Troubleshooting the Wire Feed System

Accurately troubleshooting problems with the delivery of the welding wire to the weld pool and the current to the wire when they arise or, better yet, avoiding them before they arise, is crucial to maximizing the benefits that these processes offer.

MIG (GMAW) and flux-cored (FCAW) welding, commonly referred to as “wire welding,” offer the potential for significant gains in productivity compared to stick welding. However, the wire feed system uses a more complicated mechanical system than the others to deliver the welding wire to the weld pool and the current to the wire, resulting in more potential problems in the functioning of the welding equipment.

Accurately troubleshooting these problems when they arise or, better yet, avoiding them before they arise, is crucial to maximizing the benefits that these processes offer.

For troubleshooting purposes, the wire welding system can be divided into three distinct categories based on function – wire delivery, gas supply and electricity transmission. Failure within any of these systems will result in sub-optimal welding performance, including reduced productivity and increased downtime for reworking bad welds.

WIRE DELIVERY
Regardless if you are using one-pound spools, large drums, or larger coils of wire, the mechanical feedability of the wire plays an important role in determining arc quality and weldability. You should begin troubleshooting the system by ensuring the wire is not obstructed anywhere along its path from the spool tension to the contact tip and everywhere in-between.

Hub Tension
It is important not to over-tighten the hub tension, which allows the spool of wire to turn. On a large drum of wire, this function may be served by a mechanism that traces around the spool. The hub tension is simply a means to keep the wire from de-coiling off of the spool when wire feeding stops. This should be just tight enough to keep the wire from de-coiling when you stop feeding a full spool at maximum wire feed speed. Over-tightening this will force the drive motor to work harder just to get the wire off of the spool and will lead to welding problems.

Check Drive Roll Pressure
Drive roll pressure is a very common problem in wire welding. Too loose and you have no wire feeding into the puddle. Too tight and you can crush the wire and flake off the coating, deform the wire, wear out the rolls, and damage the motor.

Flaked coating will cause these small flakes to enter the liner, further limiting the simple feedability of the wire to the puddle. Deformed wire will wear grooves into the contact tip limiting electrical conductivity and also causing poor feedability. Wear out the grooved surfaces of the rolls and you have poor friction to feed the wire properly. Over-tightened drive roll tension causes all of these problems in addition to placing excessive pressure on the drive shaft that could wear out a gear box or drive motor by misaligning it.

There is no cut-and-dry answer as to the exact pressure needed to ensure proper drive roll pressure. Drive roll tension should be adjusted so that it is not too tight, but not too loose. Start with the drive roll pressure very loose. Increase the pressure only until it is very difficult to stop the wire from feeding out of the contact tip. Use a pliers or block of wood to try and stop the wire from feeding. Go perhaps one-half of a turn beyond this point. When the wire is actually stopped, the drive rolls should spin on the wire and no bird nesting should occur.

Check Drive Roll Alignment
The drive rolls can be adjusted side-to-side to ensure they are in line with the inlet guide to the GMAW gun.

Check Inlet Guides
The inlet guides should be of the proper size for the wire used. They should not have grooves in them – often caused by misalignment or improper size.

Check Liner Condition
The gun liner should be the proper size for the wire being used and should also be clean and free of dust and debris. Over-tensioned wire will flake off and place excessive particles into the liner, clogging it up. Using special wire lubricants can also cause the wire to become ‘wet’ and dust can collect on the wire dragging it into the liner as well.

Wire manufacturers have already properly prepared the surface of the wire for maximum feedability and adding or even subtracting that can affect the weld quality. If you insist on using something to “lubricate” or “wipe off” the wire before it goes into the system, a cotton cloth with a clothes pin would be best so that there is no contamination of the wire and dust cannot collect on a ‘wet” surface. Liners are wear items and should be replaced on a regular schedule.

Contact Tip Condition
Many times feeding problems can be fixed by replacing the contact tip. The contact tip can get clogged up from spatter or from touching it to the weld puddle. If the wire is wearing grooves into the contact tip you need to check your drive roll tension.

GAS SUPPLY
In MIG and gas-shielded flux-cored arc welding, a number of problems can occur that interfere with the delivery of shielding gas to the weld pool, leading to porosity, excess spatter, an unstable arc and other defects. The smallest pinhole in a gas hose can act like a carburetor and draw in air, contaminating the weld. Here are several steps that should be taken to troubleshoot suspected shielding gas problems:

Check the Regulator/Flow Meter
The glass tube-and-ball type of flow gauge can be used as an indicator of gas leaks. If the ball does not drop to the bottom of the gauge when not welding, it is an indicator that gas is still flowing, indicating a leak. If a dial-type regulator/flow meter is used, a leak can be detected by applying a soap and water solution to all hoses and connections. Escaping gas will cause bubbles to form in the soap and water solution at the point of the leak.

Remember, gas connections and hoses downstream from the gas valve have to be checked with the gas flowing. Use the purge function during this process. Also, turning off the cylinder and watching the high pressure side slowly fall will also indicate that there is a leak in the system.

Check Gas Flow
More is not necessarily better here. Gas flow rates will typically be between 30 and 50 CFH (cubic feet per hour). Flow rates lower than this can provide inadequate shielding, resulting in porosity. Flow rates higher that this can cause problems where the surrounding atmosphere can be drawn into the shielding gas, providing a contaminated shielding gas supply, also resulting in porosity.

Check Gun Condition
Check the O-rings found on the end of the welding gun where it attaches to the wire feeder guide. If one or both O-rings are missing, cracked, gouged or worn, shielding gas can leak out or the atmosphere can be drawn in, with both instances resulting in reduced welding performance.

Check the gas ports found in the diffuser, and on some consumables brands, in the nozzle. These holes can also become clogged with spatter and restrict shielding gas flow to the weld pool. These components should be checked several times throughout the day, even if a shielding gas problem isn’t suspected.

Inside the gun cable is a hose that contains both the liner and the shielding gas. This hose can also fail from overuse and holes can be created inside of the cable, through which gas can escape and you would never see it. This problem is mostly caused by using too small of a gun for the amperage being used to weld and from the constant flexing of the gun during use.

The inside diameter of the welding gun nozzle can also have an impact on shielding gas delivery. If the nozzle diameter is too small and gas flow is set too high, a venturi-type of effect can occur, drawing in the atmosphere and contaminating the gas supply. Also, if the nozzle is too large in diameter or the contact tip extends too far from the end of the nozzle or if the contact tip-to-work distance is too great, shielding gas coverage will be impacted.

ELECTRICITY TRANSMISSION
Without good electrical flow between the power source, wire feeder, lead cable and work cable, you could experience a variety of problems, including a sputtering arc, excessive spatter and reduced equipment longevity. The best way to avoid these problems, or troubleshoot them when they occur, is to verify that all of the electrical connections between the welding components are tight and secure.

Resistance is the “unknown” welding variable and the largest cause of inconsistencies in any welding system. The MIG gun is constantly bent and twisted during normal use. This, combined with heat from the welding application, breaks down the copper in the gun over time. If you find yourself turning up your machine from the day when everything was new and correct to achieve the same result, you likely have a resistance issue.

All electrical connections for the welding cables and work cables need to be checked. All connections need to be clean and tight. No paint, rust or washers of any type should be between the copper lugs and the connection surfaces. Check to ensure that all weld cable-to-lug crimps are tight.

A good indication of a poor electrical connection is heat. After the cell has been welding for a while, check all connection points and welding cables for heat. If either the connections or cables feel hot, it is a likely indication that there is too much electrical resistance in the circuit. This could be caused by loose or faulty connections, cables that are too small for the application or an internal break in a cable. A cable that is too small for the application will likely be hot along its entire length, where as a break in the cable will result in a specific point along the cable becoming hot.

The contact tip is another common source of interruptions in the electrical current. The weld current needs to pass through this connection and into the wire, so it must be held tightly to the diffuser and it must make good contact with the welding wire. An indication of a loose connection is a discolored contact tip where it makes its connection to the diffuser. If this occurs, replace the tip with a new one and ensure that it is tightly fastened to the diffuser.

Although, it would take an entire book to list all of the problems that could potentially arise in wire welding and their possible causes, following the guidelines above should put you well on your way to renewed welding success.

Nick Peterson

Nick Peterson is a welding engineer and curriculum developer for Miller Electric Mfg., 1635 West Spencer Street, P.O. Box 1079, Appleton, WI 54912-1079, 920-734-9821, www.millerwelds.com. For more information, email Nick at npeter@millerwelds.com.

14 Comments



  • saleh wrote:

    I have a Miller S-74S wire feeder. When I start welding with wire feeder, the speed is okay after about one minute or less, but then the wire feed speed increases by itself?  What is happening?  How do I correct this?  Thank you.

    • Nick Peterson wrote:

      Saleh,
      Sorry to hear you are experiencing some issues with your wire feeder. Wire feeders have a “run-in” feature where the wire feeds slower than the set speed to help the arc start. When the arc is established the speed increases up to the set speed. This happens very quickly. Additionally a surge in shielding gas may also contribute to a change in the arc after a short time. Your issue is interesting and may be caused by something other than the basics and your issue requires more answers to questions.

      The best course of action would be to call a Miller Service Technician, and be sure you have the model name and serial number of both your wire feeder and the welding power source you are connected to. If you do not have the serial numbers the service technicians will not be able to accurately troubleshoot what may be happening. The Miller Service technicians do an excellent job of troubleshooting issues like this over the phone to determine if it may need to go into a repair shop or if it is something simple to resolve. Miller Service Technicians can be reached at 920−735−4505 and follow the prompts (be sure to get to a service/warranty/repair technician). We hope you find a solution that solves your issue.

      Thank You!
      Nick

  • deron wrote:

    My drive rollers are not turning to feed the wire, it is clicking when I pull the trigger. I have the tension all the way off and no spin at all, help!

    • Nick Peterson wrote:

      Hi Deron,
      It sounds like something that may not be financially fun.
      It could be as simple as a fuse for the drive motor, or something worse like a control board or a burned out motor.
      The best advice I can give you is to call Miller Customer Service (Assuming you have a Miller Wire Feeder) at 920−735−4505.
      Be sure to have your serial number for your wire feeder and if it is a separate unit have the welding power supply serial number as well. Select the service/warranty group and when prompted tell them the wire feeder product name so that you can get to the right technician. 
      Our technicians are excellent at figuring out the specific issue over the phone. They will help you determine if it is something simple or if it will need to go to a service shop for repair.
      Sorry to hear you are having an issue, I hope it is a simple one that will not be too costly. Unfortunately mechanical instruments made by humans can (and usually will) at some point break or malfunction.
      Good luck!
      Thank you!
      Nick

  • Tom Kruer wrote:

    Nick: Designing a robotic system fed by MIG wire drawn from bulk drums. The wire feeder is located at top of drum with six feet of liner to robot mounted torch. Is there anything else I can do to ensure proper consistent tension from the drum? Especially concerned with performance with small diameter aluminum wire. Would you recommend the drum be mounted on a Lazy Susan, use a push-pull feeder, other suggestions?

    • Galen White wrote:

      Tom,
      It should take very little effort to pull the aluminum wire out of a drum but there are several things you need to do to ensure consistent payout of aluminum drums:
      1) Buy a high quality aluminum filler metal—Hobart/Maxal is a good example
      2) Use the payoff devices recommended by the manufacturer of the drum

      1XXX/4XXX alloys use a ‘tophat spinner’                5XXX alloys use drum ring
                              
      The top hat sits down into the drum and the wire pays off around the outside of the drum, with the drum ring the wire feeds up through the center.

      3) Use a drum cone and proper connection kit with conduit 
      Make sure the conduit sticks below the connection kit 1/8″-1/4″ to eliminate wire shaving.
       
      4) Use the correct sized U-grooved drive rolls and eliminate any metallic inlet or intermediate wire guides as these can cause shaving of the wire and plugging of the liners which lead to feeding problems, burn backs, bird nests and down time.
      5) Use the least amount of drive roll tension that you need to consistently feed the wire without slippage with the robot arm/gun in the most twisted/bent configuration you expect to be using it in.  This will minimize how much the drive rolls deform the wire and allows the wire to rotate through the drive rolls as its fed.  If the wire cannot rotate as its fed, the twist will be forced back into the drum and cause a tangle.
      6) Use the largest diameter wire that the parts and welding process will allow.  This is normally determined by base metal thickness, position of the welds, fitup accuracy and whether you are able to use a GMAW-P process.  Feeding small diameter aluminum wires with just a push gun can be problematic especially if your filler metal is one of the softer alloys (1XXX or 4XXX).

      Hope this helps.

      Best regards,

      Galen White
      HOBART ALUMINUM 
      Senior Welding Engineer
      1631 International Drive
      Traverse City, MI  49686
      Phone: 231−933−1234  Ext. 23106  Office
      847−917−1575  Mobile

  • Tony Lowery wrote:

    Hey Nick, the company I work for purchased 4 Miller r-115 feeder heads in the last year and a half and we all seem to be having the same problem. At times the wire comes out like a tuning fork shaking terribly, other days it runs great. We had a Miller rep come in and swear it was the liner, however the same day he was there and we changed out everything my machine went right back to the chatter. Any suggestions?

    • Nick Peterson wrote:

      Tony,

      Sorry to hear you are experiencing some issues with your wire feeder. The liner or contact tips would be an initial thought and the fact it appears to happen on more than one unit as well as intermittently makes it even more difficult to diagnose. Your issue is interesting and may be caused by something other than the basics and your issue requires more answers to questions. The best course of action would be to call a Miller Service Technician, and be sure you have the model name and serial number of both your wire feeder and the welding power source you are connected to. If you do not have the serial numbers the service technicians will not be able to accurately troubleshoot what may be happening. The Miller Service technicians do an excellent job of troubleshooting issues like this over the phone to determine if it may need to go into a repair shop or if it is something simple to resolve. Miller Service Technicians can be reached at 920−735−4505 and follow the prompts (be sure to get to a service/warranty/repair technician).
      We hope you find a solution that solves your issue. 

      Nick

  • Jared wrote:

    Hi!
    I have a Miller STH 160 & am trying to use a Tec arc Tig feed 40 with it, when I attempt to weld the feed goes in reverse, the feed worked okay with a invert-r tig 180p, I could change the cables round on the feed motor but then I can’t use the delay function on the feeder witch I do require.

    Regards
    Jared

  • steve wrote:

    The wire continues to feed after I release the trigger when welding.

  • Brandin wrote:

    Hi. I have a Miller 24 V wire feeder 60 series and every time we plug a whip in to it, it just starts feeding wire out. We tried it with 3 different whips, but the same results. And no, the auto feed is not on.

  • Brandin wrote:

    Hi I have a miller 24v wire feeder 60 series and eevery time we plug a whip in to it. It just starts feeding wire out we tried it with 3 different whips same results an no the auto feeds not on.

  • Steven Salt wrote:

    I have an old mig welder without a spot weld facility. I am intending to fit a spot weld selection switch and timer relay to turn on and off the wire feeding motor for a adjustable period following the gas and welder being started by the gun trigger. I intend to set a small delay to allow the gas to reach the nozzle following the trigger activation. Is it necessary to start and stop the power to the welder circuit at the same time? I can see that by stopping the wire feeder I will cause the weld to cease as the welding wire is separated from the spot weld, Should I have any concerns this may damage the welder. Should the welding supply also be switched with the feeder? My thinking is that by allowing the arc to remain would give a better finish to the weld as the wire would be fully melted into the pool. Is the wire feeder consistent enough to be used in this way? The welder is an old but reliable AGA Viking 90 Ampere. Sadly not much info around for this relic nowadays.
    Regards Steven

    • Nick Peterson wrote:

      Hello Steven,

      I cannot answer your inquiry about these electronics and equipment, as I am not familiar with the equipment you are asking about. I can only answer your inquiry with the text from the Gas Metal Arc Welding Spot welding process from the Miller Publication:

      Gas Metal Arc Welding — Welding Publication — $50.00 
      English — # 250834.

      Newly updated, the Gas Metal Arc Welding Publication was the first available of a series of updated publications from Miller Electric. This is a comprehensive text on all aspects of the GMAW process. Hundreds of full-color illustrations are used to help describe the fundamentals of GMAW, metal transfer modes, equipment, electrode wires, shielding gas, and much more, with 124 full color pages (spiral bound) — 8 1/2″ x 11”. Here is a link to the web page for that book: https://www.millerwelds.com/resources/tools/.

      I apologize that I cannot provide you with the images referred to in the book text below. This book will be available in a free e-publication format soon that will have all of the images with the full text, so keep watching Millerwelds.com to see when those are available.

      Here is the relevant copyrighted text ©2015 Miller Electric Mfg. Co. for your Spot Welding project:
      As in all occupations, safety is paramount. Because there are numerous safety codes and regulations in place, we recommend that you always read all labels and the Owner’s Manual carefully before installing, operating, or servicing the unit. Read the safety information at the beginning of the manual and in each section.

      Also read and follow all applicable safety standards, especially ANSI Z49.1, Safety in Welding, Cutting, and Allied Processes.

      ANSI Z49.1:, Safety in Welding, Cutting, and Allied Processes is available as a free download from the American Welding Society at: http://www.aws.org.

      Here is a list of additional safety standards and where to get them:
      Safe Practices for the Preparation of Containers and Piping for Welding and Cutting, American Welding Society Standard AWS F4.1, from Global Engineering Documents (Phone: 1−877−413−5184, website: http://www.global.ihs.com).

      National Electrical Code, NFPA Standard 70, from National Fire Protection Association, Quincy, MA 02269 (Phone: 1−800−344−3555, website: http://www.nfpa.org and http://www.sparky.org).

      Safe Handling of Compressed Gases in Cylinders, CGA Pamphlet P-1, from Compressed Gas Association, 4221 Walney Road, 5th Floor, Chantilly, VA 20151 (Phone: 703−788−2700, website:www.cganet.com).

      Safety in Welding, Cutting, and Allied Processes, CSA Standard W117.2, from Canadian Standards Association, Standards Sales, 5060 Spectrum Way, Suite 100, Ontario, Canada L4W 5NS (Phone: 800−463−6727, website: http://www.csa-international.org).

      Safe Practice For Occupational And Educational Eye And Face Protection, ANSI Standard Z87.1, from American National Standards Institute, 25 West 43rd Street, New York, NY 10036 (Phone: 212−642−4900, 
      website: http://www.ansi.org).

      Standard for Fire Prevention During Welding, Cutting, and Other Hot Work, NFPA Standard 51B, from National Fire Protection Association, Quincy, MA 02269 (Phone: 1−800−344−3555, website: http://www.nfpa.org.)

      OSHA, Occupational Safety and Health Standards for General Industry, Title 29, Code of Federal Regulations (CFR), Part 1910, Subpart Q, and Part 1926, Subpart J, from U.S. Government Printing Office, Superintendent of Documents, P.O. Box 371954, Pittsburgh, PA 15250–7954 (Phone: 1−866−512−1800) (There are ten OSHA Regional Offices—phone for Region 5, Chicago, is 312−353−2220, 
      website: http://www.osha.gov).

      Booklet, TLVs, Threshold Limit Values, from American Conference of Governmental Industrial Hygienists (ACGIH), 1330 Kemper Meadow Drive, Cincinnati, OH 45240 (Phone: 513−742−3355, 
      website: http://www.acgih.org).

      Towing a Trailer — Being Equipped for Safety, Publication from U.S. Department of Transportation, National Highway Traffic Safety Administration, 400 Seventh Street, SW, Washington, D.C. 20590 

      U.S. Consumer Product Safety Commission (CPSC), 4330 East West Highway, Bethesda, MD 20814 (Phone: 301−504−7923, 
      website: http://www.cpsc.gov).

      Applications Manual for the Revised NIOSH Lifting Equation, The National Institute for Occupational Safety and Health (NIOSH), 1600 Clifton Rd, Atlanta, GA 30333 (Phone: 1−800−232−4636, website: 
      http://www.cdc.gov/NIOSH).

      This document contains general information about the topics discussed herein. This document is not an application manual and does not contain a complete statement of all factors pertaining to those topics.
      The installation, operation, and maintenance of arc welding equipment and the employment of procedures described in this document should be conducted only by qualified persons in accordance with applicable codes, safe practices, and manufacturer’s instructions.
      Always be certain that work areas are clean and safe and that proper ventilation is used. Misuse of equipment and failure to observe applicable codes and safe practices can result in serious personal injury and property damage.

      GMAW Spot, Plug, and Slot Welding
      Gas Metal Arc Welding Spot, Plug, and Slot are variations of the GMAW process. They do not carry a letter designation from the AWS like short circuit transfer (GMAW-S) and Pulse Spray transfer (GMAW-P) do. However, each use terms than can be easily confused with each other or other welding processes.

      GMAW Spot Welding
      When people say spot welding they are usually referring to Resistance Spot Welding (RSW). However, the term spot weld is defined by the American Welding Society as: A weld made between or upon overlapping members in which coalescence may start and occur on the faying surfaces or may proceed from the outer surface of one member. The weld cross section is approximately circular.

      When using GMAW the process is called arc spot welding and when using resistance welding the process is called Resistance Spot Welding (RSW). Arc spot welding can be performed with any arc welding process such as GMAW, FCAW, GTAW, SMAW, etc.

      Standard GMAW equipment can be used for arc spot welding by adding a special spot control and using a different gun nozzle (Figure 111). The equipment can then be used for GMAW arc spot welding. Most applications will use DCEP. Figure 110 shows an example of a GMAW arc spot weld. Two views are shown. One view is on the top of the weld where the gun nozzle was placed. The other view shows a spot weld cut through the middle and etched to reveal penetration and weld bead profile.

      GMAW arc spot welding is different than regular GMAW in one fundamental way. For arc spot welding, the gun’s nozzle should not be moved once it is in place upon the pieces to be welded.

      GMAW arc spot welding should not be confused with Resistance Spot Welding (RSW). As can be seen from Figure 112, the resulting nuggets from the two kinds of spot welding are different.

      GMAW arc spot welding adds filler metal to the weld. Resistance spot welding is done only by resistance heating, which melts the two pieces together using no filler metal. Slang terms that refer to GMAW arc spot welding are MIG Spot and Button Welding. Button welding is a reference to the resulting crown on top of  an arc spot weld.

      Figure 112 also illustrates one of the major advantages of GMAW arc spot welding. With RSW, the welder needs access to both sides of the joint. This is because the welding current passes through the pieces to be welded, from one spot tong to another on the RSW equipment. For GMAW arc spot welding, an operator really only needs access to one side of the joint to make a spot weld.

      Some welding operators feel that because of this difference, more pressure is needed on the semiautomatic gun to hold the base metal pieces together. Actually, with proper fit-up and pieces that aren’t warped, little pressure is needed with GMAW arc spot welding compared to RSW.

      There is no movement of the gun when GMAW arc spot welding. Therefore, there is no travel speed involved. Instead, a timer regulates how long the arc is on. The amount of time depends upon parameters such as voltage and amperage, electrode wire size, nozzle size, base metal type, any metal thickness.

      Because there is no movement of the gun, special nozzles are used for semiautomatic spot welding so the nozzle can be placed directly on the base metal. Usually, depending upon the joint to be spot welded, only slight pressure upon the gun is needed for a good weld. This is the case unless, for example, a lap spot weld is made upon slightly warped materials.

      Obviously, trouble can occur if there is a gap between two pieces of metal to be spot welded. Filler metal could be trapped between the two pieces of metal, and penetration into the bottom piece would be poor. Figure 113 shows an illustration of poor penetration into the bottom piece due to a gap between the pieces of metal. From this illustration it can be seen that the strengths of a GMAW or FCAW spot weld is determined by the penetration into the bottom piece of this type of lap joint. The weld must first penetrate completely through the top piece of metal and then fuse to the bottom piece of metal. Therefore, if different thicknesses of metal are being joined, the thinner piece should, if possible, be the top piece and closest to the nozzle.

      GMAW Arc Spot Welding Applications
      GMAW arc spot welding is sometimes used as a means of tacking metals together for longer, continuous welds. It can also be used alone as a means of joining metals at certain determined points. Spot welds are made for such applications as automotive work, structural building, and frame assemblies.

      Parameters And Weld Variables
      Since good penetration is necessary for a good spot weld, the parameters of welding current and welding time are very important. Under normal conditions, higher values of voltage and amperage (wire feed speed) are used for GMAW arc spot welding. Higher values are used to assure complete penetration. The heat input and weld size can be controlled to another degree by the amount of time the arc is on. The weld reinforcement (or “button” size) will increase as the weld time is increased. The button will not be wider since the weld metal does not extend beyond the sides of the gun nozzle. However, the button size may become higher with increased weld time.

      Remember that the button size on top of the weld is not always a true indication of penetration. For good penetration, amperage and voltage have more effect on penetration than the weld time.

      Arc voltage can affect the shape of the spot weld. This is important the voltage must be high enough to maintain the proper arc length. Should the voltage be too high, there could be excess spatter. Too low of a voltage can cause improper fusion. It could also result in a slight depression or hole in the center of the reinforcement (button).

      Electrode extension for GMAW arc spot welding is determined by the distance the contact tip is recessed inside the nozzle, which rests on the workpiece. A normal electrode extension is about 1/2 in., but this can change depending upon the spot nozzle used, and electrode wire type and size. The electrode extension should not be too long or it may cause stubbing at the start of the weld cycle. Electrode extension should not be too short or the wire may melt back to the contact tip.

      Figure 114 shows the importance of the shielding gas used for arc spot welding. Two GMAW arc spot welds on mild steel show that CO2 gas can help provide better fusion than an Ar-O2 mixture where the two pieces of metal come together. An Ar-CO2 mixture is sometimes used on very thin materials with the smaller sized electrode wires. Usually CO2 gas is used for better fusion on thicker materials when using steel solid wires or flux-core wires.

      Typical gases for different base metal spot welds include the following:
      Mild Steel — CO2; 75% Ar-25% CO2; 95% Ar-5% O2
      Stainless Steel — 90% He-7 1/2% Ar-2 1/2% CO2 (tri-mix); 98% Ar-2% O2
      Aluminum — Argon

      Shielding gas flow rates are generally the same for spot welding and continuous welding. Some spot control units may have pre-and post-flow timers for the shielding gas. These times may be needed for more critical applications, and will be discussed later in this section.

      Electrode wires for GMAW arc spot welding will be the same as for continuous GMAW. However, it may be helpful to use electrode wires with added deoxidizers in them. This is especially true with short weld times. The added deoxidizers will help avoid porosity when the weld bead solidifies quickly.

      Joint design is usually not a problem with GMAW arc spot welding. This is due to the different sizes and shapes of gun nozzles for the various joints. Figure 116 shows examples of spot welds on different joints with the particular type of spot nozzle needed.

      For some GMAW arc spot welds a backing block of some kind may be needed. A backing block is usually made of copper and is put at the bottom of the weld area underneath the pieces to be welded. It is used to help prevent excessive melt-through on the bottom piece of metal.

      If at all possible, GMAW arc spot welding should be done in the flat position. However, some out-of-position spot welds can be made on thinner materials, especially if backing blocks are used.

      Weld time, amperage (wire feed speed), voltage and burnback are all variables that can be set and maintained very accurately on most spot welding equipment. Run-in speed, pre– and post-flow can also be pre-set on more sophisticated equipment.

      Therefore, the only variable that cannot be pre-determined is the heat build-up in the parts to be spot welded. In some cases, pre-set weld settings are determined on sample pieces that are at room temperature. Different results can occur if a number of spot welds are made in one area of a production weld.

      When the production spot welds are made on room temperature material, they should be very much like those made on the sample pieces. But as heat builds up in the production pieces, the same time, voltage and amperage (WFS) settings may produce excessive melt-through. Heat build-up must be considered and appropriate action taken, such as reducing weld time. Or, spot welds can be made in an intermittent pattern to allow more uniform heating. Keeping in mind the heat factor of the base metals, it is always good to try sample welds when setting up for spot welding. The samples should be performed on similar material that the finished welds will be made on.

      A visual test can be made by looking at the back of a lap joint. The weld may melt through slightly if penetration is adequate. If melt-through does not appear on the reverse side of the weld, penetration may be insufficient. If a dark blue circle and slight hump appear on the reverse side of the weld metal, a good weld is usually present. Too large of an area of weld metal burning through indicates either too long of a weld time, or voltage and amperage settings that are too high.

      Testing GMAW Arc Spot Welds
      Judging the shape or size of the weld reinforcement (button) is not a good test for adequate fusion of an arc spot weld. The reinforcement may look satisfactory, but this is not a reliable indicator that proper fusion is present where the pieces of metal came together.

      One method of testing arc spot welds is to use a shear test. The members can be pulled apart using a tensile test machine to determine shear strength.

      Another more common test is the peel test, where a weld specimen is placed in a vice as shown in Figure 115.

      Depending upon material thickness, the specimen is bent or hammered into position 2, where the top piece of the weld is peeled back. This is often done with a chisel, pliers or vice grips. For a good arc spot weld, a nugget of deposited weld metal is “pulled” from the bottom member to indicate a good weld is (position 3). If a nugget is not consistently pulled from the bottom member, or if only a very small nugget is pulled, weld quality (fusion) may be poor. If this is the case, increasing the weld time or increasing the voltage and amperage can improve results.

      Aluminum GMAW Arc Spot Welds
      Aluminum is usually not as easy as steel or stainless steel to arc spot weld with GMAW. Backing blocks are usually needed on aluminum for GMAW arc spot welding. This is to avoid excessive melt-through due to high heat input. All metals to be arc spot welded should be properly cleaned to provide good fusion, and especially aluminum. If oxides are present on the two surfaces where fusion takes place, porosity may form and reduce weld strength.

      Starting Parameters
      Figure 117 shows some typical “ballpark” settings for GMAW arc spot welds on steels. Naturally there are many factors involved in these settings, so they should only be looked upon as a starting point. The actual, best settings must be determined on sample pieces that are tested.

      In addition to the parameters in Figure 117, a burnback setting will be needed. Depending upon the equipment used, trial and observation can be used to determine burnback time. As a general rule, don’t have too much burnback time to start with, or you could destroy the gun’s contact tip. It is better to start with too little burnback time (wire freezing in the puddle) and work up to a correct setting.

      When using welding power sources with outputs of about 200 amps and less, some welding operators prefer an easier method for setting parameters for spot welding. Since spot weld times are usually quite short, this method calls for setting the volt and WFS controls near max., and using the time control to change conditions. Or, the machine may have a heat setting, such as a jack plug which changes voltage and amperage (WFS).

      If using this spot weld method, the jack plug, is set to the highest setting, and the spot time control is used to change the spot weld condition. If this method is tried and good spot welds cannot be obtained, an operator should go back to the previous method of trying various volt-amp (WFS) or heat settings. Also, if using this max. settings technique for spot welding, be careful not to overload the welding power source by making too many spot welds in a short time. Welding power sources equipped with solid state contactors are also recommended to keep maintenance costs down.

      GMAW Arc Spot Welding Equipment
      GMAW arc spot welding is usually done with a Constant-voltage (CV) welding power source, a constant-speed wire feeder, special gun nozzle, and a weld-timer control unit. Also, special timer control units can be used on constant-current (CC) welding power sources with voltage-sensing feeders.

      Welding Gun Spot Nozzles
      Spot welding gun nozzles come in many sizes and designs. When the thickness of metals increase, larger diameter spot nozzles may be called for. Figure 116 showed some of the basic spot nozzles for different joints. These nozzles are somewhat longer than standard, continuous-welding gun nozzles. This is because spot nozzles actually touch the base metal, and the weld is made within the nozzle diameter. The slots on the sides of the nozzle are for the gases to escape the weld area.

      Spot Welding Controls
      Spot welding controls are built into many basic all-in-one welding power sources and can be added to some industrial wire feeders if it is not already a built-in feature.

      The burnback adjustment control helps to keep the welding wire from sticking to the workpiece after the timed arc spot weld is completed. This timed burnback control allows you to vary the amount of time weld current is present on the wire after the wire has stopped feeding. A typical time range for a control of this type could be a min. of 1/64 second (0% on dial) to a max. of 1/4 second (100% on dial).

      Figure 118 shows three different results of burnback settings. The proper burnback time will keep current on the wire for a small fraction of time after the wire has stopped feeding. The correct setting will allow the electrode wire to melt back only a slight distance from the weld puddle.

      Too high of a burnback setting can cause a contact tip to be fused to the wire and destroyed. When setting up to deposit a GMAW arc spot weld, it is better to use lower burnback settings. If the electrode wire sticks to the weld puddle following a weld, increase the burnback time slightly and pull the trigger.

      For spot welding, the burnback control allows you to select the spot weld time within a certain range. This is the time that the wire is feeding (WFS time). In some cases the timer is calibrated from zero to 100 percent. When this is the case, the welder must check the machine’s (or control’s) owner’s manual to see what the amount of time is allowed within the range. Or, the times might be written on the nameplate. For example, if the timer min. is near zero seconds and the max. is two seconds, the 50% on the percent selection would mean approximately one second of weld time. The min. on the timer could be a specific value, such as a fraction of a second.

      In some cases, weld specifications for a job may call for arc spot welds with a cycle designation for time, instead of a time designation. This means that if 60 cycle (Hz) power is being used, a spot weld specification may call for 30 cycles of weld time, meaning 1/2 of a second. 90 cycles would be 1 1/2 seconds, and so on.

      The run-in speed control allows selection of WFS from the moment the pre-flow time times out until the arc is started. When the arc is started, control of WFS is transferred from the run-in control to the WFS control on the wire feeder. Without run-in control, weld quality can suffer if electrode wire stubbing occurs before the arc is started. This is especially possible with larger size wires. The problem occurs because arc spot welding is often done with high wire feed speeds and short weld times. If no Run-In Speed control is present, any wire-stubbing time can take away from the actual weld time. With a Run-In Speed control, wire stubbing is less likely because a slower run-in speed can be used to help avoid stubbing.

      Usually the run-in speed control will be set at a lower rate (percentage) than the wire feed speed set on the wire feeder. The run-in speed control adjusts the WFS to start slower than the welding WFS and as soon as the arc is established, the WFS ramps up to the welding WFS to deposit the weld.

      A lower setting on the run-in speed control provides a slower run-in speed for the electrode wire until the wire touches the base metal. As soon as the wire touches the base metal, the spot time starts. This actually times the weld once the wire touches the base metal.

      When the spot weld is to be made, of course, the welding gun trigger must be pressed. Some spot controls are wired so that the gun’s trigger does not have to be held down the entire time of the weld. The trigger needs to be pressed down and then the control takes over so that the weld is made even if the trigger is released.

      On other types of spot controls, the gun’s trigger needs to be pressed the entire time of the weld. If released early, the weld will not be completed.

      Two other controls that may be available for GMAW arc spot welding are for the adjustment of pre-flow and post-flow of shielding gas. These controls allow for adjusting the period of time that shielding gas will flow before the weld cycle begins and after it has been completed. Pressing the gun trigger starts pre-flow time. Post-flow time begins after the spot time is completed. Proper shielding gas coverage is especially important for arc spot welds because of the short weld times used. Without proper shielding gas coverage, porosity and other defects could occur.

      A small 90 Amp welding machine may not have the capacity to perform production welding using Gas Metal Arc Spot Welding. If you plan to do any type of production welding, I would recommend investing in new equipment that has spot timers already built in and using industrial equipment to ensure high weld quality as well as consistent functionality.

      Some Miller Equipment with Spot timers built in that would work great of Gas Metal Arc spot welding on sheet metal:
      Millermatic® 252 MIG Welder
      http://www.millerwelds.com/products/mig/product.php?model=M00218
      Millermatic® 350P MIG Welder
      http://www.millerwelds.com/products/mig/product.php?model=M00151\

      I hope that helps you with your project.

      Nick