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Plasma Technology

Getting more from plasma consumables

By Rick Mann

No matter how you cut it, by plasma, oxy fuel, laser, shear or band saw, steel has a cost per linear foot when it needs to be cut into a shape. Each one of these processes requires expensive equipment, an operator, handling expenses and time to do the job.

As a job shop doing cutting for customers, or if the customer is the shop itself, when companies bid and allocate these jobs, time is the biggest expense. At hourly shop rates often beginning at around $75 per hour, companies need to get the cutting done quickly before the welders can produce the finished product.

For plasma cutting, a cutting table should be a modern system in good condition and maintained by a conscientious and trained crew to get the best cut quality. This article addresses the consumables for that system, the parts within the plasma torch that get replaced almost every shift and the need to run as long as possible while maintaining high cut-quality standards. Depending on the design and manufacturer of the torches, these consumables include the plasma electrode, nozzle, swirl ring or baffle, shield and retaining cap.

When your car isn't running perfectly, do you change the muffler? No, the muffler is on the receiving end of a complicated combustion and electronic management process. A plasma electrode is analogous to this muffler, a lot of complicated events, electronics, parts and gases have to be delivered at the right time and volume for it to deliver full and accurate power. If one contributing element is out of sync, changing electrodes and nozzles will not solve the problem. The equipment's performance needs to be diligently maintained and monitored to get the most value and productivity out of the expensive plasma consumables. Like a laser, if the power being delivered is at only 60 percent output, don't expect to cut at the maximum machine rating until the source of the problem is fixed.

Get processes in order
Cutting with plasma is a simple process when all is new and running well, but complex when even one factor is off. Diligence, preventative maintenance, operator and management understanding, and training combine to keep the system operating properly. The end product is the sum of many contributing factors, each by itself having the ability to optimize or degenerate performance. To get the most out of plasma consumables, the support system must be kept in proper shape.

Select appropriate consumables
Plasma consumables are specifically rated to a particular range of cutting amperage and gas, depending on the thickness and type of metal that needs to be cut. The highest volume of steel cutting is done on low-carbon or mild steels, such as for building bridges, ships, garbage-truck bodies and cars. Carbon steels are best cut using oxygen as the plasma gas. The oxygen increases the temperature of the cut, allowing faster cut speeds to save time while cutting. Remember, time is the biggest expense. A shop can use less-expensive compressed air, but it is very difficult and expensive to keep the air perfectly dry and clean as required, and the cut speeds are slower with air.

Silver content in electrodes
A plasma electrode is normally a copper body that supports, cools and locates a Hafnium emitter at the end that delivers the plasma cutting arc. For the process of cutting carbon steels with oxygen plasma gas, the entire industry has moved to adding silver to the copper electrodes as an improved cooling medium to withdraw the heat generated by passing up to 600 amperes of current through a small piece of metal.

Liquid cooling must be used in these high-power torches, and silver transfers heat and electricity better than any other metal. Being a precious metal, it also costs a lot more than copper to produce. This is why the consumable manufacturers offer various amounts of silver content in plasma electrodes, from all copper for cutting amperages below 100 amperes to silver foil, slugs or heads for higher cutting amperages to all silver like the Silver-Ex by Thermacut for high-power jobs which receive more heat. The more silver present in the electrode, the longer the electrode will last. Cooler operating temperatures represent reduced resistive power losses through the electrode. This produces greater power outputs from all silver electrodes that enable higher cut speeds using the same standard operating parameters. The higher cost of the precious-metal consumable is offset by the increased cut speeds and its longer life because of the cooler operating temperatures.

Speed equals profit
A lot of jobs are run at cut speeds below book specifications, because operators might think they're being careful or it adds to quality. But reduced speeds actually cause poor cut quality and introduce more heat into the cut. A lot more steel gets melted in the process, significantly increasing cleanup requirements.

Increase speeds to meet shop rates and then bid for the job. Cut as fast as you can. Any time a shop can increase cut speeds, it's the best possible way to meet and exceed bids and standards. This is where the profits lie.

How do you know if you're cutting too slow? If a conical wear pattern has evolved, focusing to the depth of the center of a burnt electrode, it's a sign of a wandering arc caused by cutting at slow speeds. This eats into the copper or silver electrode body and rapidly decreases electrode life.

Heat affected zone
One advantage of cutting with plasma is that its inherent speed allows faster cutting than oxy fuel, thereby introducing less total heat into the part being cut. This results in a smaller heat affected zone (HAZ) in the area of the cut. Slowing the machine down not only increases the HAZ, but increases the amount of heat added to the plate, causing thin sheets to warp or thicker materials to grow, which can exceed the acceptable tolerance for the job. Cutting an 8-ft.-by-8-ft. piece of 2-in.-thick steel plate at slow speeds can make the starting corner move by over an inch by the time the torch returns to it. Cut it fast and it won't move at all. All of this heat reflects up towards the torch, cooking it, the coolant and consumables. But if it moves fast enough, it won't be exposed to as much heat, and consumable life will increase.

Operators and training
Plasma operators get moved around a lot. Shops often need a man welding one day, driving the delivery truck the next and subbing for the plasma operator on the third. This creates a variety of operating styles, however, the operating manual only describes one. The manual for any system as the primary guideline needs to be used, adjusting as necessary for unique situations and environments, but without getting too creative and out of control. If the supervisor and all operators are similarly trained on proper standard operation, untrained creativity can be controlled to a minimum and other operators can work together to create a system that works. Only with process control and stability can consistent consumable-parts life be delivered by the system.

Cleanliness is important to machine performance
The Japanese style of 15-min. cleaning exercises before starting a machine do a tremendous benefit to the work area. Cutting steel is a messy job, but when that mess gets inside the torch, it causes leaks from O-rings not being able to seal, consumable-part misalignments, short-circuits and nozzle failures.

O-rings
Two-piece torches have become more popular, because they allow an operator to remove the business end of the torch to change the consumable parts. However, the problem with water-cooled torches is that they all rely on O-rings, which suffer hardening, abrasion or cuts, and dirt in a hot, grimy environment. A two-piece torch requires about five O-rings compared to a one-piece torch, generally at the point of intersection. Upon inspection, expect to find hard, inflexible, flattened, torn O-rings that are supposed to keep coolants and gases in control.

These leaking O-rings cause parasitic losses of fluids into the interface, which may then find their way into your plasma gas. They can also decrease the pressure that the operator thinks he's operating at. This kills the plasma power by a lack of gas or introduction of contaminants.

Quick-disconnect torches
Some shops may have several torch heads set up with new consumables, binned according to amperage and gas selection, so that consumable changes are accomplished by merely swapping torch heads. Errors have occured because of this, as once the parts are installed, they cannot be verified without removing them again immediately before using, thereby negating some of the savings.

Coolant care
Coolant is frequently neglected. In fact, if dust is floating on its top, from leaving the fill cap off, it is severely contaminated and therefore conductive. This can cause intermittent arc-starting problems, because the starting juice can stray off through the coolant. The coolant is a dielectric (electrically non-conductive) white water or propylene glycol mixture and needs to be changed when it is contaminated, becomes electrically conductive or exceeds six months in the system. No steel plumbing elbows or fittings may be used in the coolant path or the steel will rust, thereby ionizing the coolant and making it electrically conductive. The coolant might be in contact with both negative and positive electrical potentials within the torch. If it becomes conductive, arc starting will suffer parasitic losses when the current flows to unintended destinations and the torch begins to unsolder itself. Tap water is full of minerals and contaminants and will kill your cooling system. Water is not recommended by any manufacturer, nor is automotive anti-freeze.

Maintenance and up-time
Plasma equipment is high-tech, therefore it's maintenance must be of the highest level. Cutting-table rails must be clean, square, flat, even and level. A machine doesn't make any money unless the plasma is running. Using the best parts to keep the arc on will result in being immediately paid back in productivity for any cost difference. Ten minutes of downtime to replace a worn-out consumable will kill the $3 saved versus a part that lasts longer. Likewise, failure to maintain equipment results in downtime far more expensive than the cost to maintain it.

Get the facts
Not knowing performance levels means not knowing when production is better or worse. Opinions are nice, facts are better. Efficient job bidding includes tooling expenses. Knowing exactly what they are allows better tracking and controlling of consumable expenses as well as bidding jobs more accurately. Efficient shops keep records of tool life and changes, allowing expenses to be tracked and performance to be monitored.

Gases, condition and flow
For gases, the Ts and reducing connections can be the hidden bane of the operator. As leaks can be hard to detect if more than one plasma system shares the same gas supply, such as oxygen for plasma, care must be taken to ensure that one doesn't rob from the other, especially noticeable when startup pulses in demand occur. Setting up gas-flow requirements for a single-plasma system won't suffice when a second is added. Beware of those handy but potentially problematic quick-disconnect fittings which are everywhere in a shop. Many of these connectors are big on the outside but have small 1/4-in. bores that will kill the output on a 3/8-in. supply hose. Ninety-degree elbows, even if sized correctly, can also reduce flow rates. Check the gas-supply volume at the power supply to verify adequate gas delivery in accordance with specifications in the service manual. Often it's not what a gauge claims to be putting out that matters, but what's really delivered at the receiving end.

By training employees, maintaining equipment, operating it properly and following the manufacturer's specifications, thousands of dollars per year can be saved on consumable costs, resulting in more profit to the bottom line. FFJ

Rick Mann is International Engineering Manager for Thermacut Inc.

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