Training Articles

Upgrading and Retrofitting PAC Machines
By Dave Cook, Centricut Technical Services Director

This article was published in "Welding Design & Fabrication" - October 1998

Are you spending a lot of time and money on secondary operations to clean up dross, remove bevel angle, or rework out of tolerance parts that were cut using plasma?

Does your cutting machine vibrate or occasionally act possessed?

Does it break down and stay down for days or weeks?

Is your CNC control outdated and slow?

If the answer to any of these questions is yes, you are not alone. Many fabrication shops are plagued by chronic problems with their cutting operations. Many of these problems can be eliminated through operator training and proper machine maintenance; others are best solved by equipment upgrades.

But before you relegate the whole cutting machine to the scrap heap, consider this: you may be able to get more out of your old machine by refurbishing or retrofitting critical components.

Plasma System Upgrades
A new plasma system retrofitted onto an older cutting machine or punch press can mean improved cut quality, increased cutting capacity, and faster cutting speeds.

Today's plasma systems offer significant improvements in power supply and torch technology including microprocessor-controlled power supplies, sophisticated gas flow controls and optimized torch and parts designs.

Consumable parts life has improved significantly with the use of inert start gases for oxygen plasma cutting.

Current and gas pressure ramping or stepping devices soften the plasma arc during ignition and cut off.

Optimized consumable designs specialized for plasma gas, amperage, and application along with improved manufacturing techniques have led to better cuts and the longer life from the torch parts.

Variables to consider
Material type will determine the plasma gas used.

• If you only cut stainless steel or aluminum, an older nitrogen plasma system may still be adequate for your needs. Air plasma may also be a viable lower cost alternative.
• For mild steel cutting, oxygen plasma is becoming the industry standard. Oxygen gives excellent cut quality and weldibility and also increases cutting speeds due to its reactivity with carbon steels.
• Air plasma offers similar cut quality on carbon steels, but may produce nitriding along the cut surface. Multi-gas plasma systems let you choose a plasma gas that is best for your material and application.

Material thickness will determine the size of plasma system needed

• The machine should have sufficient power to handle your typical cutting task with ease.
• Find out the thickest material that you regularly cut and select a system that will cut this material at a reasonably fast speed.
• Manufacturers rate their systems by maximum cut capacity from an edge start; pierce ratings are typically half the cut capacity.

Hint: Don't skimp on power: In order to maintain fast cutting speeds make sure the system is big enough to handle your typical cutting jobs. For example, today's typical 100-amp air plasma system will cut mild steel at about 30-35 ipm. A 200-amp air system will pierce and cut the same material at 80 IPM and a 260-amp oxygen water injection system will cut it at approximately 100 IPM.

Cutting Machine Upgrades
Mechanical reconditioning
Although many advances have been made in cutting machine design in the last 25 years, many of the older gantry and cantilever style plasma cutting machines were well designed and remain viable. A good machine technician may be able to refurbish these machines and bring them back to life. Rack and pinion drives are now the most commonly used method of motion transmission. New gears and racks can be installed on older machines. Bearings to support the moving elements of the machine in longitudinal (the bridge moving along the rails) and transverse (the torch carriage moving along the bridge) axes are critical to smooth motion and accurate positioning of the cutter.

New drive package:
New servo motors, drive amplifiers, and encoders can be retrofitted to an old machine to replace outdated, worn, or unserviceable drive packages. PWM (Pulse width modulation) DC servo motors are typically used for plasma machines and are available as retrofit kits from automation manufacturers. These systems can be configured to retrofit many existing cutting machines. The newer drive packages offer greater positional accuracy, higher torque motors, and faster acceleration times.

New torch height controls:
Many older plasma machines were not equipped with adequate torch height control (THC) or if they were so equipped, the THC is no longer functional. Many shops simply do without this valuable tool. Most THC's are voltage regulated-that is they regulate position of the torch by sampling the arc voltage and comparing it to a preset value then driving the torch up or down to maintain that voltage and corresponding standoff. The latest THC's can control the standoff distance to within .005". Because the plasma arc column can be fixed to its optimum position to deliver a straight jet, this leads to substantial increases in cut quality, particularly bevel angle. Height controls also pay for themselves in reduced consumable useage. Most have an initial height sensing (IHS) function that allows an initial height to be set independently of arc voltage. This allows the operator to "pierce high and cut low", a practice that greatly enhances nozzle and shield life.

New CNC controls:
Retrofit options exist for most old cutting machine controls. Newer CNC controls offer more storage capacity, increased speed and accuracy, better programming features, faster communications, and more user friendly design. Most are industry standard, EIA and ESII code compatible and support third party pc software and part nesting programs. The latest generation of controls is PC-based to take advantage of the increased processing speed and lower cost of the industry standard computer processors. They use off-the-shelf computer components that are lower cost and open architecture in design. The best PC based controls offer communication rates as high as 230 kbaud, (10 times faster downloads than older models) gigabyte hard drives for file storage, and real-time graphic displays. Some can even receive software updates via e-mail (no e-proms or other firmware upgrades are necessary).

QUICK REVIEW:

• Upgrading and Retrofitting PAC Systems
• Points To Consider a Material type will determine the plasma gas used.
• Material thickness will determine the size of plasma system needed.
• Options To Consider
• Mechanical reconditioning
• New drive package
• New torch height controls
• New CNC controls

With a comprehensive refurbishment and upgrade of your existing plasma cutting system, it is possible to realize big gains in the productivity of your cutting operation and see an improvement in the bottom line without spending six figures on a new machine.