Screw-driven is the most common motion stage configuration: a motor rotates, and a screw converts that rotation into linear travel of the carriage. This article answers one question: how does a screw-driven stage achieve micron positioning, and how do you read the numbers?
The answer splits into two layers. Transmission layer: screws come in two types — trapezoidal (friction-driven, has backlash, cheap) and ball screw (ball recirculation, near-zero backlash, high precision) — with the precision gap in the micron range. Drive layer: stepper motors (open-loop, computed from step angle) and DC servos (encoder closed-loop) are the two mainstream drives, each with its own speed/precision trade-off.
This article covers the two screw types, the two drives, and how to read the numbers (step-angle conversion, ball-screw C-grade, UTS comparison). Key reminder: the motor-axis angle spec, the whole-platform positioning spec, and the workpiece dimensional tolerance are three different quantities and must not be substituted for one another.
Figure 1 | Generic ball-screw configuration (not a specific product cross-section). The nut is fixed to the carriage and does not rotate with the screw; an optional encoder on the motor shaft cannot read the lead error, backlash, or elastic deformation behind the screw. Click for full size.
1. Trapezoidal Screw vs Ball Screw
The difference between the two screw types is whether the contact between screw and nut is sliding or rolling:
| Trapezoidal screw | Ball screw | |
|---|---|---|
| Contact | Thread surfaces slide (friction drive) | Balls roll in recirculation |
| Backlash | Present (needs preloading or anti-backlash) | Near-zero (preloaded balls) |
| Self-holding | Yes (does not back-drive when powered off) | No (vertical axis needs a brake) |
| Efficiency | Low (high friction loss) | High (low rolling loss) |
| Cost | Low | High |
| Typical precision | Millimeter class | Micron class |
Trapezoidal screws suit low-precision, low-speed applications that need self-holding when powered off (e.g., clamps, lifts). Ball screws suit micron-class positioning — near-zero backlash gives good repeatability, and high efficiency gives faster speed at the same power. A vertical axis with a ball screw needs a brake or self-holding mechanism, because it does not self-hold.
2. Stepper Motor: Step Angle, Microstepping, Open/Closed Loop
A stepper motor moves in steps, each step turning by a step angle. Oriental Motor’s 2-phase PKP/PK has a basic step angle of 1.8° (standard) / 0.9° (high-resolution), and 5-phase PKP/PK is 0.72° / 0.36°[1][2].
Microstepping (subdivision) splits one full step into multiple micro-steps; e.g., 16× subdivision splits a 0.72° full step into 16 micro-steps of 0.045°. Microstepping improves smoothness and reduces low-frequency vibration, but does not improve positioning accuracy — accuracy is still limited by screw lead error and backlash.
Steppers usually run open-loop: the controller computes position from the number of pulses issued, without reading the actual position. Overload or too-fast acceleration causes missed steps, after which the computed value no longer represents the actual position. An added encoder may only monitor missed steps, or may participate in closed-loop correction — it depends on the driver and wiring[4].
Stopping accuracy (datasheet): 5-phase 0.72° type ±0.05°; the US site lists ±0.034° for the 0.36° type (unloaded); the RK II combination is ±0.05° noted as unloaded[3][5]. Do not write ±0.05° for all high-resolution types — model limits must be checked one by one.
3. DC Servo: Encoder Closed-Loop
A DC servo motor with an encoder forms a closed loop: the encoder reads the actual position, and the controller corrects the error. Newport’s UTS series provides a same-size comparison of two drives[9]:
| UTS-CC brushed DC | UTS-PP stepper | |
|---|---|---|
| Travel | 50 / 100 / 150 mm | same |
| Max speed | 40 mm/s | 20 mm/s |
| Minimum increment | 0.3 μm | 0.3 μm (with XPS; SMC100PP/ESP301 is 0.5 μm) |
| Bidirectional repeatability (guaranteed) | ±1.75 μm | ±3.0 μm |
| Guaranteed accuracy | ±2.25 / ±2.75 / ±3.25 μm | ±2.5 / ±3.5 / ±4.0 μm |
The UTS-CC rotary encoder is mounted directly on the screw (the motor drives the screw via 11:4 belt reduction); the UTS-PP drives the screw via a coupling and positions by commanded steps[9]. The UTS-CC max speed drops to 20 mm/s when the axial load exceeds 1 kg. This DC version is faster with tighter bidirectional repeatability, but you cannot generalize that all DC motors beat all steppers — the trade-off depends on the specific model’s torque-speed curve and encoder resolution.
The UTS manual lists a test environment of 20 ±1 °C, and meeting the spec also requires a mounting flatness of 5 μm[10].
4. How to Read the Numbers: Conversion, C-Grade, UTS Comparison
Step-angle conversion: direct coupling, no gearbox, ignoring screw and coupling error first, the angle converts as Δx = lead L × Δθ / 360° (millimeter lead × 1000 gives microns):
| Lead | 5-phase 0.72° full step | 5-phase 0.36° full step | ±0.05° as linear |
|---|---|---|---|
| 1 mm | 2.0 μm | 1.0 μm | ±0.14 μm |
| 2 mm | 4.0 μm | 2.0 μm | ±0.28 μm |
| 5 mm | 10 μm | 5.0 μm | ±0.69 μm |
| 10 mm | 20 μm | 10 μm | ±1.39 μm (RK II datasheet says ±1.4) |
| 20 mm | 40 μm | 20 μm | ±2.78 μm |
The RK II page’s 10 mm lead example converts ±0.05° to ±1.4 μm[3] — this only estimates the motor angle error’s contribution to linear position, not the guaranteed accuracy of the whole screw stage. Using the 0.36° PKP’s ±0.034°, the same lead converts to about ±0.94 μm.
Ball-screw C-grade: you must write manufacturer, shaft diameter, lead, and accuracy grade together. HIWIN’s external-circulation type lists shaft 8–100 mm, lead 2.5–60 mm (not every shaft has every lead), in-stock C7 marked 50 μm/300 mm[7][8]. THK per JIS B 1192 (ISO 3408) lead accuracy: C3 positioning example (effective thread ≤100 mm) represents a travel-distance error of 8 μm, varying 8 μm per 300 mm; C5 the same length 18 μm; C7 in-stock example ±50 μm/300 mm; C10 ±210 μm/300 mm[6]. Shortening the lead only shrinks the motor angle error’s converted value; it does not eliminate the screw’s own lead error.
5. Applicable Range and Limits
Screw-driven stages suit micron-class, long travel (mm to m), moderate speed, heavy load applications. Their precision ceiling is set by the ball-screw C-grade and backlash, typically in the micron class; for sub-micron or below, switch to a linear motor or piezo (see later articles in this series).
Common Misconceptions
- “Smaller step angle = more accurate”: Wrong. Step angle sets the per-step displacement; positioning accuracy is set by screw lead error, backlash, and the closed loop. A 0.36° full step at 10 mm lead is 10 μm, smaller than 0.72°’s 20 μm, but neither equals the platform accuracy.
- “Closed-loop = accurate”: A closed loop corrects motor missed steps, but cannot read the lead error and backlash behind the screw — the encoder is on the motor shaft and reads motor rotation, not the carriage’s actual position.
- “C3 is always better than C7”: C-grade is the lead-accuracy grade; C3 is tighter than C7, but the whole-platform accuracy is also affected by backlash, rails, mounting flatness, and temperature.
Self-Check for the Reader
- Can you state the difference between trapezoidal and ball screws in backlash and self-holding?
- At 10 mm lead, 5-phase 0.72° full step, what is the per-step displacement in microns?
- Can the motor-axis ±0.05° stopping accuracy be taken directly as the platform’s positioning accuracy?
Back to the opening question: how does a screw-driven stage achieve micron positioning? The answer is a ball screw (near-zero backlash) plus a closed-loop drive (stepper or DC servo), with precision set by the screw C-grade and backlash. Applicable conditions: the stepper numbers here come from Oriental Motor, the servo example from Newport, all datasheet values of specific models. Unresolved limits: a finer motor does not automatically raise the platform’s class — ±0.05° converted to ±1.39 μm is only one error component; platform acceptance reads the whole-machine spec, workpiece acceptance reads the drawing and measurement results (see “The 10 μm and 1 μm on a CNC Workpiece”).