Understanding CNC Programming and Metal Cutting Processes

Posted on May 2, 2024 in Technology

CNC Programming and Metal Cutting Processes

Axis of Spindle Rotation in CNC Programming

In CNC programming, the spindle, which holds and rotates the cutting tool, typically rotates along the Z-axis.

Understanding G-Code Blocks

Block N10 G91 G28 X0

This block commands the machine to:

  1. Set the movement mode to incremental (G91).
  2. Send the tool to its home position along the X-axis (G28) incrementally, meaning it will move 0 distance from its current location.

Block N20 G43 H2

This block instructs the machine to:

  1. Activate tool length offset (G43) using the offset value stored in register H2.
  2. Program the movement of the cutter’s center and end in the Work Coordinate System (WCS).

Cutter Radius Compensation for Down Cut

For down cut operations with M04 (clockwise spindle rotation) in effect, the appropriate G-code for cutter radius compensation is G42.

Mean Temperature in Orthogonal Cutting

The mean temperature in orthogonal cutting refers to the average temperature rise at the tool-chip interface. It is influenced by factors such as:

  • Flow strength of the work material
  • Specific energy
  • Cutting speed
  • Chip thickness
  • Volumetric specific heat of the work material
  • Thermal diffusivity of the work material

Orthogonal Cutting and its Significance

Orthogonal cutting is a machining process where a wedge-shaped tool cuts the workpiece with its cutting edge perpendicular to the direction of the cutting speed. This model simplifies the complex three-dimensional machining process into a two-dimensional representation, making it easier to analyze and understand the mechanics of metal cutting. The tooling geometry in the orthogonal model is also simpler, with only two parameters: rake angle and relief angle.

Types of Chips in Metal Cutting

Four main types of chips can form during metal cutting:

  1. Discontinuous chips: These chips break into separate segments and are typically observed when machining brittle materials at low cutting speeds.
  2. Continuous chips: These chips form a continuous ribbon without segmentation and are characteristic of ductile materials machined at higher cutting speeds.
  3. Continuous chips with built-up edge: Similar to continuous chips, but with adhesion of work material to the tool rake face due to friction at the tool-chip interface.
  4. Serrated chips: These chips have a saw-tooth appearance and are semi-continuous, formed by alternating high and low shear strain during chip formation.

Forces in Orthogonal Cutting

In the orthogonal metal cutting model:

  • Forces acting on the chip (not directly measurable): Friction force, normal force to friction, shear force, normal force to shear.
  • Measurable forces: Cutting force and thrust force.

Merchant Equation

The Merchant equation relates the shear angle to the rake angle and friction angle in orthogonal cutting. It shows that the shear plane angle increases with an increasing rake angle and a decreasing friction angle.

Chip Formation and Cutting Conditions

In a turning operation with a brittle work material and low cutting speed, a discontinuous chip is expected. Cutting speed has a stronger effect on cutting temperature than feed.

Location of Maximum Temperature

The maximum temperature in orthogonal cutting typically occurs around the middle of the tool-chip interface. This is due to the combined effects of heat generation in the primary shear zone and frictional heat at the tool-chip interface.

Consequences of High Cutting Temperatures

High cutting temperatures can lead to several undesirable consequences:

  1. Accelerated tool wear
  2. Dimensional inaccuracies in the workpiece
  3. Thermal damage and metallurgical changes to the machined surface
  4. Hot chips that pose a safety hazard

Cutting Tool Material and Temperature

Carbide tools generally exhibit lower cutting temperatures compared to high-speed steel tools due to their higher thermal conductivity and lower friction at the tool-chip and tool-workpiece interfaces.

Heat Generation and Dissipation

Heat generation during metal machining occurs due to:

  • Plastic deformation in the primary shear zone
  • Plastic deformation and friction in the secondary shear zone
  • Friction between the workpiece and tool flank

Heat dissipation occurs through:

  • The discarded chip (carries the most heat)
  • The workpiece acting as a heat sink
  • The cutting tool

The percentage of cutting energy carried away by the chip increases with increasing cutting speed because the heat generated has less time to dissipate into the workpiece, resulting in a higher heat concentration in the chip.