Since the late 18th century, the U.S. Coast Guard has stood Always Ready (Semper Paratus, its motto) to respond to public needs in a wide variety of maritime activities, including maintaining safe and secure ports, protecting the marine environment, rendering aid to people in distress, managing the waterways to ensure safe traffic and -- especially in Miami -- enforcing the law.

The Integrated Support Command (ISC) Industrial Weld Shop for the U.S. Coast Guard in Miami stands ready, too, using the latest inverter-based welding and plasma cutting power sources. Four types of inverters from Miller Electric Mfg Co have enhanced the Miami ISC's reputation as a premier station for ship alterations and repair. These products are:

• Spectrum® 2050, a 70 lb plasma cutter with 50 amp output that can cut any metal that conducts electricity.


• XMT® 304, a 76 lb CC/CV multiprocess welder, which has a dc output of 5 to 400 amps; for MIG welding aluminum, ISC pairs it with a push-pull wire feeder system, the XR-A.


• Dynasty™ 300 DX, a 106 lb ac/dc TIG welder, which has a 5-300 amp output, an extended ac balance control, and an adjustable frequency output.


• Maxstar® 91, a 28 lb dc TIG welder, which uses 110 V input power.

"This is one shop that believes in staying on the cutting edge of metalworking technology," says Kurt Knapp of Service Welding Supply, the distributor who works with ISC. "Using inverter-based power sources just makes sense because of their maneuverability, multiple-metal flexibility, and unique TIG welding capabilities."

Ship Alts

When vessels need retrofitting, modification, and repair (ship alterations, or "ship alts" in shop language), they spend three weeks dockside at ISC. Ship alts may include a total engine overhaul, reconfiguring the fuel tank, installing new piping for drains and sewer tanks, replacing fin stabilizers, mess deck repairs, reconfiguring deck lights, replacing the hydraulic lines for steering, installing new throttle cable assemblies, hull refitting (from filling in small indentations to replacing whole sections) and resurfacing of the propeller, the propeller shaft, and rudder.

The Miami ISC usually has ship alts scheduled seven months in advance. In addition to working on 110 ft cutters -- the Island-class Patrol Boats that are primary law enforcement vessels -- ISC is currently performing a complete overhaul on several 41 ft utility boats.

Ship alts often involve cutting out sections of worn, outdated, or damaged metal parts. Metals commonly encountered include marine grade (5086) aluminum for the hull, radar arch, railings and cleats; British steel (HY80), also for hulls; steel and aluminum for the mess deck; and copper-nickel alloys for the plumbing. Because of the variety of metals encountered, and the need to work quickly, ISC uses the plasma cutting process for most metal removal.

"If the metal can conduct electricity, we believe it's a job for a plasma cutter," says Nathiel Roper, welder, ISC Industrial Weld Shop. Plasma cutting is a process where an arc is constricted by passing it through a small nozzle from the electrode to the workpiece.

On Location

Plasma cutting provides numerous advantages over other cutting methods. It's faster than a saw. It cuts any electrically conductive metal, where oxyfuel cannot cut stainless steel or aluminum. Plasma cuts faster than oxyfuel. A preheat cycle is not required. The kerf width (width of the cut) is small and more precise. Its smaller heat-affected zone prevents warping or damaging paint in the surrounding areas. In addition, it is cleaner, less expensive, safer, and more convenient because it uses ordinary compressed air for "fuel" instead of potentially harmful gases (a real concern when working in a ship's hull).

Conventional plasma arc cutting was introduced in 1957. For decades, the technology was restricted to the shop floor of traditional metalworking industries because of the plasma cutter's immobility: the transformers within the machines made them heavy, bulky pieces of equipment. In recent years, however, advanced inverter technology has reduced the size and weight of the machines and freed them from the shop floor.

"We're able to bring all the inverter equipment out of the shop and onto the pier without using a fork truck like we had to for our old equipment," says David Hall, ISC Industrial Weld Shop leader. "This is great. For instance, I can set the inverter plasma cutter up on the deck or take it anywhere on the vessel because it's one-eighth the size of our previous plasma cutter. I can cut any metal on the ship and I can cut with high travel speeds (approximately 20 in./min on 3/8 in. aluminum). And while fast travel speeds are nice, I'm more concerned with the quality of the cut. With this inverter, I get a cleaner, finer cut than with our old plasma unit."

Vic Miller, a welder at ISC, points to a utility boat now under repair. He used the plasma cutter to cut away a large section of the hull where the 5/16 in. thick aluminum hull plating had eroded to less than 75% of its original thickness. Other than a brief pass with a disc sander, no additional finishing is required before fitting the new hull plating in place. The new plating sections will also be cut with an inverter plasma cutter, shaped on ISC's press brakes, and MIG welded with the XMT 304 inverter-based welder and XR-A push-pull wire feeder running .035 in. diameter 5356 wire. Notice that this entire aluminum MIG welding system fits on one small cart for easy wheeling around the shop (see picture, below).

"Another nice thing," notes Hall, "is that the new inverter plasma unit uses a lot less consumable parts like $15 tips." Inverters have a "Tip Saver" feature that shuts off the pilot arc after the arc ignites, extending the life of consumables.

Roper adds that "Cutting expanded metal -- which we do for servicing the platform on light buoys -- is no problem. We just flip a switch to keep the pilot arc in a continuous mode. This keeps the arc from going out." He says that cutting steel up to 1 1/8 in. thick isn't a problem either. "I slow down my speed and listen for the arc coming out the bottom of the plate," he states, "and that ensures that I've got a clean cut."

Portable Penetration

Using advanced TIG inverters also helps ISC maneuver its equipment around ships. Hall especially likes the 110 V dc TIG welder for pipefitting because of its light weight and ability to plug into a standard electrical outlet. He says he tacks the pipe for fit and positioning while inside the cutter, then takes it back to the weld shop for finishing.

For welding that needs to be done entirely on board, Miller says, "I can move that little ac/dc TIG inverter anywhere on a vessel. It gets into really confined spaces -- the battery room, aft steering space, near the fuel cells, you name it. But more importantly, I can adjust the ac frequency for higher penetration and more cleaning."

Miller says that frequency adjustment helps when repairing the hulls and other aluminum structures on older vessels.

"See the pitting on the hull where electrolysis has set in? Over the years," he says, "salt works its way into the metal, plus the aluminum oxides get in really deep. We need a more penetrating arc to burn down into the material. Then, when the aluminum melts, the impurities get raised to the surface of the molten metal. Then I can start adding filler rod."

Before welding, Miller uses a small die grinder to grind out the bad metal in small pits on the hull that sandblasting can't remove. For especially deep pitting, he'll cut out a small portion (perhaps 3 or 5 in. diameter circle) with the plasma cutter and weld new plate in place. Using a 2% thoriated tungsten electrode, Miller boosts the frequency beyond the "normal" 60 Hz and starts welding.

Only advanced inverter-based TIG welding machines give operators this option. Conventional ac TIG welders have a fixed output of 50 or 60 Hz, but inverters let operators adjust the welding output frequency from 20 to 250 Hz. Increasing frequency produces a tight, focused arc cone. This narrows the weld bead and creates a more forceful, deeper penetrating arc. It also lets operators direct the arc precisely at the joint and not have the arc dance from plate to plate. Inverters also feature an "extended ac balance control" that lets operators direct up to 22% more heat into the work compared to a conventional TIG machine.

Combining these two features "makes it much easier for me to put in a weld with good integrity when I'm rebuilding a stuffing tube," says Roper. This tube is a piece of schedule 120 aluminum pipe that goes through the hull and holds the propeller shaft. Good welds are mandatory, because the weld seals the stuffing tube against water entering the hull. Yet, the welding is hard. First, it involves joining new metal to old metal subjected to oil and salt. Second, since he can't weld from the inside of the pipe, Roper relies on arc force (and good joint preparation) for complete penetration. Lastly, Roper has about 2 in. of clearance between the bottom of the stuffing tube and the hull.

"You have to be a contortionist to weld a stuffing tube. I use a mirror to see where I'm welding," Roper says. For welding the top of the tube, he sets the welder's frequency control to about 110 Hz and extends the balance control to nearly its maximum penetration setting.

"When working on top and on relatively clean metal, I can boost both the frequency and the penetration because the weld pool falls right into the joint," Roper states. "For dirtier metal, I set the balance control for more cleaning. Same thing holds true for the bottom of the weld. There, I must use a lower setting so that gravity doesn't suck the weld pool out of the joint. However, I keep the frequency high. This gives me a tight arc cone for working in that small space, and it gives me good penetration."

ISC frequently uses its traditional, magnetic amplifier style welding machines, but on a more selective basis. These machines usually stay in the shop, and they do plenty of high quality work (except the old plasma cutter, which ISC quickly retired). However, when operators need a machine dockside, or for an especially tricky weld, they all reach for an inverter first.