Gear undercutting refers to the phenomenon where, during gear machining using the hobbing or gear shaping, the tip of the cutting tool removes a portion of the involute tooth profile at the root of the gear tooth. This results in a thinning of the tooth root, which severely affect the gear’s strength and the smoothness of transmission.
- Why does undercutting occur?
Undercutting primarily occurs when the gear being machined has a low number of teeth.
When using a rack-type cutter (such as a gear shaping cutter), the path of the cutter’s tip forms a line of action. If the gear has too few teeth, its base circle is small, causing the intersection point between the line of action and the cutter’s tip line (the limit point N) to fall too close to the gear’s center. If the cutter’s tip line extends beyond this limit point N, the cutter—as it continues to move—will cut away part of the involute profile at the root that had already been formed, thereby causing undercutting.
- What are the harmful effects of undercutting?
Undercutting has a serious negative impact on gear performance; it weakens tooth strength, impairs transmission smoothness, and reduces load-carrying capacity.
- How can undercutting be avoided?
Measures to avoid undercutting are primarily taken during the gear design and machining stages. The following are several common and effective methods:
(1). Increase the number of teeth
This is the most direct method. For standard spur gears with a pressure angle of 20°, undercutting can be avoided simply by ensuring the number of teeth is 17 or greater.
In certain cases, if slight undercutting at the root is permissible and strength requirements are still met, the minimum number of teeth can be reduced to 14.
(2). Use positive profile shifting (the most common method)
This method, known as positive profile shifting, involves moving the cutter outward (away from the gear center) by a certain distance to perform a “shallower cut.” Once the cutter is shifted outward, its tip line no longer extends beyond the limit point N, thereby preventing undercutting. Note: A larger positive profile shift is not necessarily better; excessive positive shift (e.g., a shift coefficient x > +0.5) can result in a pointed tooth tip, which also compromises tooth strength.
(3). Increase the pressure angle
Increasing the pressure angle enlarges the gear’s base circle and moves the limit point N further away from the tooth root, thereby reducing the likelihood of undercutting.
(4). Modify the manufacturing process
For certain specialized gears with small modules and low tooth counts, non-traditional manufacturing methods—such as wire-cut electrical discharge machining (WEDM)—can be employed to eliminate the problem of undercutting at the source.