Motion control for machine tools and CNC

In short:CNC machine tools need stiff, accurate axes: servo motors or linear motors on the feeds, torque motors on rotary and tilt tables, and linear and angle encoders that measure the slide and table directly. Measuring at the axis rather than at the motor is the single biggest step in accuracy, because ball screw heat alone can move a 1 m axis by more than 0.1 mm.

Typical requirements

Machinery we specify motion for

• CNC milling machines and machining centres
• Lathes and turning centres
• Grinding machines
• EDM (spark and wire eroding) machines
• 5-axis rotary and tilt tables
• Tool changers and tool magazines
• Coolant heads, guarding doors and loaders
• Additive manufacturing machines
• Gear cutting and honing machines

What a machine tool asks of its motion system

A machine tool is a measuring instrument that also removes metal. Every axis has to hold position against cutting forces, move fast between cuts and stay accurate as the machine warms up over a shift. That makes stiffness, thermal behaviour and feedback the three things to get right, ahead of raw speed or power.

The UK sector is represented by the Manufacturing Technologies Association, which covers machine tools, metrology, additive manufacturing and related technologies. Whether you build a special-purpose machine or retrofit an older one, the component choices below are the same.

Where motion components sit on a CNC machine

Tolomatic lists door, worktable, coolant head and tooling positioning as machine tool uses for its actuators, and SCHLEIFRING supplies slip rings and media rotary joints for machine tools and rotary tables, with components designed for 50 million revolutions.

Why closed-loop feedback matters: the numbers

HEIDENHAIN’s feed axis accuracy paper is the clearest public account of what happens when an axis is controlled only from the motor encoder (semi-closed loop):

• Heat. A 1 °C rise lengthens a 1 m ball screw by about 11 µm. At 2,000 rpm with 3 kN preload the nut alone produces roughly 100–200 W.
• Drift. On a 1 m axis with fixed-floating screw bearings, the far end drifted more than 110 µm within 40 minutes at 10 m/min. Thermal time constants were typically 1–2 hours.
• Bearing layout. Fixed-fixed mounting reduced expansion by about half, but did not remove it.
• Dynamics. Accelerating a 500 kg slide at 4 m/s² deforms the drive by 10–20 µm, which the motor encoder cannot see.
• Linear encoders. In the same tests, axes with linear encoders showed essentially no drift and noticeably better contour accuracy in circular tests.

The lesson for specifying: if the part tolerance is tighter than the expected thermal error, buy a linear encoder for the axis and a drive that accepts it as a second loop. Our guide to absolute vs incremental encoders explains the interface choice, and the encoders hub lists the types.

Accuracy and repeatability claims should be quoted to ISO 230-2, which covers linear and rotary NC axes and is used for acceptance tests and compensation. Ask suppliers which standard their published figures follow.

Ball screw or linear motor for a feed axis

Ball screws remain the default for general machining centres and lathes. Linear motors earn their cost on laser cutting, high-speed milling, grinding and other machines where acceleration and dynamic accuracy drive productivity. See the ball screw vs roller screw guide for screw selection.

Rotary and tilt tables: torque motor or worm gear

A worm gear table is self-locking and cheap, but its backlash grows with wear and its speed is limited. A torque motor table drives the table directly. An American Machinist review of one direct-drive table quotes 200 rpm and notes that, with no gearing, backlash is eliminated and fewer parts wear; a hydraulic brake provides about three times the motor’s maximum torque for clamping during heavy cuts.

For compact axes, tilt heads and tool changers, strain-wave gears give zero backlash and a high ratio in a short length. Harmonic Drive promotes hollow-shaft servo actuators and tilt-resistant gears with output bearings for milling, turning and eroding machines. Compare the options in harmonic vs planetary vs cycloidal and see the frameless torque motors hub.

How to size a torque motor for a rotary table: a worked example

Take a 400 mm steel table of 60 kg carrying a 100 kg fixture and part with a radius of gyration of 150 mm. It must index 90° in 0.5 s and also contour during 5-axis finishing with a tangential cutting force of 1 kN at 150 mm radius.

1. Inertia. Table ½ × 60 × 0.2² = 1.2 kg·m². Part and fixture 100 × 0.15² = 2.25 kg·m². Allow 0.05 kg·m² for the rotor: total about 3.5 kg·m².
2. Index acceleration. For a triangular profile, α = 4θ ÷ t² = 4 × 1.571 ÷ 0.25 ≈ 25 rad/s², with a peak speed of 6.3 rad/s (60 rpm).
3. Peak torque. 3.5 × 25 ≈ 88 Nm, plus bearing and seal friction, say 95 Nm.
4. Continuous torque. Contouring under cut: 1,000 N × 0.15 m = 150 Nm, held for minutes at a time. This, not the index, sizes the motor’s continuous rating. Heavy roughing should go through the clamp instead.
5. Encoder. For 1 µm resolution at 150 mm radius you need 1 ÷ 150,000 rad ≈ 6.7 µrad, or about 940,000 counts per revolution, so 20 bits minimum and more for a stable servo loop. Accuracy of ±3 arcsec gives about ±2.2 µm at that radius.

A housed or frameless direct-drive motor rated above 150 Nm continuous, such as the Akribis ADR-A range (published to 407 Nm continuous), with a large-bore angle encoder such as the Netzer VLZ fits this case. Check cooling: continuous torque ratings often assume a cooled mounting.

Slip rings and rotary unions on machine tools

Rotary tables with hydraulic fixture clamping, pallet changers with sensors, and continuously rotating fixtures need media and signals across the axis. A combined rotary union and through-bore slip ring keeps the table short. Specify pressure and flow per channel, the fluid (hydraulic oil, air, coolant), circuits and fieldbus. For full guidance read how to specify a slip ring and slip rings for test rigs.

Things that catch engineers out

1. Buying a high-resolution motor encoder and expecting part accuracy. Resolution at the motor is not accuracy at the tool.
2. Mounting the scale far from the cutting plane. Pitch and yaw of the slide multiply by the offset (Abbe error).
3. Sizing torque motors on index torque only. Continuous cutting torque and cooling usually decide the frame.
4. Exposing magnetic linear motor tracks to ferrous swarf. Covers and wipers are part of the design.
5. Ignoring coolant mist. Encoders, connectors and actuators need sealing, and enclosed scales usually need clean, dry purge air.
6. Mismatched interfaces. Confirm the CNC and drive accept the encoder protocol and resolution before ordering.

What to send an engineer

• Machine type and axis list, with travel, moving mass, rapid and cutting feeds and acceleration.
• Cutting forces or torque per axis, and how long they last.
• Required accuracy and repeatability, ideally to ISO 230-2.
• CNC make, fieldbus and the encoder protocols it accepts.
• For rotary tables: diameter, part mass and inertia, index time, clamp type and media to pass through.
• Coolant, swarf and temperature conditions, and the space available.

Our specialist engineering and supply partners can propose matched motor, encoder, drive and gearing sets; start in the spec builder.

Standards to have in view

• ISO 230-2:2014 (accuracy and repeatability of positioning of NC axes)
• ISO 230-3 (determination of thermal effects)
• EN ISO 16090-1 (machining centres, milling machines and transfer machines: safety)
• EN ISO 23125 (turning machines: safety)
• EN ISO 16089 (stationary grinding machines: safety)
• ISO 13849-1 (safety-related parts of control systems)
• Regulation (EU) 2023/1230 (machinery; applies from 20 January 2027)

Which standards apply depends on the end product and its market. Your engineer will confirm what the component itself needs to meet.