CNC cutting parameters

CNC cutting parameters

1. Cutting Speed (Vc)

Definition: Cutting speed is the linear speed at which the cutting tool's edge moves relative to the workpiece surface. It's typically measured in meters per minute (m/min) or feet per minute (ft/min).
Importance: The correct cutting speed is vital for achieving optimal tool life, surface finish, and material removal rate.
- Too high: Leads to excessive heat generation, rapid tool wear, and poor surface finish.
- Too low: Results in lower productivity and can sometimes cause built-up edge (BUE) on the tool.
Calculation (for turning and milling):
- Metric: $V_{c} = \frac{π \cdot D \cdot N}{1000}$
- Imperial: $V_{c} = \frac{π \cdot D \cdot N}{12}$ where:
  - $V_{c}$ = Cutting speed
  - $D$ = Diameter of the workpiece (for turning) or the tool (for milling) in mm (metric) or inches (imperial)
  - $N$ = Spindle speed in revolutions per minute (RPM)
  - $π$ ≈ 3.14159
Factors influencing selection:
- Workpiece material (hardness, machinability)
- Cutting tool material (high-speed steel, carbide, etc.)
- Coolant application
- Desired surface finish
- Type of machining operation

2. Feed Rate (f)

Definition: Feed rate is the speed at which the cutting tool advances along the workpiece. It can be expressed in:
- Millimeters per minute (mm/min) or inches per minute (IPM) - the linear speed of the tool.
- Millimeters per revolution (mm/rev) or inches per revolution (IPR) - the distance the tool advances per spindle rotation (commonly used in turning).
- Millimeters per tooth (mm/tooth) or inches per tooth (IPT) - the distance each cutting edge advances per spindle rotation (used in milling).
Importance: Feed rate affects the material removal rate, cutting forces, surface finish, and chip formation.
- Too high: Can lead to excessive tool load, poor surface finish, and tool breakage.
- Too low: Reduces productivity and can cause rubbing and increased heat.
Calculation (for milling - Feed rate in IPM):
- $FR = N \cdot n_{t} \cdot f_{t}$ where:
  - $FR$ = Feed rate in inches per minute (IPM)
  - $N$ = Spindle speed in RPM
  - $n_{t}$ = Number of teeth on the cutter
  - $f_{t}$ = Chip load or feed per tooth in inches per tooth (IPT)
Factors influencing selection:
- Workpiece material
- Cutting tool geometry and number of teeth/flutes
- Desired surface finish
- Machine rigidity and power
- Depth of cut

3. Depth of Cut (DOC)

Definition: Depth of cut is the distance the cutting tool is plunged into the workpiece in a single pass.
- Axial Depth of Cut (ADOC): The depth of the cut along the axis of the tool (vertical engagement). Often referred to as stepdown in milling.
- Radial Depth of Cut (RDOC): The engagement of the side of the tool into the workpiece (horizontal engagement). Often referred to as stepover or width of cut in milling.
Importance: Depth of cut significantly impacts the material removal rate, cutting forces, and tool wear.
- Too high: Can overload the tool, leading to breakage, increased heat, and poor surface finish.
- Too low: Requires more passes to remove the desired material, reducing productivity.
Calculation: Depends on the operation. In turning, it's often the difference between the initial and final radius. In milling, it's the axial or radial engagement of the tool.
- Turning: $D OC = \frac{D _{ini t ia l} - D _{f ina l}}{2}$ where $D_{ini t ia l}$ and $D_{f ina l}$ are the initial and final diameters of the workpiece.
Factors influencing selection:
- Workpiece material and rigidity
- Cutting tool strength and geometry
- Machine power and rigidity
- Desired surface finish
- Roughing vs. finishing operations

4. Constant Surface Speed (CSS) (G96/G97)

Definition: Constant Surface Speed is a mode used primarily in CNC turning where the control system automatically adjusts the spindle speed (RPM) as the tool moves towards or away from the center of the workpiece. The goal is to maintain a consistent cutting speed ( $V_{c}$ ) at the point of contact between the tool and the workpiece.
Why use CSS?
- Improved surface finish: Ensures a more uniform surface finish across the machined diameter.
- Extended tool life: Helps to maintain optimal cutting conditions, reducing tool wear.
- Consistent cutting conditions: Prevents the cutting speed from decreasing as the diameter decreases (towards the center) or increasing as the diameter increases.
How it works: The CNC control uses the programmed cutting speed and the current diameter of the cut to calculate and adjust the spindle RPM in real-time.
Programming: In G-code, G96 S<value> activates CSS mode with the desired surface speed <value> (in m/min or ft/min). G97 cancels CSS and returns to constant RPM mode.

5. Limiting Spindle Speed (G92 in conjunction with G96)

Definition: Limiting spindle speed is the maximum allowable RPM set by the programmer when using Constant Surface Speed (CSS).
Why use limiting spindle speed?
- Machine limitations: Prevents the spindle from exceeding the machine's maximum RPM capacity, which could cause damage.
- Workpiece safety: For large or unbalanced workpieces, high RPMs can generate excessive centrifugal forces, potentially causing the part to become unstable or even fly off.
- Fixture limitations: Some fixtures may have RPM limitations for safe operation.
How it works: When CSS is active (G96), the spindle speed will increase as the cutting diameter decreases. The limiting spindle speed (set with G92 S<max_RPM>) acts as an upper bound, preventing the RPM from going any higher than the specified value. Once the limiting speed is reached, the machine will operate at a constant RPM for the remainder of the cut towards the center.
Programming: Typically used in conjunction with G96, for example: G96 S200 G92 S3000 (set constant surface speed to 200 m/min, but limit the spindle speed to a maximum of 3000 RPM).

Understanding and correctly applying these CNC cutting parameters is fundamental for efficient, accurate, and safe machining operations. The optimal values will depend on a variety of factors and often require experience and experimentation to fine-tune.

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