Optimizing Automated Plasma Cut Quality

If you use automated plasma cutting in your business, it is crucial to consistently deliver accurate cut parts, with minimal bevel, and little or no dross. Automated plasma cutting systems can produce accurate cut parts with a variety of gas combinations, but it is what’s under the surface that will affect your final product quality. Different gas combinations will react with the cut metal edge and affect the weldability of the surface. Selecting the appropriate gas combination is a key component in ensuring high quality welds.

The gases that may be selected for automated plasma cutting are dependent upon the type of torch used by a plasma system. Some torches offer only a few choices and others a multitude of possibilities. This article will take the mystery out of deciding which is best for your cutting needs.

Basic Torch Design 101

Plasma torches are either a single gas or dual gas design. Typically, single gas design torches operate at up to 125 amps and are cooled by the gas flowing through them. The single gas input is split inside the torch into the plasma and shield flows. Over 125 amps, the increased heat of the arc requires that the torch is liquid cooled. Virtually all liquid cooled torches are the dual gas type. Dual gas torches have separate pathways for the plasma and shield gases allowing the torch leads to deliver the gases to the torch head through two separate hoses. The plasma and shield gases can therefore be different and delivery pressures and flows can be different as well.

Basic-Plasma-Torch-Design.jpg

Systems with Single Gas Torches

Low cost automated plasma cutting systems are configured with single gas torches designed to cut all metal types using shop compressed air. Prices for automated air plasma cutting systems range between $3,500 (40 amp) and $10,000 (125 amp). Please note that these prices are for a plasma cutting power and torch – not a complete CNC cutting machine. This type of plasma system has become extremely popular with metal fabricators doing ornamental metal work and relatively low production general purpose plate cutting.

However, many customers find that the cut quality delivered by air plasma is less than they require. For example, the cut face of steel plate cut with air includes large amounts of dissolved nitrides. Air is roughly 78% nitrogen and 21% oxygen. When a GMAW weld is applied directly to the cut surface, nitrides are often trapped inside the weld as the metal solidifies. Grinding the cut edge surface before welding will eliminate the nitriding issue. When cutting aluminum with air plasma, the cut face is heavily oxidized and very “grainy” in appearance and will require grinding before welding on the cut face. The cut surface of stainless will also be heavily oxidized. The surface will be dark gray and rather crusty from the formation of nickel oxides. Such surfaces are not weldable without grinding.  

Cylinder gases such as nitrogen or nitrogen-hydrogen (N2 95% - H2 5%) can be used with some single gas torch systems in an effort to improve the cut surface quality of non-ferrous metals. However, the required total flow rate for a 125 amp single gas plasma torch is as much as 550 CFH. This will increase gas costs because a cylinder with a 330 cubic foot capacity will be empty in 36 minutes.

Plasma systems configured with single gas torches offer significantly shorter consumable life and much higher operating cost than plasma systems configured with liquid cooled dual gas torches. Air plasma systems do not include long-life technology which ramps amperage and gas flow at the start and stop of every cut and ensures very consistent cutting performance over the life of the consumables. Without such technology, cut quality changes dramatically over the life of a consumable set. 

Systems with Dual Gas Torches

Today’s automated precision plasma cutting systems are priced between $40,000 (130 amp) and $75,000 (400 amp). Please note that these prices are for a plasma cutting power supply, gas console and torch only – not a complete CNC cutting machine. They are configured with liquid cooled dual gas torches, computer-controlled switch-mode power supplies and sophisticated automatic gas delivery systems. Cut charts embedded into today’s CNC controls automatically adjust cutting parameters and select the required gases based on the material and thickness selected. Also, most precision plasma systems include technology which ramps amperage and gas flow at the start and stop of every cut. This technology significantly extends consumable life and ensures extremely consistent cutting performance over the life of consumables

The type of metal to be cut, thickness and the cut face weld-ability required will determine which gas combinations are recommended. In an effort to deliver premium cutting performance, manufacturers of automated plasma systems make large investments developing consumable sets designed to operate at exact amperages using specific gas combinations.

Businesses seeking contract plate cutting opportunities will find that the vast majority of competing contract shops operate plasma cutting machines configured with precision high definition plasma systems. Shops pursuing contract plasma cutting will also be competing with the laser and water jet cutting processes. Today’s contract cutting firms offering plasma cutting must consistently deliver accurate cut parts, on time, at competitive prices, with minimal bevel, little or no dross and weld-able cut edges.

Hypertherm Process Core Thickness (PCT)

The cut chart for every cutting process contains a range of possible thicknesses. Process engineers work to optimize a range of thicknesses (usually in the middle of the overall range of thicknesses). This optimized range is called the process core thicknesses (PCT). Thicknesses greater and less than the PCT can have varied results relative to cut quality, cut speed, and piercing capability.

Process Categories

The Hypertherm XPR300 cut charts offer 5 process categories. Each category has a unique process category number (1 – 5) that correlates to the performance that you can expect when you select this process. The process category number for the process that you choose changes the quality-speed balance.

For best results, Hypertherm recommends that you select process category number 1 whenever possible. Category 1 represents an optimized thickness (or PCT) for that cut process with the overall best balance of cut quality and cut speed.

 

The table below describes the results that you can expect with different process category numbers.

Process Category # Process Category Condition

 Category Description 

Quality Speed
1 Process Core Thickness (PCT)

- Best overall balance of productivity and cut quality
- The process is optimized for this thickness
- Expect cut speeds that range form 80 in/min - 150 in/min
- Dross free, in most cases

Very good Very good
2 Greater than PCT

- Good choice when edge quality is more important than speed
- Expect cut speeds that are slower than 80 in/min
- Some low-speed dross is possible

Very good-excellent Lower
3 Less than PCT

- Good choice when speed is more important than edge quality
- Expect cut speeds that are faster than 150 in/min
- Dross-free results in most cases

Lower Higher
4 Edge start only

- Edge start is required
- Thick, low-speed dross is likely

Good Low
5 Severance

- This is the maximum thickness for these processes
Edge start is required
Expect cut speeds that are slower than 10 in/min
Cut-edge quality can be rough
Expect significant dross

Very low Very low

 

Mild Steel

Gas Combinations for Mild Steel

All manufacturers design their automated precision plasma cutting systems to use oxygen as the plasma gas for cutting steel. Oxygen plasma produces the best cut quality, least amount of dross, fastest cutting speed, most weldable edge and most forgiving parameter settings. At 30 and 50 amps, most precision automated plasma cutting systems use oxygen plasma and oxygen shield. Shop compressed air is used as the shield gas above 50 amps.

Plasma, laser and oxy-fuel cutting all use oxygen when cutting steel. All three processes leave a very thin film of iron oxide on the cut surface. This thin film can be removed easily with an abrasive treatment. However, if not removed, paint applied to the cut surface may simply flake off. The images below show the cut surface with the film on and removed.

Amperage and Thickness Recommendations for Mild Steel

In addition to selecting the best gas combination, selecting an appropriate cutting amperage is equally important to producing excellent plasma cutting results. The chart below includes recommendations for mild steel cutting amperage and material thickness when using a Hypertherm® XPR300™ plasma system. Each row matches amperage with thicknesses that will produce what Hypertherm calls a "Cut Category #1”.

Steel-Cut-with-O2-Plasma-Air-Shield-with-Iron-Oxide-Film.png Steel Cut with O2 Plasma/Air Shield with Iron Oxide Film.
 
Steel-Cut-Using-O2-Plasma-Air-Shield-With-Iron-Oxide-Film-Removed-From-Right-Side.pngSteel Cut Using O2 Plasma/Air Shield with Iron Oxide Film Removed from Right Side.
 

Mild Steel

Amperage Plasma Gas Shield Gas Cut Category #1 Gas Cost Description of Cut
30 A O2 O2 .105"-.135" Low Best cut, weldable edge
80 A O2 Air .250"-.375" Low Best cut, weldable edge
130 A O2 Air .250"-.500" Low Best cut, weldable edge
170 A O2 Air .375"-.625" Low Best cut, weldable edge
300 A O2 Air .625"-1.000" Low Best cut, weldable edge

 

Stainless Steel

Gas combinations for Stainless Steel

For general purpose cutting, most manufacturers of automated precision plasma systems suggest cutting stainless from thin gage to 1.5” with nitrogen plasma. Either nitrogen or shop compressed air is used as the shield gas. The cut should be free of top and bottom dross, have a relatively smooth cut surface and minimal bevel. If weld metal will be applied directly to the cut face surface grinding is required.

If weld metal will be applied directly to the cut surfaces from thin gage to ½”, consider gases such as F5 (95% N2 and 5% H2) plasma and N2 shield to produce weldable cuts. Some plasma systems include a gas delivery system with the ability to blend argon, hydrogen, and nitrogen for customized plasma gas mixtures based on thickness and cutting amperage. For plate from ½” to 1.5”, Ar-H2-N2 blended plasma gas mixtures with N2 shield produce premium results. Edges cut with these combinations are typically very smooth, very square and often a golden color. A few manufacturers offer the ability to use nitrogen plasma and tap water as the shield. The N2/water process produces a very square and weldable edge at a very low cost from thin gage to 1.5” plate.   

Note that gas combinations such as F5 plasma / N2 shield and Ar-H2-N2 plasma / N2 shield require a narrow window of parameters (speed and voltage) to produce premium results. Some adjustment to cutting speed and voltage may be required to optimize cuts.

 

Images below show representative results from the gas combinations listed. 

Steel-Cut-Using-N2-Plasma-N2-Shield.png Stainless Cut Using N2 Plasma/N2 Shield.
 
Stainless-Cut-Using-F5-Plasma-N2-Shield.pngStainless Cut Using F4 Plasma/N2 Shield.
 
Stainless-Cut-Using-N2-Plasma-H2O Shield.pngStainless Cut using N2 Plasma/H2O Shield.
 
Stainless-Cut-Using-AR-H2-N2-Plasma-N2-Shield.pngStainless Cut Using Ar-H2-N2 Plasma/N2 Shield.
 

Amperage and Thickness Recommendations for Stainless Steel

In addition to selecting the best gas combination, selecting an appropriate cutting amperage is equally important to producing excellent plasma cutting results. The chart below includes recommendations for stainless steel cutting amperage and material thickness when using a Hypertherm® XPR300™ plasma system. Each row matches amperage with thicknesses that will produce what Hypertherm calls a "Cut Category #1”.

Stainless Steel

Amperage Plasma Gas Shield Gas Cut Category #1 Gas Cost Description of Cut
40 A N2 N2 .105"-.135" Med Good face, may not be weldable, dry cut
60 A N2 N2 .135"-.188" Med Good face, may not be weldable, dry cut
60 A N2 H2O .135"-.188" Low Excellent face, weldable edge, wet cut
60 A F5 N2 .135"-.188" High Excellent face, weldable edge, dry cut
80 A N2 N2 .250"-.375" Med Good face, may not be weldable, dry cut
80 A N2 H2O .250"-.375" Low Excellent face, weldable edge, wet cut
80 A F5 N2 .250"-.375" High Excellent face, weldable edge, dry cut
130 A N2 N2 .375"-.500" Med Good face, may not be weldable, dry cut
130 A N2 H2O .375"-.500" Low Excellent face, weldable edge, wet cut
130 A Ar2-H2-N2 N2 .375"-.500" High Excellent face, weldable edge, dry cut
170 A N2 N2 .500"-.625" Med Good face, may not be weldable, dry cut
170 A N2 H2O .500"-.625" Low Excellent face, weldable edge, wet cut
170 A Ar2-H2-N2 N2 .500"-.625" High Excellent face, weldable edge, dry cut
300 A N2 N2 .750"-1.000" Med Good face, may not be weldable, dry cut
300 A N2 H2O .750"-1.000" Low Excellent face, weldable edge, wet cut
300 A Ar2-H2-N2 N2 .750"-1.000" High Excellent face, weldable edge, dry cut

NOTES:

Select gas combinations with a "High" gas cost only when a weldable edge is required.
Some gas combinations will produce cut edge faces that will require abrasive grinding to allow welding on the edge.

 

Aluminum

Gas Combinations for Aluminum

For general purpose cutting, most manufacturers of automated precision plasma cutting systems suggest cutting aluminum from thin gage to 1.5” using either shop compressed air or nitrogen plasma. Either nitrogen or shop compressed air is used as the shield gas. The cut should be free of top and bottom dross, have a relatively smooth cut surface with minimal bevel. However, the cut face will be heavily oxidized, rough and quite “grainy”. Grinding will be required if weld metal is to be applied directly to the cut face surface.

Blends of argon, hydrogen, and nitrogen plasma and nitrogen shield that deliver premium results should be considered if weld metal will be applied directly to the cut surfaces of aluminum from ¼” to 1.5”. Edges cut with these combinations are quite smooth, very square and weldable. A few manufacturers also offer the ability to use nitrogen plasma and tap water as the shield. The N2/water process produces a very square and weldable edge at a low cost from thin gage to 1.0” plate.

 

Images below show representative results from the gas combinations listed. 

Aluminum-Cut-Using-Air-Plasma-Air-Shield.png Aluminum Cut Using Air Plasma/Air Shield.
 
Aluminum-Cut-Using-N2-Plasma-N2-Shield.pngAluminum Cut Using N2 Plasma/N2 Shield.
 
Stainless-Cut-Using-N2-Plasma-H2O Shield.pngAluminum Cut Using N2 Plasma/N2 Shield.
 
Stainless-Cut-Using-AR-H2-N2-Plasma-N2-Shield.pngAluminum Cut Using Ar-H2-N2 Plasma/N2 Shield.
 

Amperage and Thickness Recommendations for Aluminum

In addition to selecting the best gas combination, selecting an appropriate cutting amperage is equally important to producing excellent plasma cutting results. The chart below includes recommendations for aluminum cutting amperage and material thickness when using a Hypertherm® XPR300™ plasma system. Each row matches amperage with thicknesses that will produce what Hypertherm calls a "Cut Category #1”.  

Aluminum

Amperage Plasma Gas Shield Gas Cut Category #1 Gas Cost Description of Cut
40 A Air Air .102" - .125" Low Oxidized face, not weldable, dry cut
40 A N2 N2 .102" - .125" Med Good face, may not be weldable, dry cut
60 A Air Air .125"-.188" Low Oxidized face, not weldable, dry cut
60 A N2 N2 .125"-.188" Med Good face, may not be weldable, dry cut
60 A N2 H2O .105"-.375" Low Excellent face, weldable edge, wet cut
80 A Air Air .250"-.375" Low Oxidized face, not weldable, dry cut
80 A N2 N2 .250"-.375" Med Good face, may not be weldable, dry cut
80 A N2 H2O .250"-.375" Low Excellent face, weldable edge, wet cut
130 A N2 N2 .375"-.500" Med Good face, may not be weldable, dry cut
130 A N2 H2O .375"-.500" Low Excellent face, weldable edge, wet cut
130 A Ar2-H2-N2 N2 .375"-.500" High Excellent face, weldable edge, dry cut
170 A Air Air .500"-.625" Low Oxidized face, not weldable, dry cut
170 A N2 N2 .500"-.625" Med Good face, may not be weldable, dry cut
170 A N2 H2O .500"-.625" Low Excellent face, weldable edge, wet cut
170 A Ar2-H2-N2 N2 .500"-.625" High Excellent face, weldable edge, dry cut
300 A N2 N2 .750"-1.000" Med Good face, may not be weldable, dry cut
300 A N2 H2O .750"-1.000" Low Excellent face, weldable edge, wet cut
300 A Ar2-H2-N2 N2 .750"-1.000" High Excellent face, weldable edge, dry cut

NOTES:
Select gas combinations with a "High" gas cost only when a weldable edge is required.
Some gas combinations will produce cut edge faces that will require abrasive grinding to allow welding on the edge.

 

High Quality Plasma Cutting

Profile metal cutting technology has undergone incredible changes in the past 50 years. The manufacturers of precision plasma equipment continue to invest in their pursuit of continuous improvement of plasma cutting. The variety of gas options offered by plasma cutting systems allow fabricators to select the gas solution that makes the most sense for their projects.

Automated plasma cutting using shop compressed air to cut steel, stainless and aluminum may be acceptable for certain applications. However, premium results require using gas combinations designed to deliver optimal results on specific materials and thicknesses. Assess the specifications of your project before deciding which combination of gases is most suitable for the job. It is important to fully understand the effects that plasma cutting with different gas combinations will have on the metal surface. Select the gas combination that will simplify the process and produce a quality cut edge that is weldable right after the cut is made. The gas combination that is right for your business will produce the best result.

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