Mr. Roboto: Welding fundamentals for managers
This article is the first in a series that addresses the fundamentals of robotic welding. The author discusses the basic process parameters in robotic welding, how those parameters affect weld quality and productivity, and how they can be optimized to make a robotic welding installation as profitable as possible.
Experience has taught me never to make assumptions about a person's knowledge of welding and welding automation. Often managers and engineers who have not been exposed to the welding process have a poor understanding of its mechanics. This can and often does lead to misunderstandings and disappointment during a robotic welding project.
Robotic Weld Process Control
First, a few words about how the welding process is controlled in a robotic welding application.
Using analog communication channels in the robot controller is the most common method of controlling welding voltage and wire feed speed. A typical analog channel can process a signal ranging from zero to 10 V. This signal range usually is programmed through an interface box between the robot and welding power source to correspond to the output from the power source. One analog channel is used to control voltage, another for wire feed speed.
One of the 10-V analog signals corresponds to the welding voltage output range of the power source. If, for instance, the maximum output range for a particular welding machine is 50 V, then the zero-to-10-V analog output from the robot corresponds to a zero-to-50 V output from the power source. If you desire an output voltage of 25 V (half its maximum) from the power source, the robot needs to send the power source an analog signal of 5 V (half its total value). In the same way, the wire feed analog signal may correspond to an output wire feed range of zero to 750 IPM.
Some newer robot controllers use digital communication to control welding parameters, which eliminates the old-fashioned analog method. This can remove the need for an interface box between power sources and robot and gives more flexibility in controlling weld parameters. However, the bottom line remains the same - the robot instructs the power source on which output to use for each weld, based on instructions that are programmed by an operator.
Fundamentals of GMAW
As in all aspects of life, if we get a good handle on the fundamentals, the rest ought to fall into place. Welding often has been considered more art than science, but reality is that welding can be boiled down to a few fundamentals and that adhering to those fundamentals properly can make life a lot easier.
The basic process parameters in gas metal arc welding (GMAW) are voltage, wire feed speed, amperage, and travel speed. Other critical aspects of the process include torch angle, lead angle, and weld position (the position of the weld relative to gravity).
GMAW is the process most often used for robotic welding applications. It relies on a continuously fed wire electrode that melts off at a controlled rate to fuse two pieces of metal together.
Depending on the total heat in the welding arc, the mode of material transfer from the welding wire to the part can vary from a cool, low-heat process called short-circuiting transfer to a hot, high-energy process called spray transfer.
Short-circuiting versus Spray Transfer
During short-circuiting mode, the welding wire itself short-circuits against the part, creating a direct short that spikes the amperage. The heat melts off the wire, which is continuously fed to the part until the arc short-circuits again. This short circuit may happen about 200 times per second. This is the cooler mode of wire transfer and is used for applications in which the welding heat needs to be limited, say, for thin materials. The short-circuiting process can produce some weld spatter because of the violence of the short circuit.
During spray mode, the amperage and voltage are high enough to convert the wire into tiny droplets of molten metal that are transferred across the electrical welding arc. The wire itself never touches the part.
Because there is no short-circuiting, the spray mode deposits clean, smooth welds with a good surface appearance. The high energy involved in the spray mode greatly increases penetration, so it typically is used on heavier materials. However, if spray mode is used on thin materials, the torch must travel quickly to prevent melting through the material.
Wire Feed Speed
Welding wire typically is supplied either on 50- or 60-lb. spools or in bulk packs up to 1,000 lbs. Bulk wire is preferred for its economical price and because it can reduce downtime for changing empty wire spools. You can see the importance of computer controls for welding automation immediately-if a robot is to provide precise, consistent torch travel speeds, the wire feed speed needs to be controlled carefully during welding to ensure high quality.
Wire feed speeds in robotic GMAW can be as high as 1,000 or more IPM. Typical speeds are 300 to 700 IPM. Wires are available in various diameters, the most popular for robotic welding being of 0.035, 0.045, and 0.052 in. Thinner materials generally require smaller diameter welding wires to prevent melt-through.
Because the wire feed provides the filler material for the weld, it is one of the primary factors that affect weld size. At a given torch travel speed, a higher wire feed speed creates a larger weld bead. But it is important to remember that wire feed speed also is directly proportional to depth of weld penetration. Higher wire feed speed increases penetration, all other factors being equal. So wire feed speed and travel speed must be balanced to produce a weld with adequate, but not too much, penetration and size.
Robot controllers typically are responsible for maintaining and monitoring wire feed speed according to the values specified by the programmer. Some wire feeders include tachometers that provide real-time feedback and closed-circuit control of wire feed speed.
A fundamental of GMAW is that smooth, steady, predictable wire feeding will contribute to consistent, high-quality welds.
Wire Feed Tips and Tricks
You can take some precautions to ensure consistent and reliable wire feeding, maximum productivity, and minimum downtime in a robotic application.
Regular Maintenance. During manual welding, your can adjust on-the-fly for poor or intermittent wire feeding, but a robot must have a smooth, predictable wire feed to produce high-quality welds. A thorough preventive maintenance program that regularly inspects, cleans, and replaces wire conduit, contact tips, and torch liners is necessary.
The liner inside a torch cable can become clogged with grime and tiny metal flakes from the welding wire. That is why many robot users change contact tips each shift, twice per shift, or sometimes more often, depending on the process duty cycle. Change the liner often, replacing it with a new or clean one, then clean the dirty liner thoroughly in a solvent intended for the purpose.
Eventually liners wear out and have to be discarded. Wire feed rollers also can wear out, causing slip and inconsistent wire feed. Replace these as recommended by the manufacturer.
Programming. While programming the robot, avoid putting the robot arm and welding torch in a position that pinches or causes a tight kink or angle in the torch cable. A tight radius in the cable can cause wire feed problems and can wear out the torch cable and liner prematurely.
The torch cable and the conduit between the wire feeder and the wire spool should be as short as possible to minimize the amount of drag the wire experiences inside the liner. If the wire must be fed through a long conduit, consider adding a second wire feed motor as a slave to the primary wire feeder to improve the wire feed.
Welding voltage provides the heat to melt off the wire. Most power sources used for robotic GMAW are of the constant-voltage (CV) style. This means you can set the voltage to a constant value and also can set the desired wire feed speed. Then the machine automatically adjusts amperage to melt the wire off appropriately to maintain the set voltage.
In spray mode, the metal from the welding wire is transferred to the part via small droplets sprayed across the arc. An electric arc is established so the wire itself never touches the part. The arc voltage is proportional to this arc length.
Higher voltage causes the arc to lengthen, which-taken to an extreme-can diminish weld quality. A too-low voltage value can cause the arc length to become so short that the process reverts to the low-energy short-circuiting mode.
Voltage Tips and Tricks
Excessive lengths of power cable or ground cable can cause excessive voltage drop between the welding power source and the workpiece.
If a robot instructs the power source to put out 25 V, an excessive length of ground cable may cause a drop of a couple volts, meaning the actual voltage in the welding arc is 23 V. This can cause inadequate penetration and cold welds. Piles of coiled-up ground cable also affect the inductance in the welding circuit. To be sure that you are getting the actual values in the welding arc that you expect, eliminate extra cable.
Check the power cable, the ground cable, and voltage sensor connections regularly. These items normally are connected directly to the lugs on the welding power source. The positive voltage sensor lead also may be connected to the positive power lug on the wire feeder itself.
Corroded or loose connections give erroneous voltage readings and cause the robot to put out faulty voltage signals. A current sensor typically is used to monitor welding amperage, so check that it is installed and fastened properly.
One other thing: The method of attaching ground cable connections to the workpiece or welding fixture can cause a phenomenon called arc blow, in which the welding arc wants to wander from one side of the weld to the other, rather than focusing on the center of the weld. This can be eliminated by changing the location of the ground attachment or by using multiple grounding points on a single fixture.