Cutting Tool Design

LECTURE PRESENTATION

Reading Assignment

  • FTD Chapter 3

Recommended Reading

Video Lectures

Outline

Machining

Machining Definition: Changing form and dimension of a component through metal removal.

Elements of Machining

Cutting tool
Tool holding and guidance
Workholding
Workpiece
Machine Tool
Note: Rigidity among these elements is of fundamental importance.

Basics of Metal Cutting

Single-Point Tooling

Cutting tool must be harder and more wear-resistant than the workpiece

Interference between tool and workpiece designated as feed and depth of cut

Rake Face

  • Surface the chip flows across

Flank

  • Surface that forms the outer boundary of the wedge

Side Cutting Angle

  • Protects the tool nose as the tool enters the cut
  • Also eases out of the cut.
  • Can allow a larger nose tip angle
  • Reduces chip thickness for a given feed
    • This can reduce heat

End Cutting Angle

  • Provides clearance
    • Reduces Radial forces, heat, and friction
    • Should be generous for thin material or small diameter
    • Heavy cuts and strong materials require less

Combination between SCA and ECA allow for a smaller nose radius, stronger/larger included angle.

Nose Radius

  • Important to strength
    • Larger nose radius, more strength
  • Small nose radius advantageous when machining work hardenable material.
  • Important to surface finish
  • Is the weakest point of the tool

Rake Surfaces

  • Rake is angle between the tool face and a line perpendicular to the cut workpiece surface
  • Positive for free machining action, soft metal, ductile matrial, aluminum
    • Better chipbreaking action
    • Increases shear angle
  • Negative rake for strong tool, interrupted cut
    • Increases mass of tool, more heat absorption
    • Reduces pullin- in

Relief and Clearance Angles

  • Prevents interference between tool flank and work
  • Avoid friction and heat
  • Secondary angles usually called “clearance”
  • Sometimes necessary for tool grinding purposes

Chip types

Type 1 – Discontinuous or Segmented

Type 2 – Continuous

Chipbreakers

Mechanisms of Chip Formation

The cutting process generates heat

The thickness of the chip is greater than the thickness of the layer being cut

The hardness of the chip is usually much harder than the parent material (strain or work hardening)

Differences in these relative values produce different chip types (plastic flow, vs. shearing)

Manipulating Factors

Velocity (Speed)

Affects temperature

Size of cut

Feed rate and depth of cut

Tool Geometry

Tool Material

Cutting Fluids

Workpiece Materials

Orthogonal and Oblique Cutting

Assumes tool is extruded straight out of the diagram

For a given material, the shear angle (theta) is a function of the tool rake angle and the coefficient of friction along the tool face.

Low and negative rake angles

  • Increase tool strength
  • Decrease the shear angle
  • Increase resistance to chip flow
  • Increase chip distortion
  • Increase chip breaking action
  • Increase work hardening of the surface
  • Can reduce chatter, pulling in

Coolant and lubrication (See 1711)

  • increase the shear angle
  • decrease cutting forces

Ideal chips are:

  • 1.5X as thick as the depth of cut
  • Smooth on the rake surface
  • Curled into the shape of a 6

Tool Failure

Failure is when the tool can no longer produce parts to the required specifications

Types of Tool Wear

See Carbide Depot Trouble Shooting Guide

  • Face Wear
  • Flank Wear
  • Nose Wear
  • Crater Wear
  • Notch Wear
  • Plastic Deformation
  • Thermal Cracking
  • Edge Chipping
  • Built-up Edge

Mechanisms of Tool Wear

  • Abrasive action
  • Plastic deformation of the cutting edge
  • Chemical decomposition of the cutting-tool contact surfaces
  • Diffusion between work and tool materials
  • Welding of asperities between work and tool

Guidelines for Cutting Tool Design

  • Rigidity
  • Strength
  • Weak Links (e.g. shear pins)
  • Force Limitations
  • Chip Disposal
  • Avoid Uneven Motions
  • Avoid Chatter
  • Fewer teeth, higher velocity
  • Consider negative rake angle (reduce pulling in)

Chipbreakers

Becker PCD Chipbreakers – Video

Indian Institute of Technology Kharagpur & MHRD, Govt. of India (Video 51:55)