What Is Ball Screw in CNC Machine?

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  How a ball screw in CNC machine axes converts rotation
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  Lead, preload, and accuracy class in a CNC machine
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  Where a ball screw in CNC machine work pays off
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  When a ball screw in CNC machine is the wrong choice
• 5
  Ball screw vs lead screw in a CNC machine

A ball screw is a screw shaft with a helical groove ground or rolled into it, a matching nut with the same groove, and a train of hardened steel balls running between the two. The balls roll along the groove, so the nut travels along the shaft on rolling contact instead of sliding contact.

When the servo motor turns the shaft, the balls circulate through a return path inside the nut and re-enter the groove at the other end. One full turn of a 10 mm lead screw moves the nut 10 mm. There is almost no stick-slip at low speed, which is why a ball screw holds a consistent finish on light finishing passes.

The preload matters as much as the screw itself. A preloaded double nut removes axial play so the axis can reverse direction without a dead band. On a machine cutting to ±0.005 mm, backlash of 0.02 mm would show up as a step on every direction change.

Friction coefficient is roughly 0.003 to 0.005 for a ball screw against 0.1 to 0.3 for a sliding lead screw. That difference is what lets a smaller servo motor drive a heavier table at the same acceleration.

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  Rolling, not slidingBalls carry the load, so wear is spread over many contact points.
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  Low backlashPreload takes up axial play for accurate reversal.
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  High efficiencyAround 90% of input torque becomes linear thrust.

Lead is the distance the nut travels in one shaft revolution. A 5 mm lead gives finer resolution but slower rapid moves. A 20 mm lead moves fast but needs more motor torque to hold the same cutting force. Most 3-axis mills sit between 5 mm and 10 mm.

Accuracy classes run from rolled screws for general positioning to ground screws for tight work. A ground C3 screw may hold 0.008 mm per 300 mm of travel; a rolled screw of the same diameter may drift 0.05 mm over the same length. The class, not the brand, sets the floor on what your machine can hold.

Preload is usually 2% to 8% of the dynamic load rating. Too little and the axis rattles on reversal. Too much and friction climbs, the screw runs hot, and service life drops. On a 4,000 mm travel axis, thermal growth along the shaft can reach 0.02 mm if the screw is not cooled or compensated.

Ball diameter and circuit count also set stiffness. More circuits share the load and raise rigidity, but they add drag torque. A screw sized for a 500 × 500 × 450 mm machine does not need the same circuit count as one driving a 4,000 × 400 × 150 mm gantry.

Any axis that reverses direction under load benefits. That covers X, Y, and Z on a mill, the Z axis on a router, and the linear axes on a laser or plasma table. The tighter the tolerance, the more the ball screw earns its cost.

It also suits high duty cycles. A machine running three shifts will wear a sliding screw fast; the rolling contact in a ball screw spreads that wear and keeps accuracy longer between adjustments.

Parts that need mirror finishes gain too. Because there is no stick-slip, the tool does not chatter when the axis creeps at 10 mm/min. That is the difference between Ra 0.2–0.8 μm and a surface that needs hand polishing.

Medical and aerospace work often calls for it. A fixture that holds a bone plate or a bracket must repeat to ±0.005 mm across thousands of cycles, and a preloaded ball screw is the usual way to get there.

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  Positioning axesX, Y, Z on mills and routers.
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  High duty cyclesThree-shift production cells.
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  Fine finishesCreep feed passes without chatter.
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  Tight repeatabilityFixtures and gauges that cycle daily.

A ball screw does not self-lock. On a vertical axis, gravity will pull the slide down when the motor is off, so you need a brake or a counterbalance. A lead screw with a steep enough lead will hold itself, which is why cheap drill presses still use one.

Contamination is the other limit. Chips and fine dust destroy the raceway. A screw without wipers or a bellows will lose preload in months. In a graphite or carbon-fiber shop, this matters more than the accuracy class you paid for.

Very long travel is a problem too. Past about 4,000 mm, shaft sag and whip force you to lower the critical speed. A rack and pinion or linear motor often wins on gantries that long.

Cost is the last filter. If the axis only needs 0.1 mm, a belt or a rolled lead screw will do the job for less. Specifying a ground ball screw there just adds money without adding capability.