Activity 002: First Cut — Supervised CNC Plasma Operation

Activity ID: U5M3-ACT-002 Duration: 45 minutes Objective: Students will execute their first CNC plasma cut under instructor supervision, cutting a simple geometric shape from mild steel and evaluating the result. Group Size: 2 students (operator + safety observer, then swap)

Overview

Each student will load a pre-programmed file, complete the setup procedure, execute a simple cut (4" square with a 1" center hole), evaluate cut quality, and perform proper shutdown. This is the first actual plasma cutting experience.

Materials & Equipment Needed

• CNC plasma table, fully set up and verified
• Plasma cutter set to 40A, air at 65 PSI
• Pre-programmed G-code: "first_cut_square.nc" (4" square with 1" center hole, 12-gauge mild steel parameters)
• 12 gauge mild steel plate, minimum 8" × 8"
• Full PPE: welding helmet (shade 9), FR jacket, leather gloves, boots, hearing protection, P100 respirator
• Fire extinguisher within 15 feet
• Welding screens deployed
• Digital calipers, angle gauge
• Pliers/tongs for handling hot material
• Cut evaluation worksheet

Instructions & Procedure

Phase 1: Student A — Setup & Cut (18 minutes)

Setup (8 minutes) 1. Complete the full pre-cut checklist (from Activity 001) 2. Load "first_cut_square.nc" into the controller 3. Preview the toolpath on screen — verify the square and hole are correctly positioned within the material 4. Set machine origin at front-left of the material 5. Verify all PPE is on — instructor checks both students 6. Safety observer confirms: fire extinguisher location, E-stop accessibility, welding screens deployed

Cut Execution (5 minutes) 1. Alert all shop personnel: "Plasma cutting starting" 2. Drop welding helmet, press cycle start 3. Observe the IHS sequence — torch probes the surface, retracts to pierce height 4. Watch the pierce — arc should transfer cleanly, molten metal ejected downward 5. Monitor the cut: listen for consistent arc, watch for proper THC tracking 6. The machine will cut the center hole first (CCW), then the exterior square (CW) 7. After program completes, torch rises and arc extinguishes 8. Wait for post-flow to complete (15-30 seconds of gas flow after arc stops)

Post-Cut (5 minutes) 1. Raise the welding helmet after arc is fully extinguished 2. Wait 3 minutes for the workpiece to cool 3. Remove the cut square from the skeleton using pliers (may still be warm) 4. Place on a heat-resistant surface

Phase 2: Cut Quality Evaluation (5 minutes) Measure and evaluate the Student A cut:

1. Overall dimensions: Measure the square — should be 4.000" ± 0.030"
2. Hole diameter: Should be 1.000" ± 0.030"
3. Kerf width: Measure the kerf on the skeleton edge
4. Dross: Rate 1-5 (1=none, 5=heavy)
5. Bevel angle: Use an angle gauge on the cut edge — should be 1°–5°
6. Drag lines: Observe angle — slight trailing (5°–15°) = correct speed
7. Top edge: Clean, slightly rounded = good
8. Surface finish: Smooth and consistent = good

Record all measurements on the evaluation worksheet.

Phase 3: Student B — Setup & Cut (18 minutes) Swap roles. Student B operates; Student A observes. Repeat Phases 1-2.

Phase 4: Comparison & Discussion (4 minutes) 1. Compare both cuts side-by-side 2. Identify any differences in quality between the two cuts 3. Discuss what could be improved 4. If time permits, attempt to remove dross (scraper or grinder under supervision)

Discussion Points

• How did the actual kerf width compare to the programmed kerf compensation?
• Which edge of the square had the squarest edge? Does it match the expected bevel direction?
• Was the hole cut quality different from the square cut quality? Why might they differ?
• What would you change about the parameters for the next cut?

Expected Outcomes

• Students execute a complete CNC plasma cutting cycle independently (with supervision)
• Students can evaluate cut quality using quantitative measurements
• Students demonstrate proper PPE usage and safety protocol compliance throughout

Assessment Rubric

Criteria	Excellent (4)	Proficient (3)	Developing (2)	Beginning (1)
Setup Procedure	Complete setup with no omissions	Setup complete with minor prompts	Multiple prompts needed	Cannot complete setup
Cut Execution	Smooth operation, proper monitoring	Successful cut with minor hesitation	Cut completed with significant guidance	Cannot execute cut
Safety Compliance	All PPE and procedures followed proactively	PPE complete with minor reminders	Multiple safety reminders needed	Significant safety concerns
Quality Evaluation	Accurate measurements, insightful analysis	Accurate measurements, reasonable analysis	Partial measurements, basic analysis	Cannot evaluate quality

Safety Considerations

• This is the FIRST time students operate the plasma cutter — instructor must be within arm's reach of E-stop
• Safety observer must be positioned with clear view of both the torch and the operator
• If the arc does not transfer on the first pierce attempt, E-stop and inspect consumables/work clamp
• Do NOT attempt to pick up the cut part immediately — hot metal causes severe burns
• If any student feels uncomfortable proceeding, they may stop at any time — there is no penalty

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Last Updated: 2026-03-19