Are ballscrews required for CNC?

Efficiency.

Acme leadscrews offer ca20% to ca40% motion in relation to torque input. Ballscrews are said to exceed 90% efficiency.

--Winston

You can't cut horizontal or vertical tangents of curves accurately with backlash, because rigidity disappears.

We don't have ball screws and it's a bitch to plan cuts that keep the deflection to one side. The software will compensate but you really have to be very aware and "feel" for the cutter.

Try a climb cut on a Bridgeport, then look at one from a CNC.

Another point, all of the CNC's I've run (15+ years) have backlash in the ballscrews. The execution program (which interprets your CNC program to the machine tool) typically adjusts for this backlash (after setting the parameter) with out any type of operator or programmer intervention. The method of incorporation is not always the same (the same machine executing different G code, will accommodate backlash differently)

i.e. When rapid positioning to a point, it may over shoot the location (small amount), then feed back so as to always load the axis in the same direction. However, when interpolating a bore, it will compensate based on the ratio and direction of the move. This is why a diameter that is interpolated on a CNC is never perfectly round.

Another point, all of the CNC's I've run (15+ years) have backlash in the ballscrews. The execution program (which interprets your CNC program to the machine tool) typically adjusts for this backlash (after setting the parameter) with out any type of operator or programmer intervention. The method of incorporation is not always the same (the same machine executing different G code, will accommodate backlash differently)

i.e. When rapid positioning to a point, it may over shoot the location (small amount), then feed back so as to always load the axis in the same direction. However, when interpolating a bore, it will compensate based on the ratio and direction of the move. This is why a diameter that is interpolated on a CNC is never perfectly round.

snipped-for-privacy@well.com (Philippe Habib) wrote in news:phabib-0505042148030001@192.168.1.185:

You don't absolutely *have* to have ballscrews. That said, however, there is a large number of reasons to use them.

On any system where the encoder is direct mounted to the axis, any significant backlash will cause some hysterisis (hunting), depending on the amount of backlash, this can become uncontrollable, and this is one reason most manufacturers use the encoder mounted either to the motor, or direct to the screw (or through a timing belt to the screw, spindle or other mechanical movement aparatus). Direct mounting of the encoder to the axis movement mechanism (screw, belt, etc) eliminates this, but you lose accuracy, since you now do not know for sure how much the physical slide has moved (due to backlash, leadscrew or movement mechanism mechanical or manufacturing errors/inaccuracies) although this can be compensated for in the controller. Note that on every controller I have ever worked, (many different brands and types), the maximum amount of backlash that can be compensated reliably by the control is 0.25 mm. As someone else pointed out, mechanical efficiency of a ballscrew is much better than say, an acme screw. You can purchase 0 backlash (preloaded) ballscrews, and these are fairly standard in the industry. This is not to say there will be 0 backlash in the movement, as you have other mechanical places that can introduce slop. (bearings, drive couplings, gearboxes, etc)

As to whether ballscrews are required for a particular application, would be almost totally dependent on the type of application, the accuracy required, cycle life, force required and many other factors.