Drilling operation (G81 canned cycle) in CNC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of a simple G81 drilling cycle

What is the expected tool movement sequence for this G81 drilling cycle snippet?

G81 X10 Y10 Z-5 R2 F100

CNC Programming

N10 G81 X10 Y10 Z-5 R2 F100

A Rapid to X10 Y10 at R plane (Z=2), feed down to Z-5, retract to R plane

B Feed to X10 Y10 at Z-5, rapid retract to R plane

C Rapid to X10 Y10 at Z-5, feed retract to R plane

D Feed to X10 Y10 at R plane, rapid down to Z-5, feed retract

💡 Hint

Remember G81 moves rapidly to R plane before feed drilling down.

G81 moves rapidly to the R plane (Z=2), then feeds down to the drilling depth (Z=-5), then retracts rapidly to the R plane.

📝 Syntax

intermediate

Identify the syntax error in G81 usage

Which option contains a syntax error in the G81 drilling cycle command?

A G81 X20 Y30 Z-10 R5 F150

B G81 X10 Y10 Z-5 F100

C G81 X15 Y25 Z-8 R3 F120

D G81 X5 Y5 Z-7 R2

💡 Hint

R parameter is mandatory for G81 to define retract plane.

Option B is missing the R parameter, which is required to specify the retract plane height.

🔧 Debug

advanced

Why does this G81 drilling cycle cause a crash?

Given this code snippet, why might the tool crash?

G81 X50 Y50 Z-20 R-5 F200

CNC Programming

N20 G81 X50 Y50 Z-20 R-5 F200

A R plane is set below the starting surface, causing rapid retract into the part

B Feed rate F200 is too high for drilling

C Z depth is positive, drilling above the surface

D X and Y coordinates are missing

💡 Hint

Check the relationship between R plane and starting surface.

R plane at -5 is below the starting surface (usually Z=0), so rapid retract moves tool into the part causing a crash.

🚀 Application

advanced

Calculate total drilling cycle time with G81

If a G81 drilling cycle drills 5 holes at different XY positions with parameters:
R=2 mm, Z=-10 mm, feed rate=100 mm/min, rapid move speed=500 mm/min,
and the tool moves 20 mm between holes rapidly, what is the total approximate time for the drilling cycle?

A About 6.5 minutes

B About 10 minutes

C About 3.5 minutes

D About 1.5 minutes

💡 Hint

Calculate time for feed down, retract, and rapid moves separately.

Feed down takes 12 mm at 100 mm/min (0.12 min per hole), retract rapid 12 mm at 500 mm/min (0.024 min per hole), rapid moves 20 mm at 500 mm/min (0.04 min each for 4 intervals). Total ≈ 5*0.12 + 5*0.024 + 4*0.04 ≈ 0.6 + 0.12 + 0.16 = 0.88 min, about 1.5 minutes.

🧠 Conceptual

expert

Effect of missing G80 after G81 drilling cycle

What happens if a G81 drilling cycle is programmed but the G80 cancel command is omitted before moving to a new operation?

A The machine ignores the G81 and moves normally

B The machine stops and waits for operator input

C The machine continues to perform the drilling cycle at every subsequent move

D The machine resets to default canned cycle automatically

💡 Hint

Consider how canned cycles persist until canceled.

G81 remains active until canceled by G80, so the machine will keep drilling at every programmed move, causing unintended holes.