Page 92 - Gear Technology Solutions

6.7 Cleanup Problems in the Case of Undercut and Holdback

In cutting and grinding developments it is common to cut or grind a first part or
several slots with a holdback amount. Bevel gear cutting and grinding
machines allow such a holdback by entering a holdback value in the summary
screen. The holdback is used for example in face hobbing (where the cutting of
a single test slot is not possible) or in face milling grinding when a first
complete test part should be ground in order to check for the correct grinding
wheel wear compensation and also for verifying runout and cleanup. The
holdback allows such tests without wasting a good part. After the verification of
runout or the development of the wheel wear compensation the same part can
be ground without the holdback to its final depth.

Figure 9: Trochoidal setback and cleanup problem

If the gear which was machined with a holdback has undercut or a large
amount of Toprem, there will be an area above the undercut which will not
clean up after finish grinding to full depth. (see Figure 9). Generally, the size of
this area in profile direction is significantly larger than the holdback amount
and looks to the eye like an increase of the undercut.

The same problem can also occur in production in the case of dual rotation
with a set-in. Even a small amount of set-in like for example 0.04mm can lead
to a visible increase perceived undercut.

It is recommended to avoid holdback test cycles in all cases of large undercut
(and high pressure angles). In the case of a dual rotation, the set-in amounts
should be very small, for example 0.02mm or even smaller.

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