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newSchoolWorks.com › VoTechWorks › Transportation › Auto Body & Collision › Welding for Auto Body & Collision Repair › Lesson 8
TRN-AUTO25-04 · Lesson 8 · 14 clock hours · lab

Plasma Arc Cutting and Grinding Preparation

Objective. Students will make a plasma arc cut along a scribed line on 18-gauge steel to within 1/8 in. of the line and prepare the cut edge to a smooth, square profile using an angle grinder.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Plasma Arc Cutting in Auto Body Repair

Plasma arc cutting (PAC) is the preferred method for cutting sheet metal in collision repair. A plasma cutter forces compressed air or gas through a small nozzle where an electric arc superheats the gas into plasma—a state hot enough to melt and blow through steel instantly. Because the kerf (the slot left by the cut) is narrow and the heat-affected zone is small compared to oxy-fuel cutting, plasma is ideal for thin body panels. Most body shop applications involve mild steel ranging from 18 gauge (approximately 0.048 in. thick) up to heavier structural sections.

Equipment and Consumables

A plasma cutter has three key consumables that wear with use: the electrode, the tip (also called the nozzle), and the shield cup. A worn electrode shows a pit deeper than about 1/16 in. at its center; a worn tip shows an enlarged or out-of-round orifice. Always compare consumables against a new-consumable reference photo or a manufacturer chart before starting a cut. Running worn consumables causes a ragged, drifting arc, uneven cuts, and premature damage to the torch body.

The air supply must be clean and dry. Moisture in the air line contaminates the plasma arc and ruins consumables quickly. Confirm the compressor delivers the minimum CFM (cubic feet per minute) at the rated pressure for your machine—check the machine nameplate. Drain the compressor tank and use an inline moisture separator before each session.

Setting Up the Plasma Cutter

1. Air pressure: Set the regulator to the manufacturer's specified pressure for your machine (commonly in the 60–90 PSI range; always defer to your specific unit's manual).

2. Amperage: Match amperage to metal thickness. For 18-gauge mild steel, set the machine to the amperage recommended on its cut chart (a typical range is 20–30 amps; use your machine's chart, not a general estimate).

3. Standoff distance: Hold the torch tip 1/16 in. to 1/8 in. above the metal for most hand-held cuts, or drag the shield lightly on the surface if your consumable set is rated for drag cutting. Incorrect standoff causes a wide, irregular kerf or double-arc damage to the tip.

4. Travel speed: Move the torch steadily along the cut line. Too slow, and you widen the kerf and overheat the panel. Too fast, and the arc fails to cut through completely. Watch the spark stream exit the bottom of the panel—it should angle slightly in the direction of travel, confirming a through-cut.

Scribing and Cutting Technique

Before cutting, scribe a clean, visible line with a scriber and straightedge. Press the guide firmly against the panel to prevent drift. When cutting:

After the cut, check its position against the scribed line using a metal straightedge. The cut must stay within 1/8 in. of the line for its full length to meet the competency standard for this lesson.

Edge Preparation with an Angle Grinder

A freshly plasma-cut edge has dross (resolidified metal droplets), surface scale, and often a slight bevel or rolled edge. Before any weld or filler can be applied, the edge must be dressed square and clean.

Step 1 – Rough dressing with a 36-grit flap disc: Run the angle grinder along the cut edge, holding the disc at a low angle (roughly 10–15 degrees from the panel face). Remove dross, scale, and any high spots. Work progressively—do not grind in one spot long enough to blue the metal, which indicates excessive heat and metallurgical change.

Step 2 – Finishing with a 120-grit flap disc: Switch to the finer disc and refine the edge to a smooth, square profile. Check flatness by laying a metal straightedge against the edge face; it should sit evenly with no rock or gap.

Record each step—starting condition, tools used, grits, and final result—on the lab log form before the instructor inspection.

Safety Requirements

Plasma cutting produces intense UV and IR radiation, molten metal spatter, metal fumes, and loud noise. Every operator and nearby person must use:

Apply lockout/tagout procedures whenever changing plasma consumables or removing a grinding disc—de-energize and disconnect the machine from its power source before servicing.

Keep a fire watch: plasma spatter can travel several feet. Clear combustibles from the work area and have a fire extinguisher within reach.

Key terms9

Plasma arc cutting (PAC)A cutting process that uses a superheated, ionized gas (plasma) generated by an electric arc to melt and eject metal along a cut line.
KerfThe width of the slot removed by a cutting process; in plasma cutting, a narrow kerf minimizes heat distortion on thin panels.
Standoff distanceThe gap between the plasma torch tip and the workpiece surface; maintaining the correct standoff produces a clean, consistent arc and protects consumables.
ConsumablesWear parts of a plasma torch—electrode, tip (nozzle), and shield cup—that must be inspected and replaced as they erode to maintain cut quality.
DrossResolidified molten metal that clings to the bottom edge of a plasma-cut workpiece; it must be removed before welding or finishing.
Heat-affected zone (HAZ)The area of metal adjacent to a cut or weld where heat has altered the grain structure without melting; plasma cutting minimizes the HAZ compared to oxy-fuel methods.
Flap discAn abrasive grinding accessory made of overlapping abrasive cloth segments mounted on a backing plate; used on an angle grinder to dress, blend, or finish metal edges and surfaces.
ScriberA hardened steel tool used to scratch a fine, precise layout line onto metal as a cutting guide.
ScaleIron oxide formed on the cut edge surface by heat during plasma cutting; it must be ground away before welding because it prevents proper fusion and adhesion of fillers.

Worked examples4

1. You scribe a 12-inch cut line on an 18-gauge panel. After the plasma cut, you measure the distance from the cut edge to the scribed line at six points and get these readings: 0 in., 1/16 in., 3/32 in., 1/8 in., 1/16 in., 1/32 in. Does the cut pass the 1/8 in. tolerance?
  1. Convert all measurements to a common unit for easy comparison. Express each as a fraction of an inch: 0, 1/16, 3/32, 1/8, 1/16, 1/32.
  2. Convert to 32nds of an inch for direct comparison to the limit of 1/8 in. = 4/32 in.: readings are 0/32, 2/32, 3/32, 4/32, 2/32, 1/32.
  3. The tolerance limit is 4/32 in. (1/8 in.). Compare each reading: 0 ≤ 4, 2 ≤ 4, 3 ≤ 4, 4 ≤ 4, 2 ≤ 4, 1 ≤ 4. Every reading is at or within the limit.
  4. Conclusion: all six measurement points are within tolerance.
Answer: The cut passes. The maximum deviation is exactly 1/8 in. (4/32 in.), which is at—but not beyond—the allowable limit of 1/8 in.
2. At a different measurement point the cut edge is 5/32 in. from the scribed line. By how much does this exceed the 1/8 in. tolerance, and what does this mean for the part?
  1. Express both values in 32nds: 5/32 in. (actual) vs. 4/32 in. (limit).
  2. Subtract: 5/32 − 4/32 = 1/32 in. over the tolerance limit.
  3. Because even one point on the cut exceeds the limit, the entire cut does not meet the competency standard.
Answer: The cut exceeds the tolerance by 1/32 in. at that point. The panel fails the quality check; the student must re-cut or, if material allows, the instructor will determine whether grinding can bring the edge into conformance without removing required material.
3. Your plasma cutter's cut chart specifies a minimum air supply of 4 CFM at 70 PSI for 18-gauge steel. Your compressor gauge reads 65 PSI. How many PSI short are you, and what is the likely effect on cut quality if you proceed?
  1. Identify the required pressure: 70 PSI. Identify the actual pressure: 65 PSI.
  2. Calculate the shortfall: 70 − 65 = 5 PSI below the specified minimum.
  3. Low air pressure means the plasma gas stream cannot fully clear molten metal from the kerf.
Answer: The supply is 5 PSI below the minimum. Proceeding will likely produce excessive dross on the underside of the cut, an irregular arc, and accelerated consumable wear. Adjust the regulator or service the compressor before cutting.
4. A cut edge measures 2 inches long and must be dressed from a rough plasma-cut condition to within a flat tolerance. You first use a 36-grit flap disc to remove dross, then switch to a 120-grit disc for finishing. After the 120-grit pass, your straightedge check shows a 3/32 in. gap under the straightedge at the center of the edge. Does the edge pass, and what should you do?
  1. The acceptable finished condition is a flat, square edge with no gap or rock under a straightedge—meaning essentially zero measurable gap for weld preparation.
  2. A 3/32 in. gap under the straightedge indicates the edge is concave (bellied inward) at the center.
  3. This does not meet the 'no rolled edge, sharp burrs, or surface scale remaining' requirement and also introduces a geometric flaw that would affect weld fit-up.
  4. Corrective action: return to the 36-grit disc, carefully grind the high areas at each end of the edge to level them with the center, then re-finish with the 120-grit disc and re-check.
Answer: The edge fails the flatness check. A 3/32 in. gap is unacceptable for weld preparation. Rework with the coarser disc to remove high spots, refinish, and re-inspect before logging the result.

The math in this lesson1

Learners convert cut-line deviations to 32nds of an inch and check each against a 1/8-inch limit.
5.NF.1
In class: Instructor demonstrates plasma cutter setup—air pressure setting, consumable inspection, standoff distance, and travel speed—then cuts a panel while narrating; students check plasma consumables on their unit against a new-consumable reference photo. · Students scribe a straight cut line on an 18-gauge panel, set plasma cutter to the recommended amperage for that thickness, and make the cut wearing full PPE; the cut is measured against the scribed line with a straightedge. · Students use a 36-grit flap disc and then a 120-grit disc to dress the cut edge square and smooth, final-checking flatness with a metal straightedge and recording the preparation steps on the lab log.
Materials: Plasma cutter with air compressor (minimum 4 CFM at rated pressure), plasma consumables (tip, electrode, shield), 18-ga mild steel panels, scriber, straightedge, ruler, angle grinder with 36-grit and 120-grit flap discs, full PPE, lab log forms
Check: Cut must remain within 1/8 in. of scribed line for full length; edge preparation inspected by instructor—no rolled edge, sharp burrs, or surface scale remaining—both pass.

Practice check8

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

Welding and Cutting Processes – I-CAR ProLevel / ASE B5
1. Which plasma torch consumable, when excessively worn, most directly causes the arc to wander and produces an irregular cut?
Welding and Cutting – I-CAR ProLevel
2. When making a hand-held plasma cut on 18-gauge mild steel, what is the correct indicator that the arc is fully penetrating the panel?
Safety – I-CAR ProLevel / ASE B5
3. Before changing plasma torch consumables, the FIRST action a technician must take is:
Panel Measurement and Layout – ASE B3 / I-CAR ProLevel
4. A student's plasma cut deviates 3/16 in. from the scribed line at one point. The lesson competency standard requires the cut to stay within 1/8 in. By how much does this single point exceed the tolerance?
Surface Preparation – I-CAR ProLevel / ASE B3
5. What is the primary purpose of switching from a 36-grit flap disc to a 120-grit flap disc when preparing a plasma-cut edge?
Surface Preparation – I-CAR ProLevel / ASE B3
6. Which condition found after grinding a plasma-cut edge would cause an instructor to reject the preparation?
Equipment Maintenance – I-CAR ProLevel / ASE B5
7. Moisture in the plasma cutter's air supply line will most likely cause:
Safety – I-CAR ProLevel / ASE B5
8. When selecting a grinding disc to mount on an angle grinder, which check is essential for safe operation?
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