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more expensive than hobbing.
17.1.4 Top generating — This is a fabrication option utilizing cutters that finisb-.cut the outside diameter of the teeth simultaneously with the cutting of the tooth profiles.
It can be used in both the hobbing and shaping processes, although more prevalent in hobbing and among the fine pitches. The main advantages of topping are:

1. Liberal tolerances can be applied to the outside diameter of the blank.
2. The deburring problem is reduced.
3. The gear can be nested on its outside diameter for machining modifications of the   
    body should such a speaal need arise.

17.2 Gear Grinding
Although grinding is often associated with quantity fabrication of high quality gears as a secondary refining operation, it is also a basic process for producing hardened gears. In addition, many high-precision fine-pitch gears have their teeth entirely ground from the blank state.
Gear-tooth grinding can involve either form grinding or the generating process. The latter is basically more accurate because the dressing of the grinding wheel involves a straight-sided tooth.
There are a number of distinct advantages to ground gears. These are listed as follows:

1. Achievement of high precision is possible because the process can remove very little
    material in the final pass.
2. Grinding results in a much finer surface finish than any machining process.
3. Hardened steel alloys can be ground.
4. Residual surface stresses are minimal.

Being able to use heat-treatable hard steel alloys raises the bending stress and surface endurance
stress levels by very significant amounts. See Table 1.23. Often it is the difference between a reliable and unreliable gear. In particular, case hardened, carburized or nitrided gears offer outstanding strengths and performance. They are typically used for the most demanding tasks, such as aircraft drives.
Ground gears’ superior load carrying capacity is not only due to the hardened alloys higher mechanical properties, but also because of the finer surface finish. A fine finish enables maintenance of a good continuous oil film versus boundary lubrication and breakthrough. The result is higher load capactry along with reduced wear and longer useful life.
Although there are distinct advantages for ground gears, there are some limitations and disadvantages. These are:

1. Grinding is limited to ferrous materials.
2. Hard metals grind better than soft ones.
3. Grinding of helicals and worms has limitations that possibly involve profile deviations
    and removal.
4. Pro-grind hobbing requires special protruberance hobs to provide grind wheel
    clearance at the root
5. Gear grinding machinery is scarcer than hobbing machines.
6. Grinding is a secondary operation which increases total gear cost