G84 tapping cycle in CNC Programming - Step-by-Step Execution

Concept Flow - G84 tapping cycle

Start G84 Cycle

↓

Position to Hole

↓

Spindle ON (CW)

↓

Feed Thread Depth

↓

Reverse Spindle (CCW)

↓

Retract to Clearance

↓

Cycle Complete

The G84 tapping cycle moves the tool to the hole, spins the spindle clockwise while feeding down to tap, then reverses spindle to unscrew and retracts.

Execution Sample

CNC Programming

N10 G90 G54
N20 G00 X10 Y10
N30 G84 Z-20 R2 F100
N40 G80

This CNC program uses G84 to tap a hole at X10 Y10 to a depth of -20 with a retract of 2 and feed rate 100.

Execution Table

Step	Action	Position	Spindle	Feed	Output/Result
1	Rapid move to hole	X10 Y10	Off	N/A	Tool moves quickly to hole position
2	Start spindle clockwise	X10 Y10	CW	N/A	Spindle starts rotating clockwise
3	Feed down to Z-20	Z from 0 to -20	CW	100	Tool feeds down while tapping threads
4	Reverse spindle CCW	Z-20	CCW	N/A	Spindle reverses to unscrew tap
5	Retract to R plane (Z2)	Z2	Off	Rapid	Tool retracts to clearance plane
6	Cycle complete	Z2	Off	N/A	Tapping cycle ends, ready for next command

💡 Cycle ends after retracting to clearance plane and spindle stops

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	After Step 5	Final
Position	X0 Y0 Z0	X10 Y10 Z0	X10 Y10 Z-20	X10 Y10 Z-20	X10 Y10 Z2	X10 Y10 Z2
Spindle	Off	Off	CW	CCW	Off	Off
Feed	N/A	N/A	100	N/A	Rapid	N/A

Key Moments - 3 Insights

Why does the spindle reverse direction after feeding down?

The spindle reverses (step 4) to unscrew the tap from the hole, preventing damage to threads.

What does the R value control in the G84 command?

The R value (step 5) sets the retract plane height where the tool pulls out after tapping.

Why is the feed rate active only during the feed down step?

Feed rate controls the tapping speed during thread cutting (step 3); rapid moves (steps 1 and 5) use no feed.

Visual Quiz - 3 Questions

Look at the execution table, what is the spindle direction at Step 3?

A Counterclockwise (CCW)

B Clockwise (CW)

C Off

D Variable speed

💡 Hint

Check the 'Spindle' column at Step 3 in the execution_table

At which step does the tool start retracting to the clearance plane?

A Step 2

B Step 3

C Step 5

D Step 6

💡 Hint

Look for 'Retract to R plane' action in the execution_table

If the feed rate F is increased, which step's output will change?

A Step 3 - Feed down tapping

B Step 4 - Spindle reverse

C Step 1 - Rapid move

D Step 5 - Retract

💡 Hint

Feed rate affects the 'Feed' column during the feed down step in execution_table

Concept Snapshot

G84 tapping cycle syntax: G84 Z(depth) R(retract) F(feed)
Moves tool to hole, spindle ON clockwise, feeds down to tap threads
Spindle reverses CCW to unscrew tap
Retracts to R plane
Cycle ends ready for next command

Full Transcript

The G84 tapping cycle is a CNC command that automates tapping holes. First, the tool rapidly moves to the hole position. Then the spindle turns on clockwise and the tool feeds down to the specified depth to cut threads. After reaching the depth, the spindle reverses direction to unscrew the tap. Finally, the tool retracts to a clearance plane before the cycle ends. The feed rate controls the tapping speed during feed down. The retract plane is set by the R value. This cycle prevents thread damage by reversing spindle and retracting safely.

Practice

1. What is the primary purpose of the G84 command in CNC programming?

easy

A. To perform a tapping cycle automatically

B. To drill a hole without threading

C. To move the tool to a safe position

D. To cancel any active canned cycle

Solution

1. Step 1: Understand the function of G84

   The G84 command is used specifically for tapping cycles, which means it automates the process of threading holes.

2. Step 2: Compare with other options

   Options A, B, and D describe other CNC functions: tool movement, drilling, and cycle cancellation, which are not the main purpose of G84.

3. Final Answer:

   To perform a tapping cycle automatically -> Option A

4. Quick Check:

   G84 = Tapping cycle [OK]

Hint: G84 always means tapping cycle in CNC [OK]

Common Mistakes:

• Confusing G84 with drilling cycles
• Thinking G84 cancels cycles
• Assuming G84 moves tool without machining

2. Which of the following is the correct syntax to start a G84 tapping cycle at X10 Y20 with a depth of Z-15 and feed rate F100?

easy

A. G84 Z-15 X10 Y20 F100

B. G84 X10 Y20 Z-15 F100

C. G84 F100 X10 Y20 Z-15

D. G84 X10 Y20 F100 Z-15

Solution

1. Step 1: Identify standard G84 syntax order

   The typical order is G84 followed by X, Y coordinates, then Z depth, and finally feed rate F.

2. Step 2: Check each option's order

   G84 X10 Y20 Z-15 F100 follows the correct order: G84 X10 Y20 Z-15 F100. Others mix the order incorrectly, which may cause errors.

3. Final Answer:

   G84 X10 Y20 Z-15 F100 -> Option B

4. Quick Check:

   Correct parameter order = G84 X10 Y20 Z-15 F100 [OK]

Hint: Use X Y Z F in order after G84 [OK]

Common Mistakes:

• Placing feed rate before coordinates
• Mixing Z depth position
• Omitting coordinates or feed rate

3. Given the following CNC program snippet, what will be the tool's final position after executing the G84 tapping cycle?

G90 G00 X5 Y5
G84 Z-10 R2 F75
G80

medium

A. At X5 Y5 Z-10 (bottom of hole)

B. At X5 Y5 Z0 (surface level)

C. At X0 Y0 Z0 (machine home)

D. At X5 Y5 Z2 (retract plane)

Solution

1. Step 1: Understand G84 with R parameter

   The R2 means retract plane is at Z=2. After tapping to Z-10, the tool retracts to Z2.

2. Step 2: Analyze final position after G80

   G80 cancels the cycle but does not move the tool. So final position is at retract plane X5 Y5 Z2.

3. Final Answer:

   At X5 Y5 Z2 (retract plane) -> Option D

4. Quick Check:

   G84 retracts to R plane = Z2 [OK]

Hint: G84 ends at retract plane (R value) after tapping [OK]

Common Mistakes:

• Assuming tool stays at bottom of hole
• Thinking G80 moves tool home
• Ignoring retract plane parameter

4. Identify the error in this G84 tapping cycle command:

G84 X15 Y10 Z-20 F150 R-5

medium

A. Negative retract plane value is invalid

B. Feed rate F150 is too high for tapping

C. Coordinates X15 Y10 are missing

D. Z depth cannot be negative

Solution

1. Step 1: Check the R parameter meaning

   R specifies the retract plane above the surface; it must be positive or zero, not negative.

2. Step 2: Validate other parameters

   Coordinates and feed rate are valid; negative Z depth is normal for drilling down.

3. Final Answer:

   Negative retract plane value is invalid -> Option A

4. Quick Check:

   R must be positive or zero [OK]

Hint: Retract plane (R) cannot be negative [OK]

Common Mistakes:

• Using negative R values
• Confusing Z depth sign
• Assuming feed rate is always wrong

5. You want to tap multiple holes at positions (X10 Y10), (X20 Y10), and (X30 Y10) to a depth of Z-12 using G84. Which program snippet correctly uses a loop to automate this in CNC with macro programming?

hard

A. #1=10
WHILE[#1 LE 30] DO1
G84 X[#1] Y10 Z-12 F100 R2
#1=[#1+5]
END1
G80

B. #1=10
WHILE[#1 LT 30] DO1
G84 X[#1] Y10 Z-12 F100 R2
#1=[#1+5]
END1
G80

C. #1=10
WHILE[#1 LE 30] DO1
G84 X[#1] Y10 Z-12 F100 R2
#1=[#1+10]
END1
G80

D. #1=10
WHILE[#1 LE 30] DO1
G84 X[#1] Y10 Z-12 F100 R2
#1=[#1+15]
END1
G80

Solution

1. Step 1: Identify correct loop range and increment

   Positions are X=10, 20, 30, so start at 10, increment by 10, and loop while less or equal to 30.

2. Step 2: Check each option's increment and condition

   #1=10 WHILE[#1 LE 30] DO1 G84 X[#1] Y10 Z-12 F100 R2 #1=[#1+10] END1 G80 uses #1=10, increments by 10, and loops while #1 LE 30, matching required positions exactly.

3. Final Answer:

   #1=10 WHILE[#1 LE 30] DO1 G84 X[#1] Y10 Z-12 F100 R2 #1=[#1+10] END1 G80 -> Option C

4. Quick Check:

   Loop increments by 10 to 30 = #1=10 WHILE[#1 LE 30] DO1 G84 X[#1] Y10 Z-12 F100 R2 #1=[#1+10] END1 G80 [OK]

Hint: Loop increments match hole spacing exactly [OK]

Common Mistakes:

• Wrong increment step size
• Incorrect loop condition (LT vs LE)
• Forgetting to end cycle with G80