Face/Hirth Gears for Locking Tool Turrets and 4th Axes

While Googling to try to determine how pneumatic indexing tables work, I came across a description of face gear rotary tables. These tables are extremely rigid, but they are limited to stopping positions at fixed locations on the circle, such as every 5 degrees. They are often pneumatic and some are operated simply by having the CNC spindle push down on a switch atop the table (with the spindle not rotating of course!).

The face gear mechanism works by meshing two face gears using axial movement. As the gears move closer to one another they are forced to engage precisely:

Image by Tedisa-Hirth…

Face gears are also referred to as “Hirth” gears.  Here is another example:

Advancing to a new index point consists of the following steps:

– Unlock the face gears by moving them apart.

– Rotate to approximately the right position. “Approximately” means close enough that when the face gears are locked up again the teeth will force the final correction to make the position exactly right.

– Lock the face gears by moving them together. This will nudget the shaft into the precise desired position and once locked will keep it there with great rigidity.

The big advantage of these face gear mechanisms is rigidity. The disadvantage is they only move in fixed increments.  Because of this, they’re most useful for indexing work to fixed positions.  One of the most common uses in machining is to use a Hirth Gear to lock a lathe turret in place when a new tool is indexed into position.  Another would be to create a horizontal tombstone arrange to increase the parts capacity of the machine:

Here is a spin indexer converted for use as a CNC 4th axis using a step motor and right angle drive:

Indexers like these have very limited rigidity, especially if you are machining near the edge of the chuck which puts leverage to work against the rigidity of the indexer. The usual answer to this problem in commercial gear is to provide a pneumatic friction lock (basically, a disc brake) to help lock the axis.  While a disc brake is not as rigid as a face gear system, it allows the system to be locked at arbitrary index positions instead of at fixed degree intervals.  For more on disc brakes, check out our 4th axis disc brake article.