List of Equipment Tested:
Brian Gutierrez, President of Equip Automotive Systems (distributor of Elektron): Multi-Spot MI100. www.equipmyshop.com
Joran Olsson, President, Pro Spot International: Pro Spot models i4, PR-2000. www.prospot.com
What made him so upset was his own lack of knowledge of the welding equipment he had just purchased. Knowing that this shop owner wasn't the only one challenged by the new welding technology, March Taylor, owner, Auto Body Hawaii in Kailua Kona, and I decided to conduct an evaluation of the inverter spot welding equipment available today.
We contacted manufacturers and distributors of inverter resistance spot welders and invited them to attend four days of testing during the last week of July. Al Estorga, owner of Estorga's Collision Center in Long Beach, California, opened up his facility for the two days of preparation and testing. Seven companies offered to bring their machines for evaluation. They came from as far away as France and New Jersey. When we lined up all these cool spot welders, it looked like something out of Star Wars - a bunch of little robots.
Get what's right for you
If you're looking for a quick answer to the question "what machine should I buy," you won't find it here. March and I decided from the outset that we were not going to recommend a particular machine. Different machines are right for different shops. What you'll find here is the information to help you make an intelligent decision about a critical - and very expensive, costing $12,000 - $30,000 - piece of equipment. We'll also discuss what you need to do to get your shop ready for that equipment.
Let the sparks fly!
I was in a collision repair shop recently conducting an I-CAR welding test when I observed a tech installing a quarter panel using a late-model squeeze-type resistance spot welder (STRSW). I asked the technician how he knew if his plug welds were any good. He looked at me as if to ask, "Why are you bothering me with such a stupid question?" What he actually said was, "I know these are good welds because of the spark the weld makes."
"So, the more sparks the better?" I responded.
"Yeah man, let the sparks fly!" He then chided me, "You should know that because you're an I-CAR welding instructor" and then repeated "Let the sparks fly!"
I took a survey that night with the seven techs and they all agreed that the sparks indicated the plug welds were good. During a break, I inspected the tips on the shop's welder; the copper electrodes had been sharpened with a grinder but not with the grinder furnished with the machine. The ends of the tips were uneven and they were about 11 mm in diameter. I pulled the trigger on the gun to see if the tips made full contact with each other. And, of course, they did not.
I proceeded to pull the tips from the machine and, with a file, I dressed them. I then aligned the tips and, using some scrap steel from a damaged door, I demonstrated to the techs how to make proper plugs. And guess what - no sparks! Yes, you read it correctly. There should be little or no spark when performing STRSW welds. To understand why, you need to know how principles of resistance spot welding work.
How does spot welding work?
Resistance spot welding is best described by Paul Wilcox, the president of Tite-Spot Welders in the company's technical bulletin. Resistance spot welding is "the joining of overlapping pieces of metal by applying pressure and electrical current." In other words, a spot weld is formed when a large amount of current is passed through copper electrodes that are held in place by pressure exerted on the electrodes. As the electricity flows through copper and comes in contact with the metal that is being welded, a resistance is encountered in the electrical path and heat is generated. As the heat builds up, the metal in contact with copper tips of the electrodes melts. The pressure that is exerted on the weld site by the pressure of squeezing the tips together causes the metal to deform, producing a "weld nugget" or a small circular dent as it cools.
When welding using STRSW machines, a number of variables need to be considered.
First, pressure is needed at the weld site to force the molten metal together and keep pressure on the nugget until it cools. Too much pressure at the tips will decrease the resistance factor. Too little pressure will cause weak and small weld nuggets.
Another factor is the amount of electrical current, which we measure in amperes or "amps," because the amps determine the amount of heat delivered to the weld point.
The next factor is weld time. Too long a weld time could lead to the base metal exceeding its melting point or even its boiling point. These conditions could lead to gas porosity or even explod- ing welds. Too short a weld time will not allow enough amps to flow to form a proper weld. So how much weld time is right? A lot depends on the type of steel you are welding.
Understanding the "new" steel
When life was simple, we only had to worry about 18 to 22 gauge metal on a car. With the introduction of ultra high strength steels, things are starting to get complicated. We're dealing with trip steel, dual phase steels, laser-welded tailor-made banks and advance steels, and advance steels with boron. We're challenged as an industry to deal with these new metals.
To make things a little simpler to understand, I took plastic cylinders and filled them with golf balls, marbles and BB pellets. The golf balls represent the molecules in mild steel, with marbles as the high strength steel, and BBs as the new, ultra-light, ultra-strong types of steel demanded by car makers to meet both fuel efficiency and safety standards. If the cylinders were filled with water, all the voids between the objects would be filled. If the cylinders with the BBs and marbles were frozen, both containers would be a lot stronger. Now, if the cylinders are warmed, a slight melting would occur along with a weakening of the structure. Add a lot more high heat and the ice melts.
This is a simplified explanation, but it is a good representation of what too much heat does to the new metals - it damages them. Heat applied to one of the new metals, advance steel with boron, reduces the strength up to 75 percent.
The effect of welding heat on these metals can have dramatic effects. Heat is generated in a STRSW by adding more electrical current, as measured in amps. Too much heat applied to the weld nugget and surrounding metal becomes brittle, loses weld strength and the weld can explode. Too little heat will cause a weak weld. So, with all these variables, how are we going to know which is the correct amount of heat? Inverter welders to the rescue!
Inverters take the incoming AC volts and, through a series of internal devices, convert the AC voltage into high amps, low voltage (13 volts) DC current. With this combination of high amps and low voltage a number of benefits can be derived.
First, DC current output is constant, unlike AC current which goes through cycles (50-60 cycles) per second. When welding thin metals, heat is generated above the zero line (positive) and cools below the zero line (negative). If we were to weld with AC current, the thin metal would begin to fuse after only a couple of current cycles, and if the metal cools significantly during the cycle, resistance is lost. To overcome this loss of resistance, a longer weld time is required and this will produce more heat to the metal surrounding the weld nugget - something we want to avoid with the new metals. By using DC current for the weld, there is a constant, uniform supply of electrical current and the problem of overheating is eliminated.
Another inverter weld-ing advantage is the ability to "fine tune" the voltage (amps). Instead of using a range, the new computer-controlled machines can dial in precisely the best amp/volts to produce a proper weld on varying kinds of metals. Yes, you read it right; these new inverter-resistance spot welders are using computers. But before you place your order for a great new spot welder, we now need to look at the electrical source that operates these machines.
Do you have 3-phase power?
While March Taylor and I were getting ready for our spot welder demonstration, I found out that all seven of the machines needed 220 volts of 3-phase power. Three-phase power uses three sine waves instead of one during one cycle. When one wave is at zero, the other 2 are still delivering power. With all three lines carrying electricity, a constant voltage is achieved. To use inverter welder technology, a constant voltage is needed and that is why your shop needs a 220v 3-phase power source. Most of the inverter welders need between 50 amps and 65 amps service.
What does that mean? While many of you certainly understand this, I'll explain it using an analogy for those who might be confused. Think about half-inch pipe and two-inch pipe. Consider the water flowing through the pipes as current. If you can get by with the amount of water that the half-inch pipe delivers, you are in good shape. But what if you need more water in the same amount of time? You need a larger pipe. In electricity, electrical wire is pipe. The gauge of the wire is represented by the diameter of the pipe.
Most of the machines need a minimum of 50 amp service which equates to a #6 wire. A 65 amp service needs #4 wire (note that #4 is heavier wire than #6). If the electrical run is over 80 feet, you should use the next larger size wire to reduce resistance. So, before you invest in an inverter spot welder, you should consult with an electrician to determine if you have 3-phase power where you will need it.
Some dos and don'ts of spot welding
While researching materials for this article, I found a good list of general rules for identifying and correcting spot weld problems. You can find this list in the Resistance Welder's Manufacturers Association's Resistance Welding Manual, written by Richard Dunbar.
• Too short squeeze time can result in metal expulsion, overheating electrodes, bad welds, marked work.
• Too long weld time will shorten electrodes life, cause excessive indentation at surfaces and cause internal cracks in the weld nugget.
• A peel destructive test on test strips of the same material and combination is recommended.
• Too short weld time will result in low weld strength, in proportion with weld heat.
• Too short hold time can result in surface expulsion, electrodes sticking, and internal cracks in the weld nugget.
• Weld pressure too low can result in expulsion of metal, electrode sticking, short electrode life, and possible internal cracks in the weld nugget.
• Weld pressure too high can result in variable weld strength, excessive weld current requirements, mushrooming of electrodes, and excessive indentation.
• With all other settings correct, adjust weld current to meet weld quality standards using recommended starting points.
• Electrode contact face too small will result in too small a spot, excessive electrode mushrooming, and excessive indentation. Too large an electrode contact area will result in too large a weld (assuming current is set accordingly). Use RWMA charts for electrode manufacturer recommendations.
• Electrodes misaligned or miss- matched will result in expulsion, and displaced weld nugget and excessive electrode wear.
• Insufficient cooling will result in mushrooming and short electrode life. Adequate water cooling of the welding system is crucial.
The testing begins
Now that you have better knowledge of STRSW, let's get into our testing of the squeeze type resistance spot welders. March Taylor and I decided we wanted to conduct a welding test that would simulate working on a vehicle instead of just welding some coupons together like they do at so many demonstrations.
Ole Vandborg, the owner of Scandinavian Coachcraft in West Los Angeles, furnished us with two new complete rear body panels and bumper reinforcements from a late model Volvo. These panels include high strength steel and advance steel with boron. We positioned the panels between two welding stands in order to simulate their position on a vehicle.
While we were getting set up the day before the testing, March took eight pieces of 22 gauge steel coupons and proceeded to weld them together. Wow! I was in awe of the power of the machine he was using. March began to laugh, "Toby, any good spot welder can perform this task. It looks impressive, but it is no big deal; just a way to impress a buyer." Our tests would do a lot more than just impress a buyer.
• Duty Cycle
The first order of business was to test the voltage. I put a volt meter into the 220 volt 3-phase socket and observed a voltage of 205 volts. All the vendors agreed this voltage was sufficient to operate their machines effectively.
First we checked out at the duty cycle. Fifty spot welds were performed on two 3- inch by 4-foot strips as fast as the machine would allow. Then the 51st weld was performed on 22-gauge mild steel coupons and a peel test was performed on that last weld. All machines produced welds that passed the peel test.
• Weld bond
Next came the weld bond test. We applied adhesive to a 22-gauge panel and allowed it to dry for 12 hours. The adhesive acts as an insulator between the 2 pieces of metal that are being welded. In making the welds, it should be noted that we used a pair of vise grips to provide for a shunt. To create an electrical path between the panels (a shunt), a self-tapping screw or pliers is needed for the first weld only because once a spot weld has been created the electrical path is set up. After the welds were completed a peel test was again done to test for a quality. Again, all the welders met the required weld diameter on the peel test.
• Multiple panel weld
The next test performed was a multiple panel weld. We took four pieces of 18-gauge zinc-coated mild steel and spot welded them together. When finished, I performed a peel test on the coupons and again all the welders functioned well.
• Reach test
The next test was the reach test. We wanted to be certain that each machine's electrodes could be maneuvered to where a weld was needed. We took a stripped-out front end of a Honda and painted the various zones different colors. Then each company's representative performed a simulated spot weld in each zone using only the electrodes that come standard with their machines. Once again, all the welders were able to reach the various zones on the Honda - some more easily than others.
• The rear body panel challenge
The final test was the rear body panel and this created the most challenges. March and I placed the Volvo panels on my welding stands to simulate the panel's position on a vehicle. The rear body panel is made up of two different metals. The upper portion is high strength steel (HSS) and the lower part is advanced steel with boron. The upper rear panel reinforcement is HSS and the outer rear bumper reinforcement is advanced steel with boron. We marked off the panels with 6 zones (1 zone for each welder to be tested). There were four designated weld areas on the panels which were as follows:
Area A: The area where the upper portion of the bumper reinforcement was attached to rear body panels (2 panels boron to boron).
Area B: Lower bumper reinforcement to rear body panel plus a piece of 22 gauge mild steel representing where the trunk floor attaches to the rear body panel. (2 boron and 1 mild)
Area C: Lock support area of 22-gauge mild steel, 18-gauge zinc-coated steel and two pieces of HHS. (2 HSS and 2 mild)
Area D: The ends of the two panels attached to two panels of 22-gauge mild steel (2 boron and 2 mild)
Each company representative set their machine for the various metal types and thickness, but March was the only person performing the welds because we wanted it done by someone who was not familiar with the peculiarities of the various welders. When all the machines had completed this test, we performed destructive testing to check for quality. To do this, I purchased a spot weld chisel from Dent Fix Corp. to separate the welds (more on this tool later).
Let's look at the results. In Area A, one of the machines had a problem with the boron to boron weld. They did not have the correct chip in their computer. The other 5 welders ranged from good to excellent on the boron to boron. When destroyed in testing, one of the first welds exploded due to too much squeeze pressure. The rest of the welds were fine after March reduced the squeeze pressure.
Next, to test the length of the arms, March changed to the longer electrodes and proceeded to weld the lower center portion of the rear body panel (Area B). Besides looking at the weld quality, we wanted to observe the process of changing to longer arms or electrodes (more on this later). The welds were good to excellent with the five welders (again, one welder had a computer problem with the boron application).
All the welders produced very good to excellent welds in Areas C and D. Some of the welds exploded and to remedy this March prepared a list of procedures that should be followed by the welding tech. Here they are:
Best practices to prevent "exploding welds"
Panel Prep: All sides of replacement panels must be buffed with all e-coat being removed, no factory primer. (caution: do not remove galvanized coatings )
Weld Primer: If weld primer is to be applied, the welding process should be completed while the primer is fresh, preferably within the first hour and not longer than 8 hours (cured weld primer is the same as the factory coating and will often restrict continuity).
Recommended Tip Type: Domed shaped tip should be utilized; the tip should be flattened 5mm at the top of the dome.
Squeeze Pressure: With most Guns/Pliers, adjust squeeze pressure to the maximum amount possible. Multi-panel compression welding requires very high gun squeeze pressure, in most repair situations 500 to 900 lbs at the contact tips (Super and Ultra High Strength Steel multilayer panels may require pressures in excess of 900 lbs)
Contact Tip Gap: Check to see if the gaps between tips are correct (the type of squeeze gun and manufacturers recommendations will determine gap distance).
Arm Angle: A bad arm or contact tip angle may cause welds to explode; arms should be square to the area being welded.
Weld Sequence: Start at one end and move in one direction: do not jump around.
Host Panel E-Coat: In areas where it is visually evident that there is excessive factory coating between the host panels, prior to clamping on the replacement panel you should hammer and dolly the affected area to break up the excessive coating.
Questionable Areas: Areas without a good flush fit, attach vice grips on both sides of spot to be welded.
Follow Your Weld: At each weld-spot place a vice grip (as close as possible) next to the weld spot; this will increase the quality of the weld, while supplementing the squeeze performance of the gun pliers.
Now that you know how to avoid exploding welds and how to correct them, take a look at pictures of good and bad welds (Courtesy of Electron).
Thoughts and observations
• Electrodes and guns
Most manufacturers produce one of three types of guns: a single-sided gun, a "C" gun, and an "X" gun. One manufacturer has a different style that goes by the name of "pliers." The "C" gun works using pressure which pushes the electrode out of a cylinder to make contact with the fixed electrode on the C arm. The "X" gun uses a center pivot (like an X) and forces the arm to pivot into the other electrode.
All of the guns require regularly changing the electrodes in order to gain access to different locations on a vehicle. On some this is easier than others.
The "C" gun arms attach to the gun with a single locking device. To gain greater access a larger "C" is employed. Due to the design, the electrodes are self-aligning. One company uses a solid electrode and the other uses a press-on tip. March and I preferred the press-on tip because no sharpening tool is needed and it easy to replace. The negative to this design is that it won't reach every spot on a vehicle.
The "X" gun electrodes come in matching pairs. They are designed to reach virtually every spot weld on a vehicle. Exchanging electrodes takes more time than on the "C" gun. I did prefer the electrodes that had an alignment pin on the part. The other two companies used set screws to attach the electrodes to the gun. Of these two, one had a cam action that locked in electrodes to a standard position (requiring virtually no alignment of the tips). The one gun that used the set screws took the most time to change and did require the technician to align the tips before welding, a real drawback.
Besides the design, "C" verus "X", there are other observations on electrodes: First, as the electrodes increase in length, the squeeze pressure at the tips decreases, increasing the possibility for a weld to fail. Then there is the cost factor. The additional electrode sets are not cheap, especially the water cooled units and the storage of these tips is an issue. Also on the matter of price, a consideration is the "X" adapter. One company charges for it as an accessory and another includes it in the overall package price. Make sure you're comparing apples to apples when you look at the price.
• Air pressure controls
Next on our list was the adjustment of air pressure that regulated the squeeze pressure. Most of the units had a dial in the front of the machine which made the operation easier. One unit had the control in the rear of the machine that took a little more time to adjust.
• On-board computers
All of the welders use a computer to monitor their welds. Two of the machines actually have a small video monitor built into the unit. Each of the company's reps went through the set up with March. He liked the programs that asked questions to perform the setup (sort of like the check-in computer at the airport kiosks). And when you have a computer, you of course have upgrades to the software/firmware.
Some of the machines had a port that would allow the manufacturer to send out a flash card which contained the new upgrades. The other welders would need to have a technician come out to upgrade the computer which could be a costly upgrade. One company plans to add a USB port to its machine so that an e-mail with a software upgrade attached can be sent to the body shop, the attachment then downloaded and saved to a memory stick. The memory stick would interface to the machine through the USB port.
• Metal options
All the welders had a boron metal option, a minimum panel thickness option, and a number of panels to be welded option. One company took a different approach to panel thickness. This company wanted the overall thickness to be entered on the control panel. I think this approach is technically somewhat better, but you may feel that to ask a tech to have a caliper and measure all of the metals to be welded is asking a bit too much.
These guns can be heavy and somewhat unwieldy, so we looked at ergonomics. March liked all of the guns, but the "pliers" was his favorite. As I mentioned previously, guns with the larger "C" attachments and longer electrodes guns were harder to manipulate. One machine incorporated a work area on top of its machine with a tool board in easy reach, a nice feature. The "C" gun machines had an excellent gun rest that allowed for easy arm changing.
• The whole package
Some other important items when selecting an inverter welder: training, warranty, service, OEM program requirements, basic price, and the cost of accessories.
It should be noted that 5 of the 6 machines are made in Europe. One company, at the time this article was written, had no repair stations in the United States. Their machines needed to be sent back to Europe for major repairs.
All the companies have a training CD, but I still want a live body to demonstrate because I learn through the hands-on approach as do so many technicians. You need to ask if the purchase price includes in-house training.
Finally, check on the cost of all the accessories. Remember the shop owner I mentioned earlier in the article had to purchase $2,000 worth of electrodes to make his machine fully operational.
Equipment we tested
March and I want to thank everyone who participated. These people are "pure class." They spent thousands of dollars to attend our little party. They came not knowing what to expect, but were willing to put their equipment to the test. That says a lot. Again, our thanks to those who came and worked with us.
Toby Chess has more than 30 years of industry experience, is an I-CAR instructor and a technical presenter at CIC.