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How To Select And Operate A Hand-Held Plasma Cutter

One cut and they were hooked. Fabricators, contractors, maintenance personnel, artists and do-it-yourselfers who experience the benefits of a hand-held air plasma arc cutting machine rarely want to return to oxy-acetylene cutting or mechanical cutting processes such as saws, cut-off wheels, shears and snips.

Plasma cutting improved their productivity and lowered the cost of cutting. It cut faster, did not require a pre-heat cycle, cut any metal that conducted electricity (unlike oxy-fuel, which cannot cut stainless steel or aluminum), permitted portability around job sites, minimized the heat-affected zone and yielded a cut with a small kerf (cut width). Their plasma units also gouged, pierced, beveled, cut holes and traced shapes.

The actual process of operating a hand-held air plasma cutter unit is relatively simple. In fact, the hardest part comes before ever striking an arc-selecting a machine that best matches your application and choosing the right accessories.

Plasma Cutting

Plasma 101

Plasma looks and behaves like a high-temperature gas, but with an important difference; it conducts electricity and cuts any electrically conductive metal. The plasma arc results from electrically heating a gas, typically air, to a very high temperature. This ionizes its atoms and enables them to conduct electricity. A plasma arc torch uses a "swirl ring" that spins the gas around an electrode. The gas is heated in the chamber between the electrode and torch tip, ionizing the gas and creating plasma. This causes the plasma gas to greatly expand in volume and pressure. The small, narrow opening of the torch tip constricts the plasma and accelerates it toward the work piece at very high speeds (20,000 ft./s) and temperatures (up to 30,000° F).

The high-intensity plasma jet melts a very localized area. The force of the jet (or arc) pushes through the work piece and removes the molten metal. This jet easily cuts through metals with poor heat conductivity (stainless steel) or excellent conductivity (aluminum). The flame created by an oxy-fuel torch lacks plasma's concentration and cuts stainless steel or aluminum so poorly (excess warping or dross) that professionals don't bother to use it; they consider plasma arc cutting the standard process for these metals.

Watch Plasma Cutting Demo Videos

Three Questions to Start

The first questions to ask when selecting a plasma cutter relate to cutting capacity:

  1. What thickness of metal will you routinely cut?
  2. What is the maximum thickness you might cut?
  3. How fast do you want to cut?

Like a welding machine, a plasma cutter's amperage and voltage capacities determine its "size." Note that the plasma process requires relatively high voltage and low amperage levels, the opposite of welding. Many people erroneously judge a plasma machine solely by amperage. While this is an important indicator, remember that total output power (in watts) equals amperage times voltage. Do the math to obtain a more accurate product comparison.

For instance, Miller Electric's Spectrum 125C is 12-amp unit rated at 110 VDC (1,320 watts total). This provides a quality cut on 3/16-in. steel and will sever 1/4-in. steel. The cutting capacity of a particular "size" plasma machine varies greatly by manufacturer. The following table provides an indication of the maximum cutting performance you can expect from a Miller hand-held unit.

Spectrum Model Machine Output Quality Cut Rating

Miller Spectrum Plasma Cutters

Cutting Speeds and Quality Ratings

Knowing cutting speeds for the thickness of metal being cut allows you to calculate production rates, typically in parts per hour. This helps ensure that the cutting portion of the operation does not become a bottleneck. While cutting speeds often become critical to throughput in an automated application, they relate most to cut quality and operator satisfaction (or frustration) in hand-held applications.

Cutting Speed Chart
To determine the maximum rated cutting thickness of mild steel, follow the line from the 10 IPM (inches per minute) point on the cutting chart. The point at which this line intersects the cutting curve determines the maximum recommended production cutting thickness of the unit.
Plasma Cutting Speed Chart
Note: the rating is based on 10 IPM because this is the minimum speed at which the operator achieves a smooth, steady cut when using a hand-held torch.

Many manufacturers provide cutting speed charts that allow you to compare cutting speed performance. While there is no universal standard, Miller facilitates an "apples-to-apples" comparison by qualifying capacity with three standards: rated cut, quality cut and sever cut.

Miller Plasma Cutting - Rated Cut

Rated Cut

Rated cut is the thickness of metal that an operator can manually cut mild steel at a rate of 10 IPM. This is considered the minimum speed at which an operator achieves a smooth, steady cut and the best possible cut quality. For example, a 55-amp plasma cutter has a rated cut of 7/8 in.; you can cut 10 in. of 7/8-in. thick steel plate in one minute.

Miller Plasma Cutting - Quality Cut

Quality Cut

With a quality cut rating, the operator can cut thicker material, but at a slower rate. For example, a 55-amp plasma cutter provides a quality cut on 1-in. steel, but at speeds less than 10 IPM.

Miller Plasma Cutting - Sever Cut

Sever Cut

A sever cut rating means the operator is pushing the machine to its maximum thickness capabilities (1-1/4 in. for a 55-amp unit). Cutting speeds will be very, very slow and the cut will require significant clean-up. Fortunately, cutting speeds increase as the material gets thinner.

As with cutting thickness, cutting speed varies greatly between models. Tests on a mechanized cutting table showed that a 55-amp Spectrum plasma cutter cuts 1/2 in. at 38 IPM with an extended tip and at 30 IPM with a drag shield. Some competitive units of the same amperage only cut at 24 or 28 IPM.

Primary Power

Plasma cutting requires two basic elements, air and electricity, so the next question to ask when selecting a plasma cutter is what type of input power is available. If you only have 115V primary service, don't worry. Several 12- and 25-amp plasma cutters, such as the Spectrum 125C or Spectrum 375, operate using 115 or 230 V power. If your input circuit has a 30-amp breaker, you even get equal cutting capacity at both voltages (with a 20-amp breaker, cutting capacity drops by 20 percent).

Miller Electric offers a primary power management technology called Auto-Line™. Auto-Line allows a machine to accept input voltages ranging 190 through 630 V, single or three-phase, 50 or 60 Hz. This means you can plug this plasma cutter in anywhere in the world. And even if the primary power spikes and dips, but stays within the 190 to 630 V range, units with Auto-Line technology provide a steady, consistent arc and full cutting power.

Miller Auto-Line Power Management Technology

When working in the field, many contractors and fabricators only source of power comes from an engine-driven welding generator's auxiliary power. Especially for cutting aluminum and stainless steel (which oxy-fuel can't cut), many mechanical contractors opt for a lightweight plasma cutter that permits mobility. Units in the 27 to 80-amp class weigh between 55 and 92 lbs., torch included.

The Spectrum 375 will run off an engine drive providing at least 4 kW of auxiliary power. Because of its 15 percent line voltage compensation feature and power factory correction technology (which reduces draw from 34 to 28 amps), this unit is much less susceptible to nuisance breaker trips. Spectrum plasma cutters provide decent cutting power (5/8-in. steel) when paired with an 8 kW engine drive. When paired with 15 kW auxiliary power, certain models provide full cutting power (approximately 7/8 in. and 1-1/8 in. capacity, respectively).

If you plan to use an engine drive's auxiliary power, strongly consider a unit with Auto-Line technology. Similar units without Auto-Line experience erratic cutting arcs, frequent breaker trips, blown circuit boards and are prone to premature transformer failure. These problems typically occur because the plasma cutter, once triggered, places such a load on the line that voltage levels drop below the plasma cutter's operating range.

Dirty Environments

In environments with heavy dust and metal shavings (such as from grinding), machines with Wind Tunnel Technology™ and Fan-On-Demand™ provide better reliability. With Wind Tunnel Technology, the cooling air flows through the machine in such a way that it doesn't blow over the electronic components, so grinding dust can't settle on the PC boards and other critical components. Fan-On-Demand means that the cooling fan runs only when needed, reducing the amount of debris entering the unit.

With traditional technology units, the fan works constantly, sucking in any air-borne particles. If you opt to purchase such a unit, position it as far away from the particle source as possible.

Air Supply

Most manufacturers of hand-held plasma cutters recommend using ordinary air as the cutting gas. In mobile applications, contractors often opt for bottled nitrogen because it costs less than bottled air. When cutting stainless steel, some people believe nitrogen produces slightly less oxidation, as it is drier than compressed air. For cutting thinner materials in the field, contractors involved with HVAC, sign making and maintenance often select a plasma cutter like the Spectrum 125C because of its built-in air compressor. This lightweight package provides superior mobility.

Take a Tip

Different consumables and torch accessories enable configuring a plasma cutter for a variety of applications and operator skill levels. For cutting, select a regular or an extended cutting tip. With plasma units greater than 40 amps, a built-in drag shield protects the regular tip and automatically holds it 1/8 in. off the surface of the work piece (with units less than 40 amps, the tip can be placed directly on the work piece). For following straight edges or tracing stencils or patterns, many people select this arrangement because it allows them to simply brace the edge of the drag shield on a guide (a ruler, piece of wood, cardboard pattern, etc.) and start cutting.

For beveling, cutting in corner and a better view of the cutting arc, some operators prefer an extended tip. This tip protrudes 1/2 to 1 in beyond the retaining cup, enabling the operator to better direct the flow of plasma jet and reach into corners. Having a steady hand is important because touching the tip to the work piece three or four times enlarges the hole in the tip to the point where it no longer constricts the arc sufficiently.

If you do not have a steady hand (remember that the cut will reflect any hand movement (consider using roller guides and circle cutting guides). These guides hold the torch in position so that it maintains consistent standoff height, prevent the tip from touching the work piece and make it easier to travel in a straight line or cut a perfect circle. For easy beveling, roller and circle cutting guides permit independently adjusting wheel height to set the bevel angle.

For removing old or imperfect welds, use a gouging tip. The hole on a gouging tip is three to four times wider than a regular tip. Its cone shape pushes out the plasma arc, which removes much more material than the constricted orifice of the regular tip.

Many people do not believe that plasma works well for gouging, but that perception is based on older machines with weaker arcs. More powerful Spectrum plasma cutters feature a much stronger design that creates an arc 1 to 1-1/2 in. long. Although carbon arc gouging removes metal faster, plasma arc gouging produces much less smoke and noise, as well as offers more control over the arc.

As with cutting capacity and speeds, consumable life varies greatly between manufacturers. In a test comparing the number of 12-in. long, 1/2-in. thick strips of mild steel cut with a single set of consumables, Miller outperforms many of its competitors. A 55-amp Spectrum plasma cutter cuts 450 strips with an extended tip and 330 with a drag shield. Miller's efficient design reduces consumables cost by up to 500 percent compared to competitive machines.

 

Plasma Cutting Roller Guide

Roller Guides - For Straight Lines

Plasma Cutting Circle Guide

Circle Cutting Guide

Plasma Cutting Bevel Cut

Beveled Cut - Adjust Wheel Height

HF or Contact Start

Plasma cutters use either "high frequency start" or "contact start" technology to initiate the pilot arc. If you plan to use a plasma cutter near telephones, computers, CNC machines or other electronic equipment, be aware that high frequency (HF) often interferes with electronic controls.

To avoid potential HF problems, all Miller machines feature contact start design that does not cause interference. Even better the contact start method creates a visible pilot arc that helps you better position the torch.

Pre-Cut Checklist

A few final words of advice before cutting:

Cutting Technique

 

Step 1. Place the drag shield on the edge of the base metal, or hold the correct standoff distance (typically 1/8 in.). Direct the arc straight down. (Dragging the tip will reduce tip life).

Plasma Cutting - Step 1

The arc starts immediately when trigger is pressed.

Step 2. Raise the trigger lock, press the trigger and the pilot arc starts immediately.

Plasma Cutting - Step 2

Step 3. Once the cutting arc starts, begin to slowly move the torch across the metal.

 

Plasma Cutting - Step 3

Step 4 . Adjust your speed so sparks go thru metal and out bottom of cut.

If the sparks are not visible at the bottom of the plate, the arc is not penetrating the metal. This can be caused by moving the torch too quickly, insufficient amperage or directing the plasma stream at an angle (not straight down). Insignificant grounding can also cause this problem.

Plasma Cutting - Step 4

Step 5 . At the end of a cut, angle the torch slightly towards the final edge or pause briefly before releasing trigger to completely sever the metal.

Plasma Cutting - Step 5

Step 6 . To cool torch, post-flow air continues for 20 - 30 seconds after releasing the trigger; pressing the trigger during post-flow instantly restarts the arc.

Plasma Cutting - Step 6

Traveling at the right speed produces a very clean cut with less dross on the bottom of the cut, as well as little or no distortion to the metal. If the travel speed is too slow, the material you are cutting may become hot and accumulate more dross. To minimize dross, increase travel speed or reduce amperage (for an indication of how fast to move the torch, refer to the machine's cutting speed graph or check the speed for a rated cut). Dross also accumulates when you push a machine to its maximum thickness. The only cure for this is a bigger machine.

Gouging and Piercing Techniques

To gouge — to remove old welds or imperfections — hold the torch at a 40 to 45° angle to the base metal. Establish an arc length of 1 to 1-1/2 in. and move the torch across the metal, adjusting torch speed, arc length and angle as needed. Direct sparks away from the torch, and do not gouge too deeply on one pass. Make multiple passes if needed.

To pierce metal — creating a hole, such as to start coping or insert a valve — place the torch at a 40 to 45° angle to the work piece. Press the trigger. After the machine initiates the cutting arc, bring the torch tip to a 90° angle and the arc will pierce the base metal. Generally, a machine can pierce metal up to one-half of its maximum cutting thickness. Powerful machines pierce 1/4 in. steel within a second or two.

If you select the appropriate plasma cutter and service it properly, you can experience years of trouble-free performance. In fact, most "problems" with plasma cutting relate to other systems (air, consumables), not the machine itself. Most importantly, almost every person who cuts with a plasma machine gets hooked on the technology. They couldn't be paid to go back to other cutting methods.

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