Internal gear is definitely a gear with its teeth cut in the inner surface area of a cylinder and meshes with spur gears.
In its manufacturing, due to its shape, the most common hobbing machine used in spur gear production cannot be used. Generally it is made out of gear shaper (or equipment shaping machine) equipped with a pinion cutter. More recently, the efficiency of inner gear cutting has been improved by a different process called skiving.
Because of the thinness of the ring, it is sometimes difficult to produce high precision. However, gear grinding to boost precision grade is bound by the number of producers and their obtainable grinding facilities resulting in very high cost.
Also, although it is generally too expensive to create helical teeth internal gears, Ever-Power has made it possible to create them by adopting equipment that can cut them by simply making pinion cutters and not needing helical guides.
Normally, internal gears can be utilized with the same module spur gears, depending on the number of teeth combinations, nonetheless it is necessary to notice the possibilities of getting involute, trochoid 
or trimming interferences. For using Ever-Power standard share gears, please seek advice from the PDF formatted “Technical Information of Internal Gears” section for usable number of teeth combinations.
Internal gears tend to be found in applications involving planetary gear drives and gear couplings. There are three major types of planetary gear mechanisms: planetary, solar and superstar types. With respect to the type and the pattern of which shafts become input and result, many variations of speed transmitting ratios and rotational directions are created.