SUPER FINISHING AND METAL COATING PROCESSES, Mechanical Engg 3rd Semester Notes, ME3001

 

5. SUPER FINISHING AND METAL COATING PROCESSES

This section covers advanced techniques used to improve the surface finish of materials and apply protective coatings for enhanced performance in manufacturing processes. Let’s explore each topic in detail.

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5.1 Finishing by Grinding (Basic Concept)

Grinding is a machining process used to remove material from a workpiece using an abrasive wheel. It is used to achieve high precision and improve surface finish, especially on hard materials.

• Basic Concept: Grinding involves rotating an abrasive wheel or belt, which moves over the workpiece. The workpiece may be stationary or may move in some cases. The abrasive grains on the wheel remove material by cutting small chips from the workpiece surface. Grinding is typically used for:

  • Smoothing rough surfaces
  • Creating fine surface finishes
  • Reducing the size of workpieces

  Diagram:

    +------------------+    
    | Abrasive Wheel   |  ← Rotates and removes material
    +------------------+   
            ↓
    +------------------+
    |   Workpiece      |  ← Surface is ground and smoothed
    +------------------+
  

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5.1.1 Honing

Honing is a superfinishing process that uses abrasive stones or sticks mounted on a rotating tool to remove material from a workpiece. It is typically used to improve the internal surface finish of holes or cylindrical shapes.

• Working Principle: In honing, an abrasive tool moves in a rotary motion and a reciprocating motion to achieve an accurate surface finish and improve the geometry of holes. It removes small amounts of material to refine the surface.

  • Applications: Honing is commonly used in engine cylinders, hydraulic cylinders, and bearing surfaces.

  Diagram:

       +------------------+
       |    Abrasive      |  ← Rotating and reciprocating motion
       |    Stones        |
       +------------------+
               ↓
    +--------------------+
    |   Workpiece (Hole) |  ← Improved surface finish of the hole
    +--------------------+
  

• Benefits:

  • Improves surface finish
  • Ensures dimensional accuracy
  • Removes burrs and imperfections

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5.1.2 Lapping

Lapping is a precision finishing process where two surfaces are rubbed together with an abrasive slurry or paste. One surface is usually stationary while the other is rotated.

• Working Principle: In lapping, the workpiece and the abrasive medium are moved relative to each other, and the abrasive particles slowly wear away the material on the surface of the workpiece. This process provides excellent surface finishes, commonly on flat and cylindrical surfaces.

  • Applications: Lapping is used in the finishing of flat parts, such as precision optical components, bearings, and mechanical seals.

  Diagram:

    +-------------------+
    |    Workpiece      |  ← Surface being lapped
    +-------------------+
            ↓
    +-------------------+
    |  Abrasive Slurry  |  ← Used for smoothing the surface
    +-------------------+
            ↓
    +-------------------+
    |  Lapping Plate    |  ← Rotating or moving tool
    +-------------------+
  

• Benefits:

  • High surface finish quality
  • Can achieve very flat surfaces
  • Used for parts requiring tight tolerances and precision

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5.1.3 Superfinishing

Superfinishing is an advanced finishing process where the surface of a workpiece is polished with a very fine abrasive to achieve ultra-smooth surfaces. It is used after processes like grinding, honing, or lapping to provide a near-mirror finish.

• Working Principle: Superfinishing utilizes a rotating abrasive tool that applies fine abrasives to the workpiece surface. The workpiece usually reciprocates back and forth while being pressed against the abrasive tool. The process removes the finest layers of the surface to achieve a smooth finish.

  • Applications: Commonly used for parts like camshafts, crankshafts, and other components requiring fine surface finishes.

  Diagram:

    +-------------------+
    |  Abrasive Tool    |  ← Fine abrasive contact
    +-------------------+
            ↓
    +-------------------+
    |   Workpiece       |  ← Surface is polished
    +-------------------+
  

• Benefits:

  • Produces ultra-smooth and reflective surfaces
  • Reduces surface roughness to micrometer levels
  • Improves the wear resistance and fatigue strength of the part

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5.2 Electroplating: Basic Principles and Applications

Electroplating is the process of depositing a thin layer of metal onto the surface of a workpiece by using electrical current.

Basic Principle:

• Electroplating involves placing the workpiece (the cathode) in an electrolyte solution that contains metal ions. An electrode (anode) made of the plating metal is also submerged in the solution. When an electrical current is passed through the solution, metal ions are reduced onto the surface of the workpiece, forming a thin metal coating.

• Common Electroplating Materials:

  • Gold, silver, copper, nickel, and chromium.

  Diagram:

       +----------------------+
       |   Electrolyte        |  ← Contains metal ions (e.g., Nickel)
       |  Solution            |
       +----------------------+
               ↓
     +---------------------+
     |  Anode (Plating Metal)|  ← Dissolves to provide metal ions
     +---------------------+
               ↓
     +---------------------+
     | Workpiece (Cathode)  |  ← Metal is plated onto surface
     +---------------------+
  

Applications:

• Corrosion resistance: Electroplating is used to protect parts from corrosion, such as in automotive or aerospace components.
• Aesthetic finishes: It is used for jewelry, watches, and decorative items.
• Wear resistance: Parts like gears and electrical contacts are electroplated for durability.

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5.3 Hot Dipping: Galvanizing, Tin Coating

Hot dipping is a coating process in which a metal (typically steel) is submerged in a molten bath of another metal to form a protective layer.

Galvanizing:

• Process: Galvanizing involves dipping steel into a molten zinc bath. The zinc forms a protective layer on the steel, preventing rusting and corrosion.
  • Applications: Used in the automotive industry, construction (e.g., fencing, roofing), and electrical equipment.

Tin Coating:

• Process: Tin coating involves dipping steel into molten tin to create a thin protective layer. The tin prevents corrosion and oxidation of the steel.

  • Applications: Often used for food cans (tinplate), electrical contacts, and machinery parts.

  Diagram:

      +---------------------+
      |  Molten Zinc/Tin    |  ← Hot dipping bath
      +---------------------+
             ↓
    +----------------------+
    |  Workpiece (Steel)   |  ← Coated with Zinc or Tin
    +----------------------+
  

• Benefits:

  • Provides excellent corrosion resistance
  • Improves the longevity of products exposed to harsh environments

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5.4 Metal Spraying: Basic Principle

Metal spraying is a process where molten metal is sprayed onto the surface of a workpiece to form a coating. The metal is heated to a molten state and then atomized by a spray gun.

• Working Principle: The metal is melted by an electric arc, gas flame, or induction coil. The molten metal is then sprayed onto the surface of the workpiece, forming a coating. The coating improves wear resistance, corrosion resistance, or thermal conductivity.

  Diagram:

       +---------------------+
       |  Molten Metal       |  ← Sprayed onto workpiece
       +---------------------+
             ↓
     +---------------------+
     |   Workpiece         |  ← Coating is formed
     +---------------------+
  

Applications:

• Protection against corrosion
• Wear-resistant coatings for machinery parts
• Thermal barrier coatings

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5.5 Finishing Specifications

Finishing specifications define the desired surface characteristics of a workpiece after it has undergone various finishing processes. These specifications ensure that the part meets the required tolerances, surface quality, and functionality.

• Key Factors:

  • Surface Roughness: Measured by parameters like Ra (average roughness), Rz (maximum height of the profile), etc.
  • Surface Hardness: Indicating the resistance of the material to deformation.
  • Dimensional Tolerances: The acceptable limits for the size and shape of the finished part.

• Common Finishing Techniques:

  • Surface roughness is often controlled using grinding, honing, or lapping.
  • Specific coatings or treatments may be required based on the operational conditions.