Spur Gears are the most common type of Gear. They run on shafts that are parallel.  There are external and Internal spur gears. Gears with the teeth cut on the outside of the cylinder are known as external gears. In turn gears with teeth cut on the internal side of the cylinder are known as internal spur gears. When two external gears mesh together they rotate in the opposite directions.  An internal gears running with an external gears run in the same Direction. 
Though the teeth are not straight-sided, the edge of each tooth is straight and aligned parallel to the axis of rotation. These gears mesh together correctly only if fitted to parallel shafts. No axial thrust is created by the tooth loads. Spur gears are excellent at moderate speeds but tend to be noisy at high speeds.
Spur gear teeth are manufactured by either involute profile or cycloidal profile. Most of the gears are manufactured by involute profile with either a 14.5 or 20° pressure angle. When two gears are in mesh at one instant there is a chance to mate involute portion with non-involute portion of mating gear. This phenomenon is known as “interference” and occurs when the number of teeth on the smaller of the two meshing gears is less than a required minimum. To avoid interference we can have undercutting, but this is not a suitable solution as undercutting leads to weakening of tooth at its base. In this situation Corrected gears are used. In corrected gears Cutter rack is shifted upwards or downwards.
Due to the close positioning of shafts internal gear assemblies are more compact then external gear assemblies.  Keep this in mind when designing.
NextDayGears.com can make any type of Spur Gear.   
You would be surprised how many types of Gears we can make. Odd shaped, Segmented, and many more. If you think it can not be made, we can probably make it!
Here are our Size Capabilities :
                                                                      .125″ in diameter to 36.00″ in diameter. 
                                                                       Diametral Pitches of 3DP to 120DP
                                                                       Metric Pitches of 8 Module to .200 Module.
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