A check list for a plasma cutting table retrofit
Preparing for the installation means the shop can be cutting parts sooner rather than later
When considering a retrofit to an older plasma cutting table, a fabricator needs to consider equipment location, state of table components, the types of shielding gases and consumables that might be needed with a technology upgrade, and any possible training that may be needed.
My dentist is a pilot, so he has flying magazines in his waiting room. I was leafing through one on my last visit and came across an article about how a corporate jet flown by two experienced pilots had crashed on takeoff because they did not use the mandated preflight check lists. This got me thinking about how helpful a check list could be when retrofitting a new plasma cutting system to an existing cutting table.
After researching and selecting the right equipment to replace your old plasma system, it is a good time to use a preflight check list to make sure you are ready for its arrival. Not being prepared in advance can cost valuable time getting the new system up and running.
Determine the Table Location
Using a tape measure, paper, and pencil, draw a simple floor plan layout around the existing machine showing where the current plasma system components are located. Using the manual and information provided by the vendor of the new system, check to see if the new equipment will fit in the same space as the old one does now.
A common plasma system comprises the power supply, arc starter, gas manifold, pressure-controlling devices, and possibly an interface panel. Review the lengths of the interconnecting cables, wires, and hoses that are on order to ensure they fit if you use the existing floor plan. Now is the time to develop a plan to rearrange the component locations if necessary.
Plan for the Power Track
The power track is the flexible carrier of cables, wires, and hoses, which are run from the stationary floor-mounted equipment to the moving cutting table. Some simple cutting machines have cables and hoses that drop down from an overhead frame. Other machines use a flexible power track.
Will the new cables, wires, and hoses work with the existing method? Check the cross-sectional area of the existing power track to see if it will hold all of the required hoses and cables (see Figure 1).
Double-check the Power
Don’t assume the new system will work just fine with your old system’s power service. Check the voltage and amperage requirements for the new system and have a qualified electrician review the equipment that will carry the power to the new plasma power supply.
If necessary, now is the time to increase the size of your fuse box or circuit breakers along with increasing the size of the wires carrying the current to the power supply.
Consider the Cutting Gases
The lifeblood of any plasma cutting system is compressed gas (see Figure 2). If adequate volumes of gas at the correct pressures can’t be provided, cut quality and performance suffer. The material and finish desired are greatly affected by the gases used for the plasma and for the shielding gas.
Again, check the manual or speak with your supplier about the gas specifications for the new system. What pressures and flows are required? What are the recommended hose sizes? Who will be providing the gas regulators? What will be the sources of these gases? How far from the plasma system are the gas sources located?
As an example, let’s say that the required air pressure for the new system is 100 PSI. Your old air compressor may be able to make 100 PSI, but can it maintain this pressure when the new plasma is cutting and when any other power components are also using air? Making 100 PSI does not mean the compressor turns off at 100 PSI and turns on at 80 PSI. The new plasma needs a continuous, steady supply of air at 100 PSI. This requires turning on at a point above 100 PSI and off at some higher pressure and then regulating this pressure back down to a nice steady 100 PSI.
If gases other than air are used, you also must review their requirements and sources. A modern plasma system monitors the supply gas pressures and shuts down the cutting process if the pressure falls outside of a specified window. A low-pressure source of oxygen, for instance, must use the appropriately designed regulator and properly sized hoses to avoid troublesome fluctuations in pressure when the plasma valves open and close quickly. Consult with your gas supplier if you have questions.
If water is used in the cutting process, what are its requirements? Does it need to be at a particular pressure? Does it need to be deionized or filtered? Is drinking water suitable? These are questions best answered before the new system arrives. City water pressure may be too high and might need to be reduced to avoid damaging the new plasma system.
If new supply lines are to be installed, make sure pipe sealant does not get inside the pipes and hoses. Before connecting hoses, old or new, to the new plasma system, flush out those lines to blow out any dirt, dust, dead bugs, and sealing materials so that you aren’t restricting any of the new plasma system’s tiny passages and control valves.
Don’t Forget the Ground
The term ground has a different meaning to the average industrial electrician than it does to an automated plasma specialist. The electrician’s main concern with grounding is that it is suitable to blow a fuse or trip a circuit break to protect people from electrical shock and to protect the electrical equipment from drawing so much current it damages the equipment or causes a fire. Most plasma systems require a good ground to capture and bleed off the high-frequency (HF) noise generated when the cutting torch is started. In the plasma cutting business, the ground is where we want the HF to go.
Just because the existing ground rod has been working fine with the old plasma table, don’t assume it is acceptable for use with the new system. Corrosion damages a ground rod over time and makes it unsuitable for use with a new system (see Figure 3).
Typically, all noise-sensitive components are connected to a central point on the cutting machine, often called the star ground. From the star ground, a single wire of suitable size (typically 1/0) is run to a ground rod dedicated for use with the plasma cutting machine.
Now is the time to check the existing ground rod. The quality of this dedicated ground can be measured using a time-tested technique of comparing this rod to the one installed by the power company.
Your plasma vendor can supply you with a diagram of how this measurement is made. Usually it involves a resistive load and a voltmeter to dynamically measure the resistance between the plasma ground rod and the power company-installed ground (see Figure 4).
If you need to install a new ground rod through a floor, drill two holes, one for the rod and one through which you can dispense water to add moisture to the soil and improve its conductivity. Adding a small amount of salt to the water further improves the conductivity of the wet soil. However, don’t get carried away with the salt, as too much hastens the corrosion of the rod (see Figure 5).
Purchase the style of ground rod that can be lengthened by screwing additional sections together. This way, if you need to go deeper to get a better ground, you do not have to weld the rods together. If a weld breaks out of sight, down in the hole, the upper rod just slides down beside the lower rod and does nothing to help the quality of the ground.
Look at the Lifter
Now is a good time to review the condition of the lifter (see Figure 6). In plasma cutting, the lifter correctly positions the torch for piercing and keeps it at the correct height while cutting. A poorly maintained lifter results in poor-quality cuts.
Inspect the lifter and clean or replace any defective parts. Be sure to acquire the correct parts to mount the new torch body to the old lifter and support its lead set.
Depending on the control system being used, the lifter’s settings can be integrated into the CNC or in a stand-alone control unit. Either way, the lifter is the middle man between the CNC and the plasma. The lifter usually is the device that tells the plasma power source to start after the pierce height is found, typically with a contact closure; it then receives a signal back from the power source indicating the plasma torch is up and running, again typically via contact closure.
The lifter also receives a voltage signal proportional to the torch height. This voltage is usually scaled down to a safer level via a voltage divider card. If the lifter system uses a voltage divider card, it needs to be removed from the old power supply and mounted and wired into the new power supply.
More times than not, the lifter has three pairs of wires used to interface with the plasma: start/stop, OK to move, and arc voltage. Prior to installing the new system, gather the documentation for the machine controller and find these six wires in preparation of running them to the new plasma system. Finding this documentation and these six wires ahead of the new plasma’s arrival saves time.
Review the CNC
Does the CNC contain parameters that are specific to the current plasma system? Does it contain feed rates, kerf width, pierce, or other machine-specific parameters? This information is typically provided by the plasma system manufacturer, and most older CNCs required the operator to enter the cutting parameters manually for each cut. Some newer CNCs hold many of the frequently used cutting parameters, while other CNCs may require you to enter new parameters for the new system, which can be gathered from the new system’s cut charts (see Figure 7).
Find out About the Consumables
Discuss with your plasma vendor the different materials to be cut. The new system may or may not come with consumables. It might arrive with only a starter set for just one material or process. Before the new system arrives, order plenty of consumables to cut the desired materials and thicknesses.
If an experienced plasma professional is on-site to assist with the retrofit, he or she needs consumables on hand so the various cutting processes can be demonstrated once the installation is complete.
Think About the Training
Having the cutting table operator and shop maintenance man assist in the retrofit of the new plasma cutting system not only makes the installation go quicker, but both gain valuable information and insight into its operation and maintenance.
Additionally, leave time in the installation schedule for some formal training once installation is complete. Have key personnel sit down in a quiet location with the plasma specialist performing the installation, and go over the operation and maintenance of the new plasma system. Then, take a few hours to perform some sample cuts on commonly cut materials to become fully familiar with the processes.
Using this check list prior to the arrival of a new plasma cutting system will help to get your installation off the ground and get that old cutting machine back into production and making money.
The FABRICATOR is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The FABRICATOR has served the industry since 1971.