What is the phenomenon of gear undercutting, and what is the reason for this phenomenon?

XNUMX. What is the phenomenon of gear undercutting?

Cut with Fan Cheng methodgearWhen the number of teeth is small, the top of the cutter will cut too much into the root of the gear, so the involute of the root is cut off and become thinner. This phenomenon is called undercutting of the gear.

Figure 1

XNUMX. What is the cause of gear undercutting?

Figure 2

As shown in the left figure of Figure 2, the gear index circle and the rack blade pitch line are tangent to point P, and the perpendicular line (meshing line) of the cutting edge passing point P is tangent to the gear base circle at point N.

If the cutting edge starts to cut from position I, when the cutting edge moves to position II, the gradual opening part of the tooth profile is completely cut out.When the intersection of the tooth top line of the rack knife and the meshing line is exactly at the point N, the rack knife and the cut gear continue to move.The cutting edge is separated from the cut involute tooth profile, so no undercut will occur.

However, when the intersection of the tool tip line and the meshing line exceeds point N, not only can the cutting edge beyond point N not become an involute tooth profile (because there is no involute in the base circle), but also when the tool continues to move from position Ⅱ , Then cut off a part of the involute tooth profile that has been cut at the root to form the tooth profile shown in Figure 1.

That is to say, the reason for the undercutting of the tooth profile is that the intersection of the tooth top line and the meshing line of the straight tooth profile part of the tooth exceeds the meshing limit point (that is, the end point N1 of the theoretical meshing line), and the undercutting will occur.

XNUMX. What are the consequences of gear undercutting?

For gears that produce severe undercutting, the bending strength of the gear teeth is reduced, and the transmission coincidence decreases, which affects the smoothness of the transmission.

XNUMX. How to avoid undercutting of gears?

The methods to avoid undercutting generally include: adopting modified gears; selecting sufficient number of teeth; using non-standard gears, that is, reducing the addendum height coefficientOr increase the pressure angle a (non-standard tools are required, not commonly used).

The minimum number of teeth of a standard gear without undercutting:

It can be seen from the above analysis that to avoid very cutting, the intersection of the tooth top line of the rack cutter and the meshing line should not exceed the tangent point N between the meshing line and the gear base circle, as shown in the right figure of Figure 2.

That should make, From this, we can find the minimum number of teeth without undercutting of the cut gear is:

This; When the gear teeth have a slight undercut, the radius of the root circle is increased, which is beneficial to the bending strength of the teeth. Therefore, the engineering also allows the teeth to have a slight undercut, which is preferable at this time.

The above is the gear undercutting related instructions shared with you today, including but not limited to, if there are any deficiencies, please add them, if you have any unclear points or related processing requirements, you can consult EP Technology for free.