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BLD-WELD01-09 · Lesson 7 · 14 clock hours · lab

Plasma Cutting Practice on Carbon Steel, Stainless, and Aluminum

Objective. Students produce plasma arc cuts on carbon steel, stainless steel, and aluminum to within ±1/8 in. dimensional tolerance with minimal dross, demonstrating correct travel speed and standoff distance.

How Plasma Arc Cutting Works

Plasma arc cutting (PAC) uses a constricted, high-temperature ionized gas column — the plasma — to melt and eject metal. Compressed air (or nitrogen, argon-hydrogen, or other gases depending on the application) is forced through a small copper nozzle around a tungsten electrode. A pilot arc ionizes the gas; the resulting plasma jet reaches temperatures far exceeding those of an oxy-fuel flame, allowing it to cut any electrically conductive metal, including carbon steel, stainless steel, and aluminum.

The two variables you control most directly at the torch are travel speed and standoff distance (the gap between the nozzle shield and the work surface). Both must fall within a narrow range for each metal and thickness. Get them right and you produce a clean, square kerf with minimal dross (resolidified metal that adheres to the bottom edge). Get them wrong and you create one of two recognizable defect signatures:

The correct speed produces a clean, slightly angled cut face (typically 3–5° from vertical on air-plasma systems), fine or no dross that breaks away with a light tap, and a top edge that remains sharp.

Standoff Distance

Standoff distance is typically specified by the consumable manufacturer for each amperage and metal thickness — commonly in the range of 1/16 in. to 3/16 in. for handheld units on sheet and light plate. Using a drag shield or standoff guide (a set of small nubs on the shield cup) removes guesswork for most portable units. Holding the torch too close risks double-arcing damage to the nozzle and contamination of the work; holding it too far weakens the arc and widens the heat-affected zone.

Cutting Carbon Steel

A36 carbon steel at 1/4 in. thickness is the standard learning material. Air plasma cuts it efficiently. Set amperage per the machine chart for the thickness. Begin the cut at the work edge or pierce slowly if starting in the middle of the plate. Watch the underside sparks: at correct speed they spray at a slight rearward angle beneath the cut. If they spray straight down, you are moving too fast; if they blow forward, slow down.

Cutting Stainless Steel

Type 304 stainless steel cuts cleanly with air plasma, but two concerns are heightened:

1. Hexavalent chromium fume. Stainless steel contains chromium; plasma cutting generates hexavalent chromium (Cr(VI)), a recognized carcinogen. Local exhaust ventilation (LEV) is mandatory — a fume extraction arm positioned at the source or a downdraft table. A respirator rated for metal fumes must be worn in addition to LEV. Do not rely on general shop ventilation alone for stainless.

2. Dross and discoloration. Stainless dross can re-weld (sinter) to the cut face, requiring grinding to remove — a sign of excessive heat input, usually from too-slow travel. A heat-tinted but clean face is acceptable; a face with adherent, glassy slag is not.

Measure your 3 × 3 in. square with a steel rule from edge to edge. Both dimensions must be within ±1/8 in. of 3.000 in.

Cutting Aluminum

6061 aluminum cuts easily; its lower melting point means the machine setting is typically reduced compared with the same thickness of steel. Travel speed is usually faster for aluminum at equivalent thickness. Dross on aluminum is softer and breaks away readily when correct parameters are used. Record amperage, travel speed (in./min if your machine displays it, or a qualitative pace), and standoff in your lab log. Rate the edge: smooth (clean, minimal dross), acceptable (light dross removable without grinding), or rough (heavy dross or gouging).

PPE and Safety Requirements

Dross Removal

A dross chisel and light tapping are the first-line removal method for carbon steel and aluminum. If a cut passes the lesson standard, grinding is not necessary. For stainless, re-welded dross that cannot be removed without grinding indicates the parameter combination was incorrect; the sample does not pass.

Completing the Lab

For each cut, photograph or sketch the result, note your settings, and enter an honest qualitative assessment in the lab log before the instructor countersigns. The instructor will measure all samples. You pass the lesson when all three materials meet the dimensional and dross standards described in the check.

Key terms9

Plasma arc cutting (PAC)A cutting process that uses a constricted, high-temperature ionized gas jet to melt and eject electrically conductive metals; capable of cutting steel, stainless steel, aluminum, and other metals.
Standoff distanceThe gap maintained between the plasma torch nozzle shield and the work surface; must match the manufacturer's specification for the amperage and material thickness to produce a clean cut.
Travel speedThe rate at which the torch moves along the cut line; too slow causes wide kerf and heavy dross, too fast causes incomplete cuts or forward-angled faces.
DrossResolidified molten metal that adheres to the bottom edge of a plasma cut; excessive dross indicates incorrect travel speed or amperage.
KerfThe width of material removed by the cutting process; in plasma cutting it is determined by nozzle orifice size, amperage, and standoff distance.
Hexavalent chromium (Cr(VI))A recognized carcinogen produced when cutting or welding stainless steel; requires local exhaust ventilation and appropriate respiratory protection to control exposure.
Local exhaust ventilation (LEV)A ventilation system that captures fumes and particulates at or near the source — such as a fume extraction arm or downdraft table — before they disperse into the breathing zone.
Heat-affected zone (HAZ)The portion of the base metal adjacent to the cut that has been heated enough to alter its microstructure or properties without being melted.
Double-arcingA damaging condition in plasma cutting where the arc jumps from the electrode to the nozzle rather than passing cleanly through the orifice; often caused by holding the torch too close to the work or worn consumables.

Worked examples3

1. You need to cut a 3 × 3 in. square from 1/8 in. 304 stainless steel. After cutting, you measure the piece and find one side is 3 1/16 in. and the adjacent side is 2 7/8 in. Does the piece pass the ±1/8 in. dimensional tolerance?
  1. Convert the tolerance to a decimal for clarity: ±1/8 in. = ±0.125 in.
  2. Check side 1: Target = 3.000 in. Measured = 3 1/16 in. = 3.0625 in. Deviation = 3.0625 − 3.000 = +0.0625 in. Is 0.0625 ≤ 0.125? Yes — side 1 passes.
  3. Check side 2: Target = 3.000 in. Measured = 2 7/8 in. = 2.875 in. Deviation = 3.000 − 2.875 = 0.125 in. Is 0.125 ≤ 0.125? Yes — side 2 is exactly at the tolerance limit and passes.
  4. Both dimensions are within tolerance, so the piece meets the dimensional standard.
Answer: Both sides pass. Side 1 is +1/16 in. over and side 2 is exactly −1/8 in. under; neither exceeds the ±1/8 in. limit.
2. You are checking whether your 2 in. wide aluminum strip meets the dimensional requirement. You measure the width at three places and get 1 15/16 in., 2 1/16 in., and 1 13/16 in. Which measurements pass and which fail the ±1/8 in. tolerance?
  1. Tolerance limits: 2.000 − 0.125 = 1.875 in. (low limit) and 2.000 + 0.125 = 2.125 in. (high limit).
  2. Location 1: 1 15/16 in. = 1.9375 in. Is 1.875 ≤ 1.9375 ≤ 2.125? Yes — passes.
  3. Location 2: 2 1/16 in. = 2.0625 in. Is 1.875 ≤ 2.0625 ≤ 2.125? Yes — passes.
  4. Location 3: 1 13/16 in. = 1.8125 in. Is 1.8125 ≥ 1.875? No — 1.8125 < 1.875, so this measurement fails the tolerance.
Answer: Locations 1 and 2 pass; location 3 at 1 13/16 in. (1.8125 in.) is outside the low limit of 1 7/8 in. (1.875 in.) and fails.
3. A plasma machine chart shows correct amperage for 1/4 in. A36 is 45 A. The nozzle manufacturer specifies a standoff of 1/8 in. You are holding the torch 1/4 in. from the work and moving at the correct speed, but the cut face is angled excessively and dross is heavy. What is the most likely parameter error and how do you correct it?
  1. Identify the parameters in use: amperage 45 A (correct per chart), standoff 1/4 in. (double the specified 1/8 in.), travel speed described as correct.
  2. Excessive standoff reduces arc focus, increasing kerf width, lowering energy density at the cut face, and allowing more resolidification as dross before the metal clears.
  3. Correction: Reduce standoff to the specified 1/8 in. using the torch's standoff guide or drag shield if equipped. Re-run the cut and observe whether dross and cut angle improve.
Answer: The standoff is too large (1/4 in. vs. the specified 1/8 in.). Reducing standoff to specification will concentrate the arc, improve cut-face squareness, and reduce dross.

The math in this lesson1

Learners check measured plasma-cut piece dimensions in sixteenths against a ±1/8 in tolerance.
N-Q.3
Brush up in the math halls at newSchoolWorks.com:
In class: Each student makes three straight cuts on 1/4 in. carbon steel coupons at slow, correct, and fast travel speeds; they photograph each cut, describe dross formation differences in the lab log, and identify the correct-speed sample. · Students cut a 3 × 3 in. square from 1/8 in. 304 stainless steel plate; dimensions are measured with a steel rule and the cut face is inspected for dross adherence and discoloration extent. · Students cut a 2 in. wide strip from 1/8 in. 6061 aluminum plate; they record amperage, travel speed, standoff used, and a qualitative edge rating (smooth/acceptable/rough) in the lab log; instructor countersigns each entry.
Materials: Air-plasma cutting unit, 1/4 in. A36 plate, 1/8 in. 304 stainless plate, 1/8 in. 6061 aluminum plate, steel rule, angle grinder, dross chisel, PPE (auto-darkening helmet shade 7–9 for plasma, leather gloves, hearing protection, respiratory protection—local exhaust ventilation required for stainless), lab log.
Check: Instructor measures all cut samples: dimensional deviation ≤1/8 in., dross removable without grinding on carbon steel and aluminum samples, cut face not re-welded on stainless sample; students must pass cuts on all three materials to complete the lesson.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Cutting Processes — Process Variables
1. When plasma cutting 1/4 in. A36 carbon steel at correct travel speed, the underside sparks should travel in which direction?
Safety & Health — Respiratory Hazards
2. Why is local exhaust ventilation (LEV) specifically required — not just recommended — when plasma cutting 304 stainless steel?
Cutting Processes — Defect Recognition
3. A student cuts carbon steel and finds the dross is thick, fused to the bottom edge, and requires grinding to remove. The most likely cause is:
Safety & Health — PPE
4. Which shade range is correct for an auto-darkening helmet used during plasma arc cutting on light-gauge material?
Quality & Inspection — Dimensional Measurement
5. You cut a 3 × 3 in. square from 1/8 in. stainless steel and measure one side at 2 13/16 in. What is the deviation from nominal and does it pass the ±1/8 in. tolerance?
Cutting Processes — Equipment and Consumables
6. What condition is most likely when double-arcing occurs in a plasma torch?
Cutting Processes — Process Variables
7. A student cutting 1/8 in. 6061 aluminum records an edge rating of 'rough' in the lab log. Which combination of parameters is the most probable cause?
Safety & Health — Electrical and Lockout/Tagout
8. Before changing a worn nozzle on the plasma torch, what safety procedure must be followed?
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