By Roger B. Hirsch, President of Unitrol Electronics Inc. of Northbrook, Illinois, and former chair of the Resistance Welding Manufacturing Alliance (RWMA), a standing subcommittee of the AWS.

Areas of potential personnel injury are examined and practical solutions are presented

Introduction

The potential dangers involved with operating punch presses or press brakes are quite obvious. But most people do not realize that an unprotected or improperly installed spot welder can potentially cause serious injury to the operator.  According to the Bureau of Labor Statistics, 3,974,700 accidents involving industrial welding were reported in 2005. Of these, 2,148,800 resulted in lost days of work.

This article will cover three aspects of injury exposure:

Metal expulsion injury

Pinch point injury

Electrical shock injury.

Metal Expulsion Injury

It is not unusual to see a shower of sparks (expulsion) coming from the electrodes on a spot welder (Fig.1). Many companies assume that this is a normal condition.  Some believe that the expulsion is actually hot oil or grease. The truth is that these sparks are actually droplets of molten metal coming from under the electrode or from between the parts being welded.

Because metal expulsion can cause permanent eye damage, it is imperative that operators wear approved safety glasses that have side shields (Fig 2). It is equally important to understand that a properly set spotwelder should not create any major expulsion under any circumstances. This surprises many long-time users 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 molten temperatures. At the same time, the metal molecules are all polarized in the same way at any one instance of time. This causes an extremely strong magnetic repulsion to literally launch metal droplets away from the part.  In extreme cases, metal expulsion can travel 10 feet or more to potentially injure other personnel in the area.

Elimination Of Expulsion: The solution is to use a proper welding schedule for the material being welded. Selection of the proper electrode force will create a mechanical barrier around the molten metal to keep the material within the nugget area and eliminate expulsion. It will also reduce the electrical resistance between the face of the electrode where it touches the outer sheet to lower surface heating under the electrode and keep it from reaching the molten state.

However if the electrode force is too high, the amount of heat created in the nugget zone will be reduced to compromise the weld strength or, in extreme cases, totally eliminate fusion. The RWMA Resistance Welding Manual Edition 4 (available at rwma.org) includes a collection of welding schedules for most metal alloys to provide high strength welds while eliminating major metal expulsion.

Welding At Metal Edge: Placing a spot weld too close to the edge of a part can also cause metal expulsion. In this case, even use of proper electrode force will not keep the molten metal from flying out of the metal edge (Fig.3). The RWMA handbook recommends minimum edge distances to minimize expulsion. However where the part design forces a weld location too close to an edge, use of pulsation in the welding schedule can be applied to minimize metal expulsion while maintaining weld strength.  For most metals being joined, divide the weld time shown on the welding chart by 3 (round up), and install three impulses of this time with two cycles of cool time between impulses. This will minimize expulsion and form a nugget with acceptable penetration.

An extensive article on electrode force and expulsion that was published in an earlier Welding Journal is available at www.unitrol-electronics.com/pdf/awspape2color.pdf.

Pinch Point Injury

OSHA 1910.255(b)(4) states: “Guarding. All press welding machine operations, where there is a possibility of the operator’s fingers being under the point of operation, shall be effectively guarded by the use of a device such as an electronic eye safety circuit, two hand controls or protection similar to that prescribed for punch press operation…”. Unfortunately most spot welding machines are operated without any operator protection (Fig 4).

Magnitude Of Force: It is important to understand the magnitude of force between electrodes.  For example, a welder with a ¼” electrode face set with 600 pounds of electrode force will develop12,230 lb/in2 .  This will cause major crushing damage to the operator’s finger if caught at this pinch point.

Two Hand Anti-Tiedown Initiation: Where the part and fixture design allows for welding without the operator holding the part, use of two-hand anti-tiedown initiation will keep the operator’s fingers out of the electrode pinch point zone during welding (Fig. 5). The initiation buttons should be located at least 18” apart and far enough away from the electrodes so that no part of the hand can be in the weld zone while pushing the initiation buttons.  Operation of the welder should only happen if both switches are pushed within less than 1 second of time. If the switch on either side is permanently closed, closing of the other switch will not cause the welder to operate (anti-tiedown function). If the space between the electrodes exceeds ¼”, the control should be set so that releasing either of the initiation switches before the electrodes close will cause the electrodes to immediately retract.

Light Curtains: Light curtains are rarely practical with hand fed welding operations since in reality the operator’s hands will normally be in the welding electrode zone (Fig. 6).  Also, light curtains cannot usually handle parts that have flanges which are in the same area as the operator’s hands. However, light curtains can be used effectively for automatic and semi-automatic welding systems if installed so that no mechanical movement can take place if the light curtain beam is broken. In this case, once the welding sequence has started, breaking of the light curtain beam should automatically cause any moving components to retract to a safe position, and then require re-initiating of the welder after the beam has been cleared.