What is Material Removal Rate (MRR)?
The **Material Removal Rate (MRR)**, sometimes called the **Volume Removal Rate**, is a key performance indicator in machining. It quantifies the volume of material removed from a workpiece per unit of time, typically measured in **cubic inches per minute (in³/min)** or **cubic centimeters per minute (cm³/min)**. A higher MRR generally means higher **productivity** but also results in higher **cutting forces** and more **heat generation**, which must be managed by the machine and cooling system.
MRR Formulas by Operation
MRR is calculated by multiplying the area of the material's cross-section being cut by the linear velocity (feed rate) of the cutting action.
Milling MRR
For milling (face or end milling), the MRR is the product of the **width of cut ($W$)**, the **depth of cut ($D$)**, and the **linear feed rate ($V_f$)** in inches per minute (ipm).
$$MRR_{\text{Milling}} = W \cdot D \cdot V_f$$ where $V_f = N \cdot Z \cdot F_z$
- $W$: Width of cut (Radial depth)
- $D$: Depth of cut (Axial depth)
- $V_f$: Linear Feed Rate (ipm)
- $N$: Spindle Speed (RPM), $Z$: Number of teeth, $F_z$: Feed per tooth
Turning MRR
For turning operations, the MRR is calculated based on the cylindrical nature of the cut, involving the initial workpiece diameter ($D_{\text{initial}}$), the depth of cut ($d$), the feed per revolution ($f_r$), and the spindle speed ($N$).
$$MRR_{\text{Turning}} \approx \pi \cdot D_{\text{initial}} \cdot d \cdot f_r \cdot N$$ where $f_r = V_f / N$
- $D_{\text{initial}}$: Initial Workpiece Diameter
- $d$: Depth of cut (Axial)
- $f_r$: Feed per revolution (in/rev)
- $N$: Spindle Speed (RPM)
Drilling MRR
For drilling, the cross-sectional area of the cut is the area of the drill bit, and the velocity is the linear feed rate ($V_f$).
$$MRR_{\text{Drilling}} = \frac{\pi \cdot T^2}{4} \cdot V_f$$
- $T$: Tool Diameter (Drill diameter)
- $V_f$: Linear Feed Rate (ipm)
MRR and Power Consumption:
MRR is directly proportional to the power required for the cutting operation. By multiplying the MRR by the **Unit Power ($P_u$)**—the power required to remove one unit volume of material (e.g., HP per in³/min)—you can estimate the total machine power required.