Pocket milling (rectangular) in CNC Programming

Introduction

Pocket milling removes material inside a rectangular area to create a flat cavity. It helps shape parts by cutting out unwanted material.

Making a flat rectangular cavity in a metal or plastic part.

Creating space for components to fit inside a part.

Removing material inside a frame or housing.

Preparing a surface for further machining or assembly.

Syntax

CNC Programming

G0 Xstart Ystart ; Move to start point
G1 Zdepth Ffeedrate ; Lower tool to cutting depth
G1 Xend Ystart Ffeedrate ; Cut along X axis
G1 Xend Yend ; Cut along Y axis
G1 Xstart Yend ; Cut back along X axis
G1 Xstart Ystart ; Cut back along Y axis
G0 Zsafe ; Lift tool to safe height

G0 moves the tool quickly without cutting.

G1 moves the tool while cutting at a set feedrate.

Examples

Cuts a rectangular pocket 50 units wide and 30 units tall, 5 units deep.

CNC Programming

G0 X0 Y0
G1 Z-5 F100
G1 X50 Y0
G1 X50 Y30
G1 X0 Y30
G1 X0 Y0
G0 Z10

Starts at (10,10), cuts a smaller pocket 30x15 units, 3 units deep.

CNC Programming

G0 X10 Y10
G1 Z-3 F80
G1 X40 Y10
G1 X40 Y25
G1 X10 Y25
G1 X10 Y10
G0 Z15

Sample Program

This program mills a rectangular pocket 60mm wide, 40mm tall, and 4mm deep. It moves the tool safely before and after cutting.

CNC Programming

G21 ; Set units to millimeters
G90 ; Use absolute positioning
G0 Z10 ; Lift tool to safe height
G0 X0 Y0 ; Move to pocket start
G1 Z-4 F120 ; Lower tool to 4mm depth
G1 X60 Y0 F150 ; Cut right
G1 X60 Y40 ; Cut up
G1 X0 Y40 ; Cut left
G1 X0 Y0 ; Cut down to start
G0 Z10 ; Lift tool to safe height
M30 ; End of program

Important Notes

Always start with the tool above the workpiece to avoid crashes.

Use appropriate feedrate and depth to avoid tool damage.

Confirm units (mm or inches) before running the program.

Summary

Pocket milling removes material inside a rectangle by moving the tool along the edges.

Use G0 to move quickly without cutting and G1 to cut at a controlled speed.

Set safe heights before and after cutting to protect the tool and workpiece.

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