Drilling operation (G81 canned cycle) in CNC Programming - Step-by-Step Execution

Concept Flow - Drilling operation (G81 canned cycle)

Start G81 cycle

↓

Move to position X,Y

↓

Drill down to Z

↓

Retract to R plane

↓

Check for next position

↓

Move to next

The G81 cycle moves the drill to each specified XY position, drills down to a set depth Z, retracts to a clearance plane R, then moves to the next position until all are done.

Execution Sample

CNC Programming

N10 G90 G81 X10 Y10 Z-5 R2 F100
N20 X20 Y10
N30 X20 Y20
N40 G80

This program drills holes at (10,10), (20,10), and (20,20) to depth -5 with retract plane 2, then cancels the cycle.

Execution Table

Step	Command	Position (X,Y)	Z Depth	Action	Output
1	G81 start at X10 Y10 Z-5 R2	(10,10)	-5	Move to (10,10), drill down to -5, retract to 2	Hole drilled at (10,10)
2	X20 Y10	(20,10)	-5	Move to (20,10), drill down to -5, retract to 2	Hole drilled at (20,10)
3	X20 Y20	(20,20)	-5	Move to (20,20), drill down to -5, retract to 2	Hole drilled at (20,20)
4	G80 cancel	(20,20)	N/A	Cancel canned cycle	Cycle ended

💡 G80 command cancels the G81 drilling cycle after all holes are drilled

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
X	undefined	10	20	20	20
Y	undefined	10	10	20	20
Z	undefined	-5	-5	-5	N/A
R (Retract plane)	undefined	2	2	2	N/A
Cycle Active	False	True	True	True	False

Key Moments - 3 Insights

Why does the drill retract to the R plane after drilling down?

The retract to R plane (see Step 1 in execution_table) prevents the drill from dragging across the workpiece when moving to the next hole, protecting the tool and part.

What happens if we omit the G80 command at the end?

Without G80 (Step 4), the machine stays in drilling cycle mode and will drill at any new XY moves, which might cause unintended holes.

Does the Z value change for each hole in this G81 cycle?

No, the Z depth stays the same (-5) for all holes as set in the initial G81 command (see Steps 1-3). Only XY changes.

Visual Quiz - 3 Questions

Look at the execution table, what is the Z depth during Step 2?

A -5

B 2

C 0

D N/A

💡 Hint

Check the 'Z Depth' column for Step 2 in the execution_table

At which step does the drilling cycle end?

A Step 3

B Step 4

C Step 2

D Step 1

💡 Hint

Look for the G80 cancel command in the execution_table

If the R plane was changed from 2 to 5, how would the execution_table change?

A The XY positions would change

B The drill depth Z would change to -5

C The retract height in 'Action' would be 5 instead of 2

D The cycle would end earlier

💡 Hint

R plane controls retract height shown in the 'Action' column

Concept Snapshot

G81 canned cycle drills holes at specified XY positions
Moves drill down to Z depth, then retracts to R plane
Repeats for each XY move until G80 cancels cycle
Use G81 with X, Y, Z, R, and feed rate F
G80 stops the drilling cycle

Full Transcript

The G81 canned cycle automates drilling multiple holes. It starts by moving the drill to the first XY position, drills down to the specified Z depth, then retracts to the R plane to avoid dragging. It repeats this for each new XY coordinate until the G80 command cancels the cycle. The retract plane R is important to protect the tool and workpiece during moves. The Z depth remains constant for all holes unless changed. Omitting G80 keeps the cycle active, which can cause unintended drilling. This visual trace shows each step's position, depth, and action clearly.

Practice

1. What does the G81 canned cycle do in CNC programming?

easy

A. Controls the spindle speed during milling

B. Measures the tool length automatically

C. Sets the tool change position

D. Automates drilling holes by moving down, drilling, and retracting

Solution

1. Step 1: Understand the purpose of G81

   G81 is a canned cycle used specifically for drilling operations in CNC. It automates the process of moving the tool down to the hole position, drilling to a set depth, and then retracting.

2. Step 2: Compare options with G81 function

   Options A, C, and D describe other CNC functions unrelated to G81. Only Automates drilling holes by moving down, drilling, and retracting correctly describes the drilling automation.

3. Final Answer:

   Automates drilling holes by moving down, drilling, and retracting -> Option D

4. Quick Check:

   G81 = Drilling automation [OK]

Hint: Remember G81 = drill down, drill, retract cycle [OK]

Common Mistakes:

• Confusing G81 with spindle speed commands
• Thinking G81 changes tools automatically
• Assuming G81 measures tool length

2. Which of the following is the correct syntax to start a G81 drilling cycle at X10 Y20 with a depth of Z-5 and retract height R2?

easy

A. G81 X10 Y20 R2 Z5 F100

B. G81 X10 Y20 Z-5 R2 F100

C. G81 X10 Y20 Z-5 F100

D. G81 X10 Y20 Z5 R2 F100

Solution

1. Step 1: Identify correct parameter order and signs

   The G81 cycle requires X and Y for position, Z for drilling depth (negative for downward), R for retract height, and F for feed rate. The depth must be negative to drill down.

2. Step 2: Check each option for correct syntax

   G81 X10 Y20 Z-5 R2 F100 has correct order and negative Z depth. G81 X10 Y20 Z5 R2 F100 has positive Z (wrong direction). G81 X10 Y20 Z-5 F100 misses R parameter. G81 X10 Y20 R2 Z5 F100 has positive Z depth (wrong direction).

3. Final Answer:

   G81 X10 Y20 Z-5 R2 F100 -> Option B

4. Quick Check:

   Negative Z depth and R retract needed [OK]

Hint: Use negative Z for depth and include R for retract [OK]

Common Mistakes:

• Using positive Z for drilling depth
• Omitting R retract height
• Mixing parameter order incorrectly

3. Given the following CNC program snippet, what is the final Z position of the tool after executing the G81 cycle?

G90 G00 X10 Y10
G81 Z-5 R1 F50
G80

medium

A. Z1 (at retract height)

B. Z-5 (at drilling depth)

C. Z0 (machine home position)

D. Z10 (rapid move height)

Solution

1. Step 1: Understand G81 drilling cycle behavior

   G81 moves the tool down to Z-5 to drill, then retracts to the R height, which is Z1 here.

2. Step 2: Analyze the program flow

   After drilling, the tool retracts to Z1 (R value). Then G80 cancels the canned cycle but does not move the tool further. So final Z is at retract height Z1.

3. Final Answer:

   Z1 (at retract height) -> Option A

4. Quick Check:

   G81 retracts to R height after drilling [OK]

Hint: After drilling, tool retracts to R height, not depth [OK]

Common Mistakes:

• Assuming tool stays at drilling depth
• Confusing R retract with zero position
• Ignoring G80 cancel effect

4. Identify the error in this G81 drilling cycle command:

G81 X20 Y30 Z5 R2 F75

medium

A. Z value should be negative for drilling depth

B. R value cannot be greater than Z

C. F feed rate is missing

D. X and Y coordinates are not allowed in G81

Solution

1. Step 1: Check the Z parameter meaning

   Z specifies drilling depth and must be negative to move tool down into the material.

2. Step 2: Review the given command

   Z is positive 5, which means moving up, not drilling down. This is incorrect for drilling.

3. Final Answer:

   Z value should be negative for drilling depth -> Option A

4. Quick Check:

   Drilling depth Z must be negative [OK]

Hint: Always use negative Z for drilling depth in G81 [OK]

Common Mistakes:

• Using positive Z depth
• Misunderstanding R retract role
• Omitting feed rate F

5. You want to drill three holes at positions X10 Y10, X20 Y10, and X30 Y10 with a depth of 4 mm and retract height of 1 mm using G81. Which program snippet correctly uses G81 and cancels it after drilling?

hard

A. G81 Z4 R1 F60\nX10 Y10\nX20 Y10\nX30 Y10\nG80

B. G81 X10 Y10 Z4 R1 F60\nX20 Y10\nX30 Y10\nG80

C. G81 Z-4 R1 F60\nX10 Y10\nX20 Y10\nX30 Y10\nG80

D. G81 X10 Y10 Z-4 F60\nX20 Y10\nX30 Y10\nG80

Solution

1. Step 1: Understand G81 multi-hole drilling syntax

   G81 is started with Z, R, and F parameters once, then X and Y positions for each hole follow. The cycle ends with G80 to cancel.

2. Step 2: Evaluate each option

   G81 Z-4 R1 F60\nX10 Y10\nX20 Y10\nX30 Y10\nG80 correctly starts G81 with Z-4, R1, F60, then lists all hole positions, and ends with G80. G81 X10 Y10 Z4 R1 F60\nX20 Y10\nX30 Y10\nG80 uses positive Z4 (wrong). G81 Z4 R1 F60\nX10 Y10\nX20 Y10\nX30 Y10\nG80 uses positive Z (wrong). G81 X10 Y10 Z-4 F60\nX20 Y10\nX30 Y10\nG80 omits R retract height.

3. Final Answer:

   G81 Z-4 R1 F60 X10 Y10 X20 Y10 X30 Y10 G80 -> Option C

4. Quick Check:

   Start G81 with Z, R, F; list holes; end with G80 [OK]

Hint: Start G81 with Z, R, F; list holes; end with G80 [OK]

Common Mistakes:

• Using positive Z depth
• Omitting retract height R
• Not cancelling G81 with G80