G85 boring cycle in CNC Programming - Mini Project: Build & Apply

G85 Boring Cycle CNC Program

📖 Scenario: You are programming a CNC lathe to perform a boring operation on a metal workpiece. The boring cycle will drill a hole to a specific depth at a controlled feed rate.

🎯 Goal: Create a CNC program using the G85 boring cycle command to bore a hole at a given position with a specified depth and feed rate.

📋 What You'll Learn

Create variables for boring position coordinates X and Z.

Set a variable for boring depth depth.

Set a variable for feed rate F.

Write the G85 boring cycle command using these variables.

Output the final CNC program block with the boring cycle command.

💡 Why This Matters

🌍 Real World

CNC machinists use boring cycles like G85 to precisely drill holes inside parts with controlled feed and depth.

💼 Career

Understanding how to program G85 cycles helps CNC programmers automate boring operations efficiently and safely.

1

Set boring position coordinates

Create two variables called X and Z with values 25.0 and -10.0 respectively to represent the boring position coordinates.

CNC Programming

# Define boring position coordinates
# Your code here

Hint

Use simple assignment statements like X = 25.0 and Z = -10.0.

2

Set boring depth and feed rate

Create two variables called depth with value 15.0 and F with value 0.2 to represent boring depth and feed rate.

CNC Programming

X = 25.0
Z = -10.0
# Define boring depth and feed rate
# Your code here

Hint

Assign depth = 15.0 and F = 0.2.

3

Write the G85 boring cycle command

Write a variable called g85_command that stores the string of the boring cycle command using G85, the X and Z coordinates, the boring depth as Z offset, and the feed rate F. Format it exactly as: G85 X25.0 Z-25.0 F0.2 where the Z value is Z - depth.

CNC Programming

X = 25.0
Z = -10.0
depth = 15.0
F = 0.2
# Create the G85 boring cycle command string
# Your code here

Hint

Use an f-string to combine variables: f"G85 X{X} Z{Z - depth} F{F}".

4

Print the boring cycle command

Print the variable g85_command to display the final boring cycle CNC command.

CNC Programming

X = 25.0
Z = -10.0
depth = 15.0
F = 0.2
g85_command = f"G85 X{X} Z{Z - depth} F{F}"
# Print the boring cycle command
# Your code here

Hint

Use print(g85_command) to show the command.

Practice

1. What is the primary purpose of the G85 command in CNC programming?

easy

A. To perform rapid positioning without cutting

B. To drill a hole with peck drilling cycles

C. To perform a boring cycle without dwell for smooth hole finishing

D. To execute a tapping cycle for threads

Solution

1. Step 1: Understand the function of G85 and compare with other cycles

   G85 is a boring cycle used to create holes smoothly without dwell time, unlike other cycles that may pause. Drilling uses G81 or peck drilling uses G83; tapping uses G84. G85 is specifically for boring without dwell.

2. Final Answer:

   To perform a boring cycle without dwell for smooth hole finishing -> Option C

3. Quick Check:

   G85 = boring cycle without dwell [OK]

Hint: G85 = boring without pause, not drilling or tapping [OK]

Common Mistakes:

• Confusing G85 with drilling cycles like G81 or G83
• Thinking G85 includes dwell time
• Mixing G85 with tapping cycles

2. Which of the following is the correct syntax for a G85 boring cycle command?

easy

A. G85 X20.0 Y10.0 Z-15.0 R5.0 F100

B. G85 X20.0 Y10.0 Z-15.0 R5.0 S100

C. G85 X20.0 Y10.0 Z-15.0 F100 D5.0

D. G85 X20.0 Y10.0 Z-15.0 R5.0 T100

Solution

1. Step 1: Identify required parameters in G85 and check each option

   G85 requires X, Y, Z for position, R for retract height, and F for feed rate. G85 X20.0 Y10.0 Z-15.0 R5.0 F100 uses X, Y, Z, R, and F correctly. Others use incorrect parameters like S, D, or T which are not valid here.

2. Final Answer:

   G85 X20.0 Y10.0 Z-15.0 R5.0 F100 -> Option A

3. Quick Check:

   Correct G85 syntax includes R and F [OK]

Hint: Look for R (retract) and F (feed) parameters in G85 [OK]

Common Mistakes:

• Using S or T instead of F for feed rate
• Omitting retract height R parameter
• Confusing feed rate with spindle speed

3. What will be the tool movement sequence for the command G85 X30 Y20 Z-10 R2 F80?

medium

A. Feed directly to Z-10, dwell, then rapid retract to R2

B. Rapid to retract height 2, feed to Z-10, then rapid retract

C. Rapid to Z-10, dwell, then feed to retract height 2

D. Feed to retract height 2, rapid to Z-10, then feed retract

Solution

1. Step 1: Understand G85 movement sequence and match to options

   G85 moves rapidly to retract height (R2), then feeds down to final depth (Z-10) without dwell, then retracts rapidly. Rapid to retract height 2, feed to Z-10, then rapid retract matches this. Others mention dwell or wrong order.

2. Final Answer:

   Rapid to retract height 2, feed to Z-10, then rapid retract -> Option B

3. Quick Check:

   G85 = rapid to R, feed to Z, rapid retract [OK]

Hint: G85 always rapid to R, feed to Z, then rapid retract [OK]

Common Mistakes:

• Assuming dwell occurs in G85
• Mixing feed and rapid moves order
• Confusing retract height with final depth

4. Identify the error in this G85 command: G85 X25 Y15 Z-12 F120

medium

A. Missing retract height parameter R

B. Incorrect feed rate parameter F

C. Z value should be positive

D. X and Y coordinates cannot be used together

Solution

1. Step 1: Check required parameters for G85 and analyze given command

   G85 requires retract height R to define safe rapid retract position. The command lacks R parameter, which is mandatory. Feed rate F and coordinates are correct; Z can be negative for depth.

2. Final Answer:

   Missing retract height parameter R -> Option A

3. Quick Check:

   G85 needs R parameter [OK]

Hint: Always include R for retract height in G85 [OK]

Common Mistakes:

• Omitting retract height R
• Thinking Z must be positive
• Believing X and Y cannot be combined

5. You want to bore a hole 20 mm deep with a retract height of 3 mm and feed rate of 90 mm/min using G85. Which command correctly achieves this if the hole center is at X50 Y40?

hard

A. G85 X50 Y40 Z-20 F90

B. G85 X50 Y40 Z20 R3 F90

C. G85 X50 Y40 Z-20 R30 F90

D. G85 X50 Y40 Z-20 R3 F90

Solution

1. Step 1: Set correct depth and retract height, then verify feed rate and coordinates

   Depth is negative for downward movement, so Z-20 is correct. Retract height R3 means rapid retract to 3 mm above surface. Feed rate F90 is correct. X50 Y40 sets hole center. G85 X50 Y40 Z-20 R3 F90 has all correct parameters.

2. Final Answer:

   G85 X50 Y40 Z-20 R3 F90 -> Option D

3. Quick Check:

   Negative Z for depth, R for retract, F for feed [OK]

Hint: Depth is negative Z, retract R positive, feed F set [OK]

Common Mistakes:

• Using positive Z for depth
• Setting retract height too large
• Omitting retract height R