Pocket milling (rectangular) in CNC Programming - Step-by-Step Execution

Concept Flow - Pocket milling (rectangular)

Start at initial corner

↓

Move tool to start position

↓

Mill along X axis to opposite corner

↓

Step over in Y axis

↓

Mill back along X axis

↓

Repeat stepping and milling until pocket is done

↓

Retract tool and end

The tool moves back and forth along the X axis, stepping over in Y after each pass, to clear a rectangular pocket area.

Execution Sample

CNC Programming

G0 X0 Y0 Z5
G1 Z-2 F100
G1 X50 Y0 F200
G1 X50 Y10
G1 X0 Y10
G1 X0 Y20
G1 X50 Y20
G0 Z5

This CNC code mills a rectangular pocket by moving the tool in straight lines along X and Y axes with step overs.

Execution Table

Step	Command	Tool Position (X,Y,Z)	Action	Output/Result
1	G0 X0 Y0 Z5	(0,0,5)	Rapid move to start above material	Tool above start corner
2	G1 Z-2 F100	(0,0,-2)	Plunge into material	Tool at cutting depth
3	G1 X50 Y0 F200	(50,0,-2)	Mill along X axis to right edge	Material removed along bottom edge
4	G1 X50 Y10	(50,10,-2)	Step over in Y axis	Tool moves up to next pass line
5	G1 X0 Y10	(0,10,-2)	Mill back along X axis	Material removed along next pass
6	G1 X0 Y20	(0,20,-2)	Step over in Y axis	Tool moves up again
7	G1 X50 Y20	(50,20,-2)	Mill along X axis	Material removed along top pass
8	G0 Z5	(50,20,5)	Retract tool	Tool lifted out of pocket
9	End	(50,20,5)	Pocket milling complete	Rectangular pocket cleared

💡 Tool retracts after completing all passes to clear the rectangular pocket.

Variable Tracker

Variable	Start	After Step 3	After Step 5	After Step 7	Final
X	0	50	0	50	50
Y	0	0	10	20	20
Z	5	-2	-2	-2	5

Key Moments - 3 Insights

Why does the tool move back along the X axis after stepping over in Y?

Because the tool mills the pocket in a zigzag pattern, moving forward along X, stepping over in Y, then moving back along X to cover the entire rectangular area (see steps 3,4,5).

Why is the Z axis lowered only once at the start and not during each pass?

The tool plunges to the cutting depth once at the start (step 2) and stays there while moving in X and Y to mill the pocket, avoiding unnecessary up/down moves (see steps 2-7).

What happens if the step over in Y is too large?

If the step over is too large, some material will be left uncut between passes, resulting in an incomplete pocket (refer to step 4 and 6 for step over moves).

Visual Quiz - 3 Questions

Look at the execution table, what is the tool position after step 5?

A (0,10,-2)

B (50,10,-2)

C (50,0,-2)

D (0,0,-2)

💡 Hint

Check the 'Tool Position' column for step 5 in the execution_table.

At which step does the tool retract out of the pocket?

A Step 7

B Step 6

C Step 8

D Step 9

💡 Hint

Look for the command that moves Z back to 5 in the execution_table.

If the step over in Y was doubled, how would the execution_table change?

A More X axis moves per pass

B Fewer Y step over steps, fewer passes

C More Z plunges per pass

D No change in passes

💡 Hint

Step over in Y controls how many passes are needed to cover the pocket height (see steps 4 and 6).

Concept Snapshot

Pocket milling (rectangular):
- Tool moves in zigzag along X axis
- Steps over in Y after each pass
- Plunges once to cutting depth
- Retracts after all passes
- Step over size controls finish quality

Full Transcript

Pocket milling for a rectangular area involves moving the cutting tool back and forth along the X axis while stepping over in the Y axis after each pass. The tool starts above the material, plunges to the cutting depth once, then mills along the X axis to the opposite edge. It steps over in Y, mills back along X, and repeats until the entire rectangular pocket is cleared. Finally, the tool retracts out of the pocket. This zigzag pattern ensures efficient material removal. The step over distance in Y affects how well the pocket is cleared without leaving uncut material. The execution table shows each step with tool position and action, helping visualize the process clearly.

Practice

1. What is the main purpose of pocket milling in CNC programming?

easy

A. To drill holes in a workpiece

B. To cut the outer shape of a part

C. To remove material inside a rectangular area

D. To polish the surface of a workpiece

Solution

1. Step 1: Understand pocket milling definition

   Pocket milling is used to clear out material inside a defined area, usually rectangular.

2. Step 2: Compare with other machining operations

   Drilling makes holes, contouring cuts outer edges, polishing smooths surfaces, so they differ from pocket milling.

3. Final Answer:

   To remove material inside a rectangular area -> Option C

4. Quick Check:

   Pocket milling = remove inside rectangle [OK]

Hint: Pocket milling clears inside areas, not edges or holes [OK]

Common Mistakes:

• Confusing pocket milling with drilling
• Thinking pocket milling cuts outer edges
• Assuming pocket milling polishes surfaces

2. Which G-code command is used to move the tool quickly without cutting during pocket milling?

easy

A. G1

B. G3

C. G2

D. G0

Solution

1. Step 1: Identify G-code commands for movement

   G0 moves the tool rapidly without cutting; G1 moves with controlled feed for cutting.

2. Step 2: Differentiate G2 and G3

   G2 and G3 are used for circular interpolation, not rapid moves.

3. Final Answer:

   G0 -> Option D

4. Quick Check:

   Rapid move = G0 [OK]

Hint: G0 is rapid move; G1 is cutting move [OK]

Common Mistakes:

• Using G1 for rapid moves
• Confusing G2/G3 with rapid moves
• Mixing up G0 and G1 commands

3. Given this CNC code snippet for pocket milling, what is the tool's final position?

G0 Z5
G0 X0 Y0
G1 Z-2 F100
G1 X50 Y0 F200
G1 X50 Y30
G1 X0 Y30
G1 X0 Y0
G0 Z5

medium

A. X0 Y0 Z5

B. X0 Y0 Z-2

C. X50 Y30 Z-2

D. X50 Y30 Z5

Solution

1. Step 1: Trace the tool movements

   The tool starts at Z5 (safe height), moves to X0 Y0, then lowers to Z-2 to cut the pocket edges in sequence.

2. Step 2: Identify final move

   After completing the rectangle, the tool moves back up to Z5 (safe height) at the last position X0 Y0.

3. Final Answer:

   X0 Y0 Z5 -> Option A

4. Quick Check:

   Ends at start point with safe height Z5 [OK]

Hint: Final move is rapid up at start point [OK]

Common Mistakes:

• Ignoring the final Z5 move
• Assuming tool stays lowered after cutting
• Confusing last XY position with intermediate points

4. Identify the error in this pocket milling code snippet:

G0 Z5
G0 X0 Y0
G1 Z-2 F100
G1 X50 Y0 F200
G1 X50 Y30
G1 X0 Y30
G1 X0 Y0
G1 Z5

medium

A. Starting at Z5 instead of Z0

B. Using G1 to move up to Z5 instead of G0

C. Missing feed rate on last move

D. No error, code is correct

Solution

1. Step 1: Check movement commands for safe height

   Raising the tool to safe height (Z5) should use rapid move G0, not cutting move G1.

2. Step 2: Understand feed rate relevance

   Feed rate missing on last move is acceptable for G1 if previous feed rate applies; starting at Z5 is correct.

3. Final Answer:

   Using G1 to move up to Z5 instead of G0 -> Option B

4. Quick Check:

   Safe height moves use G0, not G1 [OK]

Hint: Use G0 for rapid moves up/down [OK]

Common Mistakes:

• Using G1 for rapid moves
• Ignoring feed rate importance
• Confusing start height with error

5. You want to program a rectangular pocket 40mm wide and 25mm tall. The tool diameter is 10mm, and you want to mill in two passes to reach a depth of 4mm. Which approach correctly plans the passes?

hard

A. Cut 2mm depth in first pass, then 2mm in second pass along same path

B. Cut full depth 4mm in one pass along the rectangle edges

C. Cut 1mm depth in first pass, then 3mm in second pass along same path

D. Cut 4mm depth in first pass, then retract and cut again at 2mm

Solution

1. Step 1: Understand depth per pass for tool safety

   Cutting full 4mm depth at once risks tool damage; splitting into equal passes is safer.

2. Step 2: Plan passes to reach total depth

   Two passes of 2mm each sum to 4mm total depth, following same rectangular path for pocket milling.

3. Final Answer:

   Cut 2mm depth in first pass, then 2mm in second pass along same path -> Option A

4. Quick Check:

   Equal depth passes sum to total depth safely [OK]

Hint: Split total depth evenly for multiple passes [OK]

Common Mistakes:

• Cutting full depth in one pass
• Unequal depth passes causing tool stress
• Retracting unnecessarily between passes