Setting MIG and Flux-Core Parameters From a WPS

A Welding Procedure Specification (WPS) gives you ranges. Your machine gives you knobs. Getting from one to the other is daily work for a mid-level welder, and it is where a lot of good-looking welds go wrong. Setting the dial to a number from the WPS does not prove you are inside it, because the arc can run at something different from what the panel shows. You already know what a WPS is and why you must stay inside it. Now you will learn how to do that when someone is measuring.

What You Will Be Able to Do

• Convert WPS ranges into a starting setup for a constant-voltage machine.
• Explain how wire speed, voltage and stickout change the amps you really get.
• Measure amps, volts and travel speed the way an inspector does.
• Calculate heat input and compare it to a limit.
• Sort changes into those you may make and those you must take to your lead.

How the Settings Really Work

MIG and flux-cored welding use a constant-voltage (CV) power source. That shapes how the settings behave:

• Voltage is set at the machine. It controls arc length and bead width.
• Wire feed speed (WFS) is set at the feeder. The machine delivers whatever current it takes to burn off that much wire.
• Amps are a result. You do not set them directly on a CV machine. They follow from the wire speed, the wire size and the stickout.

Stickout, also called contact tip to work distance, is the piece that surprises people. The wire between the tip and the arc has electrical resistance. A longer stickout preheats the wire, so it takes less current to melt it. At the same wire speed, a longer stickout means fewer amps, and a shorter stickout means more. The machine dial has not moved, but your weld has changed.

This is why two welders on one machine can run different amps, and why a WPS may list a stickout along with its amps and volts. If your WPS gives amps and you only have a wire speed dial, you have to find the wire speed that produces those amps at your stickout.

Reading a WPS for Machine Settings

Here is a made-up WPS, for practice only. Real ones come from your employer.

Set up in this order:

1. Confirm the job fits. Check base metal, thickness, joint and position against the WPS, and check your own qualification.
2. Confirm the consumables. Read the wire label and the gas cylinder tag. Do not trust the last person who loaded them.
3. Set the polarity. Check it at the machine terminals each time the wire changes.
4. Set the gas flow. Read it at the flowmeter with the gas flowing.
5. Pick a starting point in the middle. The middle of each range gives you room to adjust without leaving it. Here that is about 185 amps and 23 volts.
6. Find the wire speed. Use the wire maker's chart for a first guess. Then weld a test and measure the actual amps.
7. Weld a test in position. Adjust one setting at a time.
8. Write down what worked. Note the wire speed, volts and stickout, so that you can repeat it after lunch.

Measuring What the Arc Really Sees

Dials and panel readouts are not always accurate. On code work, an inspector may check you with a separate meter. You should know the numbers before they do.

Amps. Clamp a DC-capable ammeter around the welding lead while you weld. It must be rated for DC, because a basic AC clamp meter will not read a DC welding lead. Take the reading once the arc is steady. A hold or peak function helps, since you cannot read a meter under a hood.

Volts. Voltage is lost along the cables. The longer and smaller the leads, the more you lose, so a machine reading 25 volts might give you only 23 at the arc on a long run. The honest reading is taken as close to the arc as is practical, such as from the wire feeder to the work. Read your WPS or shop procedure to see where it wants the reading taken.

Travel speed. Mark a known length, weld it and time it.

• Mark 12 inches on the joint.
• Time the weld with a stopwatch, or have a helper do it. Say it takes 100 seconds.
• Speed = 12 / 100 x 60 = 7.2 inches a minute.

Gas flow. A flowmeter reading only tells you what left the regulator. A leak or a clogged nozzle can still starve the weld. If your shop has one, a nozzle flow gauge checks what actually reaches the arc.

Calculating Heat Input

Heat input is the energy the arc puts into each inch of weld. It affects the strength and toughness of the weld and the metal next to it, and it drives distortion. Some procedures set a limit, especially on steels that need toughness.

Heat input (joules per inch) = volts x amps x 60 / travel speed (inches per minute)

The 60 turns minutes into seconds. Divide the result by 1,000 to get kilojoules (kJ) per inch.

This formula suits steady, ordinary welding. Pulsed and other waveform-controlled machines need a different method, such as the machine's energy readout, so follow what your WPS says for those.

Worked Example: Checking a Weld Against a Limit

You are on the WPS above. You measure 23 volts and 185 amps, and you timed 7.2 inches a minute.

• Heat input = 23 x 185 x 60 / 7.2
• Heat input = 255,300 / 7.2 = 35,458 joules per inch
• That is about 35.5 kJ per inch, under the 40 kJ limit.

Now the bead looks thin, so you slow down to 5.5 inches a minute to build it up.

• Heat input = 255,300 / 5.5 = 46,418 joules per inch
• That is about 46.4 kJ per inch, which is over the limit.

You are also below the 6 inch minimum speed. Nothing on the machine changed, and the weld may look better than before, but it is now out of procedure.

How slow can you go at these settings before you hit the limit? Rearrange the formula:

• Minimum speed = 23 x 185 x 60 / 40,000 = 6.4 inches a minute

So at these settings you must stay above 6.4 inches a minute, which is tighter than the 6 inch floor in the travel speed range. All limits apply at the same time, and the tightest one wins.

What You May Change

Staying inside the WPS leaves you some room. Knowing where that room ends is the skill.

Gas is an easy one to miss. One wire maker says to drop about 1 volt when moving from CO2 to 75/25 on the same wire, which shows how much the gas changes the arc. A shop that runs out of one gas cannot simply hook up the other unless the WPS covers both.

When the Weld Will Not Come Right Inside the Range

Sometimes you can stay in range and still not get a good bead. Work through these before you touch anything outside the WPS.

1. Change one thing at a time. If you change two, you will not know which one helped.
2. Check your stickout. A drifting stickout is the most common reason the amps wander.
3. Check the consumables. A worn contact tip, a dirty liner or a loose work clamp all make the arc erratic.
4. Check the fit. A root opening wider than the sketch may need more metal than the range allows.
5. Check the cables. A long lead or a hot, loose connection steals voltage.
6. Measure, then report. If the range still will not work, tell your lead what you measured. A procedure can be wrong for a job, but changing it is the job of the person responsible for the WPS.

Keeping It Repeatable

Parameters drift, so check them more than once.

• After a break. Someone may have borrowed the machine.
• After a wire change. A new spool can run differently, and a new diameter needs a new setup.
• After moving the feeder. Longer leads change your volts at the arc.
• Between passes. A WPS can list different ranges for the root, fill and cap passes.
• As the joint heats up. Watch your interpass temperature, because a hot joint can make the puddle harder to hold.

Keep a small notebook. Write the WPS number, wire speed, volts, stickout and travel speed that worked. It saves setup time, and it protects you if anyone asks how the weld was made.

Common Mistakes

• Setting the panel and assuming you are done. A clamp meter may tell a different story.
• Letting your stickout grow. Amps fall, and fusion can go with them.
• Slowing down to make a bigger bead. You can break the speed range and the heat input limit together.
• Changing voltage and wire speed together. You lose track of what fixed the problem.
• Reading volts at the machine on a long lead. The arc may be running lower than you think.
• Treating the WPS as a suggestion when a weld looks good. Appearance does not bring a weld back into procedure.

Check Your Understanding

Your amps read low at the same wire speed you used this morning. Nothing on the machine has changed. What is the first thing to check?

Your stickout. A longer stickout lowers the amps at the same wire speed on a constant-voltage machine. After that, check the contact tip and the work connection.

You run 24 volts and 180 amps at 9 inches a minute. What is your heat input?

24 x 180 x 60 / 9 = 28,800 joules per inch, or about 28.8 kJ per inch.

The machine panel shows 24 volts, but your feeder is 75 feet from it. Should you trust that number against a 22 to 24 volt range?

Not without measuring. Some voltage is lost in the leads, so the arc may be running lower than the panel shows. Measure as close to the arc as your procedure says.

The WPS calls for 0.045 inch wire. The shop has 0.052 inch, and the foreman says it will weld the same. What do you do?

Stop and ask. A different wire diameter is not covered unless the WPS lists it. Tell your lead and wait for a WPS that fits.

Key Takeaways

• On a CV machine you set voltage and wire speed, and the amps follow.
• Longer stickout means fewer amps at the same wire speed.
• Measure amps, volts and travel speed instead of trusting a dial.
• Heat input = volts x amps x 60 / travel speed, and slowing down raises it.
• Every listed limit applies at once, so the tightest one wins.
• Inside the range is your call. Outside it, stop and ask.