G85 boring cycle in CNC Programming - Step-by-Step Execution

Concept Flow - G85 boring cycle

Start at safe position

↓

Rapid move to hole position

↓

Feed move to boring depth

↓

Perform boring at depth

↓

Retract to safe position

↓

End or move to next hole

The G85 boring cycle moves quickly to the hole, feeds to the depth while boring, then retracts safely before ending or moving on.

Execution Sample

CNC Programming

N10 G90 G54
N20 G00 X50 Y50
N30 G85 Z-20 R5 F100
N40 G00 Z100
N50 M30

This program uses G85 to bore a hole at X50 Y50 to a depth of Z-20 with a retract plane at Z5.

Execution Table

Step	Line	Action	Position	Feed Mode	Output
1	N10	Set absolute positioning and work offset	Position unchanged	None	Mode set: G90 G54
2	N20	Rapid move to hole position	X50 Y50 Z current	Rapid (G00)	Moved quickly to X50 Y50
3	N30	Start G85 boring cycle: feed to Z-20	X50 Y50 Z-20	Feed (F100)	Boring hole to depth Z-20
4	N30	Retract to R plane at Z5	X50 Y50 Z5	Rapid (G00)	Retracted to safe height Z5
5	N40	Rapid move to safe Z100	X50 Y50 Z100	Rapid (G00)	Moved up to Z100 safe position
6	N50	Program end	Position unchanged	None	Program ends

💡 Program ends after retracting to safe position and executing M30

Variable Tracker

Variable	Start	After N20	After N30 (feed down)	After retract	Final
X	0	50	50	50	50
Y	0	50	50	50	50
Z	100	100	-20	5	100
Feed Mode	None	Rapid	Feed 100	Rapid	Rapid

Key Moments - 3 Insights

Why does the tool retract to the R plane after boring instead of going straight up?

The retract to the R plane (Z5) after boring is shown in step 4 of the execution_table. This ensures the tool clears the hole safely before moving rapidly to the final safe height.

What does the feed rate F100 control in the G85 cycle?

In step 3, the feed rate F100 controls the speed at which the tool moves down to the boring depth Z-20, ensuring a controlled boring operation.

Why is the initial move to X50 Y50 done with G00 and not feed?

Step 2 shows a rapid move (G00) to the hole position to save time. Feeding is only used during boring (step 3) to carefully cut the material.

Visual Quiz - 3 Questions

Look at the execution_table, what is the Z position after the boring feed move at step 3?

A Z-20

B Z5

C Z0

D Z100

💡 Hint

Check the Position column at step 3 in the execution_table.

At which step does the tool retract to the R plane (Z5)?

A Step 3

B Step 4

C Step 2

D Step 5

💡 Hint

Look for the retract action and Z5 position in the execution_table.

If the feed rate F100 was changed to F50 in line N30, how would the execution_table change?

A The rapid moves would slow down

B The tool would retract to a different Z position

C The feed mode at step 3 would be Feed 50 instead of Feed 100

D The initial move to X50 Y50 would be slower

💡 Hint

Feed rate affects the feed move speed during boring, check step 3 Feed Mode.

Concept Snapshot

G85 boring cycle:
- Rapid move to hole (G00)
- Feed boring to depth (Z) at feed rate (F)
- Retract to R plane
- Then rapid retract to safe height
Used for precise boring with controlled feed.

Full Transcript

The G85 boring cycle starts by moving the tool rapidly to the hole position. Then it feeds down to the boring depth at a set feed rate, cutting the hole. After reaching the depth, the tool retracts to a safety plane (R) before moving rapidly to a safe height. This cycle ensures efficient and safe boring operations. The example program shows these steps with positions and feed modes changing accordingly.

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