On 2008-12-20, Ignoramus25214 wrote:
I went to an auction recently and picked up a set of screws that
turned out to be ball screws for a 9x42 Bridgeport.

http://igor.chudov.com/tmp/Bridgeport-Ball-Screws/

Nice find!

I am a little surprised about it, as, I thought, ball screws could
only be used in CNC mills whose screws were held by stepper
motors.

Stepper or servo motors. Note that the shanks have two spaces
for keys -- one for the timing belt gear which drives it from the
stepper or servo motor, and the other for the handwheel (with a
spring-loaded folding crank handle, so you don't get beat to death when
the thing starts up unexpectedly. :-)

So, do they have use in a manual mill and are they in any way better
than regular drive screws? Or are they for CNC conversion only?

The problem with ball screws in a manual-only lathe is that they
are too easy to reverse drive. Replace your current leadscrews with
these, and you can grab hold of the vise on the table and pull it to
where you want it -- driving the screws from the table's motion.

A stepper naturally holds in specific positions -- like detents.
A servo with no power is a drag on the screw, so you can use the machine
in manual mode if you so desire.

Note that the X-axis ball screw used by Bridgeport in the Series
I (BOSS-3 and later) does not rotate. Instead, the ball nut is mounted
in a pair of opposed high-precision preloaded bearings, and driven by a
cog (timing) belt. This way, you don't have the screw whipping during
rapid motions. But -- you also don't have manual control of the machine
at all (other than with the jog switches).

As for benefits of the ball screws -- consider near zero
backlash, and very long life, as well as the low torque needed for the
motor to move the table.

If you set up an adjustable clutch on each screw, you *could*
use them for a manual machine, I guess. But normally that is too
expensive -- unless you luck into a set of the screws like this. :-)

Oh yes -- also the Bridgeport Series I BOSS-3 and later have a
large diameter hollow ball screw around the quill, and the nut is driven
by a motor outside the headstock housing. This way, the Z-axis force is
truly concentric with the quill, unlike the aftermarket adaptations
which I have seen, which replace the depth stop screw with a ball screw,
which provides the force off center, and I expect hastens the wear of
the quill and housing in certain directions.

Enjoy,
DoN.

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