HOW TO EVALUATE ENTRY LEVEL PLASMA CNC MACHINES

OTHER CONSIDERATIONS
Height Control. Some of these low cost machines – typically the lowest cost machines – come without a full feature height control system. Height control is very critical to plasma cutting in terms of cut edge angularity. Maintaining the correct height during a cut must be controlled to ±.010 in for good edge angularity. There is not a sheet or plate of steel that is as flat as the plasma torch needs, so an accurate method of maintaining the height during the cut is critical for best performance.

Pierce height – which is different (higher) than cut height – is critical to control the life of the plasma torch consumables. It is also controlled by the height control system. Can you cut without height control? Yes, but expect lesser cut quality, shorter consumable life (higher cut cost) and expect to stand by the machine to make adjustments and watch for torch plate collisions. With a good height control you can walk away and let the machine cut multiple parts on its own.

Plasma System. There are many plasma choices, but better quality plasma systems from major manufacturers produce better cut quality and provide longer consumable life. With most of the low cost CNC machines make sure the plasma is a “blowback” start. Many low cost import plasmas (as well as older technology air plasma systems) use a high voltage or high frequency start that can have an electrical noise effect on the PC-based machine control and electronics. This will disrupt cutting and possibly damage electronic components on the machine.

With a good height control and a good quality, new technology (45 amp) air plasma you can expect to cut and pierce up to ½ in steel with excellent cut quality at about 20 ipm. A 45 amp air plasma with a machine torch adds about $2200 to the cost of these machines. Air plasma systems of good quality are available up to about 105 amps, with ¾ in piercing capability.

Software. Most of these machines come with CAD drawing software and post processor or CAM software that takes the drawing file from the CAD and turns it into machine code that gives the cutting machine its operating directions. Look carefully at each machine’s software: some are easy to learn, some a little more difficult. The ease or difficulty, in my opinion, is based on your needs . . . as well as what types of software are easier for you to learn.

All systems come with various levels of instruction and tutorials. I don’t consider myself especially computer savvy, however, I had no problem learning the software for three different brands of these low cost machines. Usually, though, I find that it is an operator error, not something wrong with the software or system, when something does not go just right!

Tolerances, Productivity, Cut Quality. These are all areas where you will see differences between the more costly large industrial systems versus these low cost entry level machines. For example, on a large industrial machine the operator does not set the cut speed, the amperage, the arc voltage or about 20 other parameters that involve height control functions, motion control, acceleration, etc. On more automated industrial systems, the operator loads the plate, ensures the correct plasma consumables are in the torch (prompted by the screen on the CNC control), then pushes the start button and watches the machine cut . . . perhaps on ½ in material at 150 ipm at 260 amps.

On the low cost machine the operator is responsible for setting parameters, which can be taken from the plasma system operator’s manual. But there is room for error: the more experience, the better the operator does, the better the cut quality off the machine. A 40 amp plasma cutting at 20 ipm, assuming everything is set perfectly, will only cut one part for every 7.5 parts that the industrial machine cuts. The industrial system has more consistent cut quality and its cost per foot of cut is lower, based on longer consumable life and dramatically faster speeds.

Today, high end industrial plasma machine operators are typically expected to spend their time cutting metal, not drawing and adjusting parameters at the CNC control. With low cost machines, the exact opposite is often desired. The ability to walk up to the machine in a small shop, draw a part necessary for the current project and immediately cut one or two of these parts is important – and that is where these machines fit in very well. It can be expected that with a good quality plasma, a machine with a torch height control could produce cut part accuracy in the range of ±.020 in for most cases and produce literally hundreds of parts per shift, and at cut quality that is amazing by standards from even a few years ago on industrial machines.

THE BOTTOM LINE
These relatively new, low cost CNC plasma machines are filling a market niche that could not have been filled a few years ago. They allow the hobbyist, the homeowner, the small welding shop, and even the maintenance departments of larger companies a way to cut precise parts in house, without breaking the bank. Great technology, great prices. Just be sure it fits your needs!

Happy cutting!