G81 simple drilling cycle in CNC Programming - Step-by-Step Execution

Concept Flow - G81 simple drilling cycle

Start G81 Cycle

↓

Move to Hole Position

↓

Drill Down to Z Depth

↓

Retract to Clearance Height

↓

Check for Next Hole

↓

Move to Next Hole

The G81 cycle moves the drill to each hole position, drills down to a set depth, retracts, then moves to the next hole until all holes are drilled.

Execution Sample

CNC Programming

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

This program drills three holes at specified XY positions down to Z -5 with a retract height of 2, then cancels the cycle.

Execution Table

Step	Line	Action	Position (X,Y,Z)	Feed Rate	Cycle State	Output
1	N10	Start G81 cycle at X10 Y10	(10,10,0)	100	Cycle started	Move to (10,10) at rapid
2	N10	Drill down to Z-5	(10,10,-5)	100	Drilling	Drill hole at (10,10) to depth -5
3	N10	Retract to R2	(10,10,2)	100	Retracted	Retract to clearance height 2
4	N20	Move to next hole X20 Y10	(20,10,2)	100	Cycle continuing	Rapid move to (20,10)
5	N20	Drill down to Z-5	(20,10,-5)	100	Drilling	Drill hole at (20,10) to depth -5
6	N20	Retract to R2	(20,10,2)	100	Retracted	Retract to clearance height 2
7	N30	Move to next hole X20 Y20	(20,20,2)	100	Cycle continuing	Rapid move to (20,20)
8	N30	Drill down to Z-5	(20,20,-5)	100	Drilling	Drill hole at (20,20) to depth -5
9	N30	Retract to R2	(20,20,2)	100	Retracted	Retract to clearance height 2
10	N40	Cancel G81 cycle	(20,20,2)	100	Cycle ended	Stop drilling cycle
11	-	End of program	(20,20,2)	100	Idle	Program ends

💡 G80 command cancels the G81 drilling cycle, stopping further drilling.

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 6	After Step 9	Final
X	0	10	10	20	20	20
Y	0	10	10	10	20	20
Z	0	0	2	2	2	2
Cycle State	Idle	Started	Retracted	Retracted	Retracted	Ended

Key Moments - 3 Insights

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

The drill retracts to the R height (clearance) to safely move to the next hole without hitting the workpiece, as shown in steps 3, 6, and 9 in the execution table.

What happens if the G80 command is missing?

Without G80, the G81 cycle would continue drilling at each new position, possibly causing unintended drilling. Step 10 shows G80 stopping the cycle.

Why does the Z position jump from -5 to 2 after drilling?

After drilling to depth -5, the tool retracts to the R plane at Z=2 for clearance, as seen in steps 3, 6, and 9.

Visual Quiz - 3 Questions

Look at the execution table, what is the Z position after retracting from the first hole?

A 2

B -5

C 0

D 10

💡 Hint

Check step 3 in the execution table where the tool retracts to clearance height.

At which step does the drilling cycle end?

A Step 9

B Step 11

C Step 10

D Step 8

💡 Hint

Look for the G80 command in the execution table that cancels the cycle.

If the feed rate F100 was changed to F200 in line N10, what would change in the execution table?

A Positions would change

B Feed Rate column would show 200 for all drilling steps

C Cycle state would change to 'Faster'

D Retract height would change

💡 Hint

Feed rate affects speed, so check the Feed Rate column in the execution table.

Concept Snapshot

G81 simple drilling cycle syntax:
G81 X... Y... Z... R... F...
Moves to XY, drills to Z depth,
retracts to R height, repeats for holes.
Use G80 to cancel cycle.
Feed rate F controls drilling speed.

Full Transcript

The G81 simple drilling cycle automates drilling multiple holes. The cycle starts by moving the drill to the first hole position at rapid speed. Then it drills down to the specified Z depth at the feed rate. After drilling, it retracts to the clearance height R to avoid collisions. The cycle then moves to the next hole and repeats drilling and retracting. This continues until all holes are drilled. The G80 command cancels the cycle to stop drilling. The example program drills three holes at positions (10,10), (20,10), and (20,20) down to Z -5 with a retract height of 2 and feed rate 100. Variables X, Y, and Z track the tool position, and the cycle state shows if drilling is active or ended. Key points include retracting after drilling for safety and using G80 to end the cycle. Changing feed rate affects drilling speed but not positions or retract height.

Practice

1. What is the main purpose of the G81 command in CNC programming?

easy

A. To perform a simple drilling cycle at specified XY positions with a set depth

B. To move the tool rapidly without cutting

C. To set the spindle speed

D. To cancel all active cycles

Solution

1. Step 1: Understand the function of G81

   The G81 command is used to automate drilling holes at multiple XY positions with the same depth.

2. Step 2: Compare with other options

   The other options describe other commands or actions unrelated to drilling cycles.

3. Final Answer:

   To perform a simple drilling cycle at specified XY positions with a set depth -> Option A

4. Quick Check:

   G81 = Simple drilling cycle [OK]

Hint: G81 always means drilling holes at set depth [OK]

Common Mistakes:

• Confusing G81 with rapid move commands
• Thinking G81 sets spindle speed
• Forgetting G81 automates repeated drilling

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 R2 Z-5 X10 Y20

B. G81 Z-5 X10 Y20 R2

C. G81 X10 Y20 R2 Z-5

D. G81 X10 Y20 Z-5 R2

Solution

1. Step 1: Identify the correct order of parameters in G81

   The standard order is G81 followed by X, Y, then Z for depth, and R for retract height.

2. Step 2: Check each option

   G81 X10 Y20 Z-5 R2 follows the correct order: G81 X10 Y20 Z-5 R2. Others mix the order incorrectly.

3. Final Answer:

   G81 X10 Y20 Z-5 R2 -> Option D

4. Quick Check:

   Correct parameter order = G81 X10 Y20 Z-5 R2 [OK]

Hint: Use G81 X Y Z R in that order for drilling [OK]

Common Mistakes:

• Placing Z before X and Y
• Mixing R and Z order
• Omitting required parameters

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

G90 G00 X0 Y0 Z5
G81 X10 Y10 Z-4 R1 F100
G80
G00 Z5

medium

A. Z-4

B. Z1

C. Z5

D. Z0

Solution

1. Step 1: Analyze the drilling cycle commands

   The G81 command drills to Z-4 from retract height R1. After drilling, G80 cancels the cycle.

2. Step 2: Check the final move command

   After G80, the program moves the tool to Z5 with G00, which is a rapid move to safe height.

3. Final Answer:

   Z5 -> Option C

4. Quick Check:

   Final Z after G00 Z5 = Z5 [OK]

Hint: Look for last Z move after G80 to find final position [OK]

Common Mistakes:

• Assuming tool stays at drilling depth
• Ignoring G80 cancels drilling cycle
• Missing final rapid move to safe height

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

G81 X20 Y30 Z5 R2 F80

medium

A. Z value should be negative for drilling depth

B. R value cannot be used with G81

C. F value is missing

D. X and Y coordinates are not allowed

Solution

1. Step 1: Understand the meaning of Z in drilling

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

2. Step 2: Check the given command

   Z5 is positive, which means moving above the surface, not drilling down.

3. Final Answer:

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

4. Quick Check:

   Drilling depth Z must be negative [OK]

Hint: Drilling depth Z is always negative below surface [OK]

Common Mistakes:

• Using positive Z for drilling depth
• Confusing R with retract height
• Ignoring feed rate F

5. You want to drill three holes at positions X10 Y10, X20 Y10, and X30 Y10 with a depth of 6 mm below the surface, retract height 2 mm, and feed rate 120 mm/min. Which of the following programs correctly uses G81 to do this efficiently?

hard

A. G81 X10 Y10 Z-6 R2 F120
G81 X20 Y10 Z-6 R2 F120
G81 X30 Y10 Z-6 R2 F120
G80

B. G81 Z-6 R2 F120
X10 Y10
X20 Y10
X30 Y10
G80

C. G81 X10 Y10 Z6 R2 F120
X20 Y10
X30 Y10
G80

D. G81 Z-6 R2 F120 X10 Y10 X20 Y10 X30 Y10
G80

Solution

1. Step 1: Understand how to use G81 for multiple holes

   Start G81 with drilling parameters, then list XY positions on separate lines to drill multiple holes efficiently.

2. Step 2: Evaluate each option

   G81 Z-6 R2 F120 X10 Y10 X20 Y10 X30 Y10 G80 correctly sets G81 once with parameters, then drills at three XY points, then cancels with G80. G81 X10 Y10 Z-6 R2 F120 G81 X20 Y10 Z-6 R2 F120 G81 X30 Y10 Z-6 R2 F120 G80 repeats G81 unnecessarily. G81 X10 Y10 Z6 R2 F120 X20 Y10 X30 Y10 G80 uses positive Z depth (wrong). G81 Z-6 R2 F120 X10 Y10 X20 Y10 X30 Y10 G80 tries to list all XY in one line, which is invalid.

3. Final Answer:

   G81 Z-6 R2 F120 X10 Y10 X20 Y10 X30 Y10 G80 -> Option B

4. Quick Check:

   One G81 + multiple XY lines = G81 Z-6 R2 F120 X10 Y10 X20 Y10 X30 Y10 G80 [OK]

Hint: Use one G81 with parameters, then list XY positions, end with G80 [OK]

Common Mistakes:

• Repeating G81 for each hole
• Using positive Z for depth
• Trying to put all XY in one line