Ball Screw CNC Essentials

Inside the nut there is a track of steel balls. The shaft carries a helical groove, and the nut has a matching groove, so 60 or 90 balls roll between the two surfaces instead of sliding. A return tube or deflector catches them at one end and feeds them back to the start, which keeps the circuit closed.

Rolling contact changes the friction picture. Sliding friction between threads is replaced by rolling friction, and the coefficient drops by roughly an order of magnitude. A typical rolled ball screw runs at 90% mechanical efficiency or better. A trapezoidal lead screw under the same load sits closer to 30–50%.

That efficiency gap is the reason machine builders use them. Less torque is lost to friction, so a smaller servo can drive the same load, and the slide follows the command signal with very little lag. Backlash is controlled by preloading the nut, usually with a double nut or an offset internal design.

A rolled screw is formed by cold rolling the groove into the shaft. Ground screws are cut and then ground to a finish. Ground screws hold lead accuracy tighter, often 5 μm per 300 mm or better, and they cost more. Rolled screws are common on general-purpose mills and routers.

Not every axis needs a ball screw. A manual machine, a low-speed positioning slide, or a clamping axis can run a lead screw and never show a problem. The trade-off shows up when the axis is fast, frequently reversing, or holding position under load.

Sliding threads wear faster and lose position as they wear. They also require more torque, so motors run larger and hotter. On a slow feed axis with low duty cycle, none of that matters much, and the lower cost and self-locking behavior of a lead screw can be an advantage.

For a CNC axis that reverses thousands of times a shift, rolling contact wins on repeatability. Heat generation is lower, so thermal drift over a long cut is smaller. Wear is spread across many balls, and the nut keeps its preload far longer before it needs adjustment.

There is one more factor. A ball screw has low internal damping, so a badly tuned servo can chatter or ring. Preload, shaft diameter, and support bearing stiffness all feed into that. On short, heavily loaded axes a lead screw can actually damp vibration better.

• 1
  Choose rolling contact whenthe axis is fast, reverses often, or must hold position precisely under load.
• 2
  Choose a lead screw whenthe axis is slow, lightly loaded, and self-locking is useful.
• 3
  Check preload firstbacklash on a used ball screw usually comes from lost preload, not worn balls.
• 4
  Watch the dampinglow internal friction means servo tuning matters more than on a sliding axis.

Simultaneous five-axis motion asks each linear axis to track a moving tool tip while two rotary axes tilt the part or the head. Any lost motion on X, Y or Z shows up as a surface step or a witness mark on a contoured face. That is why the linear drives on these machines are usually preloaded ground screws.

The rotary table is a separate case. A tilting or rotating axis is often driven by a worm gear, a torque motor, or a roller cam, not a ball screw. So when someone says a five-axis center has ball screws, they mean the three linear axes. The rotary axes have their own backlash budget.

Thermal behavior compounds the problem. A screw that stretches as it warms shifts the zero point. On long cuts, machine builders compensate by cooling the screw, anchoring it at one end, or mapping the error in the control. A screw supported at both ends with a pre-tensioned shaft resists growth better than a floating one.

For the part itself, none of this changes the drawing. What changes is what the shop can hold. Roundness, flatness, and true position on a contoured feature are functions of the whole loop: screw, bearings, servo, and thermal state. A tight CNC tolerance is only meaningful if the machine can repeat it.

A drawing calls out a tolerance, but the machine has to deliver it. When we quote a part at ±0.005 mm, the number rests on the linear axes holding position and reversing without lost motion. Preloaded screws, matched support bearings, and a rigid frame are part of that chain, not separate items.

Consider a long aluminum bracket with a bored bore at each end. Position error between the two bores comes from thermal growth over the cut and from backlash on the axis that moves between them. A screw with a loose preload will show that error as a consistent offset in one direction.

Five-axis contouring exposes it differently. Ripples on a curved surface usually trace back to the linear axes, not the toolpath. The control interpolates in small segments, and the nut has to follow each one. Low friction helps here, but low damping can hurt if the drive gains are set too high.

We machine aluminum, stainless steel, titanium and engineering plastics on 127 CNC machines, including 16 simultaneous five-axis centers. The bulk of that work is on the linear axes described above. Every part is inspected before shipment, and reports are available on request.