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(RonC9876) wrote in
:

You are engaging the back gear drive when you pull this lever. You
must then retract the bull gear drive pin to allow this gear to rotate
on the spindle. Otherwise you are locking the drive as you have
experienced. Good luck. Ron Colonna

Open the top cover.

The drive pin is located on the large gear on the main
spindle, which is the "bull gear". Turn the spindle over
by hand and you should see a round flat disk about 3/4 - 1"
or so in diameter (I've not run an Atlas, so I don't know
exactly, on some it's a knurled knob). When pulled out, it
enables the belt pulley to run at a different speed from
the big gear, which drives the main spindle. Now when you
engage the back gears, the small gear attached to the other
end of the belt pulley drives the bull gear through a double
reduction. This is also why not pulling the pin locks it
up, you're trying to drive everything at two speeds at once.

As for what use it is, turning large diameters comes to mind,
as well as threading. The slower the main spindle turns, the
more time you have to crank the cross slide out at the end of
your thread when threading against a shoulder.

To go back to regular speeds, disengage the back gear lever
and hold the pin in, while rotating the belt pulley slowly by
hand until it arrives at the hole, which should now allow the
pin to sink in until you only see the head. Some lathes use
a clutch or cam mechanism instead, on those the pin will
engage anywhere (again, I don't know which the Atlas uses).

While the concise description above is accurate, it sounded
like you needed a tad bit more info than that. :^)

Hope that helps,
--Glenn Lyford