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Section: Machining
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Machining

Types of Chips

Quick Cheat Sheet

Summary

Chip morphology depends on work material, cutting speed, rake angle and tool–chip friction. Three primary types: continuous, continuous with built-up edge (BUE), and discontinuous (segmented).

Key Points

  • Continuous chip: ductile materials, high speed, large positive rake, low friction → smooth finish
  • BUE chip: low–medium speed, ductile materials, high friction → poor finish, fluctuating force
  • Discontinuous chip: brittle materials (CI), low speed, small rake → rough surface
  • Shear angle φ controls chip thickness and cutting force
  • Chip-thickness ratio r = t₀/tc < 1 always
  • Merchant's force circle relates Fc, Ft, F, N, Fs, Fn through tool & shear angles

Remember This

  • 1tan φ = (r·cos α) / (1 − r·sin α)
  • 2Chip-thickness ratio r = t₀/tc and is always < 1
  • 3BUE → wavy, poor surface; reduce by raising V, raising rake, using coolant
  • 4Lee–Shaffer: φ = 45° − β + α Merchant: 2φ + β − α = 90°
  • 5Cast iron always gives discontinuous chips

Quick Formulas

Chip ratio

r = t₀ / tc

Shear angle

tan φ = (r cos α) / (1 − r sin α)

Merchant's relation

2φ + β − α = 90°

Chip Formation Mechanism

During metal cutting, the work material ahead of the cutting tool is subjected to:

  • Shear deformation in the primary shear zone
  • Material separates to form a chip
  • Chip slides along rake face (secondary shear zone)

Three Main Types of Chips

1. Continuous Chip

Characteristics:

  • Long, continuous ribbon-like chip
  • Smooth, uniform appearance
  • Steady chip flow

Formation Conditions:

  • Ductile materials (low carbon steel, aluminum, copper)
  • High cutting speeds
  • Small feed rates
  • Sharp cutting edge
  • Positive rake angle
  • Good lubrication

Advantages:

  • Good surface finish
  • Steady cutting forces
  • Less tool wear
  • Better dimensional accuracy

Disadvantages:

  • Chip disposal problem (long chips tangle)
  • Can be hazardous
  • May scratch finished surface

Applications: Finishing operations, high-speed machining of ductile materials

2. Discontinuous Chip (Segmented Chip)

Characteristics:

  • Small segments or particles
  • Irregular shape
  • Chips break into pieces

Formation Conditions:

  • Brittle materials (cast iron, bronze, hard steels)
  • Low cutting speeds
  • Large feed rates and depth of cut
  • Small rake angles or negative rake
  • Poor lubrication
  • Rigid machine setup

Advantages:

  • Easy chip disposal
  • Less chip tangling
  • Safer operation

Disadvantages:

  • Poor surface finish
  • Fluctuating cutting forces
  • Increased tool wear
  • Vibration and chatter

Applications: Machining of brittle materials, rough machining operations

3. Continuous Chip with Built-Up Edge (BUE)

Characteristics:

  • Continuous chip with layers of work material welded to rake face
  • BUE forms, grows, becomes unstable, breaks off
  • Cycle repeats continuously

Formation Conditions:

  • Ductile materials at moderate speeds
  • High friction between chip and tool
  • Moderate cutting speeds (not too high, not too low)
  • Inadequate lubrication
  • High pressure and temperature at tool-chip interface

Built-Up Edge (BUE) Effects:

  • Changes effective rake angle (usually increases it)
  • Protects tool from wear temporarily
  • Causes poor surface finish (BUE fragments embed in surface)
  • Dimensional inaccuracy
  • Unstable cutting process

How to Minimize BUE:

  • Increase cutting speed (above BUE formation range)
  • Decrease cutting speed (below BUE formation range)
  • Use sharp tools
  • Apply effective coolant/lubricant
  • Use positive rake angle
  • Reduce feed rate

Chip Thickness Ratio

Chip Thickness Ratio (r) = t₁ / t₂

Where:

  • t₁ = Uncut chip thickness (feed)
  • t₂ = Chip thickness after cutting

Characteristics:

  • r < 1 always (chip is thicker than uncut thickness)
  • Smaller r → more deformation, higher forces
  • Larger r → less deformation, better cutting

Shear Angle

Shear Angle (φ) = Angle between shear plane and cutting velocity direction

Relationship with chip thickness ratio:

tan φ = (r cos α) / (1 - r sin α)

Where:

  • φ = Shear angle
  • r = Chip thickness ratio
  • α = Rake angle

Significance:

  • Larger φ → less deformation, lower forces, better cutting
  • Smaller φ → more deformation, higher forces, more heat

Factors Affecting Chip Formation

  1. Work Material Properties

    • Ductility
    • Hardness
    • Strength

  2. Cutting Conditions

    • Cutting speed
    • Feed rate
    • Depth of cut

  3. Tool Geometry

    • Rake angle
    • Cutting edge sharpness
    • Tool material

  4. Cutting Fluid

    • Lubrication
    • Cooling

  5. Machine Rigidity

    • Vibration
    • Stability

Chip Breakers

Purpose: Convert long continuous chips into short, manageable segments

Types:

  1. Groove-type chip breaker - Groove ground on rake face
  2. Obstruction-type chip breaker - Separate piece clamped to tool
  3. Step-type chip breaker - Step ground into rake face

Benefits:

  • Easier chip disposal
  • Safer operation
  • Prevents chip tangling
  • Allows higher cutting speeds in automated operations

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