G85 boring cycle in CNC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the G85 code represent in CNC programming?

G85 is the code for the boring cycle, a canned cycle used to bore holes with a fixed spindle and feed the tool in a controlled way.

intermediate

How does the G85 boring cycle differ from G81 drilling cycle?

Unlike G81 which retracts the tool rapidly after each hole, G85 moves the tool in and out with a controlled feed without rapid retract, making it smoother for boring.

beginner

Which parameters are typically used with G85?

Common parameters include X, Y for position, Z for depth, R for retract plane, and F for feed rate during boring.

intermediate

What happens if the feed rate (F) is not specified in a G85 cycle?

The machine may use the last programmed feed rate or default, which can cause improper boring feed and affect hole quality.

beginner

Write a simple example of a G85 boring cycle command to bore a hole at X10 Y5 to a depth of Z-20 with feed rate 100.

G85 X10 Y5 Z-20 R2 F100 ; Boring cycle to position X10 Y5, depth -20, retract plane 2, feed 100

What does the G85 code do in CNC programming?

A Performs a boring cycle with feed in and out

B Drills a hole with rapid retract

C Starts the spindle

D Moves the tool to home position

G85 is used for boring cycles where the tool feeds in and out smoothly without rapid retract.

Which parameter sets the depth of the hole in a G85 cycle?

A Z

B Y

C X

D R

Z specifies the depth to which the boring tool will feed.

What is the purpose of the R parameter in G85?

A Feed rate

B Tool number

C Spindle speed

D Retract plane height

R sets the retract plane height above the part before and after boring.

If you omit the feed rate (F) in G85, what might happen?

A The tool will not move

B The spindle stops

C The machine uses the last feed rate or default

D The program ends

Without F, the machine uses the last feed rate, which may not be suitable for boring.

Which cycle is smoother for boring: G81 or G85?

A G81

B G85

C Both are the same

D Neither

G85 provides a smooth feed in and out, better for boring than G81 which uses rapid retract.

Explain the purpose and basic usage of the G85 boring cycle in CNC programming.

Think about how boring is different from drilling and what G85 controls.

Describe what happens during a G85 cycle and why feed rate is important.

Consider the tool movement and cutting conditions.

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