G81 simple drilling cycle in CNC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the G81 command in CNC programming?

G81 is used to perform a simple drilling cycle, which automates the drilling of holes by moving the tool down to a specified depth and then retracting it.

beginner

Which parameters are typically required in a G81 drilling cycle command?

The typical parameters are X and Y for the hole position, Z for the drilling depth, R for the retract plane, and F for the feed rate.

intermediate

Explain the role of the R parameter in the G81 drilling cycle.

The R parameter sets the retract plane, which is the height above the workpiece where the tool stops before and after drilling to avoid collisions.

beginner

How do you cancel a G81 drilling cycle in a CNC program?

You cancel the G81 cycle by using G80, which stops the canned cycle and returns control to normal programming.

intermediate

What happens if you omit the feed rate (F) in a G81 drilling cycle?

If the feed rate is omitted, the machine uses the last programmed feed rate, which may cause incorrect drilling speed and affect hole quality.

What does the G81 command do in CNC programming?

A Performs a simple drilling cycle

B Starts a milling operation

C Moves the tool to home position

D Sets the spindle speed

G81 is specifically used for simple drilling cycles.

Which parameter in G81 defines the depth of the hole?

A X

B Z

C R

D Y

Z specifies the drilling depth in the G81 cycle.

How do you stop a G81 drilling cycle?

A Use M30

B Use G90

C Use G80

D Use G00

G80 cancels any active canned cycles like G81.

What is the purpose of the R parameter in G81?

A Set retract plane height

B Set spindle speed

C Set feed rate

D Set tool diameter

R sets the retract plane height above the workpiece.

If you omit the feed rate (F) in G81, what happens?

A Machine drills at max speed

B Machine stops immediately

C Machine throws an error

D Machine uses last feed rate

The machine defaults to the last programmed feed rate if F is omitted.

Describe how to program a simple drilling cycle using G81, including key parameters and their roles.

Think about the hole position, depth, safety height, and speed.

Explain why the retract plane (R) is important in a G81 drilling cycle.

Consider what happens when the tool moves up and down.

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