Common conventional gear finishing processes include gear shaving, grinding, honing, lapping, burnishing, and skiving. These are suitable for finishing cylindrical gears like spur and helical gears. However, for conical gears, only grinding and lapping are considered appropriate finishing methods. Each technique has its specific applications and is chosen based on the gear type and required finish quality.

1. Gear Shaving

Gear shaving is a precision finishing process used in manufacturing of gears. It involves a shaving cutter that removes material from the gear work piece in fine chips, typically between 100-400 µm in length and 50-150 µm thick. The process’s effectiveness depends on various factors, including the shaving cutter’s type, geometry, and material, the shaving allowance, and the fundamental parameters of both the shaving process and the shaving machine tool. Additionally, the material and geometry of the work piece gear play a crucial role. Gear shaving is particularly favoured in the automotive and construction machinery industries for its cost efficiency and the quality of the finish it provides.

Figure 1: Gear Shaving

 

  • Cutter Types:
    • Helical Shaving Cutter: Ideal for a range of gear types, with a helical tooth form.
    • Rack Type Shaving Cutter: Resembles a rack gear, used for linear shaving actions.
    • Worm Shaving Cutter: Suitable for worm gears, featuring a worm-like tooth form.
  • Shaving Methods:
    • Rotary Shaving: Uses a helical gear-like cutter, ideal for high-volume production.
    • Rack Shaving: Involves a reciprocating rack-type cutter, typically for gears up to 150 mm in diameter.
    • Conventional or Axial Gear Shaving: The cutter and gear blank rotate in parallel or crossed axes, suitable for gears with extended face widths.
    • Tangential or Underpass Gear Shaving: The gear blank is moved radially, effective for gears with a shoulder, offering uniform cutter wear and higher productivity.
    • Plunge Gear Shaving: A direct plunging motion into the gear blank, providing line contact and suited for large production volumes.
    • Diagonal Gear Shaving: Involves a special relationship between the face widths of the shaving cutter and gear, used mainly for medium to high-production volumes.
  • Process Parameters and Efficiency:
    • Material Removal: Achieved through fine, controlled shaving, resulting in high-quality surface finishes.
    • Cutter Material: Varies based on gear work piece hardness, with hardened high-speed steel (HSS) commonly used.
    • Shaving Allowances: Critical for achieving desired accuracy and finish.
    • Machine Tool Design: Influences the efficiency and suitability for different gear sizes and volumes.
  • Advantages and Limitations:
    • Advantages: Gear shaving is cost-efficient, enhances surface finish, and reduces pitch errors.
    • Limitations: Not suitable for correcting cumulative pitch or index errors and can introduce grooves on gear teeth flanks.

2. Gear Grinding

Gear grinding is a precision finishing process for high-strength and hardened gears. It uses a grinding wheel with specialized abrasive grains like alumina oxide, silicon carbide, and cubic boron nitride. This process effectively corrects thermal distortions from case hardening, enhances gear surface finish and micro geometry, and thus improves overall gear quality. Grinding involves three phenomena; rubbing, ploughing, cutting with actual metal removal occurring beyond a threshold force. 

Figure 2: Gear Grinding

Gear grinding is mainly divided into two categories: form grinding, and generative grinding, each with distinct mechanisms and applications.

  • Form Grinding: 
    • It involves using a grinding wheel specifically designed to match a gear’s involute profile. 
    • This requires unique wheels for different gear specifications like module, pressure angle, and tooth count. 
    • The grinding can use single, multiple, or straddle form wheels, and while the process is simpler and can accommodate complex gear forms, it tends to be slower and less accurate. 
    • It’s suitable for grinding various gear types, including spur gears and worm wheels, and employs either ceramic abrasives for very hard gears or cubic boron nitride abrasives for other applications.
  • Generative grinding:
    • It involves using a standard rotating grinding wheel to create the required gear profile through synchronous rolling with the work piece gear. 
    • This process can use various wheel shapes like threaded, dish-shaped, cup-shaped, or rack-tooth worm wheels.
    • Indexing the gear is necessary to finish all teeth.
    • It’s commonly used for gears with modules ranging from 0.5 to 10 mm.

3. Gear Honing

Gear honing is a finishing process for high-strength and hardened gears, using an abrasive-impregnated helical gear as the honing tool. 

Figure 3: Gear Honing

  • Process Mechanics:
    • Uses an abrasive-impregnated helical gear as the honing tool.
    • Work piece gear is driven at high speeds (up to 300 m/min) and reciprocated along its axis.
    • Rotary and reciprocating motions yield a cross-hatch pattern on gear flank surfaces.
    • Typically removes 0.013 to 0.05 mm from the work piece.
    • Removes nicks, burns, and minor irregularities.
  • Benefits:
    • Improved lubrication retention due to cross-hatch pattern.
    • Reduces friction, evenly distributes load.
    • Enhances gear surface finish, dimensional accuracy.
    • Improves noise and wear characteristics of gears.
  • Types of Honing:
    • External Honing: Work piece and honing gears meshed in cross-axes alignment.
    • Internal Honing: Uses a large internal helical gear as the honing tool.
  • Advantages of Internal Over External Honing:
    • Higher traverse contact ratio, more balanced honing pressure.
    • Better quality finish with fewer pitch errors.
    • Higher accuracy and stability.
  • Process Variables:
    • Abrasive size and type.
    • Honing pressure, rotational speed.
    • Speed and length of reciprocation.
  • Honing Gear Characteristics:
    • Made of strong material to minimize deformation under load.
    • Abrasive grains bonded in resinoid, vitrified, or metallic materials.
    • Abrasive types include alumina, SiC, CBN, diamond.

4. Gear Lapping

Gear lapping is a finishing process conducted at a slow speed and under low pressure. It’s used for refining cylindrical and conical gears made of high-strength or hardened materials. The method involves the continuous application of a pressurized, abrasive-laden lapping compound, which abrades the gear surfaces to achieve a high level of precision and smoothness.

Figure 4: Gear Lapping

          1.Process Overview:

    • Low-speed (less than 80 rpm), low-pressure process.
    • Utilizes abrasive-laden lapping compound under continuous pressure.

          2.Tool Material:

    • Typically softer than the work piece gear material (commonly fine-grained cast iron).
    • Allows abrasive grains to embed into the tool, minimizing wear.

          3.Lapping of Cylindrical Gears:

    • Involves meshing the work piece with a gear-shaped lapping tool.
    • Sliding velocity varies along the tooth profile, requiring auxiliary sliding in the axial direction for uniform lapping.

           4.Lapping of Conical Gears (like straight bevel, spiral bevel, hypoid gears):

    • Involves running mating gears together under controlled load.
    • Both gears finish simultaneously, removing the need for a finishing allowance.

          5.Lapping Medium:

    • A chalky paste of abrasive grains (300 to 900 mesh size) in a carrier fluid.
    • Common abrasives: aluminium oxide, SiC, boron carbide, diamond powder.
    • Different mesh sizes used for different gear types and pitches.

          6.Key Lapping Parameters:

    • Lapping pressure, 
    • Abrasive grain size
    • Concentration in the medium
    • Speed.

          7.Process Benefits:

    • Improves surface finish, accuracy, and contact pattern of gears.
    • Effective in noise level reduction.

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