Quick answer: A linear encoder, also called a linear scale, measures the true position of a machine axis directly rather than letting the control infer it from how far the ball screw has turned. Mounted along the axis, it reports exactly where the slide actually is, so the CNC can close the loop on real position. That removes ball-screw backlash, pitch error, wear and thermal growth from the result, which is why full-closed-loop machines with linear encoders hold tighter accuracy than machines that rely on the screw alone. It is one of the quiet differences between a general machine and a precision one.
This guide explains what a linear encoder is, how it works, the difference between full-closed-loop and semi-closed-loop control, absolute versus incremental scales, how accurate they are and when you actually need one. It connects to the ball screw, servo motor and CNC controller that together form the machine's motion system, and underpins precision CNC machining.
How a Linear Encoder Works
A linear encoder has two parts: a scale (a precise graduated strip, usually glass or steel) fixed along the axis, and a read head that travels with the slide and reads the graduations. As the axis moves, the read head detects the markings — optically or magnetically — and reports position to the control, typically resolving to fractions of a micron. Because the scale is fixed to the machine bed and the read head to the moving slide, the encoder measures the actual relative position of the two, independent of whatever the ball screw and motor are doing. The control compares this real position against the commanded position and corrects any difference, closing the loop on the truth rather than an estimate.
Full-Closed-Loop vs Semi-Closed-Loop
This is the core concept, and it decides how accurately a machine can position.
| Aspect | Semi-closed-loop | Full-closed-loop |
|---|---|---|
| Position measured by | Rotary encoder on screw/motor | Linear encoder on the axis |
| Sees screw backlash/pitch error | No | Yes, and corrects it |
| Sees thermal growth of screw | No | Yes, and corrects it |
| Accuracy | Good, depends on screw | Highest |
| Cost | Lower | Higher |
In semi-closed-loop control the only position feedback is a rotary encoder on the screw or motor, so the control assumes the slide moved exactly as much as the screw turned — true only if the screw has no backlash, pitch error, wear or thermal growth. In full-closed-loop control a linear encoder measures the slide directly, so all of those errors are seen and corrected. A quality ball screw makes semi-closed-loop good; a linear encoder makes full-closed-loop best.
Absolute vs Incremental Encoders
Linear encoders also differ in how they report position, which affects convenience and robustness.
- Incremental: counts graduations from a reference mark, so the machine must be homed after power-up to establish position. Common, accurate and cost-effective.
- Absolute: each position on the scale is uniquely coded, so the control knows the exact position the instant it powers on, with no homing. More convenient and robust, and increasingly the default on precision machines.
Glass vs Magnetic Scales
Two scale technologies dominate. Optical glass scales offer the highest accuracy and resolution and are the standard for precision machining, but they must be protected from contamination. Magnetic scales are more rugged and tolerant of coolant, chips and vibration, at slightly lower resolution, which suits harsher environments. Sealed encoder units protect the scale and read head from the shop environment so accuracy holds over time.
Accuracy and Thermal Stability
Glass-scale linear encoders commonly resolve to fractions of a micron and come in accuracy grades matched to precision work. Their biggest practical benefit beyond raw resolution is thermal: because the encoder measures the slide directly, it sees and corrects the positioning shift that thermal growth of the ball screw would otherwise introduce during a long run. Since heat is the largest enemy of accuracy — as covered in precision CNC machining — this is a major reason full-closed-loop machines hold tolerance over a shift while semi-closed-loop machines can drift.
When You Need a Linear Encoder
A linear encoder is not free, so match it to the work. It earns its cost when accuracy and consistency are critical, and is optional when they are not.
- Strongly recommended: molds and dies, aerospace and medical parts, optics, and any work holding tight tolerances over long runs.
- Valuable: 5-axis and high-precision contouring, where positioning error compounds across axes — see 5-axis machining.
- Often optional: general machining and lower-precision parts, where a quality ball screw in semi-closed-loop is sufficient and more economical.
HYR Machines and Closed-Loop Accuracy
HYR machines are built for accuracy with quality feed systems, and linear scales for full-closed-loop control are available where precision demands it.
- HYR 5 Axis Machining Center — +/-0.006 mm accuracy with closed-loop control for the most demanding aerospace, medical and mold contouring.
- HYR VMC1060 — +/-0.008 mm positioning, with linear-scale options for precision work.
- HYR VMC1165 — rigid larger-travel VMC configurable for closed-loop accuracy on bigger parts.
- HYR VMC range — feedback and accuracy options matched to your tolerance requirements.
Need full-closed-loop accuracy for tight tolerances? Use the HYR Machine Selector — tell us your tolerance, part type and material and get a matched machine and feedback recommendation, a technical proposal and a quotation path in minutes, plus the option of accuracy reports and a free sample cutting.
Frequently Asked Questions
What is a linear encoder on a CNC machine?
A linear encoder, also called a linear scale, is a position-measuring device mounted along a machine axis that reports the true slide position directly to the control. This lets the CNC close the loop on actual position rather than inferring it from the ball screw, improving accuracy.
What is the difference between full-closed-loop and semi-closed-loop?
In semi-closed-loop control the position is inferred from a rotary encoder on the ball screw or motor, so screw backlash, pitch error and thermal growth are not seen. In full-closed-loop control a linear encoder measures the slide directly, so those errors are detected and corrected, giving higher accuracy.
What is the difference between absolute and incremental linear encoders?
An incremental encoder counts movement from a reference and must be homed after power-up; an absolute encoder reports the exact position immediately without homing. Absolute encoders are more convenient and robust, while incremental types are common and cost-effective.
Do I need a linear encoder on my CNC machine?
For high-precision work, molds, aerospace and medical parts, a linear encoder (full-closed-loop) is strongly recommended because it removes screw and thermal error. For general machining, a well-built semi-closed-loop machine with a quality ball screw may be sufficient and more economical.
How accurate are linear encoders?
Glass-scale linear encoders commonly resolve to fractions of a micron and are available in accuracy grades suited to precision machining. Real machine accuracy still depends on rigidity, thermal control and the overall build, but the encoder removes a major error source.
Do linear encoders help with thermal growth?
Yes. Because a linear encoder measures the slide position directly, it sees and corrects positioning shifts that thermal growth of the ball screw would otherwise cause, which is a key reason full-closed-loop machines hold accuracy better during long runs.