Spindle Orientation, Drive Dogs, and Repeatable Runout

All CNC machinists have encountered the ever-present "drive dogs" that are prominently visible on a multitude of tapers:

Spindle Drive Dogs...

They're the two little "ears" that engage notches in the toolholder.  Not every taper has them, but many do.  Getting the drive dogs positioned to line up with the toolholder in the changer carousel before a toolchange can happen is called "spindle orientation" or sometimes "spindle positioning".

You might think spindle orientation is only about lining up the drive dogs with toolholder notches, but I recently learned differently.

Dave Decaussin, one of the original founders of Fadal, recently remarked on a video I was watching that spindle orientation is also essential to maintaining tolerances while machining.  The reason is runout.  When you maintain the same spindle position relationship to the toolholder, you ensure that whatever runout there is will at least be consistent-either consistently good or consistently bad.

Consistency is the main thing, because if it is consistent, we can compensate for it.  If you think about it, the runout just makes the endmill act like it is a slightly larger diameter as it wobbles in the cut.  If that effective diameter changes every toolchange, the CNC machinist will have a tough time maintaining tolerance.  But if it is consistent, there is hope.  You can use your wear offsets to just enter the effective diameter of the tool and cut accurately despite that runout.

It's good to learn at least one new thing every day!