Theory of Metal Cutting: Introduction and Material Removal Processes

Theory of Metal Cutting

• Metal cutting is a process of gradual removal of excess material from the preformed blanks in the form of chips with the help of cutting tools moved past the work surface(s).
• The purpose of machining is to impart required dimensional and form accuracy and surface finish to enable the product to fulfill its basic functional requirements, provide better or improved performance, and render long service life.

Types of Cutting Tools

• Cutting tools can be classified according to the number of major cutting edges (points) involved:
• Single point: e.g., turning tools, shaping, planning, and slotting tools, and boring tools.
• Double (two) point: e.g., drills.
• Multipoint (more than two): e.g., milling cutters, broaching tools, hobs, gear shaping cutters, etc.

Geometry of Single Point Cutting Tools

• The concept of rake angle and clearance angle will be clear from some simple operations:
• Rake angle: Angle of inclination of rake surface from the reference plane.
• Clearance angle: Angle of inclination of clearance or flank surface from the finished surface.
• Rake angle may be positive, zero, or negative, depending on the machining operation.
• Clearance angle is essentially provided to avoid rubbing of the tool (flank) with the machined surface.

Systems of Description of Tool Geometry

• Tool-in-Hand System: Only the salient features of the cutting tool point are identified or visualized.
• Machine Reference System (ASA system): The geometry of a cutting tool refers mainly to its several angles or slopes of its salient working surfaces and cutting edges.
• Tool Reference System: Orthogonal Rake System (ORS) and Normal Rake System (NRS).
• Work Reference System (WRS): The planes and axes used for expressing tool geometry in ASA system for turning operation.

Mechanism of Chip Formation

• In machining ductile materials, the chip forms through a process of gradual compression, shear deformation, and slip.
• Piispannen's model explains chip formation using a card analogy.
• The geometry of chip formation is influenced by:
• Work material
• Material and geometry of the cutting tool
• Levels of cutting velocity and feed
• Machining environment or cutting fluid
• Machining of ductile materials generally produces flat, curved, or coiled continuous chips.

Chip Thickness Ratio

• Geometry and characteristics of chip forms are assessed and expressed by some factors, the values of which indicate about the forces and energy required for a particular machining work.
• The chip thickness ratio (a2/a1) is a significant parameter in chip formation.
• The reasons for the chip thickness ratio being greater than unity are:
• Compression of the chip ahead of the tool
• Frictional resistance to chip flow
• Lamellar sliding according to Piispannen### Chip Reduction Coefficient
• The chip reduction coefficient (rc) is the ratio of chip thickness after cutting (a2) to chip thickness before cutting (a1)
• rc = a2 / a1
• Larger values of rc indicate more thickening, requiring more force or energy to accomplish machining
• Desirable to reduce rc without sacrificing productivity (metal removal rate, MRR)

Factors Affecting Chip Reduction Coefficient

• Tool rake angle: larger positive rake angle reduces rc
• Chip-tool interaction: reducing friction by using a lubricant reduces rc
• Equation 1.4: rc = cos(β - γ) / sinβ

Shear Angle

• Shear angle (β) is the angle of inclination of the shear plane from the direction of cutting velocity
• β depends on:
  • Chip thickness before and after cutting (rc)
  • Rake angle (γ)
• Equation 1.10: tanβ = cos / rc - sin
• Equation 1.11: tanβ = rcos / 1 - rsin
• Increasing rc decreases shear angle, and vice-versa

Cutting Strain

• Cutting strain (ε) is the magnitude of strain that develops along the shear plane due to machining
• Equation 1.12: ε = cotβ + tan(β - γ)

ASA System for Tool Geometry

• The ASA system is used to define tool geometry
• Axes: R, X, Y, Xm, Ym, Zm
• Planes: Reference plane (R), Machine longitudinal plane (X), Machine transverse plane (Y)
• Geometrical features and angles:
  • Shank: portion of the tool bit not ground to form cutting edges
  • Face: surface against which the chip slides upward
  • Flank: surface that faces the work piece
  • Heel: lowest portion of the side cutting edges
  • Nose radius: conjunction of the side cutting edge and end cutting edge
  • Base: underside of the shank
  • Rake angles: side rake angle (x), back rake angle (y)
  • Clearance angles: side clearance angle (ax), back clearance angle (ay)
  • Cutting angles: side cutting edge angle (fs), end cutting edge angle (fe)

Designation of Tool Geometry

• Tool geometry is designated by a series of values of the salient angles and nose radius
• Example: y, x, ay, ax, fe, fs, r (in inch)

Types of Metal Cutting Processes

• Orthogonal cutting process: cutting edge or face of the tool is 90° to the line of action or path of the tool
• Oblique cutting process: cutting edge or face of the tool is inclined at an angle less than 90° to the line of action or path of the tool

Built-up-Edge (BUE) Formation

• Causes of BUE formation:
  • High stress and temperature at the chip-tool interface
  • Mutual affinity or solubility of the chip-tool materials
• Characteristics of BUE:
  • Shape, size, and bond strength
  • Depend on work-tool materials, stress, and temperature
• Effects of BUE formation:
  • Unfavorably changes rake angle at the tool tip
  • Increases cutting forces and power consumption
  • Reduces tool life
  • Deteriorates surface finish

Types of Chips

• Continuous chips without BUE: formed when the cutting tool moves towards the work piece, and the metal is separated without discontinuity
• Discontinuous chips: formed when the metal is fractured like segments of fragments and passes over the tool faces
• Factors affecting chip formation:
  • Type of cut (continuous or intermittent)
  • Work material (brittle or ductile)
  • Cutting tool geometry (rake, cutting angles)
  • Cutting velocity and feed
  • Cutting fluid (type and method of application)

This quiz covers the fundamental concepts of metal cutting theory, including the classification of metal working processes and material removal processes. Learn about non-cutting shaping and cutting shaping processes in the context of metal component production.