Depth of cut and step-over in CNC Programming

Introduction

Depth of cut and step-over control how much material a CNC tool removes in one pass. They help make cutting safe and efficient.

When setting up a CNC milling operation to remove material layer by layer.

When adjusting the tool path to avoid overloading the tool or machine.

When planning finishing passes to get a smooth surface.

When optimizing machining time by balancing speed and tool wear.

Syntax

CNC Programming

Depth of cut: The vertical distance the tool cuts into the material per pass.
Step-over: The horizontal distance the tool moves over between passes.

Depth of cut is usually set based on tool size and material hardness.

Step-over is often a percentage of the tool diameter to ensure proper surface finish.

Examples

The tool cuts 2 mm deep each pass and moves 1 mm sideways between passes.

CNC Programming

Depth of cut = 2 mm
Step-over = 1 mm

Using imperial units, the tool cuts half an inch deep and moves a quarter inch sideways.

CNC Programming

Depth of cut = 0.5 inch
Step-over = 0.25 inch

Sample Program

This simple script calculates and shows depth of cut and step-over based on tool diameter.

CNC Programming

TOOL_DIAMETER = 10  # mm
DEPTH_OF_CUT = 2     # mm
STEP_OVER = TOOL_DIAMETER * 0.5  # 50% of tool diameter

print(f"Tool diameter: {TOOL_DIAMETER} mm")
print(f"Depth of cut per pass: {DEPTH_OF_CUT} mm")
print(f"Step-over per pass: {STEP_OVER} mm")

Important Notes

Choosing too deep a cut can break the tool or damage the machine.

Too small a step-over increases machining time but improves surface finish.

Always check manufacturer recommendations for your specific tool and material.

Summary

Depth of cut controls how deep the tool cuts each pass.

Step-over controls how far the tool moves sideways between passes.

Balancing these helps make CNC machining safe and efficient.

Practice

1. What does the depth of cut control in CNC machining?

easy

A. How fast the tool moves along the path

B. How deep the tool cuts into the material each pass

C. The speed of the spindle rotation

D. The temperature of the cutting tool

Solution

1. Step 1: Understand depth of cut meaning

   Depth of cut means how far down the tool goes into the material per pass.

2. Step 2: Match with options

   How deep the tool cuts into the material each pass matches this definition exactly, others describe different parameters.

3. Final Answer:

   How deep the tool cuts into the material each pass -> Option B

4. Quick Check:

   Depth of cut = tool depth per pass [OK]

Hint: Depth of cut = vertical cut depth per pass [OK]

Common Mistakes:

• Confusing depth of cut with feed rate
• Mixing depth of cut with spindle speed
• Thinking depth of cut controls sideways movement

2. Which of the following is the correct way to specify a step-over of 0.5 mm in a CNC program line?

easy

A. STEP_OVER 0.5

B. STEP_OVER = 0.5

C. STEP_OVER(0.5)

D. STEP_OVER:0.5

Solution

1. Step 1: Identify common CNC parameter syntax

   CNC programs often use keyword followed by value without symbols for parameters.

2. Step 2: Compare options

   STEP_OVER 0.5 uses keyword and value separated by space, which is standard. Others use symbols not typical in CNC code.

3. Final Answer:

   STEP_OVER 0.5 -> Option A

4. Quick Check:

   Parameter syntax = keyword + space + value [OK]

Hint: Parameters usually use space, not symbols, between name and value [OK]

Common Mistakes:

• Adding equals sign or parentheses incorrectly
• Using colon instead of space
• Confusing step-over syntax with programming languages

3. Given a CNC program snippet:

DEPTH_OF_CUT 2.0
STEP_OVER 1.0
MILLING_PASS

What will be the depth and sideways move per pass?

medium

A. Depth 1.0 mm, Step-over 2.0 mm

B. Depth 1.0 mm, Step-over 1.0 mm

C. Depth 2.0 mm, Step-over 2.0 mm

D. Depth 2.0 mm, Step-over 1.0 mm

Solution

1. Step 1: Read parameter values

   DEPTH_OF_CUT is set to 2.0 mm, STEP_OVER is set to 1.0 mm.

2. Step 2: Match values to depth and step-over

   Depth of cut = 2.0 mm, step-over = 1.0 mm as per the program lines.

3. Final Answer:

   Depth 2.0 mm, Step-over 1.0 mm -> Option D

4. Quick Check:

   DEPTH_OF_CUT=2.0, STEP_OVER=1.0 [OK]

Hint: Match parameter names to values directly [OK]

Common Mistakes:

• Swapping depth and step-over values
• Ignoring parameter names and guessing
• Assuming default values instead of given

4. A CNC program uses DEPTH_OF_CUT 3.0 and STEP_OVER 1.5 but the tool only cuts 1.5 mm deep per pass. What is the likely error?

medium

A. The machine overrides depth to 1.5 mm for safety

B. The step-over value is too large and limits depth

C. The depth of cut parameter is ignored due to syntax error

D. The tool diameter is smaller than depth of cut

Solution

1. Step 1: Analyze parameter effect

   If depth of cut is set to 3.0 but actual cut is 1.5, likely the program did not apply 3.0 correctly.

2. Step 2: Consider syntax errors

   A syntax error in the depth of cut line can cause the machine to ignore it and use default or previous value (1.5).

3. Final Answer:

   The depth of cut parameter is ignored due to syntax error -> Option C

4. Quick Check:

   Syntax error causes parameter ignore [OK]

Hint: Check syntax if parameters don't apply as expected [OK]

Common Mistakes:

• Blaming step-over for depth issues
• Assuming machine overrides without checking code
• Ignoring syntax errors in program

5. You want to machine a 10 mm deep pocket with a tool that can safely cut 2 mm depth per pass and 40% step-over of its 5 mm diameter. How many passes and what step-over distance should you program?

hard

A. 5 passes, 2 mm step-over

B. 2 passes, 2 mm step-over

C. 5 passes, 2.5 mm step-over

D. 5 passes, 0.4 mm step-over

Solution

1. Step 1: Calculate number of passes

   Depth per pass = 2 mm, total depth = 10 mm, so passes = 10 / 2 = 5 passes.

2. Step 2: Calculate step-over distance

   Step-over = 40% of 5 mm diameter = 0.4 * 5 = 2 mm.

3. Final Answer:

   5 passes, 2 mm step-over -> Option A

4. Quick Check:

   Passes = 10/2=5, Step-over = 0.4*5=2 mm [OK]

Hint: Divide depth by cut depth; multiply diameter by step-over % [OK]

Common Mistakes:

• Confusing step-over percentage with depth
• Calculating passes incorrectly
• Using wrong tool diameter for step-over