CNC Test: General Machinist Skills Quiz

Q: Why do “mystery moves” happen after a tool change or restart?

They usually come from modal state carryover : a prior tool left a mode active (distance mode, compensation, or a canned cycle), and the next block behaves differently than you expect. A consistent known-state line for each tool and a deliberate restart procedure reduce surprises.

Q: When is a machine-coordinate move appropriate, and why is it risky near the part?

Machine-coordinate moves are best treated as clearance/retract logic because they ignore the part zero you set with work offsets. If you accidentally program a part-based value as a machine-based value, the machine can head toward home or overtravel. Use machine coordinates only when you can visualize the machine reference position and the path is clear.

9 – 52 Questions 12 min

This quiz targets the CNC decisions that prevent scrap and crashes: controlling modal states, selecting the correct work offset, and applying tool length and cutter compensation correctly. It also checks practical use of canned cycles, safe rapid strategy, and feeds/speeds reasoning so your program behaves the same on the machine as it does in your head.

Start

Choose quiz length

1In a CNC milling program, which G-code sets the control to inch units?

2G53 commands are in machine coordinates and are not affected by work offsets such as G54–G59.

True / False

3After a tool change on a mill, what does G43 typically do when used with an H register?

4Which canned cycle is most commonly used for peck drilling (chip breaking)?

5What is the primary purpose of a work offset such as G54 on a milling machine?

6G41 selects right-side cutter compensation relative to the direction of tool motion.

True / False

7A program was written expecting inch mode (G20), but the control is actually in metric mode (G21). If the program commands F10, what happens compared to an intended 10 in/min feed?

8Which CNC commands are typically modal (stay active) until canceled or changed? Select all that apply.

Select all that apply

9Arrange a typical CNC startup “safety line” checklist in a sensible order (top to bottom) before cutting.

Put in order

1Set plane (G17/G18/G19)

2Set distance mode (G90/G91)

3Set units (G20/G21)

4Select work offset (G54–G59)

5Cancel modes (G40 G49 G80)

10You just finished cutting in G54 and want to retract to machine Z home regardless of part zero. Which line explicitly uses machine coordinates for the retract?

11Which code cancels tool length compensation on many CNC mills?

12Which guideline best describes safe use of rapid moves (G0) near a part and fixture?

13You switch from a 4-flute to a 6-flute end mill and keep RPM the same. To maintain the same chip load per tooth, what must you update? Select all that apply.

Select all that apply

14You’re adding cutter compensation to a contour that previously ran without it. Which practices help avoid alarms and bad geometry? Select all that apply.

Select all that apply

15Once G90 is commanded, the control stays in absolute distance mode until G91 is commanded.

True / False

16Arrange the typical steps for using cutter radius compensation on a profile, from approach to finish.

Put in order

1Cancel cutter comp with G40

2Feed to the lead-in start point

3Begin a lead-out move

4Rapid to clearance near the start

5Start the lead-in and enable G41/G42

6Machine the contour

17Right after an M06 tool change to T07, the program rapids to a Z value that should be safely above the part but instead hits the fixture. Which missing call is a common root cause?

18Arrange these actions into a safe, practical first-article prove-out sequence for a new CNC program.

Put in order

1Verify program header/safety line

2Single-block through the first cutting entry with hand on feed hold

3Measure the first feature and adjust wear/work offset if needed

4Dry run above the part with overrides reduced

5Confirm tools and offsets (T/H/D) match setup

6Run graphics/backplot to sanity-check motion

19The formula Feed = chip load per tooth × number of flutes × RPM calculates which value?

20Which practices reflect safe, correct use of G53 on a CNC mill? Select all that apply.

Select all that apply

21Arrange these high-level steps for drilling holes using a canned cycle in a safe, typical sequence.

Put in order

1Cancel the cycle with G80 before leaving the pattern

2Rapid to a safe clearance Z

3Set safety line (units/plane/modes)

4Call the drilling cycle with Z, R, and F

5Rapid to the first hole XY position

22During setup, the first positioning move heads toward the vise jaw instead of above the part. Which clues suggest the wrong work offset (G54–G59) is active? Select all that apply.

Select all that apply

23When should you use G80 (canned cycle cancel)? Select all that apply.

Select all that apply

High-Impact CNC Programming & Setup Errors (and How to Prevent Them)

Most CNC incidents come from small assumptions that compound during tool changes, restarts, and edits at the control. Use these pitfalls as a checklist while you practice.

Modal state carryover after tool changes

Mistake: Assuming G-codes only apply to the current line; motion, distance mode, plane, compensation, and canned cycles can stay active.
Avoid it: Start each tool with an intentional “known state” line (units, plane, G90/G91 choice, cancels like G40/G49/G80).

Confusing machine coordinates with work offsets

Mistake: Using a machine-coordinate move (typically G53) with values intended for the part zero (G54–G59), sending the machine toward home unexpectedly.
Avoid it: Treat machine-coordinate moves as clearance/retract logic only; verify the active work offset before any approach to the part.

Tool length and cutter compensation misapplication

Mistake: Forgetting to call tool length compensation or calling the wrong H register, causing Z errors that look like “mystery offsets.”
Avoid it: Standardize Txx↔Hxx pairing and confirm the offset register during prove-out.
Mistake: Turning on G41/G42 without a lead-in, or choosing the wrong side relative to the programmed direction.
Avoid it: Use a clean lead-in/lead-out and verify the D register and intended climb/conventional direction.

Unsafe rapid strategy

Mistake: Rapid (G0) at part level without a clearance plane, assuming fixtures and tool length are “close enough.”
Avoid it: Rapid only at a verified safe Z; feed into the work.

Units and feed mode surprises

Mistake: Inch/mm mismatch or mixing feed-per-minute with feed-per-rev expectations.
Avoid it: Confirm units early and sanity-check feed, RPM, and chip load before Cycle Start.

Printable CNC “Known State” + Compensation Quick Reference

Printable note: Print this section or save it as a PDF for a quick shop-floor reference.

Startup “known state” (typical safe defaults)

Units: Set intentionally (inch vs mm) before any motion.
Plane selection: Choose the correct plane for arcs/canned cycles (commonly XY for milling).
Distance mode: G90 (absolute) or G91 (incremental) — never assume.
Cancel stack: G40 (cutter comp off), G49 (tool length off), G80 (canned cycles off).
Work offset: Select the correct part coordinate system (G54–G59) before approaching the part.

Coordinates: safest mental model

Work coordinates (G54–G59): Values are relative to your part zero; use for cutting and approach moves.
Machine coordinates (often used with G53): Values are relative to machine home; use for clearance/retract logic only.

Tool length compensation (milling)

Call it: Apply tool length compensation with the correct H register after tool change.
Best practice: Keep a consistent mapping (e.g., Tool 7 uses H07) unless your shop standard differs.
Safety habit: First Z move with a new tool should be conservative and in a known safe area.

Cutter compensation (G41/G42) essentials

Lead-in required: Turn comp on during a lead-in move long enough for the control to shift the path.
D register matters: Verify the active diameter/wear offset matches the tool actually loaded.
Cancel cleanly: Cancel on a lead-out, not mid-contour.

Canned cycles: make them predictable

Always cancel: Use G80 when finished; otherwise the next positioning move may repeat the cycle.
Verify retract behavior: Know whether the cycle retracts to a clearance plane or to a reference plane in your setup.

Feeds/speeds sanity check (before proving out)

Confirm units and expected feed mode at the control.
Compute a rough chip load check: feed ≈ RPM × flutes × chip load.
During prove-out, start conservative and adjust in controlled steps.

General Machinist Tasks Mapped to CNC Programming & Setup Skills

This quiz mirrors the decisions a general machinist makes while setting up, proving out, and running CNC work safely. Use this map to connect quiz topics to the moments they matter on the floor.

1) Job setup: establishing a trustworthy coordinate system

Select and verify the correct work offset (G54–G59) for the part and fixture.
Understand when a machine-coordinate move is appropriate for clearance versus dangerous near the part.
Set a clearance strategy: safe Z, approach points, and where rapids are allowed.

2) Tool loading and length setting

Apply tool length compensation correctly (right tool, right offset register, right timing).
Recognize symptoms of a wrong length value: unexpected air cuts, plunges, or incorrect depth across tools.

3) Prove-out and first-article confidence

Control modal states deliberately so the program behaves consistently after edits, optional stops, or restarts.
Validate arcs/cycles by confirming plane selection, start/end logic, and retract behavior.
Use safe rapids: rapid only in verified clearance, then feed into the work.

4) Dimensional control during production

Use cutter compensation as an intentional adjustment method (correct side, clean lead-in/out, correct wear register).
Adjust feeds and speeds with a chip-load mindset instead of guessing, especially when tool geometry changes.

5) Troubleshooting alarms, gouges, and “mystery moves”

Trace the active modes: distance mode, compensation states, and any leftover canned cycle.
Isolate coordinate confusion (machine vs work) before changing offsets that could mask the real problem.

CNC Machinist Skills Quiz FAQ: Modes, Offsets, Compensation, and Safe Motion

Why do “mystery moves” happen after a tool change or restart?

They usually come from modal state carryover: a prior tool left a mode active (distance mode, compensation, or a canned cycle), and the next block behaves differently than you expect. A consistent known-state line for each tool and a deliberate restart procedure reduce surprises.

When is a machine-coordinate move appropriate, and why is it risky near the part?

Machine-coordinate moves are best treated as clearance/retract logic because they ignore the part zero you set with work offsets. If you accidentally program a part-based value as a machine-based value, the machine can head toward home or overtravel. Use machine coordinates only when you can visualize the machine reference position and the path is clear.

What causes cutter compensation alarms or bad size when turning on G41/G42?

Common causes are turning compensation on without a sufficient lead-in, selecting the wrong side for the direction of travel, or using an incorrect diameter/wear register. Compensation should be activated on a controlled entry move and canceled on a controlled exit so the control can resolve the offset geometrically.

Why do canned cycles sometimes repeat or drill in the wrong place?

Canned cycles persist until canceled. If the cycle isn’t canceled when you’re done, the next positioning command can inadvertently trigger another cycle at a new location. Always cancel canned cycles when finished and verify the cycle’s retract behavior matches your clearance plan.

What’s the quickest way to sanity-check feeds and speeds before pressing Cycle Start?

Confirm units and the expected feed behavior at the control, then do a rough chip-load check: feed should align with RPM, flute count, and a reasonable chip load for the tool and material. If you’re also responsible for safe operating practices during prove-out, the habits in Operator Skills Assessments align well with controlled first-run decision making.

How should I recover safely after an E-stop or an unexpected stop mid-program?

Re-establish a known state (units, distance mode, cancels), verify the active work offset, and move to a clear, high-confidence position before re-approaching the part. Treat the restart like a mini prove-out: confirm tool length compensation and avoid rapids near the work until you’ve re-verified clearance. If your shop emphasizes formal restart procedures and hazard controls, Workplace Safety Quiz Questions covers complementary safety fundamentals.