CNC screws basic knowledge: what engineers actually need

• 1
  Key takeaways
• 2
  What an engineer means by a CNC screw
• 3
  How thread geometry turns rotation into precise travel
• 4
  Backlash, preload, and why the nut design decides repeatability
• 5
  Accuracy classes, pitch error, and what the number really covers
• 6
  Materials, hardening, and the loads that end a screw's life
• 7
  Which screw for which job

Ask five people what a CNC screw is and you get five answers. In a machine shop, the phrase usually means the threaded fastener that clamps a fixture plate, a vise jaw, or a workholding block. In a machine tool catalog, it means the drive screw that converts motor rotation into axis travel. Those are different components with different failure modes, and mixing them up leads to wrong specifications.

The fastener side is straightforward. A socket head cap screw, a flat head screw, or a shoulder screw holds parts together. What matters is thread form, class of fit, material, and preload. A shoulder screw also locates, so its shank diameter matters more than its thread.

The drive side is where the engineering gets interesting. A lead screw is a threaded shaft running inside a threaded nut. A ball screw replaces sliding contact with recirculating steel balls. Same job, very different friction, efficiency, and stiffness numbers.

This page treats the drive screw as the main subject because that is where design decisions have consequences. The fastener section stays short and practical, since most engineers already know how to call out a thread.

One more term worth clearing up: people say screw when the catalog says screw, and say screw when the drawing says shaft. Both appear in CNC work. Use the drawing, not the shop-floor shorthand.

Every drive screw obeys the same relationship. One full turn moves the nut by one lead. Lead is not the same as pitch: a two-start screw with a 2 mm pitch has a 4 mm lead, so one turn moves 4 mm. That distinction matters when you size a motor or calculate resolution.

The angle of the thread flank decides how much of the motor torque becomes axial force. A shallow flank angle gives high thrust but high friction. A steeper angle is more efficient but produces less force per unit of torque. Ball screws sidestep this tradeoff by rolling instead of sliding, which is why they reach 90% efficiency where an acme lead screw sits near 30 to 50%.

Contact geometry drives stiffness. In a lead screw, the nut thread and shaft thread touch over a broad area, and that area grows under load. In a ball screw, each ball touches the raceway at two small elliptical points. Small contact areas mean high local stress but also very high stiffness, provided the raceway is hard enough.

Resolution is not accuracy. A 5 mm lead screw on a 1.8 degree stepper gives 0.025 mm per full step before microstepping, but the axis may still be 0.1 mm off after a meter of travel. Resolution describes the smallest commanded move; accuracy describes where the nut actually ends up.

Thermal growth belongs in the same conversation. A steel screw heated 5 °C over a 1,000 mm span grows roughly 0.06 mm. On a warm machine running all day, that shifts the zero point slowly and quietly.

Backlash is the lost motion when you reverse direction. In a lead screw it comes from the clearance between the two thread flanks. In a ball screw it comes from the gap between the balls and the raceway. Either way, the axis moves a little before the load moves, and any bi-directional positioning job suffers.

Preload removes that gap by forcing the nut and screw into permanent contact. Ball screw makers do it in three common ways: oversized balls, a double nut with a spacer, or a shifted lead on one nut half. Oversized balls are cheap and compact, but the preload fades as the balls wear. A double nut holds preload longer and lets you adjust it with shims.

Preload is not free. A preloaded nut has higher drag torque, generates more heat, and wears faster than a zero-preload nut. That is the tradeoff. Light preload on a fast, lightly loaded axis; heavy preload on a slow, stiff, high-force axis.

For lead screws, an anti-backlash nut with a spring or a split nut does the same job at lower cost. It works well at low speed and light load. It does not survive high reversing loads for long, because the spring or the plastic insert takes the wear.

Measure backlash the same way every time. Zero an indicator against the nut, jog one direction, then reverse by a known amount and read the lag. Do it at three positions along the travel, because wear is rarely uniform.

Screw accuracy classes describe allowable travel deviation over a defined length, not just a single-point tolerance. A rolled screw typically lands in a mid class, a ground screw in a tighter one. The class covers cumulative pitch error, plus a allowance for variation within one revolution.

Two errors hide inside that number. Cumulative pitch error accumulates along the screw and shows up as a position offset at the far end. Periodic error repeats once per turn and shows up as a ripple in surface finish or as a small oscillation in a positioning loop. They have different causes and different fixes.

Rolled screws are formed by pressing a die against a bar. It is fast and inexpensive, and the grain flow follows the thread, which helps fatigue life. Ground screws are cut and then ground after heat treatment, which removes distortion and produces tighter lead accuracy. Cost reflects the extra operations, not marketing.

For most positioning work, a rolled screw in a mid class plus a good feedback scale beats a top-class screw running open loop. The scale sees the real position, including thermal drift and pitch error. Closed loop does not fix a mechanically loose nut, though. Feedback cannot remove backlash.

Ask for the lead deviation chart when the axis is long. A single class number hides how the error is distributed, and where it sits matters if you only travel over part of the screw.

Most ball screws and lead screws start as medium-carbon or alloy steel, then get induction hardened or through hardened for the raceway. Surface hardness in the high 50s to low 60s on the Rockwell C scale is normal for a rolling element raceway. Below that, balls dent the surface and the preload goes away.

Stainless grades appear where corrosion matters, but they usually carry lower load ratings for the same size, because the alloy is softer and harder to harden uniformly. On a food or medical machine, that tradeoff is often worth taking. On a heavy press axis, it is usually not.

Aluminum and brass screws exist for light-duty adjustment and for non-magnetic or non-sparking environments. They wear quickly under repeated motion. Use them for setup adjustments, not for production cycling.

Lubrication is part of the load path. Grease stays put on slow vertical axes and carries the film through shock loads. Oil runs cooler on fast horizontal axes and flushes debris. Mixing the two, or letting a greased nut run dry, shortens life more than a one-class accuracy downgrade ever would.

Protect the screw from chips and coolant. Bellows, wipers, and a felt seal at the nut face add a little drag but keep abrasive debris out of the raceway. On any machine cutting metal nearby, that is the cheapest life extension you can buy.

When a screw fails, look at the failure pattern first. Flaking on one side of the raceway points at misalignment. Uniform wear points at lubrication or duty cycle. Cracking at the thread root points at a stress riser or an overload.