Depth of Cut in Machining: Importance and How to Calculate

What Does Depth of Cut (DOC) Mean in Machining?

The depth of cut refers to how deep the CNC cutter digs the workpiece and cuts through it to create a chip in the machining process. Cutting tools move to the left with a specific depth in the workpiece in an ideal cutting process to create chips. This depth is simply the Depth of Cut or DOC. Designers measure the depth of cut values in millimeters or inches, which ranges from 0.1 to 1mm.

Radial depth of cut (RDOC) and axial depth of cut (ADOC) are the backbones of every machining process. The radial depth (Stepover or Cut width) refers to the distance a tool engages a workpiece perpendicular to its axis direction. It is the distance from the tool’s centerline to the deepest points of the cut in a radial path. Conversely, axial depth (Stepdown or Cut depth) is the distance a tool engages a blank along its centerline. It simply is the vertical distance from the workpiece’s surface to the deepest point of the cut along the tool’s axis.

Why Is Depth of Cut Important in Machining?

Cut depth directly influences machining parameters, including tool wear rate, quality of machined surface, heat rise at the tool’s tip, and strength of the processed components. For instance, annealed metals are more susceptible to Built-up Edge (BUE) formation in the cutting process than cold-worked metals. BUE chip type comprises layers of chips accumulating at the cutting tool’s tip. Hence, uncontrolled BUE adversely affects the machined parts’ surface finish if the cut is extremely deep.

How to Calculate the Depth of Cut?

You’ll discover the importance of quantitative relationships among machining parameters when calculating the cut’s depth. Below are parameters to specify when you need to calculate the depth of cut:

• The chosen machining process (Turning, milling, drilling, etc.)
• Machine capabilities
• Workpiece material
• Tool tip properties
• Required surface finish on tolerance

Depth of Cut in CNC Turning

The chuck rotates the workpiece in the CNC turning process while the cutting tool engages the workpiece as it moves along its length. The feed rate can be the same as the depth of cut since it is the distance the cutting tool covers along the blank at each revolution over time, measured in mm/min. Designers define the depth of cut based on the thickness of the material removed. Below is a formula for this calculation:

Depth of Cut in CNC Milling

The CNC tool rotates while the blank is fastened to a worktable in the CNC milling process. How deep the tool cuts in the workpiece material in one turn refers to the depth of the cut in the milling process. The cutting depth is usually 4 times the cutting tool’s diameter for large diameter greater than 20mm and 10 times the diameter size for smaller cutter diameter.

How Does Depth of Cut Influence Other Machining Factors?

Although managed by machine operators, the depth of cut is one of the independent parameters that impact the intricacies of CNC machining processes. However, the depth of the cut relates to all the parameters in the machining process and affects the resultant workpiece surface quality and properties.

Depth of Cut, Rake Angle, and Spindle Speed

Heat fluctuations at the tool tip significantly affect the workpiece when machining thermoplastics due to their low elastic modulus and thermal conductivity. It is essential to adjust the max depth of cut, rake angle, and cutting speed relative to each other to mitigate the temperature rise and prevent gumming and sticky chips forming at the tool chip. It is worth mentioning that depth of cut is one of the three primary parameters that affect tool life after feed rates and cutting speed.

Depth of Cut and Cooling Fluid

Chip curvature reduces, and the chip turns curly when the depth of the cut decreases. Hence, you might think that supplying cutting fluid to the cutting zone will cool the machining process in this situation and when heat increases at the tool tip. However, chips become even curlier, the contact between the tool and the chip decreases, and the heat is concentrated at the tool, leading to greater tool wear when you add CNC coolant fluid. You may consider using a chip breaker in this case.

How Is Depth of Cut Different from Chip Thickness Value?

Understanding the difference between the depth of cut and chip thickness is paramount since they differ in meaning and value.

The image above shows an ideal cutting process indicated in ‘t_o‘˳ is different from the chip thickness (indicated in ‘t_c’). The tool cutter removes material at a well-defined shear plane and with a specific shear angle ϕ, as shown in the image below. Hence, chip thickness is often greater than the value of the depth of cut.

Cutting Ratio vs. Chip Compression Ratio

The cutting ratio or chip thickness describes the ratio between the cut depth and the chip thickness. Below is a formula for this calculation:

      r= t_о/t_c

Below is a formula on how to calculate the cutting ratio or chip thickness ratio from the shear angle ϕ and the rake angle α:

Φ: represents shear angle

α: represents rake angle

Chip-compression ratio or chip-compression factor is this value’s reciprocal, indicating the chip thickness’s value compared to the depth of cut (DOC).

Depth of Cut vs. Cutting Forces and Power

It is essential to know the cutting force and power when calculating the depth of cut since they can provide critical parameters, including:

• CNC tools with the required strength to prevent rapid wear and ensure optimal efficiency and desired surface finish.
• Workpiece holders and fixtures that can handle these forces with mitigated distortion.

The product of the cutting force multiplied by the spindle speed is the power required and can be calculated using this formula:

       P= F_сV

‘F_c’ represents the ‘cutting force’ and refers to the force exerted by the tool to cut through the raw material and acts in the same direction as the tool velocity ‘V.’ The total energy required to execute the shearing force needed to cut through the workpiece surface (u_s) is the total specific energy. Also, friction-specific energy is necessary to maneuver the friction between the tool and the workpiece surface (u_f).

Shear-specific energy and friction-specific energy are linked to the depth of cut and can be calculated using these formulas:

Hence, it is essential to note that product designers and engineers use these formulas and vice versa to determine the required depth of cut when there are specific machine parameters:

Helpful Considerations to Determine the Optimal Depth of Cut in Machining

Various factors influence the depth of cut in the machining process. Hence, considering these interwoven factors when choosing the ideal cut depth is essential. These factors include:

Material Properties

CNC machining materials are indispensable elements in product manufacturing. A workpiece material’s properties significantly impact the chosen cut depth. How the workpiece interacts with the tool cutter depends on properties such as toughness, thermal conductivity, and toughness. Product engineers use smaller depths of cut for harder materials to avoid rapid tool wear, while softer materials necessitate larger depths of cut.

Material properties like chip formation and workpiece temperature sensitivity are critical since brittle material may require meticulous handling to prevent chipping. Also, heat-sensitive materials may require moderate cut depth to avoid thermal damage.

Type of Machine Operation

There are different machining processes, each with distinct procedures and outcomes. The machining operation type is a crucial determinant of the appropriate cut depth. Roughing operations may allow for larger depths of cut and multiple passes for rapid material removal. However, this focuses on efficiency rather than fine surface finish.

Conversely, smaller depths of cuts are ideal for finishing operations since the focus is on superior surface quality and dimensional accuracy. Other precision machining processes like profiling and contouring may demand meticulous depth of cut control to achieve intricate geometries.

Machine Tool Capability

The rigidity, power, and capabilities of the CNC machine tool fundamentally influence the depth of cut in machining. A versatile and rigid machine tool can manage higher cutting forces involved in larger depths of cut without sacrificing stability. In contrast, a less robust machine tool may necessitate conservative depth of cut strategies to ensure stability and accuracy.

Modern CNC machines are sophisticated and have advanced control systems, enabling adaptive machining where machinists can optimize cut depths in real time based on actual cutting conditions.

Cutting Tool Characteristics

The characteristics of CNC tools are essential determinants of the depth of cut in machining since these tools help the machinists transform ideas into physical parts. Cutting tool material, geometry, and coatings dictate how well the cutter endures the rigors of machining at varying depths.

Machinists employ advanced tool materials and high-performance coatings to extend tool life and allow more aggressive depth of cut. Tool geometry (number and arrangement of cutting edges) can affect chip formation, heat distribution, and cutting forces.

Conclusion

In machining, the depth of cut is the measure of how deeply the cutter penetrates the workpiece during each feed or pass. The concept of depth of cut in machining plays a significant role in modern manufacturing, where utmost precision and efficiency are paramount. Hence, it is essential to understand this parameter when machining since it dictates tool life, surface quality, and influences machining processes.

AT-Machining is a top-precision CNC machining service provider. We excel at providing professional assistance in determining the ideal depth of cuts for your machining project, whether you need to increase production rates and quality or achieve the finest surface finish. We leverage our state-of-the-art CNC machines and machining expertise to deliver solutions that match your project’s specific machining requirements. Contact us today with more inquiries about your CNC machining projects!