Practical Welding Today - May/June 2009

The following are some tips to consider when cutting aluminum with a manual or mechanized air plasma unit.

Use Compressed Air

Compressed air is a good choice for your cutting gas. Because of its approximately 20 percent oxygen and 80 percent nitrogen content, air provides for good cut speeds with minimal dross and only a small amount of edge oxidation.

The air plasma process can result in a fairly porous edge and some level of oxidation that may affect secondary welding processes. However, these problems generally can be resolved by sanding or grinding 0.004 to 0.006 in. off the plasma cut edge. This will provide a very weldable surface.

Using nitrogen as the plasma gas in a gas-cooled torch generally will not provide any advantages in cut quality, as ambient air still can contact the cut face and create some oxidation. Nitrogen may, however, increase electrode and nozzle life in some older plasma system designs.

Many other gas combinations are available for cutting aluminum, most of which are reserved for use with mechanized plasma systems. These gas mixtures include methane-air/nitrogen as well as argon-hydrogen/nitrogen combinations that must be used only in liquid-cooled plasma

Aluminum treadplate, which is soft and easily scratched, should be cut using a shielded front end in conjunction with a cutting template.

torches with appropriately designed gas flow control systems.

Use a Torch Shield

Older plasma torches that have an exposed nozzle (nonshielded) are prone to spatter buildup. This buildup bolsters a phenomenon known as double arcing, in which an arc develops from the internal electrode to the inside of the nozzle, and a second arc develops from the outside of the nozzle to the workpiece. Double arcing affects cut edge angularity, effectively reduces the

power and cutting speeds, and causes
premature nozzle orifice wear.
Adding a shield—an electri-
cally isolated copper part that
allows the torch to drag on the
plate without double arcing—
to the front end of the plasma
torch improves nozzle life,
eliminates “stiction” of the
torch when hand cutting, and
provides for better cut edge
angularity. Further enhance-
ments to the shield allow for
accurate template cutting,

reducing some of the operator skill that was once necessary.

Use a Cutting Template or Guide

Sometimes appearance is critical, especially on applications such as cutting decorative aluminum treadplate, which often is used for truck accessories such as toolboxes, trim pieces, and bed rail protectors. This material must be handled carefully during the cutting process as it is relatively soft and can easily be scratched. Newer plasma torch designs with shielded front-end technology can be used with templates and cutting guides to cut decorative aluminum parts accurately without touching the actual material. The front end of the torch is designed to guide against a template, allowing for accurate cuts, long nozzle life, and good cut quality that requires very little secondary cleanup. ■