Unit 5 Comprehensive Quiz: Plasma Cutting

Unit: 05 - Plasma Cutting Duration: 30-45 minutes Passing Score: 70% Format: Multiple choice covering all modules Questions: 12

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Instructions

This comprehensive quiz covers all modules in the Plasma Cutting unit. You should complete all module assessments before attempting this unit quiz. The quiz tests both factual recall and application of concepts across modules.

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What is a plasma arc, and how does it cut through metal?

Plasma is a chemical acid sprayed through the torch that dissolves metal

Plasma is an electrically conductive ionized gas heated to 20,000-40,000°F that melts metal while a high-velocity gas stream blows the molten material out of the cut

Plasma is a mechanical blade that vibrates at ultrasonic frequencies to cut metal

Plasma is a laser beam focused through a gas lens to increase power

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Explanation: Plasma cutting uses an electrical arc between the torch electrode and the workpiece to ionize a gas (compressed air, nitrogen, or argon/hydrogen mix) into plasma — the fourth state of matter. At 20,000-40,000°F, the plasma arc melts virtually any conductive metal. Simultaneously, the high-velocity gas stream (exiting at near-sonic speed) blows the molten metal through the kerf, creating the cut. This combination of extreme heat and kinetic energy is what makes plasma effective on thick metals.

Why does mild steel cut more easily than aluminum on a plasma cutter, despite aluminum having a lower melting point?

Mild steel is thinner than aluminum sheet stock

Aluminum cannot be cut with plasma — only mild steel works

Aluminum has extremely high thermal conductivity (167 W/m·K vs. 50 for steel), which rapidly dissipates heat away from the cut zone, requiring more energy input to maintain the melt

Mild steel is softer than aluminum

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Explanation: Although aluminum melts at 1,220°F (660°C) compared to mild steel's 2,750°F (1,510°C), aluminum's thermal conductivity is over 3x higher. This means heat is conducted away from the cutting zone much faster, requiring either higher amperage, slower travel speed, or both to maintain a through-cut. Additionally, aluminum's reflective oxide layer can interfere with arc initiation. Mild steel's moderate conductivity and exothermic oxidation reaction (when cutting with compressed air) make it the easiest plasma cutting material.

What determines the maximum metal thickness a plasma cutter can sever?

The length of the plasma torch

The type of gas used (air vs. nitrogen)

The amperage rating of the power supply — higher amperage delivers more energy to the arc, enabling cuts through thicker material

The diameter of the nozzle orifice only

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Explanation: Amperage is the primary determinant of cutting capacity. A 40A plasma cutter typically severs up to 1/2" (12mm) mild steel; an 80A unit handles 1" (25mm); industrial 200A+ systems cut 2"+ (50mm+). However, "sever" capacity (maximum thickness it can get through) differs from "quality cut" capacity (maximum thickness with clean edges and minimal dross) — quality cut is typically 60-70% of sever capacity. Nozzle size, gas type, and travel speed also affect the achievable thickness.

What is "dross" in plasma cutting, and what causes excessive dross on the bottom edge of a cut?

Dross is the smoke produced during cutting — it indicates the exhaust system is working

Dross is the heat-affected zone adjacent to the cut

Dross is resolidified molten metal that adheres to the bottom edge of the cut — it is caused by traveling too slow (molten metal pools and re-welds), too fast (incomplete melt-through), or incorrect amperage for the material thickness

Dross is a protective coating applied to plasma-cut edges to prevent rust

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Explanation: Ideal plasma cuts produce minimal dross — the gas stream should blow all molten metal cleanly through the kerf. Excessive dross forms when: (1) speed too slow — over-melting creates pools that re-solidify on the bottom edge ("low-speed dross," typically heavy and hard), (2) speed too fast — incomplete melt-through leaves partially cut material clinging to the bottom ("high-speed dross," typically small beads), (3) amperage too low — insufficient energy to fully melt the material. Optimal settings produce dross-free cuts that require no post-processing.

What PPE is required for plasma cutting operations?

Only safety glasses and work gloves

The same PPE as MIG welding — full welding helmet required

Shade 5-8 safety glasses or face shield (for UV/IR radiation), leather or heat-resistant gloves, long-sleeved fire-resistant clothing, steel-toed boots, and hearing protection — the arc produces intense UV light, sparks, noise, and molten metal spatter

No PPE is needed if the plasma cutter has a built-in shield

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Explanation: Plasma arcs produce intense UV and infrared radiation (though less than welding arcs), requiring shade 5-8 eye protection (vs. shade 10-13 for welding). Molten metal spatter and sparks are ejected at high velocity from the bottom of the cut. Fire-resistant clothing prevents ignition of synthetic fabrics. Hearing protection is needed because plasma torches operate at 95-115 dB. Steel-toed boots protect against dropped hot workpieces. All combustible materials must be cleared from the cutting area.

What is "pierce delay" and why is it a critical setting for thicker metals?

Pierce delay is the time between pressing the trigger and the plasma arc starting

Pierce delay is the cooling time needed between cuts

Pierce delay is the time the torch remains stationary while the arc penetrates through the full thickness of the metal before travel begins — without adequate pierce delay on thick material, the torch moves before breakthrough, resulting in a failed cut start

Pierce delay controls the ramp-up speed of the plasma arc

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Explanation: When starting a cut from the interior of a plate (not from an edge), the arc must first melt through the full material thickness — this takes time proportional to thickness. On 1/4" mild steel, pierce delay is typically 0.5-1 second; on 1/2", 1-2 seconds; on 1", 3-5 seconds. If the torch begins traveling before full penetration, the arc loses contact with the bottom surface and the cut fails. CNC plasma tables program pierce delay automatically; manual operators must develop the feel for when breakthrough occurs (visible change in arc sound and color).

Why should the plasma torch be held perpendicular (90°) to the workpiece during cutting?

Tilting the torch will damage the nozzle

The plasma gas only flows straight down

A perpendicular torch produces a square-edged cut; tilting causes one side of the kerf to be beveled (angled) and the other undercut, resulting in parts that don't fit together properly

It doesn't matter — the torch can be at any angle

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Explanation: The plasma arc column is conical, slightly wider at the top than bottom. When the torch is perpendicular, this cone is centered in the kerf, producing nearly square edges on both sides. Tilting the torch shifts the cone, creating a bevel on one side (typically 1-3° even at perpendicular; worse when tilted). For manual cutting, maintaining perpendicularity is one of the hardest skills to master — drag tips/guides, straight edges, and CNC tables all help maintain the correct angle.

What is the function of the pilot arc in a plasma cutter?

The pilot arc is used only for welding, not cutting

The pilot arc heats the workpiece before cutting begins

The pilot arc is a small, low-power arc between the electrode and nozzle inside the torch that ionizes the gas — when brought near the workpiece, the ionized gas provides a conductive path for the main cutting arc to transfer to the metal

The pilot arc is the visible light from the plasma that illuminates the cut line

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Explanation: Modern plasma cutters use a "non-contact start" method: the pilot arc creates a small plasma flame at the torch tip without touching the workpiece. When the ionized gas stream contacts the conductive metal, the main cutting arc transfers from the nozzle to the workpiece (called "arc transfer"). This is safer than older "contact start" methods that required touching the torch to the metal. The pilot arc also allows cutting expanded metal, grating, and other materials where contact start would be impractical.

A student is cutting 1/4" mild steel and notices the cut edges have a blue/purple discoloration extending 3-4mm from the cut. What does this indicate?

The metal is contaminated with impurities

The cut was made at the correct speed — this is normal

The heat-affected zone (HAZ) is excessive — travel speed is too slow, causing heat to spread further into the base metal and changing its temper/hardness in that zone

The plasma gas is contaminated with moisture

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Explanation: The heat-affected zone (HAZ) is the area adjacent to the cut where the metal's microstructure has been altered by heat exposure. Blue/purple discoloration on steel indicates temperatures of 500-700°F in that zone. Excessive HAZ weakens the metal and can cause warping. Faster travel speed concentrates heat in the kerf rather than allowing it to spread. For 1/4" mild steel on a 60A cutter, optimal travel speed is typically 40-60 inches per minute — significantly faster than most beginners attempt.

What is the correct standoff distance (torch-to-workpiece gap) for most handheld plasma cutting?

The torch nozzle should be pressed firmly against the metal surface

1/8" to 3/16" (3-5mm) gap between the nozzle and workpiece — many torches include a drag shield or standoff guide to maintain this distance automatically

1" (25mm) to prevent the nozzle from overheating

The distance doesn't matter as long as the arc transfers

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Explanation: Standoff distance affects cut quality, nozzle life, and arc stability. Too close: the nozzle contacts molten spatter and degrades rapidly; double-arcing can occur (arc jumps from nozzle to workpiece, bypassing the orifice). Too far: the arc column widens, reducing energy density and cut quality; the arc may extinguish. Most manufacturers recommend 1/8"-3/16" for handheld work. Drag shields (ceramic or metal cups) allow the torch to rest on the workpiece while maintaining correct standoff — essential for beginners and straight cuts.

What ventilation or fume extraction is required for indoor plasma cutting?

No ventilation is needed if the cutting area is larger than 500 square feet

A standard household fan pointed at the cutting area is sufficient

Downdraft or backdraft fume extraction is required — plasma cutting produces metal fumes, ozone, and nitrogen oxides that are hazardous respiratory irritants, especially when cutting galvanized, stainless, or coated metals

Opening a window is adequate ventilation for all plasma cutting operations

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Explanation: Plasma cutting generates: (1) metal fume — fine particulate from vaporized metal, a chronic inhalation hazard (welding/cutting fume is classified as a possible carcinogen), (2) ozone (O₃) — produced by UV radiation interacting with atmospheric oxygen, (3) nitrogen oxides (NOx) — produced at plasma temperatures. Cutting galvanized steel releases zinc oxide fume (causes "metal fume fever"); cutting stainless produces hexavalent chromium (carcinogen). A downdraft table or snorkel-style fume extractor positioned at the source is the minimum standard for indoor plasma cutting.

What is a CNC plasma table, and what advantage does it offer over handheld plasma cutting?

A CNC plasma table is simply a larger plasma power supply

CNC plasma is only used in industrial factories, not makerspaces

A CNC plasma table uses computer-controlled motors to move the plasma torch along programmed toolpaths — it produces precise, repeatable cuts from DXF/SVG designs with consistent speed, standoff, and perpendicularity that handheld cutting cannot match

CNC plasma tables are slower than handheld cutting because the computer adds delay

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Explanation: CNC plasma combines plasma's metal-cutting capability with computer numerical control for precision. The torch is mounted on a gantry that moves in X/Y axes; Z-axis controls torch height (THC — Torch Height Control automatically maintains optimal standoff). Advantages over handheld: (1) precision — ±0.5mm vs. ±2-3mm handheld, (2) repeatability — cut 100 identical parts from one design file, (3) complex geometry — curves, small holes, and intricate patterns that are impossible by hand, (4) consistent quality — no operator fatigue affecting cut quality. Design workflow: CAD → CAM (nesting, toolpath) → G-code → CNC controller. Many community makerspaces now include CNC plasma tables for metal fabrication.

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Last Updated: 2026-03-19