I have yet to come across any written instructions for the proper use of a
spin index, so it seems that knowledge of their operation is assumed to be
passed down from one machinist to the next in professional shops. Which is
fine if you work in a shop, but not so helpful otherwise.

I understand that they are used to rotationally index every 10 degrees
directly off the 36 hole plate, and every 1 degree via the 10 vernier holes
(and I fully understand how the vernier works). I also understand that they
are designed for 5C collets (or work-holders with 5C shanks). So there's no
need to dwell on those points. But there are some additional features of the
spin index that are not intuitive.

For example, the spindle is considerably longer than the body. There is a
retaining ring (with three set screws) that keeps the spindle snug against
the bearing surfaces of the body, but if this ring is loosened or removed,
then the spindle can slide axially in and out of the body. According to
marketing literature for a typical spin index, the "spindle travels 2-1/16
inches for flute grinding" which would explain the length of the spindle.
But what isn't clear is how one maintains the index position when the
spindle is slid out of the body. Because once you slide the spindle out, the
indexing pin is no longer able to engage the hole in the indexing plate and
the entire spindle turns freely.

The next question is, what is the purpose of the locking screw (on the top
of the body, it screws down against the spindle)? Since the spindle is held
in position by the indexing pin, the locking screw seems redundant? Unless
the spindle is supposed to be locked with the screw, then the indexing pin
removed to prevent wear or damage to the indexing holes. But if that's the
case, then the locking screw seems woefully under-designed, as it has a very
small bearing face upon which to make contact with the spindle. (In fact,
since the locking screw acts radially against the spindle, under ideal
conditions it will only make line contact against the spindle.)

Finally, I'm just curious about the original intended purpose of the spin
index. I realize that it's pressed into many applications where you need to
machine a part in increments of degrees. But then again, there are several
other tools available for that purpose such as the dividing head and rotary
table (and those tools are typically more versatile). Was the spin index
purposely built specifically for end mill or reamer flute grinding, or was
it always intended to be a general purpose rotary index?

Thanks in advance for any info.

- Michael