Training Articles

Cost of Operation in Mechanized Plasma Cutting
By Dave Cook, Centricut Technical Services Director

Article originally appeared in "Welding Design & Fabrication" - May 2000

How to calculate cost of operation and establish metrics for improvement
There are many costs associated with a mechanized plasma-cutting machine beyond the capital equipment purchase. There are general overhead costs, maintenance costs, service call charges, gas costs, consumable and torch costs, and electricity charges. The plasma-profiling machine is also likely to have a host of auxiliary equipment that may also be considered: material handling equipment, environmental control equipment, safety gear etc. The labor component for plasma cutting may include machine operators, helpers, maintenance personnel, secondary operation workers and others. The intent of this article is to review the most significant variables affecting annual cost of operation and to establish metrics for improvement.

In typical plasma cutting operations there are four major ongoing costs: power, gas, consumables, and labor.

Power Cost:
Power Cost = Consumption X Arc-on time X power cost
The major power consumer in a cutting machine is the DC power supply. Most of the energy consumed by the system is put directly to work on the material in a very hot energy-dense arc. To get a rough idea of the power consumption of a plasma system multiply the amperage output by the average operating voltage. To calculate kilowatts of input consumed, multiply by a power supply efficiency factor of around 85%. Ex. A 200A plasma system has an average operating voltage of about 140V. This means the power supply puts out 28 kVA. 28kVA X .85 = 23.8 kW

To arrive at daily or yearly power consumption multiply times the average up-time or arc-on time in a day. Arc-on time as is the amount of time actually spent cutting over a given time interval. This can be measured by a pierce and arc-on time counter, or calculated from programming distances and speeds and daily throughput. Arc-on time will vary with material type and thickness, size of cut pieces, material handling, machine speed, torch height control speed, and many other factors. Most shops average about 35% actual arc-on time. That means in a given 8-hour shift only 2.8 hours are spent cutting and in a 2080-hour year 728 hours are spent cutting. Multiplying this times a typical power cost of $.10/kWH gives you an annual power cost. Annual Power Cost = 23.8 kW-h X 728 hours/yr X $.10/kWH = $1,730 /year

Gas Costs:
Gas Cost = Gas consumption X Arc-on time X gas cost

Plasma systems use oxygen, air, nitrogen, argon-hydrogen, and other gases.

The consumption rate varies with the size of the plasma system and various operating conditions. Generally the operations manual will provide consumption rates in cubic feet per hour for a given nozzle size and operating pressure or flow tube setting. For example a 200A oxygen plasma system consumes 70 cfh of oxygen when cutting. To find the cost of operation multiply the consumption rates of plasma gas by the arc-on time and cost of the gas, which is often measured in dollars per hundred cfh.

Annual Gas Cost = 70 ft3/hour X 728 hours/year X $10/100 cfh = $5,096 /year

The same system may use 300 cfh of shield air. Shop air is generally considered free other than associated maintenance costs to keep it clean. Cut-water or water shield are also inexpensive. But shield gases such as nitrogen, CO2, and mixes can be costly and should be calculated as above.

Consumable Costs:
Consumable Costs = Consumption rate X Arc-on time X consumable cost

Consumable costs can be tracked on a weekly, monthly or yearly basis. These costs vary widely depending not only on the cost of the parts but on the performance and life of the parts, which is dependent on many factors. Consumable and plasma torch life varies with application, operating parameters, duration of cuts, number of pierces, operator skill etc. The best way to capture and begin to control consumable costs is to keep daily logs of parts life measured in number of pierces and arc hours. Over time, in a production environment, it is possible to closely track the number of pierces and the total arc-hours for a given set of parts on a given cutting job. If a plasma torch is operated and maintained correctly the annual cost of torches, gas swirling devices, shields, retaining caps and other parts should be low compared to the nozzle and electrode cost. But the reality in many shops is that overall consumable cost is 2 X the nozzle and electrode cost.

Annual Consumable Cost = 1 set / 3 arc-hours X 728 arc-hours/year = 243 sets/year X $20/set = $4,853 / year X 2 = $9,706

Labor costs:
Labor cost = Working hours per year X shop hourly rate X number of operators & helpers

Most shops have 1 operator and 1 helper per machine per shift. Depending on the quality of the cuts from the machine there may be multiple workers on secondary operations or none. A typical fully burdened shop rate is $35 /hour.

Annual labor cost = 2080 hours per year X $35/hour X 2 workers = $145,600

Obviously labor is the major cost of operation for plasma cutting. To get the most out of the plasma cutting operation the fabricator must use his labor wisely. That doesn't mean that each operator needs to run three plasma tables. A better solution is to pay for and train a good operator who can keep the machine running and produce good parts.

Here are some recommendations for optimizing your cutting machine to lower cost of operation and increase productivity:

1) Maximize up-time on the machine: A cutting machine should be cutting. Preventative maintenance is essential to prevent costly downtime for repairs. Material handling solutions such as multiple cutting beds, overhead cranes, and plate handlers can minimize manual loading and offloading and keep the operator focused on the cutting process. Motion matters as well: If the torch height controls or machine traverse speed is slow the machine spends more time positioning the torch than cutting metal.

2) Minimize secondary operations: Controlling costs of secondary options is achieved by optimizing cut quality. To do this requires not only a well-maintained machine but also a well-trained operator. The highly skilled operator produces more cut pieces, of higher quality, with less scrap material and less rework down the line. Getting good cut quality from the PAC process requires careful control over process parameters and attention to detail.

3) Control Consumable Costs: Controlling consumable costs, like controlling cut quality is part equipment and part operator. A good operator will get the most out of a set of parts and prevent catastrophic failures.