I don't have a reference drawing to point you to, although someone else
may know of one.

The RT worms that I've seen are mounted in an eccentric sleeve, so that
both ends of the worm shaft are moved the same amount of distance,
simultaneously. The worm shaft is mounted within the sleeve with
provisions for eliminating/adjusting-to-minimum the axial movement of
the worm shaft.
The eccentric sleeve better describes the visualization of the two
eccentric bushings question.

The eccentric engage-disengage sleeve/worm assembly rotates to move the
worm on/off the table gear. The rotational movement of the eccentric
sleeve may have a stop adjustment for the engage position, so that the
sleeve is set to the same position easily each time it is engaged. The
stop would be set to a point where most or all of the worm-gear
backlash is eliminated, but stops short of the point where the worm and
gear might bind.
A stop for the disengaged point isn't really required.

With the table disengaged, the table is moved freely to any point,
quickly. The degree scale around the edge of the table could be used
for low accuracy positioning without needing to dial in accurate
positions.
In theory, the table locks would prevent the table from shifting while
the machining operation is taking place, but engaging the worm before
machining would give the operator the assurance that the table will
remain locked in position.

The precision fit of parts in quality RTs allows them to perform well
with power applied to the worm shaft, from either a motor mounted to
the RT, or from a power feed shaft driven by the machine the RT is used
on.
A low quality RT will most likely present problems for the user. The
first one I bought was a cheap import that was difficult to operate.
Some other types of RTs don't utilize a worm shaft. They might be
actuated to index a certain number of stops, sometimes with pneumatic
power.

WB
.....................

jtaylor wrote:

On Mon, 13 Feb 2006 10:39:02 -0400, "jtaylor"
wrote:

In my head I have a kind of picture how a rotary table might be made.

You have a base, with a central pivot (could be bored for a taper).
Around
the pivot is a gear & the table, driven by a worm.

Some of these tables are advertised to have a method by which the table
can
be decoupled from the gear, to allow quick rotation without turning the
worm. I have been turning this picture around and around in my head and
cannot figure out how they do this.

Someone tell me or point to a drawing, please...



Do that have just one bushing at one end of the worm, or is the bush really
a pair, one each end, connected along the back side of the worm (so they'll
turn together)?