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The potential dangers involved with operating punch pressesor press brakes are quite obvious. But most people do notrealize that an unprotected or improperly installed spot

welding machine can potentially cause serious injury to the op-erator. According to the Bureau of Labor Statistics, 3,974,700accidents involving industrial welding were reported in 2005. Ofthese, 2,148,800 resulted in lost days of work. Spot welding safetyshould be as important to the manufacturer as are safety con-cerns for presses and other industrial machinery.

Metal Expulsion Injury

It is not unusual to see a shower of sparks (expulsion) comingfrom the electrodes on a spot welding machine — Fig. 1. Many

companies assume that this is a normal condition. Some believethat the expulsion is actually hot oil or grease. The truth is thatthese sparks are droplets of molten metal coming from under theelectrode or from between the parts being welded.

Because metal expulsion can cause permanent eye damage, itis imperative that operators wear approved safety glasses that haveside shields — Fig. 2. It is equally important to understand that aproperly set spot welding machine should not create any major ex-pulsion under any circumstances. This surprises many long-timeusers and gets to the heart of the resistance welding process.

Cause of Expulsion

When a spot weld is being made, the metal heats up to moltentemperatures. At the same time, the metal molecules are all po-

Making ResistanceSpot Welding SaferMaking ResistanceSpot Welding Safer

ROGER B. HIRSCH (roger@unitrol-electronics.com) is president of Unitrol Electronics Inc., Northbrook, Ill., and first vice chair of the Resist-ance Welding Manufacturing Alliance (RWMA), an AWS standing subcommittee.

Areas of potential

injury on spot

welding machines

are detailed, and

practical solutions

are presented

Fig. 1 — Metal expulsion typically caused by low electrode force.

BY ROGER B. HIRSCH

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larized in the same way at any one instantof time. This causes an extremely strongmagnetic repulsion between these moltendroplets to literally launch them awayfrom the part. You see this as expulsion.In extreme cases, metal expulsion cantravel 10 ft or more to potentially injureother personnel in the area. It can also setfire to gloves, clothing, and other flamma-ble material in the area.

Elimination of Expulsion

The solution is to use a proper weld-ing schedule for the material beingwelded. Selection of the proper electrodeforce will create a mechanical barrieraround the molten metal to keep the ma-terial within the nugget area and elimi-nate expulsion. It will also reduce the elec-trical resistance between the face of theelectrode where it touches the outer sheetto lower surface heating under the elec-trode and keep it from reaching themolten state.

However, if the electrode force is toohigh, the amount of heat created in thenugget zone will be reduced to compromisethe weld strength or, in extreme cases, to-tally eliminate fusion. The RWMA Resist-ance Welding Manual, Edition 4 (availableat www.rwma.org), includes a collection ofwelding schedules for most metal alloys toprovide high-strength welds while eliminat-ing major metal expulsion1.

Welding at Metal Edge

Placing a spot weld too close to theedge of a part can also cause metal expul-sion. In this case, even use of proper elec-trode force will not keep the molten metalfrom flying out of the metal edge. TheRWMA handbook recommends mini-mum edge distances to minimize expul-sion. However, where the part designforces a weld location too close to an edge,use of pulsation in the welding schedulecan be applied to minimize metal expul-sion while maintaining weld strength. Formost metals being joined, divide the weldtime shown on the welding chart by three(round up), and install three impulses ofthis time with two cycles of cool time be-tween impulses. This will minimize expul-sion and form a nugget with acceptablepenetration.

Pinch Point Injury

OSHA 1910.255(b)(4) states, “Guard-ing. All press welding machine operations,

1. An extensive article on electrode forceand expulsion was published in the Welding Journal and is now accessible at www.unitrol-electronics.com/pdf/awspape2color. pdf.

Fig. 3 — Pinch point injury at electrodes can be severe.

Fig. 2 — Wearing of approved safety glasses with side shields is mandatory for eye protection.

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where there is a possibility of the opera-tor’s fingers being under the point of op-eration, shall be effectively guarded by theuse of a device such as an electronic eyesafety circuit, two hand controls, or pro-tection similar to that prescribed forpunch press operation….” Unfortunately,most spot welding machines are operated

without safeguards to protect the opera-tor from serious finger injury — Fig. 3.

Magnitude of Force

It is important to appreciate the mag-nitude of force between electrodes. Forexample, a welding machine with a ¼-in.

electrode face set with 600 lb of electrodeforce will develop 12,230 lb/in2. This cancause major crushing damage to the op-erator’s finger if caught at this pinch point.

Two-Hand Antitiedown Initiation

Where the part and fixture design al-lows for welding without the operatorholding the part, use of two-hand anti-tiedown initiation will keep the operator’sfingers out of the electrode pinch pointzone during welding — Fig. 4. The initia-tion buttons should be located at least 18in. apart and far enough away from theelectrodes so that no part of the hand canbe in the weld zone while pushing the ini-tiation buttons. Operation of the weldingmachine should only happen if bothswitches are pushed within less than onesecond of time. If the switch on either sideis permanently closed, closing of the otherswitch will not cause the welding machineto operate (antitiedown function). If thespace between the electrodes exceeds ¼in., the control should be set so that re-leasing either of the initiation switches be-fore the electrodes close will cause theelectrodes to immediately retract.

Light Curtains

Light curtains are rarely practical withhand-fed welding operations since the op-erator’s hands will normally be in thewelding electrode zone. Also, light cur-tains cannot usually handle parts that haveflanges that are in the same area as theoperator’s hands. However, light curtainscan be used effectively for automatic andsemiautomatic welding systems when in-stalled so that no mechanical movementcan take place if the light curtain beam isbroken — Fig. 5. In this case, once thewelding sequence has started, breaking ofthe light curtain beam should automati-cally cause any moving components to re-tract to a safe position, and then requirereinitiating of the welding machine afterthe beam has been cleared.

Continuity Monotoring

This type of system measures electricalcontinuity between the electrodes to verifythey are actually touching the part to bewelded — Fig. 6A. If anything, such as theoperator’s finger, blocks the movement ofthe electrode, this system will not detectcontinuity. The continuity signal is pickedup from points on the welding transformersecondary pads. This eliminates the needfor wire connections at the electrodes — 6B.

In operation, the electrodes are closedunder low force that is accomplished by amodified pneumatic valve system. A groupof precision pressure regulators in a pad-

Fig. 4 — Two-hand antitiedown switch welding machine initiation.

Fig. 5 — Light curtain guarding on all sides protects the operator on this hand-loaded semi-automatic dial table welding machine.

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locked box provides pressure needed tocounterbalance heavy welding rams to lessthan 50 lb of force at the electrodes. Thesame components, arranged differently,can be used to furnish low pressure for op-erating welding machines with lightweightrams, or for operation of rocker arm ortransgun welding machines — 6C. If conti-nuity is detected before a maximum timehas been reached, the electrodes arebrought to full welding force. However, ifcontinuity is not detected before that time,the electrodes are retracted and will notcome forward until the initiation switch hasbeen released and closed again.

This type of safety system does nothave any operator-set components anddoes not require any adjustment whennew setups are made. If designed prop-erly, the continuity system cannot be over-ridden and is in place at all times that thewelding machine is under power. Becausethis is a fully passive process, it can be usedas the primary pinch point safety systemfor a spot welding machine.

Ring Guards

These devices typically utilize a rodwith a horizontal ring installed at one endthat encircles the moving electrode. Alimit switch in this system is adjusted everytime a new setup is made on the weldingmachine so that it closes if the rod travelsto a position less than ¼ in. from the partbeing welded. When the welding machineis initiated, the rod lowers before the elec-trode starts movement. If the limit switchcloses within a timer setting, the rod re-tracts while allowing the electrode totravel forward for the weld.

This type of system is not usable wherethe parts being welded are not flat in thearea of the weld. More importantly, thefact that the level of safety is determinedby how the limit switch is adjusted duringeach setup makes this process nonpassiveand, therefore, ring guards should not beused as the primary safety system for aspot welding machine.

Ram Limit Switches

With this system, a limit switch is in-stalled on the frame, and a cam moves withthe ram. The limit switch cam is adjustedto close when the electrode is at a posi-tion less than ¼ in. from the metal sur-face. The welding machine’s pneumaticsystem is designed to lower the electrodesunder less than welding force until thelimit switch has been closed, and retractthe electrodes if this switch is not closedwithin a timer setting. Like the ringguards, this type of guarding requires ad-justment every time a new setup is madeand is, therefore, not passive. All safetyof the welding machine is controlled by

the actions of the last person who adjustedthe limit switch cam. Therefore, this typeof system cannot be considered as the pri-mary guarding method.

Barrier Guards

On automatic or semiautomatic weld-ing machines, systems can be installed thatwill automatically close a mechanical bar-rier at the start of each sequence. Themoving barrier contains an interlockinglimit switch to prevent electrode move-

ment if the barrier is not fully closed. Themechanical barrier should be designed topreclude any ability for the operator toreach into the electrode area when it is inplace. Closing force for the barrier shouldbe selected to prevent injury to the oper-ator from the moving barrier.

Interlocked Cage Doors

Systems, such as robotic welding cells,should have a cage installed to preventmovement within the cell when an entry

Fig. 6 — A — This system from Unitrol Electronics provides passive electrode pinch-pointprotection at Ada Metal Products, Inc., Lincolnwood, Ill. B — continuity measuring wiresconnected to welding transformer secondary pads; C — padlocked NEMA-4 enclosure withprecision regulators supply pressure to new solenoid and quick exhaust valves. This arrange-ment counterbalances an 160-lb ram to less than 40 lb.

A

B C

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door is open. If an entry door is openedduring a welding sequence, all movementmust stop. Once the door is again closed,movement can only be started when aninitiation switch on the outside of the cageis activated. Under no circumstancesshould an initiation button station be in-stalled on the interior of the cage.

Laser MeasurementSystems

In this type of guard, a laser beam isaimed alongside the electrode at the partbeing welded. When the welding machineis initiated, the laser measurement systemchecks to verify that the distance beingmeasured is no less than ¼ in. from thetop of the metal being welded before al-lowing any movement of the electrode.Like the ring guards, this type of guard-ing requires adjustment every time a newsetup is made. Laser beam measurementsystems are not passive and thereforeshould not be used as the primary safetysystem.

Electrical Shock Injury

By design, a resistance welding ma-chine uses low voltage at the electrodesto produce an intrinsically safe process.However, an incorrectly grounded weld-ing machine can pose a serious shock dan-ger to the operator. A potentially fatalshock can occur if the primary winding ofthe welding transformer shorts to thetransformer frame, or if a line voltage con-nection comes loose and shorts to thebody. If a proper ground is in place on thewelding machine, the line voltage will havea good path to ground and clear the con-trol’s circuit breaker, line fuses, or mainpower circuit breaker.

Incoming Ground Wire

Proper grounding of all types of weld-ing machines depends on the quality ofthe incoming ground wire to the machine.

Grounding a welding machine using theconduit or BX cable is not an acceptablemethod. The separate ground wire shouldenter the welding control enclosure andbe connected to a properly bonded groundstud — Fig. 7. Wire size should be in linewith NEC250.122 standards.

Transformer SecondaryGrounding

OSHA Standard 1910.255(b)(9) coversgrounding as follows: “Where technicallypractical, the secondary of all welding trans-formers used in multispot, projection andseam welding machines shall be grounded.This may be done by permanently ground-ing one side of the welding secondary cur-rent circuit. Where not technically practi-cal, a center tapped grounding reactor con-nected across the secondary….”

Press and Rocker Arm Welding Machines

Floor- or bench-mounted welding ma-chines are normally manufactured withthe stationary arm bolted to the trans-former secondary pad and also connectedto the frame. The frame is grounded tocomplete the path from secondary toground. The movable arm of the weldingmachine is not grounded.

Portable Gun Welding Machines

Welding machines that use a perma-nently mounted transformer that connectsto a portable welding gun by a long kick-less cable require a ground strap to con-nect either side of the transformer sec-ondary to the transformer frame. With thetransformer frame properly connected toa ground wire, the path from the portablewelding gun to ground is fully established.

Portable Transguns

This type of welding machine includesin one package a welding transformer andarms typically suspended by a counterbal-anced mount and positioned by an oper-ator. A flexible cable enters into the trans-former carrying line voltage from the con-trol. Because this portable device cannotbe hard-connected to a suitable ground,the ground wire comes into the weldingmachine in the same flexible cable thatcontains the line voltage wires.

While many older transguns were sup-plied simply with a double-pole contactorbetween the control and the transgun, thisform of protection is not adequate for per-sonnel protection and can expose the op-erator to potentially fatal electrical shockeven when the welding machine is underpower but not being operated.

The RWMA bulletin 5, section 5-015.68.04, covers the special requirementsfor portable transgun grounding. Becausethe transgun is not hard-grounded, safetyof the operator depends on the presenceof a reliable ground path. However, sincethe ground path comes from a long flexi-ble wire, there are additional require-ments for this type of welding machinethat monitor ground wire integrity andground fault leakage current.

Grounding Integrity

The RWMA standard requires that“the welding gun transformer case andsecondary shell be grounded and pro-tected by fail safe circuitry designed toimmediately disconnect line voltage fromthe transgun via a circuit breaker withunder-voltage trip. The combined clear-ing time shall not exceed 60 ms. A sensedvalue of grounding conductor resistancein excess of 1Ω by the ground integrity

Fig. 7 — A separate, properly sized incom-ing ground wire is required to protect fromelectrical shock.

Fig. 8 — On a dual secondary welding transformer, one grounding reactor is required for eachsecondary. The center tap from both devices is connected to the building ground.

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monitor would be considered an inade-quate ground.”

Ground Fault Current Relay

If the transformer primary shorts tothe secondary, line voltage will be presenton the case of the transgun. With the op-erator maneuvering the transgun whileholding parts of the welding machine case,this voltage can find a path through theoperator and to ground. This leakagethrough the operator reduces current re-turning to the control and can be meas-ured by a ground fault relay. The RWMAstandard continues: “A sensitive, fail safe,ground fault relay with a maximum trippoint of 15 mA must be used to provideprotection against ground fault leakagecurrents. The ground fault relay must im-mediately disconnect line voltage from thePortable Transgun via a circuit breakerwith shunt trip or a circuit breaker withundervoltage trip. The combined clearingtime shall not exceed 60 ms.”

If purchasing a new transgun for hand-held application, verify that that the weld-ing control contains systems to match theabove requirements. If operating an ex-isting hand-held transgun, check to besure that the welding control includes thisfull protection.

Robotic Transguns

Since the frame-grounding path to arobotically moved transgun is throughhinge points, grounding must be doneusing a separate ground wire similar tothat for the portable transgun above. Be-cause it is possible for an operator to touchthe frame of a transgun, the same type ofgrounding integrity and ground fault cur-rent relay applied to portable transgunsshould be used for robotic transguns.

Fixture Transformers

Welding machines that utilize multipletransformers on the same structure re-quire special grounding. Connection ofone side of the welding machine’s second-ary on multiple transformers to the com-mon ground causes ground paths throughthe metal being welded and through thefixture. These ground paths can easilyweld or tack the metal parts to the fixture.There can also be electrical paths backthrough the metal being welded to othergrounded electrodes to cause arcingthroughout the system. For this applica-tion, a grounding reactor must be installedon the welding transformer secondary inplace of a hard-wire ground.

This grounding reactor, containing aninternal saturable coil, is connected acrossthe weld transformer secondary with acenter tap on the saturable coil connected

to ground (Fig. 8). The coil has high im-pedance when voltage applied across it isless than the device’s rating (24–34 V).However, if the transformer secondary be-comes connected to line voltage, the coilsaturates and becomes a very low-imped-ance device. This drains the high voltagefrom the transformer secondary throughthe ground wire to produce high currentthat should trip the control’s circuitbreaker or blow the line fuses. If the trans-former has two secondary circuits, onegrounding reactor must be installed oneach circuit.

Permanently Mounted Transguns

These welding machines are struc-turally like the portable transguns dis-cussed earlier, but are permanentlymounted to a properly grounded steelpedestal. Either side of the machine’stransformer should be connected to thetransformer frame to complete the groundpath. Where multiple transguns are usedin a system, or where arcing to the fixtureis a potential problem, a grounding reac-tor, as presented above, should be used inplace of a hard-wire ground connection.

Conclusion

Resistance welding can be one of thesafest processes in the factory whenproper personnel protection, correctequipment design, and appropriate elec-trical grounding are utilized. ◆

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