and allow for at least one arm of the weld gun to be "closecoupled" to the welding transformer. S.A.E. Grade 8 bolts arerecommended for attaching the shunt to its mounting points.

Automotive Welding Handbook, RESISTANCE WELDING, Shunts


Resistance Welding

Electrode Backup

To provide a counter force for the welding pressure and to complete theelectrical circuit, a water-cooled electrode backup button is needed for eachelectrode tip. The backup button retainer is also insulated from the fixture and,therefore, potential ground.

Fig. 2-33

Automotive Welding Handbook, RESISTANCE WELDING, Electrode Backup


Just as electrode tips wear out, BACKUP BUTTONS BECOME WORN ANDMUST BE REPLACED AS REQUIRED.

Automotive Welding Handbook, RESISTANCE WELDING, Electrode Backup


Resistance Welding

Electrode Holders

An electrode holder has two main functions. It transmits force and conductscurrent. It is the mechanical link between the cable and the electrode tip. It isalso the electrical junction between the tip and the secondary of the transformer.

Fig. 2-34, Typical Electrode Holders

Holders insulate the welding circuit from the weld gun cylinder. The holdersthemselves are water-cooled by the "in" and "out" routing of water to the tips.The holders are designed to have a large enough mass to carry the secondarycircuit.

Automotive Welding Handbook, RESISTANCE WELDING, Electrode Holders


Resistance Welding

Transformers

A welding transformer transforms the primary high voltage/ low current powersupply to low voltage/high current secondary weld service.

A simple transformer contains a primary winding, an iron core and a secondarywinding.

Current flowing through the primary winding creates a magnetic field. Thismagnetic field is coupled through the iron core to the secondary windings andinduces secondary voltage.

If there are twice as many turns in a primary coil as there are in the secondarycoil, the secondary voltage will be one-half the primary voltage and thesecondary current will be twice the primary current.

Automotive Welding Handbook, RESISTANCE WELDING, Transformers


Fig. 2-35, Cutaway View of a Transformer

A transformer's primary voltage relates to the secondary voltage as the numberof turns in the primary winding to the number of turns in the secondary winding.

This is referred to as the TURNS RATIO.



Fig. 2-36, Basic Transformer

The transformer in the illustration has 88 turns in the primary and 1 turn in thesecondary. The turns ratio is 88:1.

The product of voltage times current is the same for primary and secondary.

The transformer manufacturer attaches a nameplate to the transformer whichgives the manufacturer's name, the GM transformer number (TR), the GM serialnumber and the date the transformer was shipped.



Fig. 2-37

The following ratings are usually specified for any given transformer:

The PRIMARY VOLTAGE specified for the welding transformer is the normalvoltage for which the transformer has been wound. This specified primaryvoltage will be lower than input buss voltage due to voltage drop in the control,the wiring between the buss and transformer and the resistance voltage drop inthe transformer itself. The transformer's primary voltage is supplied through theweld control panel. Standard single-phase, resistance welding transformers arerated at 440V U.S. pdmary and 550V Canadian primary.

NOTE: There is a table in Appendix A1 which lists new model transformerdata. The table lists 480V/575V primary rating instead of the 440V/550V. Afew years ago, based on assembly plant power service and distributionimprovements, along with improved weld control capability, it was decidedto spec new models for the higher primary supply voltage. Older styletransformers are still assembled to meet the 440V/550V primary rating.

FREQUENCY of the alternating current line to which the transformer is to beconnected is specified in the rating. The transformer should not be operated atother than its rated frequency.

The DUTY CYCLE of a welding transformer is the ratio of the time that thewelding current flows within the sequence of a complete system cycle.

The duty cycle is important because it indicates the relative amount of timeallowed for cooling between operations. Off-time permits excess heat to beremoved trom the transformer by water and air cooling.



KVA RATING of welding transformers is a thermal rating based on a duty cycleof 50%. A lower duty cycle allows the transformer to operate at a higher KVAwithout overheating.

Because of the amount of heat developed while operating, all weldingtransformers are water-cooled. If there is insufficient water flow, the effectiveKVA rating will be reduced. Without proper water-cooling, a transformer willoverheat and transformer damage could take place.

SECONDARY VOLTAGE is voltage that is produced across the secondaryterminals with no load on the secondary and with rated voltage applied to theprimary. Secondary voltage can be varied by the tap switch. When a weldcannot be made because of insufficient secondary current, even with thetransformer set at its maximum secondary voltage, the first thought is to use atransformer with a higher secondary voltage and possibly a higher KVA capacity.

Other notations are also used for transformer identification.

A The letter "A" as part of the transformer number indicates a tap switchwhich can be remounted in five different positions on the transformer itself.

-- Without the letter "A," the tap switch or tap terminals are on the top (primaryconnection) of the transformer only.

X The "X" which is part of the transformer number and/or painted on thehousing indicates that it is fully "potted." See the following page.

Y Another transformer option is available just for robotic operations. Thetransformer is potted at the secondary end only. The potting covers aboutone third of the total transformer internal volume. Secondary end pottedtransformers are noted by the letter "Y" as part of the transformer number.They can be used on robotic operations where it is necessary for thetransformer secondary connection to be turned in the up position.Secondary end potting is much less expensive than fully potting atransformer.



Fig. 2-40, Portable Gun Transformers

A portable gun welding transformer has two secondary lugs where the kicklesscable is connected by using adapter plates.

The adapter plates eliminate some of the stress of the inductive "kick" betweenthe secondary lugs from being reflected back into the internal structure of thetransformer.

Not using adaptor plates may void the transformer warranty.

The "X" marking on a transformer denotes that the transformer is fully "potted".

A fully "potted" transformer is one that is protected from moisture andcontaminants (water, oil and weld flash) with an epoxy-type resin addedinternally to the complete transformer by its manufacturer.

A black "X" appears on transformers that have a primary rating of 440 volts.

A red "X" appears on transformers that have a primary rating of 550 volts.

Removing the cover from the portable/robot gun transformer's primary endreveals the primary wire and tap link connections. The primary wires areconnected to the L1 and L2 primary terminals. The voltage tap connections aremade by a bolted link arrangement. Be sure the primary service is turned off andlocked out before modifying link arrangements.



Fig. 2-41a, TR-48 thru TR-70

Below is another arrangement for secondary voltage tap setting. See appendixA1 for transformer data.



Fig. 2-41b TR-70AX, 70BX

The high voltage setting is the most common connection. It is used whereverhigher welding currents and required for heavy metal, coated metal, long cablesand/or large throat guns.

Fig. 2-42a, Double Secondary (D.S.T.)



Fixture Type Transformers

In certain welding transformers, there are two secondary coils of one turn eachconsisting of water-cooled copper castings. These transformers are referred toas double secondary transformers (D.S.T.s). The primary coil is divided intosections. Various numbers of turns can be selected by means of a tap switch.The tap switch is used for selection of higher or lower welding voltages.

The transformer's tap switch should never be moved while the transformer isenergized. Any movement of this "heat selector" during the weld cycle couldseverely damage the transformer, possibly injure the person doing it, and voidthe transformer warranty.

Fig. 2-42b, Single Secondary (S.S.T.)Fixture Type Transformer

Single secondary transformers (S.S.T.s) are approximately 3/4 the width of themore commonly used double secondary transformer. They have a set voltagewith no tap switch. The set voltage is approximately the same as tap 3 on theequivalent D.S.T.



Fig. 2-43, Welding System Grounding

This shows a robot installation with a remotely mounted AC portable gunwelding transformer.

The ground conductor in the transformer is continuous to the buss ground only.It does not continue through the weld cable to the gun.



Fig. 2-44

This shows a portable gun assembly AC welding transformer with its coverremoved.

When used with a robot or manual application the transformer's metal case isgrounded. The transformer has a double turned secondary connected in seriesand internally connected at the midpoint of the secondary to the case. Bygrounding the case, the transformer's secondary is grounded.

The actual, final connection to ground can be done in two different ways. Thefirst is connecting the 1/0 service cable ground conductor from the transformerground all of the way to the ground conductor provided in the weld distributionbuss.

Some of the older weld buss installations do not carry a ground conductor,however. For that case, the transformer service cable ground must be attachedwith a proper lug to an actual portion of the plant's steel building structure. Thesteel structure itself, where the ground connection is made, should be checkedto verify that it does provide a direct path to earth ground.

It should be noted also that only one end of the ground conductor should be.connected to earth ground. The other end terminates at the last point of contacton the equipment being grounded.



Fig. 2-45, Air-Operated Pinch Gun With a TypicalFab Type Single Secondary Transformer

When a fabricating type ungrounded transformer is used, an isolation contactoris required.

The isolation contactor is the mechanical switching device used to isolate theprimary circuit (both conductors are opened) from the welding transformer whenwelding is not in process. It can be part of the main weld control cabinet asshown above, or in its own enclosure mounted near the transformer at thefixture.



Fig. 2-46, Integral Trans-Guns

Welding transformers used with commercial integral trans-guns are sized andselected for specific individual robotic or hard automatic applications.

Integral gun transformers are generally smaller and lighter than conventionaltransformers. Resistive losses are reduced by the close coupling of thetransformers to the weld gun. With HF/DC welding, inductive losses are alsogreatly reduced.



Resistance Welding

Controls

Present-day resistance welder controls use microprocessor technology tocontrol the flow of electrical current from the welder service buss to the weldingtransformer.

Multiple spot weld, high-volume tooling requires specialized welder controls withmultiple welding SCRs and contactors. They control the simultaneous weldingthat is generally done in fabricating plants.

Single spot weld, lower-volume tools use welder controls with a single SCR.They are applied to sequential welding usually associated with assemblyplants.

Latest style weld control specifications provide advanced methods of weldcontrol programming, operating and monitoring. Included in the list of newrequirements are things like: weld cycle current data monitoring and recording,weld monitoring, and weld program upload/download capability through a weldcontrol network.

Automotive Welding Handbook, RESISTANCE WELDING, Controls


Fig. 2-48

The welder control model number, the manufacturer's name, the CPC/MDSspecification number and the latest revision number appear on the front of theenclosure door of the main cabinet.

A service manual containing wiring diagrams, replacement information,photographs, descriptive literature and information regarding installing,operating, troubleshooting and maintaining the control is provided by thecontrol's manufacturer.

WELD CONTROL MDS NUMBERS

MDS NUMBER TOOLING APPLICATION

MDS 366MDS 555

Assembly plant controls used for sequential welding with manual,robotic or hard automatic applications.



MDS 526 High frequency' DC control used for sequential welding withrobotic or hard automatic applications.

MDS 599 Previously titled RWC1. Weld control for sequential welding withmanual, robotic or hard automatic applications.

MDS 601 Weld stand control primarily for fabricating plant use where up to12 contactors are required.

MDS 601A Auxiliary cabinet for MDS 601 containing 3 contactors.MDS 614 Weld press control used with welding presses requiring up to 36

contactors.MDS 614S Auxiliary cabinet for MDS 614 containing 6 contactors.MDS 646 Designed to be used with MDS 601A and/or MDS 614S, this is a

firing and timing control only.

* An extended list appears in Appendix A1.

Assembly style weld controls are single-output systems. Each control has onlyone SCR pack. Generally, each control also requires its own data entry panel.Later-style systems may have DEPs capable of interfacing up to 32 controlpanels. Refer to the control manufacturer's specific manual for more detailedinformation.



Fig. 2-50

Manual welding application with a weld station having a single weld control.



Fig. 2-51, MDS 366

The assembly-style welder control is compatible with manual, robotic or hardautomatic applications.

It is built to operate on a 60-Hz welder buss supply voltage of 480 volts or 575volts.

Fig. 2-52, MDS 555



Fig. 2-53

Robotic welding application using a high frequency/direct current (HF/DC) integral trans-gunwith control.



Fig. 2-54, MDS 526

Fig. 2-55, High Frequency/DC Block Diagram



High frequency refers to the ability of the control to increase the frequency of the powersupplied by the plant's welder service buss.

The sketch shows how the control system increases the operating frequency to 1200 Hz. Thesystem rectifies the 3-phase, 60-Hz line frequency input and then feeds it into a bank ofswitching transistors which pulse-train generate and modulate the 1200 Hz AC voltage. It isthen directed to the welding transformer primary. The transformer secondary output passesthrough a diode bridge to provide the DC welding current.

Fig. 2-56, HF/DC Trans-Gun

Welding transformers used for HF/DC welding are smaller in size and lighter in weight thenconventional AC trans-gun welding transformers.

Because the welding current is DC, the HF/DC welding transformer is not affected bymagnetic fields or metal in the weld gun's throat.

To guard against moisture and contaminants the welding transformers are "potted."

It should be noted that the secondary voltage of an HF/DC transformer is less than standardAC transformers. This can be a problem on metals of poor fit or very dirty surfaces.

Also a point of concern is the proper attachment of secondary jumpers to the transformer.Anchor bolts must be of the proper length. If they are too long, and the jumper is not firmlyattached, arcing and damage to the transformer will occur.

The following pages highlight another style of weld control approved for use. It followsadvanced guidelines which began in the original RWC1 specification. The control is now



identified as MDS-599.

A few major points of description of the MDS-599 are:

can be applied to robotic, automatic or manual welding operationl

operates from 480V or 575V, I ph, 60 Hz supplyl

can be applied with either AVC or CC modesl

capable of at least 15 selectable weld schedulesl

each schedule has it's own stepper counterl

each function within a weld program has a max. of 99 cycles (wrt 60 Hz) availablewithin it's adjustment range

l

capable of single, dual or poly pulse weldsl

upslope and downslope weld current programming is availablel

fault-to-standby and fault-to-alert responses and signal reactions through the controlare available

l

functional ACR coolant failure monitoringl

NEMA Size 5 isolation contactor availablel

capable of interfacing to a welder program network or master program work stationl



Fig. 2-56B MDS-599(RWC-1)



Fig. 2-56C MDS-599 (RWC-1) With I/O Module -Control Module Swing - Mount Open



Fig. 2-56D

Fig. 2-57, Weld Stand Control

The MDS 601 has a basic system consisting of the main cabinet, the data entry panel and thepush-button panel.

To add up to 12 SCRs to the basic system, either the MDS 601A and/or the MDS 614Sauxiliary SCR cabinets may be added.

Fig. 2-58, MDS 601 Weld Stand Control



Fig. 2-59, MDS 601A Auxiliary SCR Cabinet (Three Pack)

Fig. 2-60, MDS 614S Auxiliary SCR Cabinet (Six Pack)

Fig. 2-61, Fab. Data Entry Panel (DEP) and Push-Button Panel

Located remotely from the main cabinet, the fab.-style control data entry panel can programup to 36 SCRs and display the control machine functions.

Refer to the control manufacturer's service manual for operating and programminginformation.



Fig. 2-62, Weld Press Control

The MDS 614 is a basic system consisting of the main cabinet, the data entry panel and theMDS 601A auxiliary SCR cabinet.

To add more SCRs to the basic system, the MDS 614S auxiliary SCR cabinet must be added.

Fig. 2-63, MDS 614 Weld Press Control



Fig. 2-64, Firing and Timing Control

The MDS 646 basic system consists of the main cabinet and the data entry panel.

The firing and timing control is a welder control only, with no SCRs or machine controlcapability. It is to be used with the MDS 601A and/or the MDS 614S.



Fig. 2-65, Multiple SCR Firing and Timing Control

The MDS 646 cabinet contains the main weld processor, the weld initiation and completionrelays and the user terminal strips.

In a resistance welding control, power from the primary service to the welder transformer mustbe handled very quickly and accurately. The latest method of control is done throughsilicon-controlled rectifiers. They are known as "SCRs." Shown below is a picture of an SCRpackage used in later-model, single-phase AC weld controls.



Fig. 2-66

Often referred to as "an SCR" or the "weld contactor," the package actually contains twoSCRs connected in an inverse-parallel arrangement.

Note that each SCR is signaled, through its own gate, to conduct during the correct half of theprimary current cycle. Signaling of the SCR gates is done by the weld control logic itself. Thisis shown in the following set of pictures.



Fig. 2-67

Prior to SCR applications, but still in use today, the ignitron tube handles the switching andcontrol of primary current to the welding transformer. Ignitron tubes, like SCRs, will conductcurrent in only one direction. Two tubes are connected in reverse order to provide voltage ofthe proper polarity to the specific tube. The tubes are then "triggered" to conduct primary



current at the proper portion of cycle.

Fig. 2-68, Ignitron Tube Connections

Ignitron tubes are constructed differently than SCRs. Each tube has: a. a cathode (pool ofmercury) b. an anode c. the ignitor (same as SCR gate) d. a sleeve (the double-walled jacketcarrying cooling water)

Although ignitron tubes provide the same function as SCRs, they consume about three timesthe physical space. They can be purchased at different ratings to handle different levels ofprimary current.



Fig. 2-69

Ignitron tubes have somewhat different failure characteristics than SCRs. When an SCR fails,it simply will not conduct current. An ignitron tube, however, can:

arc back or1.

have faulty ignition2.

Of the two, arc back is the most serious. In an arc back, the tube will carry currentcontinuously even though the ignitor is off. This can cause the weld gun to stick to the metalbeing welded and fail to open. The main line switch will have to be pulled, or primary fusesblow, to stop the welding current. Portions of the welding equipment may be seriouslydamaged. Causes of arc backs can be:

high momentary current overload1.

excessive duty cycles2.

inadequate cooling water3.

Faulty ignition is a result of damage to the ignitor rod itself. The result will be erratic welding:full heat, partial heat or no heat. Damage to the ignitor rods can come from:

failure of rectifiers in the ignitor circuit1.

"wetting" of the rod with mercury from the cathode pool2.

mechanical shock3.

excessive ignitor current4.

If the rectifiers in the ignitor circuit fail, reverse current will be allowed to flow through theignitor; that is, current will flow from the mercury-pool cathode into the ignitor rod instead of



from the ignitor rod into the mercury, which is the correct direction. This will cause "pitting" ofthe ignitor rod, which will result in a hard-starting tube. Rectifiers should be checked everytime a hard-starting tube is changed. An ohmmeter can be used to check rectifiers: theresistance in the "reverse" direction should be approximately 4 or 5 times the resistance in the"forward" direction; if it is much less than this, change the rectifier. If this is not done, the newtube may be ruined.

All ignitor rods eventually become "wetted" with age. The carbon ignitor rod attracts a film ofmercury to it and eventually it will fail to cause proper ignition. If the tube is correctly handled,installed and used, its life will be greatly influenced by the length of time it takes the ignitor tobecome "wetted." This may, however, take years to occur.

Mechanical shocks and "sloshing" of the mercury can chip the ignitor rod, which will causehard starting or complete lack of firing. Ignitors are fragile and should be protected from roughtreatment.

Ignitors are designed to handle the required amount of current for a limited time; allowing thecurrent to flow through the ignitor for too long a time will damage it. If the ignitron tubes arefired without the proper load being connected to them, ignitor currents will continue to flowthroughout the whole half cycle. NEVER FIRE IGNITRON TUBES IF THE SECONDARYWELDING CIRCUIT IS NOT CLOSED. This means that insulating material such as fiber, etc.,should not be put between the electrode tips to check gun operation. If it is desired to run theguns. without firing the tubes, pull the main line switch or remove the fuse in the ignitor circuit.This fuse is designed to prevent excessive ignitor currents and should not be tampered with.

Fig. 2-72



Resistance Welding

Densification Unit

Fig. 2-73, Densification Unit

The densification unit, often called "the dens pak," provides a properpackage-style mounting of the system service elements. It may include:

cooling water supply/return valves and manifolds1.

compressed air service valve, regulators and distribution solenoids2.

electrical control system junction box with receptables,3.

Automotive Welding Handbook, RESISTANCE WELDING, Densification Unit


switches and indicators4.

cooling water leak sensor5.

Although the dens pak function is the same for each application, certain stylesare available to provide specific functions. A table of reference numbers appearsin the appendix.

Usually used with the assembly plant style welder control, the densification unit(MDS 406 or MDS 558) is designed to mount on the side of a weldingtransformer.

It may also be mounted on its own bracket attached to a robot hip or arm.Position of dens pak components will obviously vary. The primary intent is tomount the dens pak close to the weld transformer and gun.

The densification unit may contain automatic water shutoff and/or multiplepressure air regulation as options.

AUTOMATIC WATER SHUTOFF OPTION (MDS 528)

The optional automatic water shutoff will stop the flow of water to the electrodeshould a cap come off. More importantly, it will signal the control and stop therobot from welding without a cap on the electrode shank. (WD 11111Breference)

For regular operation, water shutoff and a no-weld signal will be generatedwhenever water flow is less than .75 gpm. MDS 528 reaction to a weld capremoval during welding is generated within one second.

MULTIPLE PRESSURE OPTION

The multiple pressure option is an air pressure regulation method that providesfor three separate and adjustable welding pressures with full line pressure forreturn. (WD 20410 reference)



Fig. 2-75



Resistance Welding

AVC and C-Reg

Later-style resistance weld controls provide two different methods of handlingweld application. One method is known as "automatic voltage compensation"(AVC) or constant voltage weld control. The second method is called "constantcurrent control" or C-REG. Both weld output control approaches have specificapplications for which they are required or recommended.

AVCThis particular weld control function delivers a pre-programmed voltage level tothe weld transformer. Within the first few cycles of weld time, the controlmonitors the primary voltage input. Based on the reading, the SCR firing angle ismodified by the control to provide a steady primary voltage to the weldtransformer. By maintaining a steady primary voltage, it is assumed that asteady current, required for the specific weld, is delivered by the transformer.Most resistance weld controls are designed to use their AVC capability for everyweld unless programmed differently.

AVC weld control is recommended for general resistance weld applications,such as:

multiple guns and/or transformers firing simultaneously from the samecontrol

1.

situations where poor part fit-up or weld surface contamination is expected2.

The AVC control is capable of adjusting for service line voltage variation in therange of +10% to -20%.

C-REG. (or cc)This weld control approach is also done by adjusting the SCR firing angle. Butthe approach here uses a preprogrammed secondary circuit requirement tocontrol the SCR output to the transformer primary. The controller cancompensate, within reasonable limits, for variations in both secondaryimpedance and service line voltage fluctuations.

Some steady current controls require that a reference point be developed by aninitial firing of the weld gun for making a proper weld. The control then uses that

Automotive Welding Handbook, RESISTANCE WELDING, AVC and C-Reg


current reference as a base line for required compensation. Programmablelimits, typically +15%, define the amount of regulation which will be applied. Thebase value must be taught for each welding condition. It must be updated eachtime a secondary circuit component is modified.

Another style of C-REG. control is "self-taught" each time a weld is initiated.They do not have programmable limits on the amount of regulation which isapplied. The only limiting factor is the current capability of the transformer beingcontrolled. General system fault diagnostics can be more difficult when the levelof regulation cannot be programmed.

Some special applications for constant current control are:

integral transformer guns where transformer secondary voltage is lowerand contact resistance variation must be handled

1.

buss bar-style welding from one transformer during sequential multi-pointprocesses

2.

It should be noted that any new-style weld controls purchased under thespecifications of MDS 366,526,555 and 599 have both AVC and C-REG.capability.

HIGHLIGHTJust about the time this handbook was being published, another type ofresistance weld power application was being tested. Its name is "lowfrequency/DC." The basic reason for trying another AC-to-DC weld approachwas to reduce inductive reactance problems and losses, but retain the on-linestyle of weld control present'ly used for standard AC welding. Inductivereactance losses can be a problem when cable length and weld gun arm sizeshave to be very large in order to contact and weld a specific part. Being able toapply a DC-style welding procedure with a present AC-style, single-phasecontrol would provide another selection available for making proper resistancewelds.

The system being tested at a couple of assembly plants uses a standardEQ-5100 weld control to feed a new LF/DC transformer. The line diagramswhich follow indicate the basic differences between the systems. The LF/DCtransformer uses the same style diodes as the present HF/DC transformer.



Fig. 2-79A, Single-Phase Transformer

Fig. 2-79b, Single-Phase DC Power Supply



Fig. 2-80, Three-Phase DC Power Supply



Resistance Welding

Weld Guns

Fig. 2-81, Weld Gun Assembly

The weld gun is a cylinder (air or hydraulic) which is connected to, and alwaysinsulated from, an electrode holder.

The selection of a weld gun is influenced by the electrode force required.Electrode force is determined by the governing metal thickness (GMT) of theparts being welded.

Air-operated weld guns are designed to develop their respective electrode force(500-2000 lbs.) using 85 psi as a normal shop-air operating pressure.

Air and hydraulic guns are essentially the same, except for the cylinders andtheir pressure sources. Hydraulic guns receive their operating force from a

Automotive Welding Handbook, RESISTANCE WELDING, Weld Guns


hydraulic oil pressure supply unit. Hydraulic guns are usually used in presselectrode tooling for metal fab applications.

To aid in positioning a manual or robot weld gun, rails, cross rails and springbalancers are used.

Gun hangers with special adapters are selected to help in positioning theelectrode tips to the weld metal.

Electrode tip orientation to the parts being welded depends on the properadjustment of the gun hangers and balancers along with proper handling by theoperator as the welds are being made.

Ideally, the line of force should be as close to perpendicular to the weld metal aspossible (reference, Page 2-5).

Grounding of portable gun transformers is discussed on Page 2-44.

Weld gun force is based on 85 psi air pressure. Guns that follow specificstandards are numbered to reflect their characteristics. PED 948 outlines theweld gun numbering system, covering portable and automatic weld guns.

"Specific" abbreviations that G.M. uses to identify standard guns, electrodes,electrode arms and holders are listed below. A much longer list appears in theappendix.

AR -Electrode Arm Assembly Y - Portable Weld Gun CBL - Welding Cable (Jumper) YA - Portable Weld Gun (Air) CLA - Air Cylinder (Weld Gun) SA - Robotic Six Axis Gun CLH - Hydraulic Cylinder (Weld Gun) HA - Hanger Assembly CLD - Dual-Piston Cylinder (Weld Gun) HE - Electrode Holder Assembly

*CLP - Pre-Lube Cylinder (Weld Gun) MGA - Nonportable Weld Gun (Mounted Air) MGH - Nonportable Weld gun (Mounted Hydraulic

Documents

Welding Handbook