G85 boring cycle in CNC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of a simple G85 boring cycle command

What is the expected tool movement output of the following G85 boring cycle command?

G85 X50 Y25 Z-10 R2 F100;

A The tool moves to X50 Y25, bores down to Z-10 at feed 100, then retracts to R plane 2.

B The tool rapidly moves to X50 Y25, bores down to Z-10 at feed 100, then retracts rapidly to R plane 2.

C The tool bores down to Z-10 at feed 100 without moving in X and Y axes.

D The tool moves to X50 Y25, bores down to Z-10 at feed 100, then retracts rapidly to the start point.

💡 Hint

Remember that G85 is a boring cycle with feed on the boring stroke and a feed retract to the R plane.

G85 moves the tool to the specified X and Y, bores down to the Z depth at the programmed feed rate, then retracts at feed to the R plane. The retract is not rapid but at feed rate.

🧠 Conceptual

intermediate

Purpose of the R parameter in G85 boring cycle

What does the R parameter specify in a G85 boring cycle command?

A The retract plane height where the tool stops after boring.

B The feed rate for the boring operation.

C The rapid move height before starting the boring.

D The radius of the boring tool.

💡 Hint

Think about where the tool stops after finishing the boring stroke.

The R parameter defines the retract plane height above the part where the tool stops after boring before moving to the next position.

🔧 Debug

advanced

Identify the error in this G85 boring cycle command

What error will occur when running this G85 boring cycle command?

G85 X30 Y20 Z-5 F150;

A No error; command runs correctly with default retract plane.

B Runtime error because feed rate F150 is too high for boring.

C Error because Z value cannot be negative in G85.

D Syntax error due to missing R parameter for retract plane.

💡 Hint

Check if all required parameters for G85 are included.

G85 requires the R parameter to specify the retract plane. Omitting R causes a syntax or runtime error.

🚀 Application

advanced

Calculate total Z-axis travel in a G85 boring cycle

Given the command

G85 X40 Y10 Z-15 R5 F80;

, what is the total Z-axis travel distance of the tool during the cycle?

A 10 units (from Z0 to Z-15 and back to Z5).

B 15 units (only the boring depth).

C 40 units (from R5 down to Z-15 and back to R5).

D 25 units (from Z0 to Z-15 plus retract to Z10).

💡 Hint

Calculate distance from retract plane to boring depth and back.

The tool moves from retract plane at Z5 down to Z-15 (20 units), then back up to Z5 (20 units), totaling 40 units of Z-axis travel.

💻 Command Output

expert

Output behavior of G85 with multiple boring positions

What is the sequence of tool movements for this program snippet?

G90 G85 F120 R3
X10 Y10 Z-8
X20 Y10 Z-8
X20 Y20 Z-8
M30

A Tool moves rapidly to all XY positions first, then bores down to Z-8 at each point without retracting.

B Tool moves to X10 Y10, bores to Z-8 at feed 120, retracts to Z3, then repeats for X20 Y10 and X20 Y20 similarly.

C Tool bores down to Z-8 at X10 Y10, then moves rapidly to X20 Y10 and bores, but retracts only after last position.

D Tool bores down to Z-8 at X10 Y10, retracts to Z0, then moves to next position and repeats.

💡 Hint

G85 cycles bore at each XY position with feed retract to R plane before moving to next position.

G85 cycles move the tool to each XY position, bore down to Z depth at feed rate, then retract at feed rate to the R plane before moving to the next position.

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