High-Straightness Ground
Positioning Shafts

A positioning shaft is a precision-ground shaft used in a linear or rotary positioning system where the shaft itself determines the axis of motion. Unlike a simple shaft that transmits torque, a positioning shaft guides a carriage, probe, or tool along a defined path — so its straightness, surface finish, and dimensional accuracy directly determine the positioning accuracy of the system. Common applications: linear stage shafts, CMM probe extensions, optical bench motion shafts, ball-spline drives, and actuator rods.

Cylindrical grinding between centres achieves straightness ≤ 0.005 mm per 300 mm as standard. For longer shafts or tighter requirements, incremental grinding passes are used. Shafts are measured for straightness using a CMM in a V-block fixture — the full straightness error plot is included on the inspection report. For ultra-precision positioning stages requiring straightness below 0.001 mm/300 mm, discuss with our engineers — this requires specialised lapping operations.

Positioning shafts often run in linear bushings or ball-spline nuts. The shaft Ra directly affects: bearing preload consistency (rough shafts cause variable ball contact load); sliding friction (rough shafts cause stick-slip in plain bushings); and bore wear rate (rough shafts abrade the bushing bore faster). Ra 0.2 µm is standard; Ra 0.1 µm is available for applications requiring minimum friction variation.

A positioning shaft is held to tighter straightness tolerance (as well as diameter and finish) because it defines a geometric axis rather than just transmitting force. A standard transmission shaft needs strength and fit tolerances at its bearing seats, but its mid-span straightness is less critical. A positioning shaft may also be longer relative to its diameter, making heat-treatment distortion and straightness correction the dominant manufacturing concern — every shaft is ground between dead centres and re-measured post-HT.

Prototype (1–5 pcs, hardened + ground between centres): 5–8 days. Small batch (10–50 pcs): 8–12 days. Production (100–1,000 pcs): 10–16 days. Stainless or titanium: add 2–4 days. Shafts over 600 mm: add 2–3 days for straightness correction. Rush production available.

GCr15 (SUJ2 / 100Cr6) through-hardened bearing steel — the premium choice for ball-spline shafts and matched-bearing linear stages, HRC 60–62. 42CrMo4 (induction hardened) for general linear actuators and servo positioning. SUS440C hardenable stainless for clean-room and vacuum-chamber stages. SUS303/316L for chemical, pharma, and lab automation. Al 7075 hard-anodized for optical-bench positioning. Ti Grade 5 for aerospace actuators.

Standard ground OD: Ra 0.4 µm. Precision linear bushing surface: Ra 0.2 µm. Mirror finish for low-friction or vacuum applications: Ra 0.1 µm. All measured by contact profilometer and reported on the inspection certificate.

Yes — end-machined features such as flats, threaded ends, keyways, cross-holes, and tapered shoulders for clamp mounting can all be added in the same 5-axis setup. Concentricity between integrated features and the ground sliding length stays within 0.003 mm TIR.

Maximum length between centres: 1,500 mm. For shafts over 600 mm, straightness correction after heat treatment is performed before final grinding to keep ≤ 0.005 mm/300 mm. For shafts over 1,000 mm, specify the full-length straightness limit (typically ≤ 0.02 mm over the full length) on your drawing — long shafts are inspected on a granite surface plate with dial indicator in addition to the V-block CMM check.