RE: [CAD_CAM_EDM_DRO] ball screw vs acme crew

Ball screws aren't preferred over acme screws for mill applications because
of backlash or lack there of. There are three main things to consider when
selecting a screw: accuracy, backlash and efficiency.

When a screw is described as having a pitch of 5 teeth per inch, NO screw
actually has exactly 5 tpi. Threads will actually be a little bit closer or
father away than exactly 0.2 inches. This "drift" can add up to about
0.005" over a 12" distance in the case of an acme screw. So over the
course of 60 turns, the nut will actually travel 12" +/- 0.005". A rolled
ball screw is a little better with a drift of about +/- 0.004" over 12" of
travel. A ground ball screw can be accurate to +/- 0.0003" or better per
foot of travel (better than 10 times the accuracy of a rolled ball screw).
Obviously, none of these numbers are carved in stone. Some acme screws are
more accurate than others and some rolled ball screws are more accurate than
others. Ground ball screws are just pretty darn good.

The second thing to consider is backlash. Acme screws can be purchased with
preloaded zero backlash nuts (or pretty close to it) and so can rolled ball
screws. Both are also available with non preloaded nuts and backlashes of
0.003" to 0.010" (OUCH!). Combining non preloaded nuts on either acme
screws or rolled ball screws with the inherent inaccuracies of thread drift
can bring repeatability down to pretty near zero.

Efficiency also needs to be considered. When a motor turns an acme screw,
the screw turns within the fixed nut and the table moves. Due to the
friction of the nut sliding over the screw however, only about 50% of the
motor's torque gets translated into linear motion. A ball screw (rolled or
ground) has a nut with recirculating ball bearings, so the nut is not
actually rubbing against the screw, instead it is rolling on the bearings.
This reduced friction allows the ball screw to transfer up to about 93% of
the motor's torque into a linear force to move the table. This means that
the same motor can handle nearly twice the load if it uses a ball screw
rather than an acme screw. Alternately, with a ball screw, the motor can be
half the size (torque) and still drive the same load as opposed to using an
acme screw.


Terry Owens wrote:

>Hi,
>
>Ball-screws are important in the X and Y axis because they are designed for
>as near zero backlash as possible. Machine tools fitted with high precision
>ground ball-screws are also very accurate in the distance travelled. Due-to
>the weight of the Z axis the slide is always pressing down so backlash

isn't

>a problem, except if you want to do a movement towards the tail-stock on a
>lathe.
>