Module 3: Machine Operation & Setup Assessment Quiz

Module: U5M3 - Machine Operation & Setup Duration: 25 minutes Passing Score: 70% Format: Multiple choice and scenario-based

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What is the purpose of the Torch Height Controller (THC) on a CNC plasma table?

To automatically maintain the correct standoff distance between the torch and the workpiece during cutting

To control the temperature of the torch consumables

To set the Z-zero position before cutting begins

To monitor and display the plasma amperage

Submit

Explanation: THC monitors the arc voltage (which correlates with torch-to-work distance) and adjusts the Z-axis in real-time to maintain consistent standoff. This compensates for warped material, uneven surfaces, and thermal distortion during cutting. Consistent standoff = consistent cut quality.

Before starting any plasma cutting operation, what is the FIRST physical inspection step?

Inspect all cables, hoses, and connections for damage, and verify the work clamp is making solid contact

Turn on the machine and run a test cut

Load the G-code file

Set the torch height

Submit

Explanation: Damaged cables can cause electrical shock (200-300V DC), gas leaks can cause arc instability, and a poor work clamp connection creates arc wander and can cause shock. Physical inspection must happen before the machine is powered on.

What is the purpose of the "initial height sense" (IHS) or "ohmic sensing" before a plasma cut begins?

To detect the top of the workpiece surface and set the correct pierce height and cut height

To test the electrical resistance of the workpiece

To preheat the workpiece before cutting

To verify the work clamp connection

Submit

Explanation: IHS/ohmic sensing lowers the torch until it detects contact with the workpiece (via electrical contact or touch sensor), then retracts to the programmed pierce height. This ensures accurate and consistent torch height regardless of material thickness or surface variations. Pierce height is typically 1.5-2× the cut height.

What is the correct consumable replacement procedure?

Power off the machine, remove the shield cup, inspect and replace electrode and nozzle as a matched set, reassemble in correct order

Replace only the nozzle when cut quality degrades

Replace consumables while the machine is powered on but not cutting

Replace consumables only during annual maintenance

Submit

Explanation: Electrode and nozzle wear together and must be replaced as a set to maintain balanced performance. The machine must be powered off and the torch completely cool before disassembly. Reassembly order matters: electrode → swirl ring → nozzle → shield cup/retaining cap. Over-tightening damages threads.

When setting up a CNC plasma cut, in what order should the cutting operations be sequenced?

Interior features (holes) first, then exterior profiles last

Exterior profiles first, then interior features

Largest features first, smallest last

Order does not matter

Submit

Explanation: Interior features (holes, slots) must be cut first because the surrounding material holds the part rigid and in position. If the exterior profile is cut first, the part is free to move and may shift, making interior cuts inaccurate or impossible.

What is the water table used for in CNC plasma cutting?

To capture sparks, reduce fumes, minimize noise, and cool the workpiece during cutting

To provide water cooling for the torch

To clean the workpiece before cutting

To lubricate the cutting process

Submit

Explanation: The water table sits below the cutting surface and is filled to just below or touching the bottom of the workpiece. It traps ~90% of fumes and particulates, significantly reduces noise (by 10-15 dB), captures sparks and molten metal, and provides some workpiece cooling. Water level must be maintained — too low reduces effectiveness, too high interferes with cutting.

A student notices the plasma arc is wandering and producing an inconsistent kerf. What should they check first?

Consumable condition (worn nozzle or electrode) and air pressure/quality

The G-code file for errors

The water table level

The ambient temperature

Submit

Explanation: Arc wander is most commonly caused by a worn nozzle (orifice becomes oblong/oversized) or contaminated/low-pressure gas. A worn electrode can also contribute. Check consumables first, then verify gas pressure (should be 60-75 PSI) and air quality (moisture, oil). Replace consumables if the electrode pit exceeds 1/16" depth or the nozzle orifice is visibly worn.

What is "double arcing" and why is it dangerous?

When the arc passes through the nozzle wall instead of the orifice, damaging the nozzle and potentially the torch body

When two plasma torches are operated simultaneously

When the arc jumps between two adjacent cut lines

When the pilot arc fires twice before transferring

Submit

Explanation: Double arcing occurs when the arc finds an unintended path through the nozzle wall. This instantly destroys the nozzle and can damage the torch body (which is expensive). Common causes: piercing too thick material, standoff too low, incorrect nozzle for the amperage setting, or touching the nozzle to the workpiece while the arc is active.

After completing a plasma cutting job on the CNC table, what is the correct shutdown sequence?

Turn off arc, allow torch to cool, raise torch to safe height, turn off gas after post-flow completes, power down machine, maintain fire watch

Press E-stop, turn off power, leave immediately

Power down machine, then turn off gas immediately

Leave the machine running to keep the torch cool

Submit

Explanation: The plasma torch requires a post-flow period (15-30 seconds) after the arc stops to cool the consumables with gas flow. Cutting gas supply too early causes consumable overheating and premature wear. The torch should be raised to safe height, gas should complete post-flow, then the machine can be powered down. Fire watch must continue for 30 minutes per OSHA guidelines.

When plasma cutting on a CNC table, the cut starts correctly but gradually worsens over a 10-minute run. The kerf gets wider and dross increases. What is happening?

Consumables (electrode and nozzle) are wearing during the cut and need replacement

The material is getting harder as it heats up

The gas pressure is slowly decreasing

The water table is evaporating

Submit

Explanation: Consumable wear is progressive during cutting. The electrode pit deepens, reducing arc intensity, while the nozzle orifice enlarges, reducing arc constriction. This causes gradually wider kerf, more dross, and reduced cut quality. For long jobs, inspect consumables at regular intervals and replace when wear limits are reached.

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Last Updated: 2026-03-19