When gears are produced by a generating process, the datum line of the basic rack profile need not necessarily form a tangent to the reference circle. The tooth form can be altered by shifting the datum line from the tangential position. The involute shape of the tooth profile is retained, the effect is merely to use parts further from or nearer to the origin of the same involute. The radial displacement from the tangential position is termed addendum modification.
The displacement is considered positive when in the direction away from the centre of the gear and negative when in the direction towards the centre of the gear.
The effect of addendum modification on the tooth form is shown in figs. 1, 2 & 3.
The load carrying capacity of the teeth with no addendum modification (fig.1) can be improved by the positive addendum modification (fig.2). An extremely large addendum modification results in an unsuitable tooth form with pointed teeth as in fig.3.
The addendum modification coefficient (x) is the amount of the addendum modification divided by the module. Thus, the amount of the addendum modification = x / m.

See also Addendum

For a far more In-Depth view, see Profile and Lead Correction

Fig 1. No addendum modification




Fig 2. Favourable addendum modification



Fig 3. Excessive addendum modification