Depth of Cut in Machining: Definitions, and Calculation Methods

When you choose CNC machining to manufacture a part, several key parameters determine the final quality of the product. Among them, the depth of cut (DOC) plays a central role. Along with feed rate and cutting speed, depth of cut has a direct impact on the machining efficiency, surface finish, and tool life.

This comprehensive guide explains what depth of cut is, why it matters, how it relates to other machining parameters, and how you can calculate it accurately.

What Is Depth of Cut in Machining?

Depth of Cut describes how far a cutting tool penetrates below a workpiece’s surface on each pass. When the tool moves laterally (as in turning) or spins while the part stays still (as in milling), it removes a layer of material whose thickness equals the DOC. Think of it like shaving a thin slice of cheese—how thick that slice is, that’s your depth of cut.

In milling, the rotating tool moves downward into the material by a set amount. In turning, the fixed cutting tool advances radially into the rotating workpiece by a specified distance. CNC manufacturers express depth of cut in millimeters (mm) or inches (in). Typical values for metal cutting range from 0.1 mm to 2 mm per pass.

Types of Depth of Cut

Machining operations involve two main kinds of depth of cut.

Type	Abbreviation	Direction	Also Known As
Radial Depth of Cut	RDOC	Perpendicular to the tool’s axis	Stepover, Cut Width
Axial Depth of Cut	ADOC	Along the tool’s axis, vertical into material	Step Down, Cut Depth

Radial depth of cut measures how far the tool engages the workpiece side-to-side. Axial depth of cut measures how deep the tool plunges into the part. Both values contribute to material removal rate and cutting forces.

Why Controlling Depth of Cut Matters

Controlling the depth of cut is essential for both performance and quality. If the depth is too shallow, the process may become inefficient. If it’s too deep, it may damage the tool, overheat the material, or create a poor surface finish.

Here are several reasons why DOC matters:

• Tool Wear Increases with Greater DOC: A deeper cut requires more force and generates more heat at the tool’s tip, which can wear out the tool much faster.
• Surface Finish Is Affected: When DOC is too high, it may lead to rough or uneven surfaces due to vibration, chip buildup, or deflection.
• Built-up Edge (BUE) Risk: Especially in soft metals like aluminum, excessive DOC can lead to BUE formation, where material sticks to the tool, distorting the surface.
• Dimensional Accuracy Can Decline: A higher depth of cut increases stress on both the tool and the workpiece, which can lead to deflection or warping.

Example: In turning operations on soft metals, uncontrolled BUE formation can leave a rough finish and lead to tool failure if the DOC isn’t properly set.

How Depth of Cut Affects Other Machining Variables

Although depth of cut is set manually or through G-code, it interacts with other machining factors in meaningful ways. You must balance all these to achieve the best results.

Cutting Speed and Feed Rate

Cutting speed (surface speed of the tool) and feed rate (tool movement per revolution) both work together with depth of cut to define the material removal rate (MRR). Changing one parameter affects the others. For example, if DOC is increased, the feed rate or speed might need to be reduced to avoid tool overheating.

Cooling and Lubrication

Coolants can help manage temperature in cutting zones, but interestingly, excessive use of coolant at low depths of cut may worsen tool wear. That’s because it can cause chips to curl tightly and trap heat at the tool interface. In such cases, using a chip breaker might be more effective than just adding coolant.

Rake Angle and Material Type

Some materials like thermoplastics are heat-sensitive. When machining such materials, the tool’s rake angle and the depth of cut must be carefully balanced to prevent thermal deformation and sticky chips.

Tool Geometry and DOC

The rake angle of the cutting tool and the cutting speed must be balanced with the depth of cut. This is particularly important when machining plastics or soft metals. These materials are more sensitive to heat and deformation, and using an improper DOC can cause gummy chips or tool sticking.

How to Calculate Depth of Cut

The method to calculate DOC depends on the type of machining process—turning or milling.

Before you calculate depth of cut, you must decide:

1. Machining Process: Milling, turning, drilling, etc.
2. Workpiece Material: Steel, aluminum, plastic, etc.
3. Tool Material and Coating: Carbide, high-speed steel (HSS), diamond style, etc.
4. Machine Capability: Maximum spindle power, rigidity, and travel limits.
5. Required Surface Finish: Roughness value or tolerance range.

Turning Operations

In turning machining, a workpiece spins while the tool moves radially. The depth of cut is simply the distance from the outer surface to the cut surface. The radial engagement equals DOC. Machinists calculate DOC from the intended material removal per pass:

• Subject: Tool path
• Action: Moves into part by radial distance
• Result: Creates a chip with thickness equal to DOC

Example Calculation:
A bar turns from 50.0 mm diameter to 48.0 mm diameter. The DOC per pass is:

DOC=(50.0−48.0)/2=1.0 mm

Parameter	Value
Initial Diameter	50.0 mm
Final Diameter	48.0 mm
Depth of Cut (per pass)	1.0 mm

Milling Operations

In milling machining, the tool rotates while the workpiece remains stationary. The depth of cut is how deep the tool penetrates vertically into the material.

There are typically two directions of depth in milling:

• Axial depth of cut – depth along the spindle axis
• Radial depth of cut – depth perpendicular to the spindle axis

While there’s no fixed formula for milling DOC, a general rule of thumb is:

• For tools with a diameter greater than 20 mm, DOC is often set to 4× tool diameter
• For tools smaller than 20 mm, DOC may go up to 10× tool diameter, depending on the rigidity of the setup

Always ensure that your machine can handle the forces generated by these values.

Typical Depths of Cut in Common Processes

Depths of cut vary widely across machining operations. The following table summarizes typical ranges:

Process	Typical Depth Range (mm)	Notes
Turning	0.5 – 3.0	Depends on workpiece hardness
Face Milling	0.5 – 10.0	Determined by cutter diameter and power
Peripheral Milling	1.0 – 5.0	Higher for roughing, lower for finishing
Slotting	0.1 – 3.0	Smaller for narrow slots
Drilling	Full hole depth	Controlled by drill length
Grinding	0.01 – 0.1	Very shallow for precision
Broaching	0.05 – 0.5	Based on broach tooth geometry
Planing/Shaping	0.2 – 5.0	Varies with material and tool
EDM	Variable	Set per pulse and total depth needed

Setting the Depth of Cut: A Step-by-Step Guide

BOYI TECHNOLOGY recommends the following workflow to choose an initial depth of cut:

1. Read the tool manufacturer’s recommended range.
2. Check material properties and workpiece geometry.
3. Consider machine capabilities and recent maintenance status.
4. Input a conservative depth for an initial test cut.
5. Run the test, then measure tool wear, surface finish, and part dimensions.
6. Increase or decrease depth by small increments until results match targets.
7. Lock in the final parameters and document them for repeat jobs.

Chip Thickness vs. Depth of Cut

One common misconception is that chip thickness is equal to the depth of cut. In reality, chip thickness is usually greater than the depth of cut.

During cutting, material shears along a shear plane. This causes the chip to compress and become thicker than the uncut layer, or DOC.

Let’s use some definitions:

• Depth of Cut (t₀): The normal distance that the tool tip moves into the material.
• Chip Thickness (tₐ): The thickness of the metal ribbon or chip after it shears off the workpiece.

Due to the shearing action at the cutting edge, the chip thickness is usually greater than the depth of cut. This is because the material deforms and compresses before being separated.

Chip Thickness Ratio (Cutting Ratio):

The ratio between the depth of cut and chip thickness is called the cutting ratio (r):

r = DOC / Chip Thickness

A lower ratio indicates greater material compression, which often results in more heat and increased tool wear.

Shear Angle and Rake Angle Effects

During shearing, the chip compresses and slides over the rake face. The geometry yields:

Where:

• ϕ is the shear plane angle.
• α is the tool’s rake angle.

A larger rake angle α increases the shear angle ϕ and lowers cutting force, but it also changes the chip thickness.

The Link Between Depth of Cut, Cutting Forces, and Power

As DOC increases, so do the forces acting on the tool and machine. Calculating these forces can help determine whether the tool or machine can handle the load.

Cutting Force (Fc)

This is the main force required to shear the material.

Cutting Power (Pc)

Where:

• Pc = Power (W)
• Fc = Cutting force (N)
• V = Cutting speed (m/min)

The energy required for cutting includes:

• Shear-specific energy (us): Energy needed to deform the material.
• Friction-specific energy (uf): Energy lost to friction between the chip and tool.

These values are influenced by the depth of cut. More depth requires more energy, which demands a more powerful machine or stiffer tooling setup.

Tip: If you’re unsure why your tool is overheating or wearing quickly, check your depth of cut and recalculate the forces.

BOYI TECHNOLOGY: Your Machining Partner

BOYI TECHNOLOGY specializes in CNC machining services across Europe and North America. We combine:

• Advanced 5-axis machining centers
• Certified toolpaths optimized for chip evacuation
• In-house testing of cutting forces and power draw
• 100% dimensional inspection on premium CMM equipment

Our engineers collaborate with you to set the ideal DOC, feed, and speed for your material and tolerance demands. Whether you’re producing small prototypes or high-volume parts, our engineers ensure that depth of cut and other critical factors are perfectly tuned for quality, speed, and cost. Contact us to learn more about how we can support your manufacturing goals.

Conclusion

The depth of cut is more than just a number in your CNC program. It shapes how your part turns out and how long your tool lasts. Misjudging DOC can lead to tool breakage, poor quality, and wasted material.

When planning a machining job:

• Start with manufacturer recommendations for depth of cut based on material and tooling.
• Consider your machine’s capabilities—more depth requires more rigidity and power.
• Adjust DOC together with cutting speed and feed rate to balance tool life and productivity.

Carefully controlling depth of cut and understanding its impact on other parameters, you can create a more stable, efficient, and cost-effective machining process. By working with BOYI TECHNOLOGY, you will reduce tool costs, shorten cycle times, and achieve consistent, high-quality parts.